Corrosion Chemistry PDF
Corrosion Chemistry PDF
Corrosion Chemistry PDF
George R . B r u b a k e r ,
EDITOR
EDITOR
Corporation
89
AMERICAN
CHEMICAL
SOCIETY
WASHINGTON, D. C. 1979
620.1'6'23
ASCMC 8
78-25554
89 1-424 1979
Copyright 1979
American Chemical Society
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PRINTED IN THE UNITED STATES OF AMERICA
Advisory Board
Kenneth B. Bischoff
James P. Lodge
Donald G . Crosby
John L. Margrave
Robert E. Feeney
Leon Petrakis
Jeremiah P. Freeman
F. Sherwood Rowland
E. Desmond Goddard
Alan C. Sartorelli
Jack Halpern
Raymond B. Seymour
Robert A. Hofstader
Aaron Wold
Gunter Zweig
FOREWORD
The A C S
S Y M P O S I U M
SERIES
w a s f o u n d e d i n 1 9 7 4 to p r o v i d e
C H E M I S T R Y
SERIES
p a r a l l e l s that of the c o n t i n u i n g
SERIES
The
A D V A N C E S
except t h a t i n o r d e r to save t i m e t h e
A s a further
Papers p u b l i s h e d i n the A C S
S Y M P O S I U M
SERIES
a r e o r i g i n a l c o n t r i b u t i o n s not p u b l i s h e d elsewhere i n w h o l e o r
m a j o r p a r t a n d i n c l u d e reports of r e s e a r c h as w e l l as r e v i e w s
since s y m p o s i a m a y e m b r a c e b o t h types of p r e s e n t a t i o n .
PREFACE
his v o l u m e records the p r o c e e d i n g s of a t w e l v e l e c t u r e short c o u r s e
j o i n t l y s p o n s o r e d b y t h e C h i c a g o Sections of t h e A m e r i c a n C h e m i c a l
S o c i e t y a n d t h e E l e c t r o c h e m i c a l Society.
T h e lectures c o v e r e d s e v e r a l
aspects of c u r r e n t w o r k i n c o r r o s i o n ; t h e y w e r e a d d r e s s e d to p h y s i c a l
chemists a n d c h e m i c a l engineers as w e l l as t o c o r r o s i o n specialists a n d
surface scientists. R e a d e r s f r o m e a c h of these d i s c i p l i n e s w i l l find s e v e r a l
p a r t s of t h e w o r k d e s c r i b e d h e r e c h a l l e n g i n g a n d i n f o r m a t i v e .
T h e s t u d y of c o r r o s i o n has u n d e r g o n e s u b s t a n t i a l e x p a n s i o n i n t h e
p a s t d e c a d e i n response t o n e w t e c h n i q u e s of s t u d y a n d to d e m a n d s f o r
n e w m a t e r i a l s to w i t h s t a n
expanded applications o
ods f o r more-or-less r o u t i n e surface analysis. E x a m p l e s i n w h i c h c o n s i d e r a t i o n of c o r r o s i o n has h a d a m a j o r i m p a c t o n the d e v e l o p m e n t of n e w
m a t e r i a l s i n c l u d e h i g h t e m p e r a t u r e t u r b i n e s , a i r f r a m e structures, e l e c t r o n i c a p p l i c a t i o n s of t h i n m e t a l films, a n d e n e r g y d e v e l o p m e n t p r o g r a m s
s u c h as c o a l gasification. T h i s v o l u m e c o n t a i n s chapters r e l a t e d to a p p l i cations a n d r e v i e w s of f u n d a m e n t a l research. T h e i m p a c t of n e w e l e c t r o c h e m i c a l a n d surface c h a r a c t e r i z a t i o n m e t h o d o l o g y is seen t h r o u g h o u t
t h e text.
Sections o n e l e c t r o c h e m i s t r y a n d h i g h t e m p e r a t u r e c o r r o s i o n
This
G E O R G E
BRUBAKER
Chicago, Illinois
IBM
Corporation
B E V E R L E Y PHIPPS
S a n Jose, C a l i f o r n i a
S e p t e m b e r , 1978
vii
0-8412-0471-3/79/47-089-001$08.50/0
1979 American Chemical Society
CORROSION
CHEMISTRY
1.
Corrosion:
Problem
of
Materials
Science
s e r i o u s l y d i s a p p o i n t e d i n t h e v a l u e r e c e i v e d ? We c a n ' t
s e l l many g o l d - p l a t e d C a d i l l a c s . " Or t h e customer asks
how can I c h e a p l y save money i n t h e l o n g r u n w i t h o u t
paying the f a c t o r y ?
Ziebart i t !
W e l l , enough c o m m e r c i a l s - who i s t h i s c o r r o s i o n
e n g i n e e r and what i s he l i k e ? How many y e a r s has he
spent i n t h e i v o r y t o w e r s ?
Or has he e v e r v i s i t e d them?
W e l l , i f I had my d r u t h e r s , I would see t o i t t h a t
t h e c o r r o s i o n e n g i n e e r had a b i t o f f l a v o r i n g from
m a t e r i a l s s c i e n c e as shown i n F i g u r e 1. I n t h e s e days
of s p e c i a l i z a t i o n , i t would be n i c e t o t r a i n some generalists.
As an e x p e r t r e s p o n s i b l e f o r t h e f u t u r e
b e h a v i o r s o f most o f o u r s o l i d m a t e r i a l s , s u r e l y a
mastery o f t h e f u l l range c o v e r e d by c h e m i s t r y , p h y s i c s
m e t a l l u r g y , and j u s t a few o t h e r i n c i d e n t a l f i e l d s
would be c o n s i s t e n t w i t h a s s i g n e d r e s p o n s i b i l i t i e s . I f
he f e e l s a b i t s h o r t
t h e c o n s u l t a n t s and
ive t e c h n i c a l s o c i e t y .
What, you t h i n k t h e s u r v e y i s f a c e t i o u s and o u t o f
p l a c e ? L e t ' s t a k e a l o o k a t a c o r r o s i o n smorgasbord o f
t e r m i n o l o g y i n F i g u r e 2. The e x p e r i e n c e s o f a c o r r o
s i o n e n g i n e e r a r e r e p r e s e n t e d by t h e t e r m a r r a y . Sampl
ing
them a t random, i t seems t o be a j u m b l e .
However,
i t i s an o v e r v i e w .
Who, t h e n , i s t h e c o r r o s i o n e n g i n e e r ? F o r many
h e r e , he has an e x c i t i n g , r e s p o n s i b l e way o f d o i n g
complex m a t e r i a l s s c i e n c e i n s e r v i c e t o our s o c i e t y .
T h i s i n d i v i d u a l may n e v e r have d e l i b e r a t e l y p l a n n e d t h e
career which developed.
R a t h e r as a c i v i l e n g i n e e r , a
m e c h a n i c a l e n g i n e e r , a c h e m i s t , a p h y s i c i s t , an a n a l y s t ,
one day s u d d e n l y he has a r e s p o n s i b i l i t y t h r u s t on him
r e l a t i n g to materials r e l i a b i l i t y , to materials perfor
mance o r j u s t t o s o l v e t h a t p r o b l e m b e f o r e we a r e b r o k e .
Thus, many r o u t e s l e a d t o a c o r r o s i o n e n g i n e e r ' s
r e s p o n s i b i l i t y as i s r e f l e c t e d i n F i g u r e 3 .
He c o n
t r i b u t e s t o A P r i o r i P r o b l e m S o l v i n g b u t economics o f
f i r s t c o s t s f r e q u e n t l y l e a d t o P o s t - F a c t u m Problem
S o l v i n g . For, too o f t e n , c o r r o s i o n i s t i e d d i r e c t l y
t o systems' weakest l i n k v a r i a b l e s . Much o f t h e needed
knowledge i s absorbed a l m o s t by an osmosis p r o c e s s
because t h e r e j u s t a r e n ' t s i m p l e v a l i d d e s c r i p t i o n s o f
g e n e r a l use. A l l q u i c k l y become s p e c i f i c .
L a d i e s and gentlemen - i f you were t o examine t h e
agenda f o r t h e n e x t few weeks, q u i c k l y a r e c o g n i t i o n
w o u l d d e v e l o p t h a t t h e key-words have t o u c h e d on t h e
program e x p o s u r e s each w i l l e x p e r i e n c e - can you see
t h e beauty o f t h e t o u g h e s t m a t e r i a l s s c i e n c e problems
emerging t h r o u g h t h e program?
Chemistry
Physics
Consultants
Metallurgy
Ceramics
ENGINEER
CORROSION
Polymer
Figure 1.
Corrosion engineer
Thermo
Analysis
Design
Colleagues
Experimental
Instrumentation
Instrumental
Irreversible
Quantum Mechanics
Mechanics
Chemistry
Statistical
Electronics
S t a t i s t i c a l A n a l y s i s o f Experiments
Semiconductor P h y s i c s
D i f f u s i o n Transport
Hydrodynamics = L-G-S
Civil
Mechanical )
' Engineering
Electrical '
Chemistry
Thermodynamics
Analytical
S o l i d State
Electrochemistry
Physical
Water
Soil
i a l
Hot
Corrosion
Attack
- Cracking
Cruds
Flaking - Foliation
Layer
Crystalline
Granular
Corrosion
Inter
Trans
Stress Corrosion
Stress
Fatigue
Cracking
Inter
Trans
Oxidation
Dissolution
Salt
Atmospheric
Spray
Molten
L i q u i d Metal
Fuels
Salt
Action
Precipitation
Local C e l l
Chemical A c t i o n
Condensates
Action
CORROSION
SMORGASBORD
Electrochemical
Figure 2.
Gases
Steam
Brines
F r e s h Water
Aerated
inorganic
Organic
Pipelines
Attack
Attack
Pitting
Crevicing
Intergranular
Intercrystalline
Grooving
General
Local
Inhibition
Corrosion Potentials
Equilibrium Potentials
Mixed P o t e n t i a l s
Confused P o t e n t i a l s
A c t i v e Surface
Passive Surface
Activation
Passivation
CORROSION
C H E M I S T R Y
Aesthetics
Environments
Economics
Performance
THE PROBLEM
A P r i o r i Materials Selection
C o r r o s i o n and Weakest L i n k
Figure 3.
Variables
1.
TUOMi
Corrosion:
Problem
of
Materials
Science
L e t us l o o k a t a n o t h e r p a i r o f smorgasbord f i g u r e s
on c o r r o s i o n . The f i r s t , F i g u r e 4, f o c u s s e s l a r g e l y on ,
s u r f a c e phenomena from a p h y s i c a l c h e m i s t - e l e c t r o c h e m i s t
o r s u r f a c e c h e m i s t c o n t e x t . Do t h e words have a f a m i l i a r meaning? S h o u l d I d e f i n e a few o r perhaps we
should look at another, Figure 5 , which o u t l i n e s
a n o t h e r s e t o f , p r o b a b l y l e s s f a m i l i a r t o most, key
words. These are r e l a t e d t o t h e d e s c r i p t i o n o f s o l i d
s t a t e s t r u c t u r e s w h i c h a r e e n c o u n t e r e d on t h e s o l i d
side of the e l e c t r i c a l double l a y e r .
I t i s the d i s t u r b i n g phenomena p r o c e e d i n g w i t h i n t h e s o l i d s u r f a c e
s i d e t h a t g i v e s r i s e t o the d i v e r s e experiences of the
c o r r o s i o n engineer.
F o r an o v e r v i e w we c l e a r l y c o u l d spend much t i m e
on each word, l e a r n i n g what meaning was e x p e r i e n c e d by
each o f us.
Each would be s u r p r i s e d what a range
c o u l d be r e v e a l e d b
much o f t h e managemen
i n c l i n e d t o say, please p r a c t i c e your a r t w i t h o u t
f u l l y r e c o g n i z i n g o r e x p e r i e n c i n g t h e complex b e a u t y
present.
F o r i t i s o n l y a r t t o t h e b e h o l d e r - l e t us
continue s k e t c h i n g the canvas.
D i d you e v e r s t o p t o c o n t r a s t t h e c o n c e p t u a l i z a t i o n o f c h e m i s t s common models, o r f o r t h a t m a t t e r ,
most d e s c r i p t i o n s o f s o l i d m a t e r i a l b e h a v i o r s ?
In
F i g u r e 6 we are e x a m i n i n g the p h e n o m e n o l o g i c a l s i t u a t i o n o v e r w h i c h we spend so much t i m e p u z z l i n g .
The
m e n t a l images u s u a l l y do n o t a s s i g n d e t a i l e d s t r u c t u r e
t o t h e s o l i d phase. We can c l e a r l y see i t b e f o r e us
- i t i s so o b v i o u s .
The model we a r e u s u a l l y s p e a k i n g about i s shown
i n t h e lower l e f t c o r n e r - a s t r u c t u r e l e s s s o l i d
b e h a v i n g i n an e n v i r o n m e n t o f l i q u i d , o r gas, more
r a r e l y an i m p e r f e c t , impure vacuum. The l a t t e r e n v i ronment has t h e s p a t i a l e x t e n t .
More t r u l y as i n t h e l o w e r r i g h t c o r n e r t h e s o l i d
i s d e f i n e d by x-y-z c o o r d i n a t e s w h i c h d e f i n e what i s
a c t u a l l y happening where as a f u n c t i o n o f time i n an
experiment.
S t i l l more p r e c i s e l y , t h e s o l i d i n t h e e x p e r i m e n t
i s b e i n g space averaged t o produce the d a t a i n t h e
f i g u r e s above.
The s o l i d i s b e s t d e s c r i b e d g e n e r a l l y f o r t h e
c o r r o s i o n e n g i n e e r ' s p u r p o s e s as a
" p o l y p h a s e inhomogeneous s o l i d c o n t a i n i n g
h e t e r o g e n e i t y homogeneously d i s p e r s e d . "
The d e s c r i p t i o n i s r a t h e r p r e c i s e a c c o r d i n g t o Webster s
U n a b r i d g e d D i c t i o n a r y . T h i s r e f e r e n c e can be c i t e d i f
the statement r e q u i r e s s u b s t a n t i a t i o n .
Goodness, w i t h t h i s c o m p l e x i t y where do we go from
1
Ionic
Polarization
Fields
Fields
Kinetic
Double Layer
Planes
Potential
Ideal
Phase)
CORROSION
Content
Energy
Potentials
Loss
Open-Circuit Potentials
Short-Circuit
Potentials
Cell Potentials
Over P o t e n t i a l s
Anodic-Cathodic P o t e n t i a l s
Mixed P o t e n t i a l s
I R
Polarization Potentials
Activation Polarization
Concentration P o l a r i z a t i o n
Electrode
Entropy
Heat
Free
Oxidation-Reduction
Thermodynamic
Exchange C u r r e n t s
Potential
Electrochemical
Figure 4.
Polarizable
Non-Faradaic
Faradaic Processes
Constant
Constant C u r r e n t
Potential Transients
Diffuse
The Helmholtz
E l e c t r i c Dipole F i e l d s
Atomic
(Liquid
Model Components
oo
Phase
Solutions
Bands
Figure 5.
Impurity
Potential
Bands
Bands
Dislocations
Fermi L e v e l s
Work F u n c t i o n s
Surface P o t e n t i a l s
Conduction
States
Defects
E l e c t r o n s and Holes
Surface
Surface
Charge T r a n s f e r
Catalysis
-Metal
Insulator-Semiconductor
Imperfections
Charge T r a n s p o r t
F l a t Band
Band S t r u c t u r e
CORROSION
Space Charge P o t e n t i a l
Valence
States
States
States
Trapping
Impurity
Dopant
Ionic-Covalent
Electronegativity
Solid Solutions
Metastable
Inhomogeneous-HeterogeneousHeterogeneity
Solid
Zeolite to Perfect C r y s t a l -
Associated
Imperfections
- Interstitials
Mass T r a n s p o r t
Vacancies
10
CORROSION
Figure 6.
CHEMISTRY
1.
Corrosion:
Problem
of
Materials
11
Science
h e r e ? W e l l , l e t s j u s t acknowledge t h a t t h i s has
a l w a y s been t h e s t a g e on w h i c h most o f us a r e a c t i n g
out our p r o f e s s i o n a l l i v e s .
Thus, i n F i g u r e 7, S o l i d s a r e d e s c r i b e d as b e i n g
i d e a l l y p e r f e c t v e r s u s i d e a l l y i m p e r f e c t w i t h most
being i n the l a t t e r category.
A sharp consequence was
t a u g h t t o me many y e a r s ago by L e r o y Dunham o f t h e
E d i s o n P r i m a r y B a t t e r y . When I s m a r t l y d e s c r i b e d a
s t a t i s t i c a l quantum-mechanics model o f c h a r g e t r a n s f e r
c a t a l y s i s o f oxygen r e d u c t i o n on an e l e c t r o d e s u r f a c e ,
he s m i l e d as i f he was e n j o y i n g h i m s e l f . He had been
f o r c e d t o l e a v e s c h o o l a t t h e e i g h t h grade l e v e l . ,
L a t e r i n t h e day as he and I s t r o l l e d t h r o u g h the
p r o d u c t i o n p l a n t making h i s e l e c t r o d e s , he q u i e t l y
s a i d , "Don, I e n j o y e d y o u r comments so v e r y much.
Back i n t h e '30*s when I was d o i n g the development work
many c o n f u s i n g r e s u l t
e v e n i n g s I have e n j o y e
B r i t a n n i c a . One day i t dawned on me t h a t the d i s t r i
b u t i o n of people i n the United S t a t e s versus other
a r e a s r e s e m b l e d t h e charge t r a n s f e r b e h a v i o r .
To
change t h i n g s t h e p e o p l e must move a r o u n d , get i n t o
and o u t o f c a r s , t r a i n s , p l a n e s .
What a memorable moment - t h e t h e o r y d i d not have
t o be a b s o l u t e l y r i g h t t o be u s e f u l i n g u i d i n g e x p e r
iments.
Why have I t a k e n such a c i r c u m s p e c t p a t h t o r e a c h
t h i s point? W e l l , I wish to present a c o n c e p t u a l i z a
t i o n o f t h e p r o b l e m s w h i c h has been u s e f u l t o me.
It
has h e l p e d t o b r i d g e t h e chasm s e p a r a t i n g d i f f e r e n t
experimental s i t u a t i o n s .
The f o l l o w i n g F i g u r e 8 i n d i c a t e s t h a t the l e c t u r e s
c o u l d be p l a c e d i n a v a r i e t y o f c a t e g o r i e s . F i r s t , the
f a m i l i a r m e t a l - e l e c t r o l y t e system w i t h gas e v o l u t i o n
p o t e n t i a l l y present.
Second, t h e systems i n v o l v i n g
m e t a l s s e p a r a t e d from the l i q u i d by a c o v e r i n g f i l m
s t r u c t u r e , and t h i r d t h e m e t a l s e p a r a t e d from a gas by
an i n t e r m e d i a t e phase l a y e r . There i s some a m b i g u i t y
e v i d e n t i n t h i s d e s c r i p t i o n ; however, i n t h e n e x t few
m i n u t e s p e r h a p s some new p e r s p e c t i v e s f o r t h o u g h t
o r g a n i z a t i o n can emerge.
I n a more g e n e r a l sense s u r f a c e c o r r o s i o n p r o
c e s s e s can be d e s c r i b e d i n terms o f the phases p r e s e n t
as shown i n F i g u r e 9.
Here t h e s o l i d i s n o t i d e n t i f i e d
s i m p l y as a m e t a l but a more g e n e r a l term S i s b e i n g
used w i t h L r e p r e s e n t i n g l i q u i d phase and G gas phase.
The systems l o c a l l y p r e s e n t can be r e p r e s e n t e d by t h e
n o t a t i o n s S - L , S - L - G , S-G or, i f a c o v e r i n g c h e m i c a l l y
a l t e r e d l a y e r e x i s t s on t h e s u r f a c e , then S i - S i s
present w i t h S i the s t a r t i n g m a t e r i a l .
2
12
CORROSION
CHEMISTRY
Solids
Ideally Perfect
R e a l i t y + I d e a l l y Imperfect
THEORY :
i f i t leads t o experiments
does n o t need t o be
RIGHT
t o be v e r y v a l u a b l e .
Figure 7.
Modeling
TUOMi
Corrosion:
Problem
of Materials
Science
Metal-Electrolyte-Gas
Metal-Solid-Liquid
Corrosion E l e c t r o chemistry
Iron D i s s o l u t i o n
Mechanisms
Corrosion I n h i b i t i o n
and I n h i b i t o r s
Stress Corrosion
Cracking
Solid Electrolyte-Ionic
E l e c t r o n i c Transport
Iron Passivation
Valve Metals D i e l e c t r i c
Layers
Treatments
LECTURE SERIES
CORROSION
Metal-Solid-Gas
High-Temperature
Corrosion
Low-Temperature
Atmospheric
Corrosion
C o r r o s i o n Phenomena
N o v e l Energy
Conversion Processes
Iron P a s s i v a t i o n
Figure 8.
14
CORROSION
CHEMISTRY
Pb,
+ SO*
{ S )
PbS0iw
+ 2e
+ 0.335
( S }
Cathodic
Positive:
Pb0 , x + 4H
2
Pb
( s )
+Pb0
_
+ SO * + 2e
2 ( s )
+ 2 50>
2
Cathodic
?
PbSO* + 2 H 0
Anodic
2
PbSO^
( g )
+ 2H 0
2
1.685
^ 2
1.
Corrosion:
Problem
of Materials
15
Science
Solid-Liquid-Gas
S-L-G
S o l i d Gas
SG
S i ~ S 2 """ L
Si
Figure 9.
Corrosion system
Pb0
+ 2H + H SO + 2e
2 ( s )
Positive
Pb0
2 ( s )
+2H +H SO
2
PbSOu
Potential
E
{s)
PbSOu
Anodic
l(
+2e -
+ 2 H 0
2
(s)
+ 2e - P b
( s )
+ SO,
Negative
Pb
( s )
S 0 ; - PbSO,
(s)
log i
Figure 10.
reactions
+ 2e
16
CORROSION
CHEMISTRY
I f the two e l e c t r o d e s a r e s h o r t - c i r c u i t e d t o g e t h e r ,
t h e c a t h o d i c p r o c e s s o f t h e p o s i t i v e combines w i t h t h e
a n o d i c p r o c e s s o f the n e g a t i v e as shown i n F i g u r e 11.
The b a t t e r y now has a s i n g u l a r p o t e n t i a l - t h e s h o r t
circuited potential.
T h i s c l e a r l y i s a mixed p o t e n t i a l
o r c o u l d be v i e w e d as a c o r r o s i o n p o t e n t i a l o f the
system.
L o o k i n g a t F i g u r e 12, a s k e t c h o f a s i n g l e c e l l
shows t h a t t h e p o s i t i v e i s a p a s t e o f P b 0 i n a l e a d
grid.
T h i s e l e c t r o d e i n t h e e l e c t r o l y t e c l e a r l y must
have a mixed p o t e n t i a l s i n c e m e t a l l i c l e a d i n t h e
p r e s e n c e o f l e a d d i o x i d e and s u l f u r i c a c i d i s thermodynamically unstable.
At the n e g a t i v e the l e a d g r i d
contains pasted l e a d ! This i s a l s o unstable w i t h
respect to charging the b a t t e r y . Both e l e c t r o d e s
represent a c o r r o s i o n c o n t r o l problem!
Really, this i
the e l e c t r o d e s are f u l l
i n c r e a s e d so oxygen and hydrogen a r e e v o l v e d a t t h e
p o s i t i v e and n e g a t i v e , r e s p e c t i v e l y . T h i s i s i n d i
c a t e d by t h e p o t e n t i a l - l o g c u r r e n t l i n e s i n F i g u r e 13.
These r e a c t i o n s c o n v e r t t h e b a t t e r y i n t o a p o t e n t i a l
e x p l o s i v e by t h e H2-O2 r e c o m b i n a t i o n t o form w a t e r
i f a match i s used t o a i d i n s e e i n g how much w a t e r
needs t o be added. I f t h e 0 r e d u c t i o n c o u l d be made
t o p r o c e e d on t h e n e g a t i v e s u r f a c e as a l o c a l c o r r o
s i o n c u r r e n t , and i f t h e H c o u l d be made t o undergo
o x i d a t i o n on t h e p o s i t i v e s u r f a c e , a h e r m e t i c a l l y
s e a l e d c e l l c o u l d be made. These e l e c t r o c h e m i c a l
r e a c t i o n s , however, r e p r e s e n t forms o f l o c a l c e l l
r e a c t i o n s f a m i l i a r to c o r r o d i n g systems.
As any o f t h e s e r e a c t i o n s p r o c e e d a t t h e e l e c t r o d e
surface, the surface chemistry i s continuously changing.
The c o m p o s i t i o n o f t h e l o c a l e l e c t r o l y t e i n c o n t a c t
w i t h t h e s o l i d s i s c h a n g i n g . Thus, t h e s i m p l e chem
i s t r y i m p l i e d by t h e r e a c t i o n
2
Pb + P b 0
+ 2 H S0i> + PbSOi* + 2 0
2
r e a l l y does n o t e x i s t .
There a r e numerous changes o f
a complex form p r o c e e d i n g on t h e m e t a l s u r f a c e s , on
t h e m e t a l l i c h e a v i l y - d o p e d o x i d e P b 0 , on t h e PbSOi+
c r y s t a l l i t e s , on t h e Pb p a r t i c u l a t e s i n t h e n e g a t i v e
g r i d , as w e l l as i n systems o f g r a i n b o u n d a r i e s w h i c h
are present.
S u d d e n l y , as we p e e r beyond t h e c a s u a l s o l u t i o n
c h e m i s t r y e q u a t i o n , r a t h e r complex s o l i d s t a t e c h e m i s
t r y i s i n t e r a c t i n g w i t h s o l u t i o n and gas phases
processes.
2
Corrosion:
Figure 11.
Problem
of Materials
Science
8C
18
CORROSION
CHEMISTRY
LOAD
Pasted
Pb
Pasted
Pb0
|charge y-
dischargj
Pb
Grids
H0
2
Figure 12.
V r
H 0
2 ( g )
Mg)
V)
+ 2 H
+2H + 2e+H
l
2e
Positive -
Mg) ^
(t)
Negative -
H2
* (g)
log i
Figure 13.
1.
Corrosion:
Problem
of Materials
Science
19
C l e a r l y t h e system has e l e c t r o c h e m i c a l - c h e m i c a l
exchange c u r r e n t s a t t h e p o s i t i v e s and n e g a t i v e s w h i c h
d e f i n e t h e p o t e n t i a l s o b s e r v e d . The m e t a l g r i d must
show c o m b i n a t i o n s o f PbSOt* f i l m , P b 0 f i l m , as w e l l as
h y d r i d e f i l m phenomena. I s n ' t c o r r o s i o n c h e m i s t r y
e x c i t i n g i n t h a t b l a c k box under t h e hood?
The b a t t e r y e l e c t r o d e mixed p o t e n t i a l b r i n g s a t
t e n t i o n t o the c o r r o s i o n engineer's problem o f c o n t r o l
l i n g e i t h e r the cathodic or anodic process t o minimize
the c o r r o s i o n c u r r e n t . The problems o f s u r f a c e p a s s i
v a t i o n , the question of i d e n t i f y i n g the d i s t i n c t i v e
s p a t i a l l o c a t i o n s of the r e a c t i o n processes are
f r e q u e n t l y p r e s e n t i n p r a c t i c a l s i t u a t i o n s - what i s
c a t h o d i c t o an a n o d i c r e g i o n o r v i c e v e r s a , what a r e
u s e f u l ways t o m o d i f y t h e s u r f a c e p r o c e s s e s ?
Thus, when a t t e n t i o n i s g i v e n t o d e t a i l what
appeared as a s i m p l
l i t t l e t h o u g h t becom
F o r a few moments l e t ' s d r o p t h e key words and
turn t o c r e a t i n g a skeleton f o r the c o r r o s i o n engi
neer's interphase t r a n s p o r t processes.
We need a l l t h e
tools possible t o bring the materials science r e
s o u r c e s t o o u r a i d . How can we e x p r e s s q u e s t i o n s so
a i d can come from o t h e r e x p e r t s ?
2
Part I I
T h i s phase o f t h e d i s c u s s i o n i s c o n c e r n e d w i t h
concepts o f s o l i d s t a t e chemistry r a t h e r than a
d e t a i l e d a n a l y s i s o f a p a r t i c u l a r case.
The o b j e c t i v e
i s t o b r i n g a t t e n t i o n t o a v a r i e t y o f s u r f a c e chemis
t r y p e r s p e c t i v e s . T h i s v a r i e t y c a n be h e l p f u l because
t h e number o f ways we can scavenge i n f o r m a t i o n from
r e l a t e d m a t e r i a l s s c i e n c e a r e a s becomes expanded.
F u r t h e r , a v a r i e t y o f s e e m i n g l y u n c o n n e c t e d phenomena
can be b r o u g h t i n t o r e l a t e d b a l l g a m e s . The m u l t i d i s c i p l i n a r y character of r e a l material science i si t s
real richness.
E a r l i e r a t t e n t i o n was b r o u g h t t o t h e c o r r o s i o n
p r o c e s s e s o f l i q u i d - s o l i d systems. L e t ' s s t a r t w i t h a
m e t a l c o n t a c t i n g an e l e c t r o l y t e as do a l l e l e c t r o
chemistry t e x t s .
The s i m p l e s t model o f t h e e l e c t r i c a l d o u b l e l a y e r
between a m e t a l and an e l e c t r o l y t e i s t h e s i m p l e c a p a
c i t o r v i s u a l i z e d by Helmholtz- as shown i n F i g u r e 14.
The d i f f u s e i o n d i s t r i b u t i o n i n t h e l i q u i d phase was
r e c o g n i z e d by Gouy and Chapman^-' - t o form a space
charge r e g i o n a d j a c e n t t o t h e e l e c t r o d e s u r f a c e .
S t e r n ^ i n 1924 combined t h e s t r u c t u r e s t o form a
compact d o u b l e l a y e r a t t h e e l e c t r o d e s u r f a c e w i t h a
1
20
CORROSION
HELMHOLTZ
1879
fee
f
i'
\Helmholtz
Plane
. STERN (1924)
i l '
STERN-GRAHAME
imD
+4
Helmholtz Plane
Figure 14.
GOUY CHAPMAN
1910-1913
CHEMISTRY
1.
Corrosion:
Problem
of
Materials
21
Science
d i f f u s e space c h a r g e .
I n 1947 D a v i d Grahame added t o
t h i s t h e s p e c i f i c a d s o r p t i o n o f i o n s f o r m i n g an i n n e r
and o u t e r H e l m h o l t z p l a n e w i t h h y d r a t e d c a t i o n s not
a p p r o a c h i n g as c l o s e l y as t h e a n i o n s a t t h e i d e a l
metal substrate surface. In t h i s i d e a l p o l a r i z a b l e
e l e c t r o d e model p e r s p e c t i v e , no charge t r a n s f e r o c c u r s
between m e t a l and l i q u i d phase - i . e . , no F a r a d a i c
processes are present. I t i s important t o v i s u a l i z e
t h i s s u r f a c e r e g i o n as formed o f atoms, m o l e c u l e s , and
ions having s i g n i f i c a n t thermal v i b r a t i o n a l , r o t a t i o n a l
and t r a n s l a t i o n a l e n e r g y .
The s k e t c h e d s t r u c t u r e s a r e
f o r o n l y a moment i n t i m e b u t a r e a v e r a g i n g o v e r space
a d j a c e n t t o t h e e l e c t r o d e s u r f a c e when measurements
are b e i n g made.
A subsequent d e s c r i p t i o n by B o c k r i s and a s s o c i ates- - drew a t t e n t i o n t o f u r t h e r c o m p l e x i t i e s as shown
i n F i g u r e 15. The m e t a
combinations of o r i e n t e
s p e c i f i c a l l y adsorbed a n i o n s , f o l l o w e d by s e c o n d a r y
water d i p o l e s along w i t h the hydrated c a t i o n s t r u c t u r e s .
T h i s model s e r v e s t o b r i n g a t t e n t i o n t o t h e dynamic
s i t u a t i o n i n w h i c h changes i n p o t e n t i a l i n v o l v e
s e q u e n t i a l as w e l l as s i m u l t a n e o u s r e s p o n s e s o f molec
u l a r and a t o m i c systems a t and near an e l e c t r o d e s u r
f a c e . Changes i n p o t e n t i a l d i s t r i b u t i o n i n v o l v e i n t e r
a c t i o n s e x t e n d i n g from atom p o l a r i z a b i l i t y , t h r o u g h
d i p o l e o r i e n t a t i o n , t o i o n movements. The e l e c t r i c a l
f i e l d e f f e c t s a r e complex i n t h i s i d e a l p o l a r i z e d
e l e c t r o d e model.
The models c l e a r l y have n o t a s s i g n e d any a t o m i c
s t r u c t u r e t o the metal s i d e . With a m e t a l l i c s u b s t r a t e
R i c e , i n 1 9 2 8 , s h o w e d the e l e c t r i c f i e l d p e n e t r a t i o n
was i n d e e d s l i g h t .
C o n s e q u e n t l y , t h i s model was ade
quate f o r t h e i d e a l p o l a r i z a b l e e l e c t r o d e w i t h o u t
F a r a d a i c charge t r a n s f e r .
A f u r t h e r c o m p l i c a t i o n i s i n t r o d u c e d i n F i g u r e 16
where t h e p r e s e n c e o f s u r f a c e adatoms i s i n d i c a t e d as
w e l l as m e t a l l a t t i c e v a c a n c i e s i n t h e s u b s t r a t e
s u r f a c e . W i t h i n t h e system a t t e n t i o n now can be drawn
to F a r a d a i c processes i n v o l v i n g the s u b s t r a t e s t r u c t u r e .
The t r a n s f e r o f an atom t o t h e s u r f a c e can be e x p r e s s e d
by the e q u a t i o n
2
M/D
+ D
/ n
where M/Qs/
the s u r f a c e adatom, i s f u r t h e r p o t e n t i a l l y
i n v o l v e d i n t h e exchange p r o c e s s
M
/Q
+
s
M +
(H 0)
2
+ e +
22
CORROSION
Figure 16.
CHEMISTRY
1.
TUOMi
Corrosion:
Problem
of Materials
Science
23
T h i s s u g g e s t s t h a t a t t e n t i o n needs t o be g i v e n t o
i n t e r p h a s e exchange c u r r e n t s w h i c h i n v o l v e n o t o n l y
e l e c t r o n t r a n s f e r from t h e m e t a l s u r f a c e , b u t may
i n v o l v e atom exchanges from t h e b u l k t o t h e s u r f a c e
r e g i o n . The p l a n e m e t a l s u r f a c e a s s o c i a t e d w i t h
s t u d y i n g t h e model i d e a l p o l a r i z e d e l e c t r o d e b e h a v i o r
now becomes p a r t o f an i n t e r p h a s e system s e p a r a t i n g
m e t a l from an e l e c t r o l y t e o r a gas phase system.
The e v o l u t i o n o f s e m i c o n d u c t o r e l e c t r o n i c s depen
ded upon d e v e l o p i n g a d e t a i l e d m a t e r i a l s s c i e n c e ,
f i r s t f o r germanium and t h e n f o r s i l i c o n .
This con
fronted the electrochemist w i t h a f u r t h e r refinement
o f t h e e l e c t r i c a l d o u b l e l a y e r systems model. A t t h e
surface of a s i l i c o n semiconductor c r y s t a l the e l e c
t r o n i c p r o c e s s now i s no l o n g e r s i m p l y an e l e c t r o n
t r a n s f e r from a m e t a l b u t i n v o l v e s two d i s t i n c t
reactants, electron
r e a c t i o n s are not equivalen
semiconductor, S :
1Si
S + OH"
>
S0H + e
24
CORROSION
CHEMISTRY
i n t e r s t i t i a l A and atoms
/,
/
i m p r o p e r s i t e A and atoms
A/G3,
B/CB
l a t t i c e v a c a n c i e s f o r A and s i t e s +1 > FT
a s s o c i a t e d A and v a c a n c i e s
1+1~1
as r e a d i l y d e s c r i b e d by a s i m p l i f i e d symbolism.
In
r e a l systems a t t e n t i o n must be g i v e n t o d e f i n i n g a t
l e a s t c o n c e p t u a l l y t h e n a t u r e o f t h e phases p r e s e n t .
F r e q u e n t l y , under o r d i n a r y c o n d i t i o n s the s o l i d does not
f i t any s i m p l e s t o i c h i o m e t r y o r e l e c t r i c a l n e u t r a l i t y
compound model but i s a s t r u c t u r e u n i q u e t o the a c t u a l
system under s t u d y .
from u n o r t h o d o x approache
I f a t t e n t i o n i s given to the combination of a
m e t a l M becoming c o v e r e d w i t h MX the model as shown i n
F i g u r e 18, s e v e r a l o b s e r v a t i o n s can be made as t o
i n t e r p h a s e systems and t h e growth o f t h e f i l m l a y e r .
At the e x t e r i o r s u r f a c e a p a r t i c u l a r combination of
i n t e r p h a s e exchange c u r r e n t s f o r s o l i d phase growth
can be f o r m u l a t e d depending on t h e d e t a i l e d c h a r a c t e r
of the s o l i d phase, the s u r f a c e s t a t e s , the allowed
e l e c t r o n i c processes.
S i m i l a r l y , a set of interphase
exchange c u r r e n t s can be f o r m u l a t e d a t t h e boundary o f
the m e t a l and t h e MX l a y e r a g a i n s u b j e c t t o c o n s t r a i n t s
d e f i n e d by t h e mass t r a n s p o r t p r o c e s s e s p e r m i s s i b l e i n
t h e c o v e r i n g l a y e r and i n t h e m e t a l .
The s i t u a t i o n i s i l l u s t r a t e d i n g r e a t e r d e t a i l i n
t h e f o l l o w i n g model s i t u a t i o n s where a t t e n t i o n i s g i v e n
t o t h e i n t e r p h a s e boundary exchange c u r r e n t s i n j e c t i n g
the l a t t i c e i m p e r f e c t i o n s which are r e s p o n s i b l e f o r
atom t r a n s p o r t t h r o u g h t h e compound MX A**
Thus i n F i g u r e 19 l a t t i c e v a c a n c y w i t h a t r a p p e d
h o l e i s i n j e c t e d a t t h e compound e l e c t r o l y t e i n t e r f a c e ,
t h e i m p e r f e c t i o n a t t h e metal-compound i n t e r f a c e r e a c t s
t o r e l e a s e a v a c a n c y i n t o the m e t a l .
Alternatively,
i n F i g u r e 20 t h e exchange p r o c e s s i n j e c t s a l a t t i c e
v a c a n c y on t h e c a t i o n l a t t i c e w h i c h a p p e a r s a t t h e
m e t a l compound i n t e r p h a s e t o r e l e a s e an e l e c t r o n and
t r a n s f e r a metal i o n i n t o the s u r f a c e r e g i o n .
T r a n s p o r t models such as t h e s e have been c r e a t e d
t o b r i n g a t t e n t i o n t o t h e p o s s i b i l i t y o f t h e boundary
exchange c u r r e n t s i n j e c t i n g i m p e r f e c t i o n i n t o a
c r y s t a l l i n e phase d u r i n g an a n o d i c p r o c e s s .
In f a c t ,
t h e y may d e t e r m i n e t h e s o l i d phase s t r u c t u r e s formed.
F u r t h e r m o r e , t h e m e t a l w i t h a s o l i d phase c o v e r i n g f i l m
1.
Corrosion:
Problem
of Materials
25
Science
B/n~
Journal of the Electrochemical Society
Figure 17.
ANION VACANCY
METAL ATOM VACANCY
CATION VACANCY
CATION
SURFACE SITE
AO-ATOMSr*y* AD-IONS
M
METAL
ANION
SURFACE SITE
SURFACE
26
CORROSION
CHEMISTRY
Journal of the
Electrochemical Society
Figure 19. Model boundary
exchange currents involving the injec
tion of a lattice vacancy with trapped
hole at compound-electrolyte interface
and vacancy release into the metal with
a hole and metal ion in the compound
(12)
METAL
j
L
. | @ _ fe>
/ p/ j!g
D
|
COMPOUND
|
METAL COMPOUND
(g) I
g'
j __? I
! P/O^" D ; @
N
| @
^g)
_ @ _ j
0~ D * ~ D | i
Journal of the
Electrochemical Society
Figure 20. Model boundary interphase
exchange currents involving httice vacancy injection at the compounds-electrolyte interface and vacancy exchange
into the metal releasing an electron and
transferring a metal ion into the compound hyer (12)
METAL
M
i
COMPOUND
MX
METAL COMPOUND
MX
M
(M)
_jgfc!
^
1.
Corrosion:
Problem
of
Materials
Science
27
can be i n r e v e r s i b l e e q u i l i b r i u m w i t h an e l e c t r o l y t e
w i t h o u t h a v i n g exposed m e t a l n e c e s s a r i l y i n c o n t a c t
w i t h an e l e c t r o l y t e . I n such a model, an i n c r e a s e i n
t h e d r i v i n g c u r r e n t s may i n c r e a s e the mass t r a n s p o r t
r a t e s ( a l t e r s o l i d phase s t r u c t u r e s ) w i t h i n l i m i t s so
t h e r a p i d phase t r a n s f o r m a t i o n s (M t o MX) can o c c u r
w i t h o u t n e c e s s a r i l y i n v o l v i n g i o n - s o l u t i o n and p r e c i p
i t a t i o n r e a c t i o n s i n t h e o r d i n a r y sense.
T h i s d i s c u s s i o n has b r o u g h t a t t e n t i o n t o t h e p o t
e n t i a l p r e s e n c e o f two c l a s s e s o f i n t e r p h a s e t r a n s p o r t
c o n d i t i o n s i n s o l i d s t a t e systems. L e t us examine
t h e s e f o r a moment f r o m t h e p e r s p e c t i v e o f f o r m i n g
s o l i d phases under model c o n d i t i o n s .
F o r t h e s i n g l e i n t e r p h a s e exchange p r o c e s s o f
s o l i d phase P2 i n t e r a c t i o n s w i t h P i shown i n F i g u r e 21,
a s e r i e s o f model c o n d i t i o n s a r e i l l u s t r a t e d
F o r each
a set of interphase
f o r m u l a t e d w h i c h hav
i n t e r p r e t a t i o n s . The f i r s t i s the l o c a l d e p o s i t i o n o f
m e t a l l i c t i t a n i u m by t h e t h e r m a l d e c o m p o s i t i o n o f T i C l i *
gas on a hot w i r e .
T h i s v a p o r p l a t i n g t e c h n i q u e can be
p e r f o r m e d t o grow a v a r i e d c r y s t a l l i t e s t r u c t u r e o f a
r e l a t i v e l y pure t i t a n i u m on t h e h o t f i l a m e n t . More
g e n e r a l l y , v a p o r p l a t i n g t e c h n i q u e s o f many complex
forms a r e used t o grow e p i t a x i a l s i l i c o n l a y e r s o f
c o n t r o l l e d impurity content onto s i l i c o n s u b s t r a t e s .
The c a r e f u l c o n t r o l o f c o n d i t i o n s r e s u l t s i n an amaz
i n g l y homogeneous f i l m growth o f h i g h s e m i c o n d u c t o r
q u a l i t y . The i m p e r f e c t i o n s t r u c t u r e i n a c o m p o s i t i o n
and s t r u c t u r a l sense depends on t e c h n i q u e d e t a i l s .
The second example i s e l e c t r o l y t i c p l a t i n g o f
copper f i l m s .
C o n t r a r y t o some e x p e c t a t i o n s growth
c l o s e t o e q u i l i b r i u m p o t e n t i a l c o n d i t i o n s does n o t
r e s u l t i n the h i g h e s t q u a l i t y d e p o s i t .
I n g e n e r a l , the
c o m p o s i t i o n and s t r u c t u r e o f the d e p o s i t depends on t h e
d e t a i l e d c o m b i n a t i o n o f t r a n s p o r t p r o c e s s e s towards and
away from t h e e l e c t r o d e s u r f a c e . A d d i n g t h e l o c a l
hydrodynamic v a r i a b l e s p r o v i d e s t h e system w i t h an
e x t r e m e l y b r o a d s t r u c t u r a l c h e m i c a l range.
The t h i r d example i n v o l v e s growth o f a c r y s t a l l i n e
s a l t phase from t h e s a t u r a t e d s o l u t i o n . The thermo
dynamic d e s c r i p t i o n a g a i n i s i n a d e q u a t e f o r d e s c r i b i n g
the d e t a i l e d s t a t e s of the s o l i d .
The r e l a t i v e l y
anhydrous c h l o r i d e i o n r e a d i l y d e p o s i t s i n t o the
s u r f a c e l a t t i c e but t h e sodium i o n must be d e h y d r a t e d
t o form N a C l . T h i s means t h a t w a t e r must be d i f f u s e d
away from t h e s u r f a c e d u r i n g s o l i d i f i c a t i o n .
The w h i t e
c a s t o f s a l t , termed v e i l i n g , i n v o l v e s s o l u t i o n i n c o r
p o r a t i o n d u r i n g growth w i t h a subsequent d i f f u s i o n o f
t h e s o l v e n t o u t o f the c r y s t a l .
28
CORROSION
CHEMISTRY
IP
Substrate
Pi
2
Filament
S2
Pi
+ +
Fe
Cu
Seed
Crystal
Na
CI
Seed
Crystal
Ge
Cu
Plating
Solution
Saturated
NaCl
Electrolytic
Plating
Solution
Precipitation
Molten
Ge
Solidification
of Melt
\
Figure 21.
1.
Corrosion:
Problem
of
Materials
Science
29
The f o u r t h example i s t h e c o n t r o l l e d s o l i d i f i c a
t i o n o f germanium (or s i l i c o n ) t o produce s e m i c o n d u c t o r
grade m a t e r i a l s . The s o l i d phase s t r u c t u r e and compo
s i t i o n depend s t r o n g l y upon the i n t e r p h a s e exchange
p r o c e s s e s w h i c h can enhance i n c o r p o r a t i o n i n t o the
s o l i d o r i n t o t h e l i q u i d d e p e n d i n g upon ???
This
example i m p l i c i t l y i n c l u d e s t h e p r o d u c t i o n o f most o f
the m e t a l l i c s t r u c t u r a l m a t e r i a l s .
The m e t a l a l l o y systems g e n e r a l l y have b o t h c o n
t r o l l e d and u n c o n t r o l l e d i m p u r i t i e s b e i n g d e p o s i t e d and
r e d i s t r i b u t e d w i t h i n the s o l i d as a r e s u l t o f complex
l i q u i d hydrodynamic i n t e r p h a s e r e g i o n p r o c e s s e s as w e l l
as s e c o n d a r y s o l i d s t a t e d i f f u s i o n and t r a n s f o r m a t i o n
processes.
This materials science i s f a m i l i a r to a l l
c o r r o s i o n e n g i n e e r s who become i n v o l v e d w i t h c o m m e r c i a l
systems.
The second i n t e r p h a s
t h e dynamic i n t e r a c t i o
phase exchange c u r r e n t s a t t h e s e p a r a t e d b o u n d a r i e s as
i n d i c a t e d i n F i g u r e 22 where a s o l i d phase P i s formed
by i n t e r a c t i o n s i n v o l v i n g the s e p a r a t e d phases P
and
Pi.
The c l a s s i c o x i d a t i o n o f aluminum t o form a p a s s
i v e s u r f a c e i s the f i r s t i l l u s t r a t i o n .
While the
second model i s t h e a n o d i c o x i d a t i o n o f aluminum, the
i n t e r p h a s e t r a n s p o r t phenomena h e r e can be d i s t r i b u t e d
w i t h i n more complex c o n t e x t s as a r e s u l t o f the
boundary l a y e r c o m p o s i t i o n changes i n the e l e c t r o l y t e .
T h i s model i s a p a r t i c u l a r l y f a s c i n a t i n g one because
s e v e r a l decades ago i t was c l e a r t h a t t h e l a r g e
n e g a t i v e l y c h a r g e d oxygen a n i o n c o u l d n o t m i g r a t e i n
such o x i d a t i o n p r o c e s s e s - the m e t a l i o n +3 aluminum
was s m a l l and o f c o u r s e had t o t r a n s p o r t a l l t h e
current through the f i l m .
Famous l a s t words t h a t
c r e a t e d a c r i s i s when good t r a n s p o r t number e x p e r i m e n t s
were p e r f o r m e d w h i c h showed b o t h atoms move i n the
film-forming process
The s i l v e r t a r n i s h i n g r e a c t i o n i n v o l v i n g hydrogen
s u l f i d e i s a c l a s s i c of s o l i d s t a t e m a t e r i a l s science
l i t e r a t u r e , and the z i n c o x i d a t i o n i s y e t a n o t h e r
example o f a more complex p r o t e c t i v e l a y e r c o r r o s i o n
p r o b l e m f o r w h i c h wide r a n g e s o f d a t a e x i s t r e l a t i n g
to p u r i t y , to k i n e t i c c o n d i t i o n s , etc.
T h i s i n t e r p h a s e exchange c u r r e n t m o d e l i n g o f the
s o l i d phase f o r m a t i o n p r o c e s s s e r v e s t o emphasize the
v a r i e d p e r s p e c t i v e s from w h i c h u s e f u l i n f o r m a t i o n can
be drawn t o a i d i n d e s c r i b i n g and u n d e r s t a n d i n g c o r r o
s i o n p r o c e s s e s i n v a r i e d systems. R a r e l y are the r e a l
systems s i m p l e and many p i e c e s o f d a t a a r e examined
b e f o r e t h e models do i n c o r p o r a t e t h e f u l l ranges of
2
CORROSION
IP
CHEMISTRY
IP
2-3
1-2
- S:
Al
gas
j
Al
lAnodic
Oxide
Ag
Ag S
2
Anodizing
Electrolyte
}
H2S
Tarnishing
1
Zn
ZnO
Alkaline
Electrolyte
- -
Figure 22.
Passivation
II
(capacitors)
Battery
Reaction
1.
Corrosion:
Problem
of
Materials
31
Science
variables present.
The c o r r o s i o n e n g i n e e r ' s e x p e r i e n c e s e x i s t i n a
complex s y n e r g i s t i c r e l a t i o n s h i p t o t h e v a r i e d i n t e r
phase exchange c u r r e n t r e l a t i o n s h i p s p r e s e n t w i t h i n
systems o f p r a c t i c a l c o n c e r n . I t i s i n d e e d r a r e t h a t
h i s problems a r e e x p r e s s e d w i t h i n any model m a t e r i a l s
science contexts.
He must n e c e s s a r i l y work w i t h the
e c o n o m i c a l l y f e a s i b l e m a t e r i a l s f o r the p r a c t i c a l
a p p l i c a t i o n s . As t h e p r o d u c t i o n e n g i n e e r s work w i t h
s t e a d i l y i n c r e a s i n g l a b o r c o s t s i n a s s e m b l y , the
m a t e r i a l s e n g i n e e r - c o r r o s i o n e n g i n e e r - wear e n g i n e e r
become i n c r e a s i n g l y on t h e s p o t t o e x t r a c t t h e r e q u i r e d
v a l u e s from cheaper m a t e r i a l s .
I t i s no j o k e t h a t
the c o n t r a c t went t o t h e l o w e s t b i d d e r .
What does t h i s mean t o a l l o f us assembled h e r e ?
F i r s t , a r e c o g n i t i o n needs t o e x i s t t h a t i n a l l
practical situation
b a l l game - i t canno
i f we are e x p e c t e d t o w a r r a n t y t h e p e r f o r m a n c e .
Second, a r e c o g n i t i o n needs t o e x i s t t h a t , though
the problems a r e complex, many new t o o l s and s k i l l s
a r e i n c r e a s i n g l y a v a i l a b l e t o c l a r i f y d i r e c t l y the
c h a r a c t e r o f t h e problems p r e s e n t . The a r t i c l e s w h i c h
follow bring attention to t h i s .
T h i r d , a r e c o g n i t i o n needs t o e x i s t t h a t o r g a n
i z e d knowledge on s o l i d s t a t e c h e m i s t r y i s becoming
i n c r e a s i n g l y a v a i l a b l e t o us.
T h i s i s i l l u s t r a t e d by
. . Hannay's m u l t i - v o l u m e T r e a t i s e on S o l i d S t a t e
C h e m i s t r y . - ^ As n o t e d by Hannay i n h i s f o r e w o r d ,
"... Yet even though the r o l e o f c h e m i s t r y
i n t h e s o l i d s t a t e s c i e n c e s has been a v i t a l
one and t h e s o l i d s t a t e s c i e n c e s have, i n
t u r n , made enormous c o n t r i b u t i o n s t o chemi
c a l t h o u g h t , s o l i d s t a t e c h e m i s t r y has not
been r e c o g n i z e d by t h e g e n e r a l body o f
c h e m i s t s as a major s u b f i e l d o f c h e m i s t r y ...
S o l i d s t a t e c h e m i s t r y has many f a c e t s , and
one o f t h e p u r p o s e s o f t h i s t r e a t i s e i s t o
help d e f i n e the f i e l d . "
I f c h e m i s t s are t o be t h e a t o m i c - m o l e c u l a r domain
custodians of s o l i d s t a t e m a t e r i a l s science, a serious
c o n c e r n w i l l need t o e x i s t f o r a c q u i r i n g u s e f u l b a c k
grounds i n t h i s a r e a .
A s i g n i f i c a n t related publica
t i o n i s t h e r e c e n t appearance o f F. A. K r o g e r s second
e d i t i o n of the Chemistry of Imperfect C r y s t a l s - i n
t h r e e volumes. The d e f e c t c h e m i s t r y c o n c e p t s from t h i s
a r e a need t o be i n c o r p o r a t e d more g e n e r a l l y i n t o the
new s u r f a c e s c i e n c e w h i c h r e l a t e s t o t h e e n v i r o n m e n t a l
1
1 5
32
CORROSION
CHEMISTRY
s t a b i l i t y of materials.
F o u r t h and l a s t , b u t n o t l e a s t , a g r o w i n g r e c o g n i t i o n needs t o e x i s t t h a t the a p p l i c a t i o n o f the new
complex i n s t r u m e n t a l t e c h n i q u e s can c l a r i f y the s u r face chemistry s p e c u l a t i o n s p r e v i o u s l y necessary.
So
f r e q u e n t l y our c o n c e p t u a l i z a t i o n s o f the r e a l p r o b l e m
a r e wrong. I p e r s o n a l l y a c c e p t t h e h y p o t h e s i s t h a t
t h e f i r s t s i x models when t e s t e d i n d e t a i l w i l l p r o v e
wrong. However, as t h e r e a l model emerges i n i t s
complex beauty, t h e number o f p a t h s t o o p t i m i z a t i o n s
have m u l t i p l i e d and s c i e n c e i s no l o n g e r dead ended.
The complex i n s t r u m e n t a t i o n s i n c l u d e LEED, low
energy e l e c t r o n d i f f r a c t i o n , w h i c h i s r e v e a l i n g a
complex model f o r s u r f a c e s t r u c t u r e when a p p a r e n t
m u l t i l a y e r a d s o r p t i o n o f oxygen p r o c e e d s on c l e a n m e t a l
surfaces.
It also include
chemical a n a l y s i s ,
t h a t can p r o v e t h a t t h e p o s t u l a t e d c o v e r i n g f i l m , by
g o l l y , i s n o t c o v e r i n g , and what's the s u r f a c e compos i t i o n ? E v e r t r y t o d e a l w i t h n i n e s u r f a c e components
a t once? W e l l , you can s e r i o u s l y e x p l o r e t h a t d i s t r i b u t i o n f o r e l e c t r o l e s s n i c k e l f i l m s b e i n g d e p o s i t e d on
a catalyzed substrate.
I t a l s o i n c l u d e s Auger e l e c t r o n s p e c t r o s c o p y
microprobe i d e n t i f i c a t i o n of the surface crud i n the
p i t , or d e f i n i t i o n of the surface composition g r a d i e n t
p r e v i o u s l y o m i t t e d from t h e s p e c u l a t i o n on atom
transport processes.
The s c a n n i n g e l e c t r o n m i c r o s c o p e p r o v i d e s us
neophytes w i t h a r e a l i s t i c l o o k a t s u r f a c e s t r u c t u r e
as e n c o u n t e r e d i n t h e system. When teamed w i t h Auger
o r ESCA a c o n f r o n t a t i o n can be c r e a t e d w i t h s e l f ,
t r y i n g t o r a t i o n a l i z e the o l d c o m f o r t a b l e models t h a t
d i d n o t have t o acknowledge t h a t s u r f a c e c h e m i c a l
s t r u c t u r e r e a l l y e x i s t e d on t h e " p o l y p h a s e inhomogeneous s o l i d c o n t a i n i n g h e t e r o g e n e i t y homogeneously
dispersed."
C o r r o s i o n i s h e r e t o s t a y . The wedding t o
m a t e r i a l s s c i e n c e i s i m p l i c i t l y r e v i e w e d i n the f i r s t
five figures.
I t i s a c l e a r l y i m p o r t a n t complex m a t e r i a l s s c i e n c e d r a w i n g on a l l t h e o t h e r d i s c i p l i n e s .
As
t h e s u r f a c e s c i e n c e o f i n t e r p h a s e mass and charge
t r a n s p o r t phenomena on s o l i d s c o n t i n u e s t o e v o l v e t h e n
more c l e a r l y , d e f i n e d new r o u t e s f o r p r o d u c t o p t i m i z a t i o n s w i l l be e v i d e n t .
Would you agree t h a t c o r r o s i o n i s t h e most g e n e r a l
problem o f m a t e r i a l s s c i e n c e ?
May a l l have f u n e x p l o r i n g t h e new v i s t a s o f our
p h y s i c a l world through the f o l l o w i n g c h a p t e r s .
For
01.
33
References
1.
CORROSION CHEMISTRY
34
Literature Cited
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
General References
1-10.
Stern, M . , et. al., J . Electrochem. Soc. (1957)
104 pp. 56-63, 559-63, 645-50.
Helmholtz, H . , Wiss Abhandlphysik. Tech. Reichenstalt I (1879) p. 925.
Gouy, G . , J. Phys. (1910) 9 p. 457.
Chapman, D. L., P h i l . Mag. (1913) 25 (6) p. 475.
Stern, ., z. Electrochem. (1924) 30 p. 508.
Grahame, D. C . , Chem. Rev. (1947) 41 p. 441.
Bockris, J . O'M., et. al., Proc. Roy. Soc.
(1963) A274 pp. 55-79.
Rice, . K . , Phys. Rev. (1928) 31 p. 1051,
MacDonald, J . R., J. Appl. Phys. (1964) 35 (10)
p. 3053.
See, for exampl
381-4 08, "The
Semiconductors," H. C. Gatos, editor, John Wiley,
New York (1960).
Rees, A. L. G . , "Chemistry of the Defect Solid
State," John Wiley, New York (1954).
Croft, G. T., and Tuomi, D., J. Electrochem.
Soc. (1961) 108 p. 915.
Davies, J. . , et. al., J. Electrochem. Soc.
(1965) 112 p. 675.
Hannay, . . , editor, "Treatise on Solid State
Chemistry," Volumes 1-7, Plenum Press, New York
(1974).
Kroger, F. . , "Chemistry of Imperfect Crystals Volume 1, Preparation, Purification, Crystal
Growth, and Phase Theory; Volume 2, Imperfection
Chemistry of Crystalline Solids; Volume 3,
Applications of Imperfection Chemistry: Solid
State Reactions and Electrochemistry," North
Holland-American Elsevier, New York (1974).
RECEIVED
September 1, 1978.
2
Electrochemical Techniques in C o r r o s i o n Studies
FRANCIS M. DONAHUE
Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109
Criterion
of Corrosion
(1)
0-8412-0471-3/79/47-089-035$10.25/0
1979 American Chemical Society
36
CORROSION
CHEMISTRY
298 =
( 2 )
AG*
= ( ^
9 8
9 8
(3)
where H
is
the standard enthalpy o f formation
of a s p e c i e s , Y, at 25C and ( ( G - H ) / T ) is a
thermodynamic f u n c t i o n which depends on t h e s p e c i e s , Y,
and t h e temperature, T. These thermodynamic q u a n t i t i e s
are t a b u l a t e d in some r e f e r e n c e books (2^3J .
E q u a t i o n 1 is u s e f u l as a c r i t e r i o n o f c o r r o s i o n
f o r p o s t u l a t e d c o r r o s i o n r e a c t i o n s . I t s main u t i l i t y
is t h a t i t p e r m i t s one t o a s c e r t a i n whether a p a r t i c u
l a r e n v i r o n m e n t a l agent ( d i s s o l v e d s p e c i e s ) o r t h e
s o l v e n t can i n t e r a c t c h e m i c a l l y w i t h t h e m e t a l l i c
s t r u c t u r e t o cause c o r r o s i o n o f t h e s t r u c t u r e . I f t h e
computed f r e e e n t h a l p y change is p o s i t i v e , i t i n d i c a t e s
t h a t c o r r o s i o n cannot o c c u r by t h e p o s t u l a t e d r e a c t i o n .
However, i t does n o t mean t h a t c o r r o s i o n cannot o c c u r
due t o t h e a c t i o n o f another e n v i r o n m e n t a l agent.
T h e r e f o r e , computations o f t h i s s o r t s h o u l d be exhaust
i v e and account f o r a l l e n v i r o n m e n t a l a g e n t s . I f t h e
computed f r e e e n t h a l p y change is n e g a t i v e , i t i n d i c a t e s
t h a t c o r r o s i o n by t h e p o s t u l a t e d r e a c t i o n is p o s s i b l e .
The computation p r o v i d e s no i n f o r m a t i o n about t h e r a t e
of t h e c o r r o s i o n r e a c t i o n .
Many c o r r o s i o n p r o c e s s e s , e.g., s t r e s s c o r r o s i o n
c r a c k i n g and o t h e r c o r r o s i o n - f r a c t u r e p r o c e s s e s , cannot
be d e s c r i b e d c o m p l e t e l y by a c h e m i c a l r e a c t i o n . These
p r o c e s s e s a r e complex i n t e r a c t i o n s among c h e m i s t r y ,
p h y s i c a l p r o p e r t i e s o f t h e m e t a l and m e c h a n i c a l s t r e s s .
T h e r e f o r e , t h e c o r r o s i o n tendency o f t h e m e t a l - e n v i r o n
ment i n t e r a c t i o n cannot be e v a l u a t e d u s i n g E q u a t i o n 1.
A t t h e p r e s e n t time, e m p i r i c a l c r i t e r i a (which a r e be
yond t h e scope o f t h i s c h a p t e r ) a r e used.
8
Electrochemical
D O N A H U E
Techniques
I l l u s t r a t i o n 1. C h e m i c a l C r i t e r i o n o f C o r r o s i o n .
Copper m e t a l is in c o n t a c t w i t h a s t r o n g a c i d
s o l u t i o n at 25C.
a) I f t h e o n l y i n f o r m a t i o n a v a i l a b l e a r e t h e
a c t i v i t i e s o f - h y d r o g e n i o n (10
) and c u p r i c i o n (10~ ) , can c o r r o s i o n o c c u r ?
b) I f oxygen at a f u g a c i t y o f 0.2 is in e q u i
l i b r i u m w i t h t h e s o l u t i o n in p a r t " a " ,
can c o r r o s i o n o c c g r ?
_
c) I f f e r r o u s (a=10
) and f e r r i c (a=10
)
i o n s a r e c o n t a m i n a n t s o f t h e s o l u t i o n in
p a r t " a " , can c o r r o s i o n o c c u r ?
Solutions :
a) Assume a c o r r o s i o
reaction
Cu + 2 H >
From P o u r b a i x (1) and i n s p e c t i o n o f
chemical r e a c t i o n ,
Y
Species
(kcal/mole)
Y
Cu
0
-1
H
0
-2
the
Cu
H
2 +
15.53
0
+1
+1
(1.987) (298)
n( ( 1 0
- 4
)/(10~ ) )
= +1.553X10 c a l / m o l e o f Cu
T h e r e f o r e , c o r r o s i o n c a n n o t o c c u r by
postulated reaction.
b) Assume a c o r r o s i o n r e a c t i o n ,
2Cu
+ 4H
+ 0
-> 2 C u
2 +
Cu
H0
2
+ 2H 0
2
i n s p e c t i o n of
0
0
2 +
the
(kcal/mole)
0
H
2
the
15.53
-56.69
Y
-2
-4
1
+2
+2
38
CORROSION
CHEMISTRY
U s i n g E q u a t i o n s 1 and 2 and t h e a v a i l a b l e
d a t a (assuming u n i t a c t i v i t y f o r w a t e r ) ,
AG = (-2) (0) + (-4) (0) + (-1) (0) +
(2) (15530) + (2) (-56690) +
(1.987) (298) n((10~ ) /(10~ ) (0.2)
4
= -8.14X10 c a l / 2 mole o f Cu
T h e r e f o r e , c o r r o s i o n can o c c u r as a r e s u l t
of the p o s t u l a t e d r e a c t i o n .
c) Assume a c o r r o s i o n r e a c t i o n ,
3+
2+
2+
Cu + 2Fe
+ Cu
+ 2Fe
From P o u r b a i x (1) and i n s p e c t i o n o f t h e
chemical r e a c t i o n ,
z
Species
(kcal/mole)
Y
Cu
0
1
3+
Fe
-2.5
Cu
15.53
+1
Fe
-20.30
+2
U s i n g E q u a t i o n s 1 and 2 and t h e a v a i l a b l e
data,
AG = (-1) (0) + (-2) (-2530) + (1) (15530)
2 +
2 +
n(10" )
= -2.27X10 c a l / m o l e o f Cu
T h e r e f o r e , c o r r o s i o n can o c c u r by t h e p o s t ulated reaction.
E l e c t r o c h e m i c a l Thermodynamics
E l e c t r o d e P o t e n t i a l Measurements.
E l e c t r o c h e m i c a l r e a c t i o n s can be w r i t t e n in t h e
g e n e r a l form
Red = Ox + ne"
(4)
2.
Electrochemical
D O N A H U E
Techniques
39
and t h e d i s p o s i t i o n o f i o n i c and d i p o l a r s p e c i e s on
the s o l u t i o n s i d e o f t h e i n t e r f a c e . Although t h i s
e l e c t r i c f i e l d is p r e s e n t , i t c a n n o t be measured d i
r e c t l y . I n s t e a d , a r e l a t i v e measurement is made w h i c h
provides u s e f u l information.
The c i r c u i t used in t h i s measurement is shown in
F i g u r e 1. The e l e c t r o d e under i n v e s t i g a t i o n , TE, is
c o n n e c t e d t o a second e l e c t r o d e , RE, by means o f a
v o l t a g e - m e a s u r i n g d e v i c e , E. S i n c e i t is n e c e s s a r y t o
make t h e s e measurements in t h e v i r t u a l absence o f a
c u r r e n t f l o w i n g in t h e c i r c u i t , t h e v o l t a g e - m e a s u r i n g
d e v i c e s h o u l d p o s s e s s a l a r g e (>10^ ohms) i n p u t imped
ance. I n a d d i t i o n , i t is u s u a l l y n e c e s s a r y t o make
measurements o v e r t h e range o f 2.0V w i t h an a c c u r a c y
on t h e o r d e r o f lmV.
The r e f e r e n c e e l e c t r o d e , RE, s h o u l d p o s s e s s a
s t a b l e , known e l e c t r o d
t h e e l e c t r i c f i e l d at
S i n c e a l l e l e c t r o d e p o t e n t i a l measurements a r e r e l a t i v e ,
i t is c o n v e n i e n t t o have an " u l t i m a t e r e f e r e n c e p o i n t " .
By c o n v e n t i o n , t h e e l e c t r o d e p o t e n t i a l o f t h e hydrogenhydrogen i o n e l e c t r o c h e m i c a l r e a c t i o n , i . e . ,
H
= 2H
+ 2e"~
(5)
*RE1,2
( 6 )
where is t h e e l e c t r o d e p o t e n t i a l r e l a t i v e t o t h e
second
r e f e r e n c e e l e c t r o d e (the d e s i r e d q u a n t i t y ) ,
is t h e e l e c t r o d e p o t e n t i a l r e l a t i v e t o t h e f i r s t
r e f e r e n c e e l e c t r o d e (the measured q u a n t i t y ) and
2
is t h e e l e c t r o d e p o t e n t i a l o f t h e f i r s t r e f e r e n c e
'
e l e c t r o d e r e l a t i v e t o t h e second r e f e r e n c e e l e c t r o d e .
2
40
CORROSION
CHEMISTRY
T a b l e I . E l e c t r o d e p o t e n t i a l s o f some common
r e f e r e n c e e l e c t r o d e s at 25C. (4)
Electrode Reaction/
Name o f R e f e r e n c e
Electrode
2Hg
+ 2C1~ = H g C l
0
+ 2e~ / C a l o m e l
Electrolyte
0.01 M KC1
0.10 M
1.0 M "
Satd.
11
0.1 M CuSO,
Cu = C u
+ 2e~ /
Copper-Copper
0.5 M
" *
Sulfate
Satd.
Ag + C l ~ = A g C l + e" 0.001 M KC1
Silver-Silver
0.01 M "
Chloride
2 +
Electrode
Potential
(V v s . SHE)
0.389
0.333
0.280
0.241
0.284
0.294
0.298
0.400
0.343
I l l u s t r a t i o n 2. C o n v e r s i o n o f R e f e r e n c e E l e c t r o d e
Scale.
I f t h e e l e c t r o d e p o t e n t i a l o f an i r o n e l e c t r o d e
is -0.528 V v s . s a t u r a t e d c a l o m e l r e f e r e n c e
e l e c t r o d e (SCE), what is i t s v a l u e r e l a t i v e t o
SHE?
Solution:
I n s e r t i n g t h e measured e l e c t r o d e p o t e n t i a l and
the e l e c t r o d e p o t e n t i a l o f t h e r e f e r e n c e e l e c t r o d e
from T a b l e I (
= 0.241 V v s . SHE) in
E q u a t i o n 6,
= -0.287 V v s . SHE.
0
Equilibrium
Electrode
Potentials.
The c o n d i t i o n f o r e q u i l i b r i u m f o r t h e e l e c t r o c h e m
i c a l r e a c t i o n g i v e n in E q u a t i o n 4 is
v
e eo
( 7 )
e o
(8)
where F is t h e F a r a d a y c o n s t a n t . The c h e m i c a l p o t e n t i a l
of a s p e c i e s , Y, is
2.
Electrochemical
D O N A H U E
41
Techniques
= + RT n a
(9)
I n s e r t i n g E q u a t i o n s 8 and 9 in 7 and r e a r r a n g i n g
(10)
I t c a n be shown t h a t
=
(l/v F)Zv
e
(11)
yy
where is t h e s t a n d a r d e l e c t r o d e p o t e n t i a l o f t h e
r e a c t i o n . Tabulated values o f standard e l e c t r o d e poten
t i a l s r e l a t i v e t o SHE f o r many e l e c t r o c h e m i c a l r e a c t
i o n s o f i n t e r e s t in c o r r o s i o n a r e a v a i l a b l e () I n
s e r t i n g Equation 1
= + (RT/v F)n(II(a ) )
e
(12)
Diagrams.
I f an e l e c t r o c h e m i c a l r e a c t i o n is p e r t u r b e d from
the e q u i l i b r i u m s t a t e , t h e r e l a t i v e s t a b i l i t i e s o f t h e
s p e c i e s in t h e r e a c t i o n a r e changed. The m a n i f e s t a t i o n
o f t h e p e r t u r b a t i o n is t h e measured e l e c t r o d e p o t e n t i a l ,
w h i c h d i f f e r s from t h e e q u i l i b r i u m e l e c t r o d e p o t e n t i a l
f o r t h e r e a c t i o n . I f t h e measured e l e c t r o d e p o t e n t i a l
is p o s i t i v e w i t h r e s p e c t t o t h e e q u i l i b r i u m p o t e n t i a l ,
t h e r e a c t i o n g i v e n by E q u a t i o n 4 p r o c e e d s i r r e v e r s i b l y
from l e f t t o r i g h t , i . e . , t h e r e d u c e d form o f t h e
c h e m i c a l s p e c i e s is u n s t a b l e w h i l e t h e o x i d i z e d form
o f t h e s p e c i e s is s t a b l e . The c o n v e r s e is t r u e when t h e
42
CORROSION
CHEMISTRY
measured p o t e n t i a l is n e g a t i v e w i t h r e s p e c t t o t h e
equilibrium potential.
Water is an e l e c t r o c h e m i c a l l y a c t i v e c h e m i c a l
s p e c i e s . The e l e c t r o c h e m i c a l r e a c t i o n s in w h i c h w a t e r
is t h e p r i m a r y r e a c t a n t o r p r o d u c t a r e
2H 0 = 4 H
+ 0
+ 4e~
(13)
and
H
+ 20H~ = 2H 0 + 2e~
(14)
=
2
"
and
* H / H 0 = -0-059PH
O /
(16)
where t h e s e e l e c t r o d e p o t e n t i a l s a r e V v s . SHE.
P o u r b a i x (1) has shown t h a t p l o t t i n g e l e c t r o d e
potentials of electrochemical reactions against solu
t i o n pH is u s e f u l in d e l i n e a t i n g r e g i o n s o f s t a b i l i t y
o f v a r i o u s c h e m i c a l s p e c i e s in and in c o n t a c t w i t h
aqueous s o l u t i o n s . These p l o t s a r e commonly c a l l e d
P o u r b a i x Diagrams. The u t i l i t y o f t h i s approach w i l l
become e v i d e n t in t h e subsequent p r e s e n t a t i o n .
The P o u r b a i x Diagram f o r t h e system, H 0-H -0 -H OH", is g i v e n in F i g u r e 2. The l i n e s "15" a n d
"16"
r e p r e s e n t p l o t s o f t h e d a t a f o r E q u a t i o n s 15 and 16,
r e s p e c t i v e l y . I n s p e c t i o n o f E q u a t i o n 13 and l i n e "15",
in c o n j u n c t i o n w i t h t h e p r e v i o u s d i s c u s s i o n o f s p e c i e s
s t a b i l i t y , i n d i c a t e s t h a t t h e s o l v e n t , w a t e r , is
s t a b l e ( w i t h r e s p e c t t o E q u a t i o n 13) at e l e c t r o d e
p o t e n t i a l s below t h i s l i n e w h i l e w a t e r is u n s t a b l e at
p o t e n t i a l s above t h i s l i n e . S i m i l a r l y , i n s p e c t i o n o f
E q u a t i o n 14 and l i n e "16" i n d i c a t e s t h a t w a t e r is
s t a b l e at e l e c t r o d e p o t e n t i a l s above l i n e "16" and
u n s t a b l e below t h e l i n e . F i g u r e 2 is a n a l o g o u s t o a
phase diagram; i t r e p r e s e n t s a form o f e l e c t r o c h e m i c a l
phase d i a g r a m . S i m i l a r t o a phase d i a g r a m , t h e r e g i o n
where a s p e c i e s is s t a b l e is i d e n t i f i e d w i t h t h e chem
i c a l symbol f o r t h e s p e c i e s .
The P o u r b a i x Diagram is p a r t i c u l a r l y u s e f u l in
d e t e r m i n i n g t h e s t a b l e s p e c i e s f o r m e t a l l i c systems in
+
2.
D O N A H U E
Electrochemical
43
Techniques
44
CORROSION
CHEMISTRY
c o n t a c t w i t h aqueous s o l u t i o n s . Two g e n e r a l c l a s s e s o f
m e t a l s a r e found when F i g u r e 2 and t h e P o u r b a i x Diagram
f o r a m e t a l a r e superimposed. T h i s c l a s s i f i c a t i o n is
based on t h e r e l a t i o n s h i p between the m e t a l - m e t a l l i o n
e q u i l i b r i u m r e a c t i o n and the r e g i o n o f s t a b i l i t y o f
w a t e r . I n t h e f i r s t c a s e , the m e t a l - m e t a l l i o n e q u i l i brium p o t e n t i a l f a l l s w i t h i n the r e g i o n of s t a b i l i t y
o f w a t e r . I n t h e s e i n s t a n c e s , i t is p o s s i b l e t o measu r e t h e e q u i l i b r i u m e l e c t r o d e p o t e n t i a l o f the m e t a l m e t a l l i o n r e a c t i o n in aqueous s o l u t i o n s and t o d e v i s e
a means whereby the k i n e t i c p r o p e r t i e s o f t h i s r e a c t i o n
may be o b t a i n e d w i t h m i n i m a l k i n e t i c c o m p l e x i t y . The
second c l a s s o f m e t a l s has m e t a l - m e t a l l i o n e q u i l i b r i u m
e l e c t r o d e p o t e n t i a l s w h i c h f a l l below t h e r e g i o n o f
s t a b i l i t y o f w a t e r . S i n c e t h e s e m e t a l s form mixed
p o t e n t i a l systems w i t h t h e s o l v e n t (see b e l o w ) , t h e
equilibrium electrod
r e a c t i o n c a n n o t be
t h e k i n e t i c s o f the c o m p l e t e r e a c t i o n can be d e t e r m i n e d
o n l y w i t h g r e a t d i f f i c u l t y . Copper and i r o n a r e examples o f t h e s e two c l a s s e s o f m e t a l s and w i l l be d i s c u s s e d below.
F i g u r e 3 is the P o u r b a i x Diagram f o r copper and
some o f i t s i o n i c s p e c i e s and compounds in c o n t a c t
w i t h w a t e r at 25C. The e q u i l i b r i u m e l e c t r o d e p o t e n t i a l
f o r t h e c o p p e r - c u p r i c i o n r e a c t i o n is l o c a t e d w i t h i n
t h e r e g i o n o f w a t e r s t a b i l i t y (dashed l i n e s ) . T h e r e f o r e , t h e measurement o f t h e e q u i l i b r i u m p o t e n t i a l is
p o s s i b l e , and t h e k i n e t i c s o f t h e c o p p e r - c u p r i c i o n
system can be s t u d i e d w i t h o u t i n t e r f e r e n c e from r e a c t i o n s i n v o l v i n g d e c o m p o s i t i o n o f the s o l v e n t . W i t h
t h e e x c e p t i o n o f E q u a t i o n 21, w h i c h is a c h e m i c a l r e a c t i o n i n v o l v i n g the h y d r o l y s i s o f c u p r i c i o n ( r e a c t i o n s w h i c h a r e p u r e l y c h e m i c a l , s i n c e t h e y b e a r no
r e l a t i o n t o e l e c t r o c h e m i c a l r e a c t i o n s , p e r s e , appear
as v e r t i c a l l i n e s on P o u r b a i x D i a g r a m s ) , a l l o f t h e
r e a c t i o n s c o n s i d e r e d in F i g u r e 3 a r e e l e c t r o c h e m i c a l
and may be s t u d i e d u s i n g t h e t e c h n i q u e s o u t l i n e d in
t h i s c h a p t e r . I f the measured e l e c t r o d e p o t e n t i a l ( w i t h
r e s p e c t t o SHE) and t h e s o l u t i o n pH a r e known, F i g u r e 3
may be used t o d e t e r m i n e the s t a b l e form o f copper o r
i t s compounds w h i c h can be e x p e c t e d under t h o s e c o n ditions .
F i g u r e 4 is the P o u r b a i x Diagram f o r i r o n and
some o f i t s i o n i c s p e c i e s and compounds in c o n t a c t
w i t h w a t e r at 25C. The e q u i l i b r i u m p o t e n t i a l o f t h e
i r o n - f e r r o u s i o n r e a c t i o n f a l l s o u t s i d e the r e g i o n of
s t a b i l i t y o f w a t e r (dashed l i n e s ) . T h e r e f o r e , any attempt t o measure t h e e q u i l i b r i u m p o t e n t i a l w i l l f a i l
s i n c e t h e s o l v e n t w i l l undergo e l e c t r o c h e m i c a l r e d u c -
2.
D O N A H U E
Electrochemical
45
Techniques
Cu 0
Cu 0
2Cu
Cu *
2
+
+
+
+
Cu =
2H =
H0 =
H0 =
H0 =
+
Cu + 2e
2Cu + H 0 + 2e~
2CuO + 2H + 2e
Cu 0 + 2H + 2e~
CuO + 2H+
2+
2+
(17)
(18)
(19)
(20)
(21)
CHEMISTRY
CORROSION
14
pH
Figure 4. Pourbax diagram for the system Fe-Fe -Fe -Fe O -Fe 0 .
Activities of ferrous and ferric ions are JO" . Temperature is 25C. The reactions considered;
2+
3+
l4
Pe
3Fe + 4H 0
3Fe + 4H O
2Pe + 3H O
Pe
2P* + 3H O
2Fe O + H 0
2
2+
2+
2+
=
=
=
=
=
=
=
Pe r2e
Fe O + 8 + 8e~
Fe O + 8H + 2e~
Fe O + 6H + 2e~
P + e
Fe O + GIF
3Fe 0
+ 2H+ + 2e~
2+J
(22)
(23)
(24)
(25)
(26)
(27)
(28)
2.
D O N A H U E
Electrochemical
Techniques
47
t i o n w h i l e t h e i r o n w i l l undergo e l e c t r o c h e m i c a l o x i d a
t i o n . T h i s t y p e o f p r o c e s s is a form o f c o r r o s i o n and
is t h e b a s i s f o r an e l e c t r o c h e m i c a l model o f c o r r o s i o n
c a l l e d t h e t h e o r y o f mixed p o t e n t i a l s (to be d i s c u s s e d
below).
I n t h e p o t e n t i a l - p H r e g i o n where i r o n m e t a l is t h e
s t a b l e s p e c i e s , c o r r o s i o n - d e f i n e d as d i s s o l u t i o n o r
o x i d a t i o n - cannot occur s i n c e these r e a c t i o n s a r e n o t
f a v o r e d t h e r m o d y n a m i c a l l y . P o u r b a i x (1) has d e s i g n a t e d
t h e s e r e g i o n s a s "immune" t o c o r r o s i o n . However, in
t h e b r o a d e r sense o f c o r r o s i o n (see d e f i n i t i o n o f c o r
r o s i o n at t h e b e g i n n i n g o f t h i s c h a p t e r ) t h i s r e g i o n
may be q u i t e c o n d u c i v e t o i r o n d e t e r i o r a t i o n . F o r e x
ample, in t h i s r e g i o n hydrogen gas is s t a b l e (see F i g
u r e 2 ) . Some i r o n a l l o y s a r e s u s c e p t i b l e t o f r a c t u r e
in t h e p r e s e n c e o f hydrogen (_5) . T h e r e f o r e , i tisn o t
sufficient for a structur
in o r d e r f o r i t t o
When a c h e m i c a l l y s t a b l e o x i d e (or s a l t ) f i l m is
p r e s e n t on t h e s u r f a c e o f a m e t a l (see t h e i r o n o x i d e
s t a b l e r e g i o n s o f F i g u r e 4 ) , t h a t m e t a l may be f r e e
o f subsequent c o r r o s i o n . The c o n d i t i o n s f o r t h i s form
of c o r r o s i o n m i t i g a t i o n a r e that the underlying f i l m
is a d h e r e n t , c o h e r e n t and p o r e - f r e e . I n e s s e n c e , t h e s e
c o n d i t i o n s m e r e l y s t i p u l a t e t h a t t h e f i l m must be an
e f f e c t i v e b a r r i e r between t h e m e t a l and t h e e n v i r o n
ment. T h i s c o n d i t i o n is c a l l e d p a s s i v i t y and is c h a r
a c t e r i z e d by measured e l e c t r o d e p o t e n t i a l s in t h e r e
g i o n s where t h e f i l m is s t a b l e . I r o n and i t s a l l o y s
have been shown t o e x h i b i t p a s s i v e b e h a v i o r (6) .
F i g u r e 5 is a s i m p l i f i e d r e p r e s e n t a t i o n o f t h e
P o u r b a i x Diagram f o r i r o n . I t d e l i n e a t e s t h e r e g i o n s
where immunity, c o r r o s i o n and p a s s i v i t y c a n be e x p e c t
ed. S i m i l a r diagrams (as w e l l a s t h e more c h e m i c a l l y o r i e n t e d diagrams) a r e a v a i l a b l e in t h e monograph by
P o u r b a i x (1).
I l l u s t r a t i o n 3. Use o f P o u r b a i x Diagrams.
I d e n t i f y t h e s t a b l e s p e c i e s and whether c o r r o s i o n
is p o s s i b l e in t h e f o l l o w i n g s i t u a t i o n s :
a) copper m e t a l at +0.150 V v s . SCE in an aqueous
s o l u t i o n w i t h a pH o f 2.5 and a c u p r i c i o n
a c t i v i t y o f 0.01 at 25C.
b) i r o n m e t a l at -0.750 V v s . SCE in an aqueous
s o l u t i o n w i t h a pH o f 5.0 and a f e r r o u s i o n
6
a c t i v i t y o f 1 0 ~ at 25C.
Solutions :
a) t h e e l e c t r o d e p o t e n t i a l ( v s . SHE) is computed
u s i n g E q u a t i o n 6 and T a b l e I , i . e . , = 0.391 V
American Chemical
Society Library
48
CORROSION
v s . SHE.
CHEMISTRY
R e f e r r i n g t o F i g u r e s 2 and 3
the
2+
+
s t a b l e s p e c i e s a r e Cu
, H and H 0.
Corrosion
is p o s s i b l e under t h e s e c o n d i t i o n s ,
b) the e l e c t r o d e p o t e n t i a l (vs. SHE) is computed
u s i n g E q u a t i o n 6 and T a b l e I , i . e . , = -0.509
V v s . SHE. R e f e r r i n g t o F i g u r e s 2 and 4, the
s t a b l e s p e c i e s a r e f e r r o u s i o n s and H^.
Corr o s i o n is p o s s i b l e under t h e s e c o n d i t i o n s .
Electrochemical Kinetics
f
P r o p e r t i e s of E l e c t r o d e
Reactions.
E l e c t r o c h e m i c a l (electrode) r e a c t i o n s are i n h e r
e n t l y h e t e r o g e n e o u s The e l e c t r o n t r a n s f e r r e a c t i o n
o c c u r s at a m e t a l (o
substrate)-solutio
f o l l o w i n g the e l e c t r o n t r a n s f e r r e a c t i o n , t r a n s p o r t o f
c h e m i c a l s p e c i e s between the b u l k o f t h e s o l u t i o n and
the i n t e r f a c e a l s o t a k e s p l a c e . F i g u r e 6 is a r e p r e
s e n t a t i o n of these processes which c o n s t i t u t e the t o
t a l i t y o f the e l e c t r o c h e m i c a l r e a c t i o n .
The t h r e e s t e p s a s s o c i a t e d w i t h e l e c t r o c h e m i c a l
r e a c t i o n s , i . e . , t r a n s p o r t o f r e a c t a n t ( s ) t o the i n t e r
f a c e , the e l e c t r o n t r a n s f e r ( s u r f a c e ) r e a c t i o n and
t r a n s p o r t o f p r o d u c t ( s ) from the i n t e r f a c e , a r e sequen
t i a l . T h e r e f o r e , the o v e r a l l r a t e o f r e a c t i o n is c o n
t r o l l e d by t h e s l o w e s t o f the t h r e e s t e p s . When t h e
t r a n s p o r t p r o c e s s e s a r e c a p a b l e o f o p e r a t i n g at h i g h
r a t e s r e l a t i v e t o the e l e c t r o n t r a n s f e r r e a c t i o n , the
r a t e o f the o v e r a l l r e a c t i o n can be d e s c r i b e d by equa
t i o n s o f e l e c t r o d e k i n e t i c s . These t y p e s o f e l e c t r o d e
r e a c t i o n s a r e s a i d t o be "under a c t i v a t i o n c o n t r o l " .
On the o t h e r hand, when the e l e c t r o d e r e a c t i o n is
c a p a b l e o f o p e r a t i n g at h i g h r a t e s r e l a t i v e t o the
t r a n s p o r t p r o c e s s ( e s ) , the r a t e o f the o v e r a l l r e a c t i o n
can be d e s c r i b e d by e q u a t i o n s o f c o n v e c t i v e mass t r a n s
p o r t . These t y p e s o f e l e c t r o d e r e a c t i o n s a r e s a i d t o
be "under t r a n s p o r t c o n t r o l " . I n the d i s c u s s i o n t o
f o l l o w , the e q u a t i o n s o f e l e c t r o d e k i n e t i c s and c o n v e c t
i v e mass t r a n s p o r t w i l l be p r e s e n t e d w i t h the c o n d i
t i o n s under w h i c h the r e s p e c t i v e e q u a t i o n s a p p l y .
Readers w i s h i n g a more d e t a i l e d p r e s e n t a t i o n t h a n is
p o s s i b l e h e r e s h o u l d r e f e r t o t h e monograph by V e t t e r
(7) .
The r a t e o f an e l e c t r o c h e m i c a l r e a c t i o n is u s u a l l y
measured by a c u r r e n t , I , f l o w i n g in an e x t e r n a l e l e c
t r i c a l c i r c u i t (see b e l o w ) . T h i s c u r r e n t is r e l a t e d t o
the f l u x o f a r e a c t i n g s p e c i e s , N_, and the f l u x o f a
D O N A H U E
Figure 6.
Electrochemical
Techniques
50
CORROSION
CHEMISTRY
p r o d u c t s p e c i e s , N , and t h e r a t e o f t h e s u r f a c e r e
a c t i o n (based on
the reacting species), r
by
R
Jf
I/nFA = j = /
= Tj
(29)
where is t h e number o f e l e c t r o n s t r a n s f e r r e d in t h e
in t h e e l e c t r o c h e m i c a l r e a c t i o n , A is t h e s u r f a c e a r e a
o f t h e m e t a l s u b s t r a t e in c o n t a c t w i t h t h e s o l u t i o n
and V j and v a r e t h e s t o i c h i o m e t r i c c o e f f i c i e n t s o f
the
r e a c t a n t and p r o d u c t s p e c i e s , r e s p e c t i v e l y .
The s i g n o f t h e c u r r e n t is dependent on t h e sense
o f t h e e l e c t r o c h e m i c a l r e a c t i o n . F o r example, when
E q u a t i o n 4 o p e r a t e s from l e f t t o r i g h t , i . e . ,
R
Red
-> Ox + n e "
(4a)
an e l e c t r o c h e m i c a l o x i d a t i o
i c a l o x i d a t i o n s are o f t e n c a l l e d anodic r e a c t i o n s . I n
t h e c o n v e n t i o n used in t h i s c h a p t e r , a n o d i c r e a c t i o n s
a r e a s s o c i a t e d w i t h p o s i t i v e c u r r e n t s . When t h e d i
r e c t i o n o f t h e r e a c t i o n is r e v e r s e d , i . e . ,
Ox + n e "
Red
(4b)
an e l e c t r o c h e m i c a l r e d u c t i o n ( c a t h o d i c r e a c t i o n )
occurs. Cathodic r e a c t i o n s are associated with negative
c u r r e n t s . The s i g n s o f t h e r a t e and f l u x terms in
E q u a t i o n 29 s h o u l d be a d j u s t e d t o accomodate t h i s s i g n
convention.
The r a t e ( c u r r e n t ) o f an e l e c t r o c h e m i c a l r e a c t i o n
is d e s c r i b e d by t h e sum o f t h e r a t e s ( c u r r e n t s ) o f t h e
a n o d i c and c a t h o d i c r e a c t i o n s w h i c h c o n s t i t u t e t h e
e l e c t r o d e r e a c t i o n . The a n o d i c and c a t h o d i c c u r r e n t s ,
representing "parts" of the o v e r a l l current (rate), are
called p a r t i a l currents.
Activation Controlled Electrode
Reactions.
A t e q u i l i b r i u m , t h e r a t e s o f t h e a n o d i c and c a t h
o d i c p a r t i a l r e a c t i o n s a r e e q u a l , i . e . , t h e r e is no
n e t change o f t h e i n v e n t o r y o f Red and Ox. When t h e
system is p e r t u r b e d such t h a t t h e e l e c t r o d e p o t e n t i a l
is p o s i t i v e w i t h r e s p e c t t o t h e e q u i l i b r i u m p o t e n t i a l ,
t h e r a t e o f t h e a n o d i c p a r t i a l r e a c t i o n is g r e a t e r
t h a n t h a t o f t h e c a t h o d i c p a r t i a l r e a c t i o n . The e l e c
trode r e a c t i o n e x h i b i t s a n e t anodic (oxidation) cur
rent. Likewise, f o r perturbations negative t o the
equilibrium p o t e n t i a l , the electrode reaction e x h i b i t s
2.
Electrochemical
D O N A H U E
Techniques
51
)-exp(-n/6 )]
(30)
where i is t h e n e t c u r r e n t d e n s i t y ( I / A ) , i is t h e
exchange c u r r e n t d e n s i t y , is t h e o v e r p o t e n t i a l and
$ and 3 a r e s o - c a l l e d T a f e l c o n s t a n t s ( s l o p e s ) .
The exchange c u r r e n t d e n s i t y is a measure o f t h e
i n t r i n s i c r e a c t i v i t y of the electrode r e a c t i o n , i . e . ,
t h e r a t e s o f t h e a n o d i c and c a t h o d i c p a r t i a l r e a c t i o n s
at t h e e q u i l i b r i u m p o t e n t i a l . The e m p i r i c a l e q u a t i o n
w h i c h o f t e n d e s c r i b e s t h e exchange c u r r e n t d e n s i t y is
a
= nFk [Red
(32)
where ^ is t h e p e r t u r b e d e l e c t r o d e p o t e n t i a l a s s o c i
ated
with a net current density, i . Overpotentials
f o r anodic r e a c t i o n s a r e p o s i t i v e w h i l e those f o r
cathodic reactions are negative.
A t l a r g e a n o d i c o v e r p o t e n t i a l s , E q u a t i o n 30
becomes
i = i exp[n/e ]
(33)
= B n(i/i )
(34)
or
a
where is t h e a n o d i c T a f e l c o n s t a n t o r s l o p e (see
a b o v e ) . E q u a t i o n 34 i n d i c a t e s t h a t - i d a t a f o r an
e l e c t r o d e r e a c t i o n are l i n e a r over a s p e c i f i e d range.
T h i s l i n e a r r e g i o n p r o v i d e s a c c e s s t o two i m p o r t a n t
e m p i r i c a l p a r a m e t e r s in E q u a t i o n 30, v i z . , t h e a n o d i c
T a f e l s l o p e and t h e exchange c u r r e n t d e n s i t y . The form
e r is t h e s l o p e o f t h e l i n e in t h i s l i n e a r r e g i o n . The
exchange c u r r e n t d e n s i t y is o b t a i n e d by e x t r a p o l a t i n g
52
CORROSION
CHEMISTRY
the l i n e f o r t h e l i n e a r r e g i o n d a t a t o t h e e q u i l i b r i u m
p o t e n t i a l , i . e . , =0. I n s p e c t i o n o f E q u a t i o n 34 shows
t h a t t h e c u r r e n t d e n s i t y o b t a i n e d by t h i s e x t r a p o l a t i o n
is n u m e r i c a l l y e q u a l t o t h e exchange c u r r e n t d e n s i t y .
At l a r g e (negative) c a t h o d i c o v e r p o t e n t i a l s ,
E q u a t i o n 30 becomes
i = -i exp[-n/B ]
(35)
= -B n(-i/i )
(36)
or
c
where 3 is t h e c a t h o d i c T a f e l c o n s t a n t o r s l o p e . The
u t i l i t y o f E q u a t i o n 36 f o r the a n a l y s i s o f e m p i r i c a l
k i n e t i c p a r a m e t e r s f o r c a t h o d i c p a r t i a l r e a c t i o n s is
identical to that fo
reactions.
When e l e c t r o c h e m i c a l r a t e d a t a , i . e . , e l e c t r o d e
p o t e n t i a l - c u r r e n t d e n s i t y d a t a , a r e p l o t t e d , i t is
o f t e n done on s e m i - l o g a r i t h m i c p a p e r . S i n c e t h e l o g
a r i t h m i c s c a l e o f t h i s paper is u s u a l l y "base 10",
t h e T a f e l s l o p e s w h i c h a r e measured a r e r e l a t e d t o
t h e T a f e l c o n s t a n t s in E q u a t i o n 30 by
c
= b/2.30
(37)
mV v s .
mV v s .
mV v s .
SCE
2
2
2
SCE
SCE
mA/cm
mA/cm
mA/cm
280
150
- 2.6
100
210
0.96
270
57
0.42
205
140
- 3.1
260
32
200
0.00
- 3.9
130
250
18
- 4.6
110
195
-0.35
245
13
190
-0.64
110
- 5.6
240
9.5
- 6.8
100
185
-0.90
235
7.2
80
-10
180
-1.15
230
5.1
175
-1.37
60
-15
225
3.8
-21
40
170
-1.60
220
20
-32
2.5
165
-1.80
215
1.76
160
-2.10
-47
0
155
-2.30
2.
DONAHUE
Electrochemical
Techniques
53
Solution:
The d a t a a r e p l o t t e d in F i g u r e 7. The a n o d i c and
c a t h o d i c T a f e l s l o p e s ("base 10") a r e 40 and 120
mV, r e s p e c t i v e l y , and t h e exchange c u r r e n t d e n s i t y
is 1.0 mA/cm . T h e r e f o r e , t h e c u r r e n t d e n s i t y electrode p o t e n t i a l behavior o f t h i s a c t i v a t i o n
c o n t r o l l e d e l e c t r o d e r e a c t i o n is d e s c r i b e d by
i = 1.0[exp(n/17.4)-exp(-n/52.2)]
2
where t h e c u r r e n t d e n s i t y is in mA/cm
o v e r p o t e n t i a l is in mV.
Transport
Controlled Electrode
and t h e
Reactions.
As n o t e d above
from t h e s u r f a c e is
r e a c t i o n . The c o n v e c t i v e mass t r a n s p o r t e q u a t i o n s
w h i c h d e s c r i b e t h i s m a t e r i a l f l u x ( i n terms o f c u r r e n t
d e n s i t y and c o n s i s t e n t w i t h t h e s i g n c o n v e n t i o n p r o posed in t h i s c h a p t e r ) a r e
i =
(n/|v
R e d
(38)
and
i = -(n/|v |)Fk ([0x] -[0x] )
0 x
(39)
54
CORROSION C H E M I S T R Y
I
0.1
1.0
10
l_
100
2
DONAHUE
Electrochemical
Techniques
CORROSION
56
CHEMISTRY
la
( n /
la
le
l Redl
) F k
[ R e d ]
(40)
(n/|v
-(n/|v |)Fk [Ox]
0 x
(42)
(43)
Ic
10
cm / s e c .
Solution:
The mass t r a n s p o r t c o r r e l a t i o n f o r a r o t a t i n g
c y l i n d e r is (90
Sh = 0.0627 R e
Sc
where, in t h i s c a s e ,
Sh = Sherwood Number = k r/D
2
Re = Reynolds Number = wr /v
2 / 3
1 / 3
2.
DONAHUE
Electrochemical
57
Techniques
2 / 3
1 / 3
= O.0627 (D/r) ( a i r / v )
(v/D)
-3
= 6.58X10
cm/sec.
The e l e c t r o c h e m i c a l r e a c t i o n f o r oxygen r e d u c t i o n
+
0 + 4H + 4e -* 2H 0
Then, t h e l i m i t i n g c u r r e n t d e n s i t y , i . e . , E q u a t i o n
42, is
he = - 4 F k [ O x ]
is
= (-4 e q u i v / m o l e ) ( 9 . 6 5 X 1 0
(6.58X10"
= -2.54X10"
cm/sec)(10~
A-sec/equiv)
7
mole/cm )
A/cm .
C r i t e r i a f o r Contro
I t is u s e f u l t o be a b l e t o a s c e r t a i n a p r i o r i
w h i c h t y p e o f c o n t r o l o p e r a t e s f o r an e l e c t r o d e r e a c t i o n . I n o r d e r t o d e m o n s t r a t e how t h i s may be
a c h i e v e d , c o n s i d e r a c a t h o d i c p a r t i a l r e a c t i o n . The
key element in a s c e r t a i n i n g c o n t r o l is t h e r a t i o o f
the h y p o t h e t i c a l a c t i v a t i o n c o n t r o l l e d c u r r e n t dens i t y , E q u a t i o n 35, t o t h e l i m i t i n g c u r r e n t d e n s i t y ,
e.g., E q u a t i o n 42. T h i s r a t i o is
i/i
(44)
F o r e l e c t r o d e r e a c t i o n s where t h i s r a t i o is 0.3 o r
l e s s , a c t i v a t i o n c o n t r o l o p e r a t e s . When t h e r a t i o is
1.0 o r g r e a t e r , t h e e l e c t r o d e r e a c t i o n is under
transport control.
I n s p e c t i o n o f E q u a t i o n 44 u n d e r l i n e s t h e r e l a t i v e
i m p o r t a n c e o f v a r i o u s p a r a m e t e r s in t h e s t u d y o f
e l e c t r o d e r e a c t i o n s . I t is e v i d e n t t h a t t h e a n a l y s i s
of a c t i v a t i o n c o n t r o l l e d e l e c t r o c h e m i c a l data f o r
r e a c t i o n s w i t h l a r g e exchange c u r r e n t d e n s i t i e s is
r e s t r i c t e d t o a s m a l l range o f o v e r p o t e n t i a l u n l e s s
the mass t r a n s f e r c o e f f i c i e n t is l a r g e . The mass t r a n s f e r c o e f f i c i e n t is g e n e r a l l y l a r g e f o r e x p e r i m e n t a l
arrangements l i k e r o t a t i n g d i s k s , r o t a t i n g c y l i n d e r s
and o t h e r systems c a p a b l e o f a c h i e v i n g h i g h f l u i d
v e l o c i t y r a t e s (90 . On t h e o t h e r hand, i f one wants t o
s t u d y t h e r e g i o n o f mass t r a n s p o r t c o n t r o l , i t is
n e c e s s a r y t o o p e r a t e at l a r g e ( n e g a t i v e , in t h e c a s e
o f c a t h o d i c r e a c t i o n s ) o v e r p o t e n t i a l s and l o w concent r a t i o n s o f t h e r e a c t a n t s p e c i e s . However, t h i s
58
CORROSION
CHEMISTRY
Processes
Mixed P o t e n t i a l Model o f C o r r o s i o n .
M e t a l l i c c o r r o s i o n processes which a r e chemical
in n a t u r e (see above) c a n be w r i t t e n in t h e f o l l o w i n g
g e n e r a l form
M + 0x
e n v
- M
Z +
+ Red
(45)
e n v
where M r e p r e s e n t s t h
d i z e d form o f t h e m e t a l ( f o r c o n v e n i e n c e , i t is w r i t
t e n as a m e t a l l i c i o n ) , Ox
represents the species
in t h e aqueous s o l u t i o n
which r e a c t s w i t h the
m e t a l in t h e c o r r o s i o n r e a c t i o n and Red
is t h e
m o d i f i e d form o f t h a t s p e c i e s . I n s p e c t i o n o f E q u a t i o n
45 shows t h a t i t is composed o f two e l e c t r o c h e m i c a l
partial reactions, i . e . ,
n
M -> M
z +
+ ze
(46)
and
0^
+ ze" + R e d ^
env
env
(47)
The a n o d i c p a r t i a l p r o c e s s , E q u a t i o n 46, g e n e r a t e s t h e
e l e c t r o n s w h i c h a r e used in t h e c a t h o d i c p a r t i a l p r o
c e s s , E q u a t i o n 47. T h i s model o f c o r r o s i o n p r o c e s s e s
is based on t h e t h e o r y o f mixed p o t e n t i a l s (11) and is
shown s c h e m a t i c a l l y in F i g u r e 9. The o r i g i n a l t h e o r y
o f mixed p o t e n t i a l s was based on t h e " s u p e r p o s i t i o n "
of p o l a r i z a t i o n curves f o r the r e s p e c t i v e p a r t i a l pro
c e s s e s (11-13). However, s i n c e many mixed p o t e n t i a l
systems ( p a r t i c u l a r l y c o r r o s i o n p r o c e s s e s ) i n v o l v e
i n t e r a c t i o n s among t h e r e a c t a n t s , t h e p r e s e n t a t i o n
o f mixed p o t e n t i a l s g i v e n h e r e w i l l c o n s i d e r t h e more
r e c e n t approach c o n s i d e r i n g t h e s e i n t e r a c t i o n s ( 1 4 ) .
M e t a l s where c o r r o s i o n p r o c e s s e s t a k e p l a c e a r e
u s u a l l y i s o l a t e d , i . e . , n o t in c o n t a c t w i t h an e x t e r
n a l e l e c t r i c a l c i r c u i t . Charge c o n s e r v a t i o n is t h e
n e c e s s a r y c o n d i t i o n f o r a mixed p o t e n t i a l p r o c e s s t o
l e a d t o c o r r o s i o n on an i s o l a t e d m e t a l , i . e . ,
.i.(
) = 0
j j corr'
V M
(48)
2.
DONAHUE
Electrochemical
Techniques
59
.j (c o r r ') =
r
E q u a t i o n 49 w i l l be used s u b s e q u e n t l y t o d e v e l o p e x
p r e s s i o n s f o r t h e c o r r o s i o n c u r r e n t d e n s i t y f o r spe
c i f i c examples o f c o r r o s i o n systems.
When a m e t a l e l e c t r o d e is made p a r t o f an e l e c
t r i c a l c i r c u i t (see b e l o w ) , t h e n e t c u r r e n t d e n s i t y ,
i . e . , i = I / A , at a p e r t u r b e d p o t e n t i a l , . , in u n i f o r m
c o r r o s i o n is
i = _. ( )
(50)
E q u a t i o n 50 w i l l be used s u b s e q u e n t l y t o d e v e l o p
e l e c t r o c h e m i c a l r a t e e q u a t i o n s f o r s p e c i f i c examples
o f c o r r o s i o n systems.
Two A c t i v a t i o n C o n t r o l l e d P a r t i a l
Processes.
corr
(51)
S u b s t i t u t i n g t h e a p p r o p r i a t e forms o f t h e r e s p e c t i v e
c u r r e n t d e n s i t i e s f o r t h e p a r t i a l p r o c e s s e s and t h i s
d e f i n i t i o n in E q u a t i o n 49, one o b t a i n s
CORROSION C H E M I S T R Y
60
oa
e x
^>c^<<W*corr>/*cc>
i c o r r = ioaexp((
) / 3 )
= i (( -
)/ )
oc
o c c o r r " ce
r
(53)
A m a t h e m a t i c a l r e l a t i o n s h i p s i m i l a r t o E q u a t i o n 31 c a n
u s u a l l y be o b t a i n e d e x p e r i m e n t a l l y f o r t h e c o r r o s i o n
current density.
F o r a c o r r o d i n g m e t a l w h i c h is c o n n e c t e d t o an
e l e c t r i c a l c i r c u i t ( i n order t o study i t s electrode
p o t e n t i a l - c u r r e n t d e n s i t y p r o p e r t i e s - see b e l o w ) ,
the o v e r p o t e n t i a l is
= .-
= (.-
corr'
)+(
V Y
- )
corr '
(54)
S u b s t i t u t i n g t h e a p p r o p r i a t e forms o f t h e r e s p e c t i v e
c u r r e n t d e n s i t i e s and t h i s d e f i n i t i o n in E q u a t i o n 50,
one o b t a i n s
i = ioa ((
3 )((.-
^
c o r r -
o a )' / aa
^ * c o r r " )a/a3 )
) / U > exp ( ( - > / * )
- e x p ( ( -
(55)
V V Y
o c
cc
* i - * c o r r
a a
cc
density,
- ^ P ^ c o r r - ^ / f W
( 5 6 )
(57)
2.
DONAHUE
Electrochemical
61
Techniques
w h i c h is t h e m i x e d p o t e n t i a l a n a l o g
I n s e r t i n g E q u a t i o n 5 7 in 5 6 y i e l d s
1
- corr
I e x
( e / e
aa ,
e x
P -
e / 6
of
cc
overpotential.
) 1
( 5 8 )
S i n c e E q u a t i o n 5 8 is i d e n t i c a l in f o r m t o E q u a t i o n 3 0 ,
the a n a l y s i s of data for mixed p o t e n t i a l systems
is
t h e same a s t h a t f o r s i m p l e e l e c t r o d e
reactions.
In s t u d i e s of r e a c t i o n mechanisms of the p a r t i a l
p r o c e s s e s , t h e f o l l o w i n g f o r m o f E q u a t i o n 50
is
useful
(15)
= nF{k[Red]
[Ox]
exp(
( -)/B
a a
-k[Red]
W h e n d a t a a r e t a k e n at f i x e d p o t e n t i a l ( s )
in
the r e
spective linear region(s)
(the s o - c a l l e d T a f e l
region)
of electrode p o t e n t i a l - c u r r e n t density
experiments
(so-called p o l a r i z a t i o n experiments) with appropriate
v a r i a t i o n o f c o n c e n t r a t i o n o f one o r more of
the
species,
the r e a c t i o n order(s) of the species
may
be computed, e . g . ,
for cathodic
data,
8
inlOx]
o)
(60)
,[Red],T
With j u d i c i o u s c h o i c e of e x p e r i m e n t a l c o n d i t i o n s and
appropriate a n a l y s i s of the data, the parameters
in
E q u a t i o n 59 c a n b e o b t a i n e d w i t h r e l a t i v e e a s e .
Once
these parameters are s p e c i f i e d , the e l u c i d a t i o n of
the r e a c t i o n mechanism(s)
c a n be a t t e m p t e d .
However,
no d i s c u s s i o n o f r e a c t i o n m e c h a n i s m s o r t h e
methods
u s e d t o d e v e l o p them w i l l be g i v e n h e r e .
I l l u s t r a t i o n 6. A n a l y s i s o f A c t i v a t i o n C o n t r o l l e d
Mixed P o t e n t i a l Data.
T h e d a t a g i v e n in t h e a c c o m p a n y i n g t a b l e s
are
f o r i r o n in s u l f u r i c a c i d . D e t e r m i n e :
a) t h e c o r r o s i o n p o t e n t i a l s a n d t h e
corrosion
current d e n s i t i e s for the three
systems;
b) t h e s p e c i f i c r a t e c o n s t a n t a n d t h e r e a c t i o n
order for hydrogen ion for free
corrosion;
c) t h e s p e c i f i c r a t e c o n s t a n t s a n d t h e r e a c t i o n
orders for hydrogen ion for the p a r t i a l
CORROSION
CHEMISTRY
processes.
+
[H ]
M
0.055
0.100
0.250
System
A
Symbol
mV v s . SHE
-300
-325
-350
-375
-400
4 -8.80X10"
4 -1.44X10"
-425
-1.52X10"
- 3.31X10"
-450
-3.06X10"
-500
-8.27X10"
-550
-600
-2.16X10"
-5.66X10"
-5.70X10"
3
-1.50X10" -3.76X10"
3
-3.94X10" -9.84X10"
2
-1.03X10" -2.57X10"
Solutions :
a) The d a t a a r e p l o t t e d in F i g u r e 10. E x t r a p o l a t
ion of the T a f e l region data to the r e s p e c t i v e
intersections yields:
System
corr
-406
corr
1.33X10 -4
1.79X10 -4
-390
C
-367
2.85X10*-4
where t h e c o r r o s i o n p o t e n t i a l s a r e mV v s . SHE
and t h e c o r r o s i o n c u r r e n t d e n s i t i e s a r e A/cm .
b) I n o r d e r t o d e t e r m i n e t h e r e a c t i o n o r d e r o f
hydrogen i o n f o r f r e e c o r r o s i o n , p l o t l o g i
v s . l o g (hydrogen i o n c o n c e n t r a t i o n )
,i.e.,
the l o w e s t l i n e in F i g u r e 11. The s l o p e o f t h e
l i n e , 0.5, is t h e r e a c t i o n o r d e r f o r hydrogen
i o n under t h e s e c o n d i t i o n s . To compute t h e
s p e c i f i c r a t e c o n s t a n t , one n o t e s t h a t
2
corr/
4
where n=2 and F=9.65X10 A - s e c / e g u i v and k
" c o n t a i n s " t h e terms a s s o c i a t e d w i t h "Red".
DONAHUE
Electrochemical
M z+
Figure 9.
Techniques
_
Red.
env
env
SOLUTION
METAL
-300
CO
>
-400
3
W
EH
U
W
Q
-500 h
U
W
W
-600
CURRENT DENSITY
Figure 10.
(A/cm )
CORROSION C H E M I S T R Y
2.
DONAHUE
Electrochemical
65
Techniques
Then,
System
A
2.94X10"
2.93X10"
2.95X10~
mean v a l u e = 2.94X10""
c) I n o r d e r t o d e t e r m i n e t h e r e a c t i o n o r d e r s f o r
hydrogen i o n f o r t h e p a r t i a l p r o c e s s e s , choose
^=-325 mV f o r t h e a n o d i c p a r t i a l p r o c e s s d a t a
and .=-500 mV f o r t h e c a t h o d i c p a r t i a l
process
d a t a . P l o t t h e l o g (^) v s . l o g [ H ] ,
i . e . , t h e upper p a i r o f l i n e s in F i g u r e 1 1 .
The s l o p e s o f t h e s e l i n e s a r e t h e r e s p e c t i v e
reaction orders i . e .
=-1.0
d
=+1.0
The s p e c i f i
(see de Bethune () f o r t h e s t a n d a r d e l e c t r o d e
p o t e n t i a l s and T a f e l s l o p e s a r e from F i g u r e 10)
+
k=i [ H ] / n F
a a
and
exp((-325-(-440))/17)
System
A/ni
^a
1.58X10"
8.65X10"
3.35X10*"
A/Cnt
5.46X10"
11
8.27X10*"
4.30X10"
5.44X10"
11
5.27X10"
11
12
1.50X10"
4.29X10""
12
3.76X10"
4.30X10"
12
The mean v a l u e s o f t h e c o n s t a n t s a r e :
k=5.39X10"
a
l:L
12
66
CORROSION C H E M I S T R Y
corr
= i
((
)/3
oa ^
corr oa"
(n/|v |)Fk [Ox]
0 x
aa
(61)
E q u a t i o n 61 d e m o n s t r a t e s t h a t the c o r r o s i o n r a t e f o r
t h i s c l a s s o f systems is c o n t r o l l e d u n i q u e l y by the
by the r a t e o f mass t r a n s p o r t . Comparing E q u a t i o n 61
w i t h E q u a t i o n 53 r e v e a l s t h a t t h e c o r r o s i o n p o t e n t i a l
is d e f i n e d by the n a t u r e s o f t h e a n o d i c and c a t h o d i c
p a r t i a l processes f o
at hand, the c o r r o s i o
magnitude o f t h e mass t r a n s f e r c o e f f i c i e n t - a p r o p e r t y
o f the c o n v e c t i v e mass t r a n s p o r t c o n d i t i o n .
The e l e c t r o d e p o t e n t i a l - c u r r e n t d e n s i t y b e h a v i o r
o f t h i s t y p e o f c o r r o s i o n system is
i - icorr^Pte/'W-
1 1
( 6 2 )
E q u a t i o n 62 p r e d i c t s T a f e l b e h a v i o r o n l y f o r a n o d i c
( p o s i t i v e ) p o l a r i z a t i o n . C a t h o d i c p o l a r i z a t i o n is
p r e d i c t e d t o be p o t e n t i a l i n d e p e n d e n t at l a r g e n e g a t i v e
p o l a r i z a t i o n s . However, f o r most c o r r o s i o n s y s t e m s ,
t h i s r e g i o n o f p o t e n t i a l independence is s m a l l due
to the presence of other c a t h o d i c p a r t i a l processes,
e.g., s o l v e n t d e c o m p o s i t i o n t o form hydrogen gas.
While these other cathodic p a r t i a l processes u s u a l l y
do n o t p a r t i c i p a t e in the c o r r o s i o n system per s e ,
t h e y a r e m a n i f e s t e d in the e x p e r i m e n t a l d a t a and can
cause d i f f i c u l t y in a n a l y z i n g t h e d a t a . Methods o f
compensating f o r t h e s e e f f e c t s have been employed
w i t h s u c c e s s (16_, 17) .
I l l u s t r a t i o n 7. E v a l u a t i o n o f Mass T r a n s p o r t
C o n t r o l l e d Data.
The d a t a g i v e n below a r e f o r c a t h o d i c p o l a r i z a t i o n
o f i r o n in 0.01 M s u l f u r i c a c i d in t h e p r e s e n c e
o f 0.53 M f e r r i c i o n . E s t i m a t e the l i m i t i n g c u r
r e n t d e n s i t y and mass t r a n s f e r c o e f f i c i e n t f o r
the r e d u c t i o n o f f e r r i c i o n under t h e s e c o n d i t i o n s .
2.
DONAHUE
Electrochemical
Techniques
67
-
mV v s . SHE
360
380
400
450
500
550
600
650
700
mA/cm'
0
1.30
4.54
4.89
5.37
5.96
7.57
11.6
22.0
Solution:
The d a t a a r e p l o t t e d in F i g u r e 12. A l t h o u g h t h e r e
is no u n e q u i v o c a l l y p o t e n t i a l i n d e p e n d e n t r e g i o n ,
t h e d a t a between -400 and -550 mV a p p r o x i m a t e
t h a t b e h a v i o r . Th
d e n s i t y over t h i
t o be t h e b e s t e s t i m a t e o f t h e l i m i t i n g c u r r e n t
density.
The e l e c t r o d e r e a c t i o n f o r f e r r i c i o n r e d u c t i o n is
mole/cm )
M u l t i p l e P a r t i a l Process
C o r r o s i o n Systems.
A l t h o u g h most c o r r o s i o n systems c a n be d e s c r i b e d
by t h e l i m i t i n g models p r e s e n t e d above, t h e r e a r e in
s t a n c e s where c o n t r o l o f t h e c o r r o s i o n system is a
combination o f both types, v i z . , a c t i v a t i o n c o n t r o l l e d
a n o d i c p a r t i a l p r o c e s s w i t h two c a t h o d i c p a r t i a l p r o
c e s s e s - one under a c t i v a t i o n c o n t r o l and a n o t h e r
under t r a n s p o r t c o n t r o l . Examples a r e i r o n c o r r o s i o n
in a c i d s o l u t i o n w i t h i n o r g a n i c c o n t a m i n a n t s (16_, 18)
and oxygen (7) . The c o r r o s i o n c u r r e n t d e n s i t y in
such systems is
corr
corr ' cc
)Pk [Ox]
c
(63)
CORROSION C H E M I S T R Y
68
-300
-500
-700
10
-3
0
10
-2
Z
2.
DONAHUE
Electrochemical
Techniques
69
Techniques
A l t h o u g h t h e o t h e r a u t h o r s in t h i s monograph have
d e s c r i b e d t h e i r e x p e r i m e n t a l systems in d e t a i l , i t is
worthwhile t o o u t l i n e the e s s e n t i a l features o f the
experimental e l e c t r o c h e m i c a l techniques which a r e
used t o measure and e v a l u a t e t h e v a r i o u s p a r a m e t e r s
d i s c u s s e d in t h i s c h a p t e r .
E l e c t r o d e P o t e n t i a l Measurements
The methods o f m e a s u r i n g e l e c t r o d e p o t e n t i a l s in
t h e absence o f an e x t e r n a l l y a p p l i e d c u r r e n t have
been g i v e n above. I n g e n e r a l , t h e s e comments a r e
a p p l i c a b l e t o p o l a r i z a t i o n e x p e r i m e n t s , as w e l l . The
m a j o r d i f f e r e n c e between " e q u i l i b r i u m " and p o l a r i z a t i o n measurements is t h a t , in t h e l a t t e r c a s e , an
ohmic v o l t a g e drop is p r e s e n t between t h e t e s t and
r e f e r e n c e e l e c t r o d e s due t o t h e f l o w o f c u r r e n t
t h r o u g h t h e r e s i s t i v e e l e c t r o l y t e (between t h e t e s t
and a u x i l i a r y - see below- e l e c t r o d e s ) . I n o r d e r t o
m i n i m i z e t h i s e f f e c t ( i t i n t r o d u c e s an e r r o r in t h e
measurements w h i c h v i r t u a l l y p r e c l u d e s t h e c o r r e c t
a p p l i c a t i o n o f , f o r example, E q u a t i o n 5 8 ) , one uses
a L u g g i n c a p i l l a r y (shown s c h e m a t i c a l l y in F i g u r e 1 3 ) .
S i n c e no c u r r e n t f l o w s in t h e v o l t a g e m e a s u r i n g c i r c u i t
(between t h e t e s t and r e f e r e n c e e l e c t r o d e s ) , t h e pot e n t i a l o f t h e s o l u t i o n at t h e t i p o f t h e c a p i l l a r y
is e q u a l t o t h e s o l u t i o n p o t e n t i a l at t h e r e f e r e n c e
e l e c t r o d e . T h e r e f o r e , i f t h e c a p i l l a r y t i p is l o c a t e d
close to the t e s t electrode, the actual i n t e r f a c i a l
p o t e n t i a l d i f f e r e n c e c a n be measured. B a r n a r t t (19)
has g i v e n a detail, ed a n a l y s i s o f t h e e f f e c t s o f such
capillaries.
E l e c t r o c h e m i c a l P o l a r i z a t i o n Systems.
F i g u r e 13 is a s c h e m a t i c r e p r e s e n t a t i o n o f t h e
t h r e e e l e c t r o d e system n o r m a l l y used in e l e c t r o c h e m i c a l
p o l a r i z a t i o n s t u d i e s . T h i s system i n c l u d e s two s e p a r a t e
e l e c t r i c a l c i r c u i t s . One o f t h e s e , between t h e t e s t
and r e f e r e n c e e l e c t r o d e s , is a v o l t a g e m e a s u r i n g
70
CORROSION C H E M I S T R Y
Figure IS.
2.
DONAHUE
Electrochemical
Techniques
71
CORROSION C H E M I S T R Y
72
2.
DONAHUE
Electrochemical
73
Techniques
is o n l y w i t h a p o t e n t i o s t a t t h a t one c a n measure t h e
passive current density, i
, (which p r o v i d e s a b a s i s
f o r d e t e r m i n i n g whether
anodic p r o t e c t i o n o f a
s t r u c t u r e is a d v i s a b l e ) .
p
Symbols/Nomenclature
Symbol
A
a
b
Cat
e"
F
Significance
Surface area
Activity
T a f e l s l o p e (base "10"
C a t a l y s t in r e a c t i o n
Electron
Faraday c o n s t a n t
Thermodynami
lation
K i n e t i c / T r a n s p o r t calcu
4
G
H
I
i
k
k
c
n
M
Ox
R
Red
l a t i o n s (=9.65X10 )
Free Enthalpy
Enthalpy
Current
Current density
S p e c i f i c rate constant
Mass t r a n s f e r c o e f f i c i e n t
Units
2
cm
V
-
A-sec/equiv
cal/mole
cal/mole
A
A/cm
(variable)
cm/sec
2
Natural logarithm
Metal
Flux
mole/cm s e c
Number o f e l e c t r o n s in
equiv
reaction
O x i d i z e d form o f c h e m i c a l
species
Gas c o n s t a n t (=1.99)
cal/mole-K
Reduced form o f c h e m i c a l
species
Temperature
K
Number o f e l e c t r o n s in
metal o x i d a t i o n r e a c t i o n -D
mole/cm
Concentration
V
T a f e l s l o p e (base "e")
Reaction order of Cat
Polarization
V
V
Electrode potential
Overpotential
V
Chemical p o t e n t i a l
cal/mole
S t o i c h i o m e t r i c c o e f f i c i e n t mole
R e a c t i o n o r d e r o f Red
R e a c t i o n o r d e r o f Ox
-
CORROSION C H E M I S T R Y
74
Symbol
Significance
Units
Superscripts.
z+
Standard p r o p e r t y
I o n i c c h a r g e on m e t a l l i o n -
Subscripts.
a
b
c
corr
e
env
J
j
satn
Y
298
P r o p e r t y o f anode o r
anodic r e a c t i o n
Property o f bulk
solution
P r o p e r t y o f cathode o r
cathodic reaction
P r o p e r t y at f r e e c o r r o s i o n Property of e l e c t r o n
Propert
Propert
species
Property o f " j t h " par
t i a l process
Property o f product
species
L i m i t i n g (maximum) p r o p
erty
Equilibrium property
P r o p e r t y at s a t u r a t i o n
P r o p e r t y o f any c h e m i c a l
species
P r o p e r t y at 298K
Literature Cited
1. P o u r b a i x , M. " A t l a s o f
Electrochemical
Equilibria",
Pergamon, O x f o r d , 1966.
2. - "JANAF Thermochemical T a b l e s " , U. S. Department
o f Commerce, Washington, 1965.
3. L e w i s , G. ., Randall, M., Pitzer, K. and Brewer,
L. "Thermodynamics", 2nd Edition, pp. 669-686,
M c G r a w - H i l l , New Y o r k , 1961.
4. de Bethune, A. J., Licht, T. S. and Swendeman, .,
J. E l e c t r o c h e m . Soc., (1959), 106, 616.
5. L o u t h a n , M. R. " P r o c e s s
Industries
Corrosion",
pp. 126-134, NACE, Houston, 1975.
6. Uhlig, H. H. " C o r r o s i o n and C o r r o s i o n
Control",
2nd
Edition,
pp. 60-91, W i l e y , New Y o r k , 1971.
7. Vetter, K. J. " E l e c t r o c h e m i c a l
Kinetics",
Academic,
New Y o r k , 1967.
8. W e l t y , J. R., W i c k s , C. E. and W i l s o n , R. E. "Fun
d a m e n t a l s o f Momentum, Heat and Mass T r a n s f e r " ,
pp. 578-589, W i l e y , New Y o r k , 1969.
2.
DONAHUE
Electrochemical Techniques
9.
75
D o n a h u e , F. M . " F u n d a m e n t a l s o f
Electrochemical
Engineering",
Chapter
IX,
Engineering
Summer C o n
ferences,
University
of
Michigan,
A n n Arbor, 1 9 7 8 .
1 0 . D o n a h u e , F. M .
"Physicochemical
Processes
f o r Water
Quality
Control",
W. J. W e b e r ,
Editor,
pp. 467-468,
Wile-Interscience,
New York, 1 9 7 2 .
11. W a g n e r , C . a n d Traud, W . , Z .
Elektrochem.,
(1938),
44, 391.
12.
Stern,
M . and G e a r y , A .
L.,
J.
Electrochem.
Soc.,
(1957),
104, 56.
13.
Stern,
M.
J.
Electrochem.
Soc.,
(1957), 104, 645.
14. Donahue,
F.
M.,
J.
Electrochem.
Soc.,
(1972), 119,
72.
15. D o n a h u e , F. M . " F u n d a m e n t a l s o f
Electrochemical
Engineering",
Chapter
X,
Engineering
Summer C o n
ferences,
University
of
Michigan,
Ann
Arbor,
1978
16.
Makrides,
A.
C.,
J.
Electrochem.
Soc.,
107, 869.
1 7 . D o n a h u e , F. M . a n d N o b e , K . " S e c o n d
International
Congress on
Metallic
Corrosion",
pp. 916-924,
NACE, H o u s t o n , 1966.
18.
Gatos,
H.,
Corrosion,
(1956), 12, 322t.
19.
Barnartt,
S.,
J.
Electrochem.
Soc.,
(1961), 108,
102.
RECEIVED September 1, 1978.
3
High-Temperature Corrosion
J. B R U C E W A G N E R , JR.
Center for Solid State Science, Arizona State University, Tempe, A Z 85281
T h e p u r p o s e of t h i s review p a p e r is to s u r v e y the
princi
ples of h i g h t e m p e r a t u r e o x i d a t i o n o r h i g h t e m p e r a t u r e
corro
s i o n . A typical s i t u a t i o
w h i c h c a n act a s a n o x i d a n t
duct f o r m s a layer w h i c h s e p a r a t e s the r e a c t a n t s , the m e t a l and
the gas a t m o s p h e r e .
U n d e r special c o n d i t i o n s , the k i n e t i c s a r e
d i f f u s i o n c o n t r o l l e d , i.e., the r a t e of the reaction (the r a t e of
o x i d e t h i c k n e s s g r o w t h ) depends on the d i f f u s i o n of s p e c i e s ,
i o n s and e l e c t r o n s , t h r o u g h the layer ( s o m e t i m e s c a l l e d a tar
nish
layer).
Actually
when a m e t a l o r alloy is e x p o s e d to a cor
rosive g a s , the r e a c t i o n k i n e t i c s m a y be c o n t r o l l e d by one o r
m o r e of the f o l l o w i n g steps:
1.
T r a n s p o r t of r e a c t a n t g a s e s to the s u r f a c e .
2.
T r a n s p o r t of r e a c t a n t s ( o r p r o d u c t s ) t h r o u g h a b o u n d a r y
layer adjacent to the s u r f a c e .
3.
A s u r f a c e c o n t r o l l e d reaction ( p h a s e b o u n d a r y r e a c t i o n ) at
the g a s - m e t a l i n t e r f a c e .
4.
T r a n s p o r t of r e a c t a n t s t h r o u g h a corrosion p r o d u c t l a y e r
e i t h e r b y b u l k d i f f u s i o n o r by migration t h r o u g h cracks and
pores.
In the p r e s e n t p a p e r , a t t e n t i o n will be f o c u s e d on the f o u r t h s t e p
i n v o l v i n g b u l k d i f f u s i o n . T h i s is a
classical
electrochemical
s i t u a t i o n i n v o l v i n g a n anode (the m e t a l ) w h e r e o x i d a t i o n o c c u r s
and a cathode (the o x i d e at the o x i d e - g a s i n t e r f a c e ) w h e r e r e d u c
t i o n of o x y g e n occurs. T h e o x i d e layer a c t s as the s o l v e n t f o r
point defects w h i c h diffuse t h r o u g h i t as will be d i s c u s s e d b e l o w .
C o n s i d e r the d i a g r a m s h o w n in Figure 1. T h e o x i d e layer
t h i c k e n s w i t h t i m e and s o the r a t e of o x i d a t i o n ( g o v e r n e d by d i f
f u s i o n t h r o u g h the o x i d e l a y e r ) d e c r e a s e s w i t h t i m e , t . T h i s s p e c c i a l s i t u a t i o n y i e l d s the p a r a b o l i c r a t e l a w f i r s t r e p o r t e d by
T a m m a n (j_) and by P i l l i n g and B e d w o r t h (2).
T a m m a n ' s rate
e q u a t i o n was s t a t e d in t e r m s of t a r n i s h l a y e r t h i c k n e s s , ,
0-8412-0471-3/79/47-089-076$05.00/0
1979 American Chemical Society
3.
WAGNER
High-Temperature
Corrosion
77
and is
whence
()
= 2k.pt
(2)
= V
(3)
CORROSION C H E M I S T R Y
film thickness,
or weight
go in per unit
area, Am/A
time
>
Figure 1. Schematic of film thickness or gaininweight per unit area vs. time for
oxidation of a pure metal where diffusion is rate controlling. The kinetics are
denoted as parabolic oxidation kinetics.
time
Figure 2.
>
3.
WAGNER
High-Temperature
79
Corrosion
remained for C a r l Wagner (4) to p e r f o r m the c l a s s i c e x p e r i ment to distinguish the mobile species in a c o r r o s i o n e x p e r i ment. H i s experimental set-up is shown in Figure 4. The overa l l reaction he studied was
2Ag(s) + S ( 1 ) = A g S ( s ) .
2
2.
(4)
^ M^ 0^
3.
(5)
Anti-Schottky Disorder:
CORROSION C H E M I S T R Y
METAL OXIDE
inert
BEFORE
OXIDATION
M A
AFTER
inert
markers
OXIDATION
Figure 3. Schematic location of inert markers before oxidation (on the surface of
the pure metal) and after oxidation (at the metal-metal oxide interface). From
this limiting case one may infer that the mobile species diffuses from the metalmetal oxide interface outward through the scale or tarnish layer. If the marker
were found at the oxide-gas interface, the inference would be that the mobile
species diffused from the oxide-gas interface to the metal-oxide interface.
S (liquid)
+ 2Ag -2e-=Ag S === S
R
2Ag+ 2e~
] Ag -108 mg
Figure 4. Schematic of the experimental setup used by C. Wagner (4) to determine the location of the reaction 2kg + S = Ag S and the migrating species (silver) through the artificially prepared tarnish layer of Ag S separating the reactants,
silver and liquid sulfur
2
3.
WAGNER
High-Temperature
81
Corrosion
4.
i ^
(*>
A n t i - F r e n k e l D i s o r d e r : E q u a l c o n c e n t r a t i o n s of a n i o n
vacancies and anion interstitials
K
5.
= t i ^
= [O';][O:']
(7)
Anti-Structural Disorder:
and anions on cation sites
K
Anti-Str.
0 ^ M ^
= o
+ v ^
+h*
(9)
w h e r e t h e n o t a t i o n o f K r 8 g e r a n d V i n k is a g a i n u s e d .
S u p e r i o r p r i m e s and heavy dots denote effective negative
and positive c h a r g e s , r e s p e c t i v e l y .
Ions o n n o r m a l l a t t i c e s i t e s
are designated with no effective charge while defects a r e d e s i g
nated w i t h e f f e c t i v e c h a r g e s r e l a t i v e to the n o r m a l i o n s .
Thus a
n i c k e l o u s i o n o n a n o r m a l s i t e in n i c k e l o x i d e is d e n o t e d a s N i - ^
a n d a n i c k e l i c i o n in N i O w o u l d b e d e n o t e d a s N i j ^ . T h e
" e x t r a " o x y g e n is a c c o m o d a t e d o n a n o r m a l l a t t i c e s i t e a n d a
cobalt i o n vacancy (with a single effective negative charge) plus
o n e c o m p e n s a t i n g e l e c t r o n h o l e is f o r m e d . A n a l t e r n a t i v e
d e s c r i p t i o n o f t h e e l e c t r o n h o l e is a c o b a l t i c i o n ( C o
or Co )
s i t u a t e d in a s u b l a t t i c e o f n o r m a l l y c o b a l t o u s i o n s . T h e e q u i l i
b r i u m c o n s t a n t f o r E q . ( 9 ) is w r i t t e n a s
+ + +
io
= [v ][h*]/
= exp(-AG^Q /RT)
p*
(10)
= e x p C f - ^ +TAS^)/RT]
(11)
CORROSION C H E M I S T R Y
82
1 0
= C v ^
/ P
'
T h u s i f one s o l v e s e x p l i c i t l y f o r the c a t i o n v a c a n c y
o r the e l e c t r o n hole
concentration
concentration,
- ^ " - - S ^
Because D
ce [ V < ^ ] t h e n i f o n e m e a s u r e d t h e r a d i o t r a c e r d i f f u s i o n o f c o b a l t in C o O , t h e i s o t h e r m a l o x y g e n p r e s s u r e d e p e n dence should exhibit a one-quarter dependence.
T h i s is e x a c t l y
w h a t C a r t e r a n d R i c h a r d s o n (6_) d i d . T h e i r r e s u l t s a r e s h o w n
in F i g u r e 5. T h e e l e c t r o n i c c o n d u c t i v i t y , , is
Q
a=[h]u q
(14)
w h e r e the s y m b o l u, d e n o t e s the m o b i l i t y of a n e l e c t r o n h o l e
a n d is h e r e a s s u m e d n o t t o b e d e p e n d e n t o n c o m p o s i t i o n .
Be
c a u s e [ h ] cc p o f t h e n t h e i s o t h e r m a l e l e c t r o n i c c o n d u c t i v i t y
s h o u l d a l s o be d e p e n d e n t u p o n the o n e - q u a r t e r p o w e r of the
oxygen pressure.
T h i s behavior was r e p o r t e d by E r o r and
W a g n e r (7_) ( s e e F i g u r e 6 ) .
T h e d i f f u s i v i t y o f o x y g e n is n e g l i
g i b l e c o m p a r e d to c o b a l t a c c o r d i n g to m a r k e r s t u d i e s
and
t o s t a b l e o x y g e n i s o t o p e d i f f u s i o n s t u d i e s ( J^O JJL)
Thus
w h e n c o b a l t is o x i d i z e d , t h e m i g r a t i n g s p e c i e s s h o u l d b e
cobalt v i a cation vacancies and electrons (as electron
holes).
F o r an oxide g r o w i n g on a m e t a l by a bulk diffusion c o n
trolled process,
the
flux as
eq
c m sec
z
RT
F
f
P
(t
1 +
t )t q
2
| |
2
u
p
ffOj
P Q
<
3. W A G N E R
High-Temperature
Corrosion
83
Journal of Metals
Figure 5. Tracer diffusion in cobaltous oxide as a function of oxygen pressure
[and hence Co/O ratio given by Equations 9 and 13]. The symbols (X) denote
data obtained by a sectioning technique while () denote data by the surface
decrease method. The slopes of the lines are approximately one-fourth, indicating
the existence of singly ionized cation vacancies (6).
CORROSION C H E M I S T R Y
84
w h e r e t h e f l u x n / A is t h e r a t e o f o x i d e f o r m a t i o n p e r u n i t a r e a ,
F is F a r a d a y ' s c o n s t a n t , N
is A v o g a d r o s n u m b e r , q is t h e
e l e c t r o n i c c h a r g e , t d e n o t e s a t r a n s f e r e n c e n u m b e r a n d the s u b
s c r i p t s 1, 2 a n d 3 d e n o t e t h e m e t a l i o n , t h e o x y g e n i o n a n d
e l e c t r o n , r e s p e c t i v e l y . T h e t o t a l e l e c t r i c a l c o n d u c t i v i t y is .
L o c a l e q u i l i b r i u m is a s s u m e d t o o c c u r at t h e m e t a l - o x i d e i n t e r
f a c e a n d a l s o at t h e o x i d e - g a s i n t e r f a c e . T h e r e f o r e , t h e c h e m i
c a l p o t e n t i a l o f o x y g e n is f i x e d at e a c h i n t e r f a c e . T h e o x y g e n
p r e s s u r e at t h e m e t a l - o x i d e i n t e r f a c e is f i x e d a s t h e d i s s o c i a
t i o n p r e s s u r e of the o x i d e a n d d e n o t e d a s P o T h e o x y g e n
p a r t i a l p r e s s u r e in t h e g a s p h a s e , p 5 > is at e q u i l i b r i u m at t h e
o x i d e - g a s i n t e r f a c e . ( S e e F i g u r e 7) T h i s e q u a t i o n m a y be
written as
k
Q
A
"
w h e r e k is t h e r a t i o n a l r a t e c o n s t a n t * e x p r e s s e d a s e q / c m - s e c .
I n o t h e r w o r d s , t h e f l u x is i n v e r s e l y p r o p o r t i o n a l t h e f i l m
t h i c k n e s s - - j u s t the r e q u i r e m e n t of the p a r a b o l i c r a t e l a w .
When t
t i o r t , ( t h e o x i d e is p r i m a r i l y a n e l e c t r o n i c c o n
d u c t o r ) the e q u a t i o n m a y be r e w r i t t e n u s i n g the N e r n s t - E i n s t e i n
equation,
r
D =
u.
B. kT = iV- kT.
|z.|q
(17)
H e r e B ^ is t h e a b s o l u t e m o b i l i t y o f t h e i t h s p e c i e s , u^ t h e d r i f t
m o b i l i t y , D f the s e l f d i f f u s i o n c o e f f i c i e n t a n d the other t e r m s
h a v e t h e i r u s u a l s i g n i f i c a n c e . It f o l l o w s t h a t
w h e r e C q d e n o t e s the n u m b e r of e q u i v a l e n t s of o x i d e p e r c c .
P a r t i c u l a r n o t e is m a d e o f t h e f a c t t h a t t h e t r a n s p o r t n u m b e r s
a n d the d i f f u s i o n c o e f f i c i e n t s a r e b e h i n d the i n t e g r a l b e c a u s e
the p a r a m e t e r s depend d e c i s i v e l y o n the m e t a l - t o - o x y g e n r a t i o
a n d h e n c e o n the e f f e c t i v e v a l u e of the o x y g e n p o t e n t i a l . T h e
v a l e n c e of the c a t i o n and a n i o n , z a n d z a r e b e h i n d the i n t e
gral.
e
T h e r a t i o n a l r a t e c o n s t a n t , 1 ^ , is r e l a t e d t o t h e T a m m a n
rate constant,
k
= k^/v.
1
2)
3.
WAGNER
High-Temperature
Corrosion
85
F r e q u e n t l y , the v a l u e s of a n d D
are very dissimilar
a n d o n e t e r m in b r a c k e t s E q . ( 1 8 ) m a y b e n e g l e c t e d .
For
e x a m p l e , in t h e c a s e o f t h e o x i d a t i o n o f c o b a l t , i t w a s n o t e d
e a r l i e r t h a t D D Q in C o O . C o n s e q u e n t l y , t h e s e c o n d t e r m
in b r a c k e t s m a y b e n e g l e c t e d a n d
(19)
O f t e n a u t h o r s p l o t the l o g a r i t h m of the p a r a b o l i c r a t e c o n s t a n t ( u s u a l l y k p o r fop) v e r s u s l o g P Q a n d i n f e r f r o m t h e
oxygen p r e s s u r e dependence ( 1/n) a m e c h a n i s m .
If o n e m e c h a n i s m d o m i n a t e s a c r o s s t h e o x i d e l a y e r , t h a t is, o n e m e c h a n i s m
is p r e d o m i n a t e b e t w e e n P Q ' a n d t h e u p p e r l i m i t f o r P Q ^ , t h e n
2
1_
[
0
J_
( 2 0 )
w h e r e the v a l u e o f a n d i t s s i g n w i l l d e p e n d o n t h e t y p e o f d e
f e c t s in t h e o x i d e .
F o r p-type oxides s u c h as cobaltous o x i d e ,
1/n = + 1/ 4 , for cuprous oxide 1/n = + 1/8, etc.
F o r an n-type
oxide such as Z n O , P Q
~ /
D e t a i l s of the d e f e c t s t r u c t u r e
o f m a n y c o m p o u n d s m a y be f o u n d , f o r e x a m p l e , in t h e b o o k b y
K r o g e r ( 12).
N o t e t h a t a n I n c r e a s e in o x y g e n p r e s s u r e (po
) r e s u l t s in
a n i n c r e a s e in o x i d a t i o n r a t e . H o w e v e r t h e s i g n o f t h e e x p o n e n t
o n t h e o x y g e n p r e s s u r e in E q . ( 2 0 ) e x e r t s a l a r g e e f f e c t .
For
c o b a l t o u s o x i d e , 1/n = + 1/4.
A n i n c r e a s e in o x y g e n r e s u l t s in
a n i n c r e a s e in t h e c o n c e n t r a t i o n o f c a t i o n v a c a n c i e s a n d a c o n
s e q u e n t i n c r e a s e in o x i d a t i o n r a t e . B u t f o r s o m e m e t a l s , t h e
c h a n g e in o x i d a t i o n r a t e w i t h o x y g e n p r e s s u r e is s m a l l .
For
example, zinc oxide growing on zinc m e t a l . The dominant de
f e c t s in z i n c o x i d e a r e s i n g l y i o n i z e d z i n c i n t e r s t i t i a l s a n d c o m
p e n s a t i n g e l e c t r o n s ( i . e. , Z n / O > 1 ) . T h e e q u a t i o n m a y be
written as
=
+ 1/2 0 .
= [Zn!][e']
(21)
p i
u
is
(22)
is
[Zn:]=[e']
(.23)
86
CORROSION C H E M I S T R Y
so that
[']
=/K
2 2
(24)
2
H e n c e , i n c r e a s i n g the o x y g e n p r e s s u r e o v e r z i n c o x i d e g r o w i n g
o n z i n c m e t a l a f f e c t s the o x i d a t i o n r a t e v e r y l i t t l e b e c a u s e d i s f u s i o n t h r o u g h z i n c o x i d e is v i a i n t e r s t i t i a l z i n c i o n s .
These
l i m i t i n g c a s e s a r e s h o w n s c h e m a t i c a l l y in F i g u r e 8 ( 1 3 ) ,
T h e t e m p e r a t u r e d e p e n d e n c e o f t h e k i n e t i c s at c o n s t a n t
o x y g e n p r e s s u r e is o f t e n p l o t t e d a s l o g k p o r l o g krp v e r s u s
1/T.
In a d d i t i o n to the m i g r a t i o n e n t h a l p y of the m o b i l e s p e c i e s , the
s l o p e of s u c h a n A r r h e n i u s p l o t m a y r e f l e c t a n e n t h a l p y f o r the
c h a n g e in c o m p o s i t i o n o f t h e o x i d e w i t h t e m p e r a t u r e .
The oxidation rat
self diffusion data. C o n v e r s e l y
to c a l c u l a t e s e l f d i f f u s i o n d a t a . E q . ( 18) m a y be r e a r r a n g e d
a n d the r a t e c o n s t a n t d i f f e r e n t i a t e d w i t h r e s p e c t to l o g o x y g e n
p r e s s u r e to y i e l d
CrfhD?
'
I
} =
f ^
eq
(25)
d l n
P<D
Di
Moreover
When
and z i = z
, the e q u a t i o n m a y be s i m p l i f i e d .
the p r a c t i c a l s c a l i n g c o n s t a n t s m a y be
into E q . (25) to y i e l d
s
P
D
= constant
d log
d
introduced
(26)
T h u s i f o n e m e a s u r e s k p ( o r k-p ) a s a f u n c t i o n o f o x y g e n p r e s
s u r e a n d p l o t s the data a s k p ( o r krp) v e r s u s l o g P Q * , the t a n
g e n t t o t h e c u r v e g e n e r a t e d y i e l d s a v a l u e o f D at a n y f i x e d
PQ S u c h p r o c e d u r e s h a v e b e e n u s e d b y F . S. P e t t i t [ F e O
( l l ) ] , K . F u e k i a n d J. B . W a g n e r [ N i O ( 1 5 _ ) , M n O ( 1 6 ) ] a n d
b y S . M r o w e c a n d c o w o r k e r s [ C o O (17_, 1 8 ) , C u ( 1 9 ) ] .
The
a g r e e m e n t b e t w e e n s e l f d i f f u s i o n data c a l c u l a t e d f r o m d i r e c t
r a d i o t r a c e r d i f f u s i o n data and those c a l c u l a t e d f r o m o x i d a t i o n
k i n e t i c s is r e m a r k a b l y g o o d i n d i c a t i n g t h e v a l i d i t y o f C .
Wagner's theory.
x
7 7
S o m e m e t a l - o x y g e n s y s t e m s e x i s t w i t h m o r e than one
stable oxide compound.
E x a m p l e s a r e C o O and C o 0 ; F e O ,
Fe 0
and F e 0 ; and C u 0 and C u O . When oxygen p r e s s u r e s
h i g h enough to n u c l e a t e a s e c o n d p h a s e a r e e n c o u n t e r e d ,
the
r a t e of o x i d a t i o n m a y be a l t e r e d d r a m a t i c a l l y . A s a n e x a m p l e ,
3
3. W A G N E R
High-Temperature
METAL
Corrosion
OXIDE
87
0 GAS
2
METAL
GAS
metal
deficit
GAS
The upper figure shows the effect of increasing oxygen pressure on a metal deficit
oxide, e.g., cobaltous oxide. See Equations
9, 10, 11, 12, and 13. The lattice defects in
CoO are V '. Parabolic oxidation proceeds
via diffusion of cobalt ions migrating by
means of V '. Thus increasing p " exerts
a large influence on the oxidation rate of
cobalt. The lowerfigureshows the effect of
oxygen partial pressure on a metal excess
oxide growing on a metal, e.g., zinc oxide.
The lattice predominant defectsinZnO are
Z n / . See Equations 21, 22, 23, and 24.
Hence increasing the oxygen pressure does
not appreciably affect the oxidation rate (13).
Co
Vo
02
CORROSION
88
CHEMISTRY
c o n s i d e r the o x i d a t i o n o f p u r e c o p p e r t o c u p r o u s o x i d e .
The
r a t e of o x i d a t i o n i n c r e a s e s w i t h i n c r e a s i n g o x y g e n p r e s s u r e
u n t i l t h e c u p r i c o x i d e p h a s e is f o r m e d o n t o p o f t h e g r o w i n g
cuprous oxide.
In t h i s e x a m p l e o x i d a t i o n p r o c e e d s v i a d i f f u s i o n
of c o p p e r ( v i a s i n g l y i o n i z e d c o p p e r i o n v a c a n c i e s ) t h r o u g h the
inner oxide.
When P Q
a t t a i n s the v a l u e f o r the c o e x i s t e n c e of
C u O a n d C u O , t h e g r a d i e n t in o x y g e n a c t i v i t y a n d c a t i o n v a c a n
c y c o n c e n t r a t i o n is f i x e d ( P Q ' e q u a l s t h e d i s s o c i a t i o n p r e s s u r e
f o r C u 0 or the c o e x i s t e n c e o x y g e n p r e s s u r e f o r C u a n d C u 0 ) .
T h e v a l u e o f t h e o x y g e n p r e s s u r e at t h e C u 0 - C u O b o u n d a r y is
f i x e d a n d b e c a u s e d i f f u s i o n o f c o p p e r t h r o u g h t h e C u 0 is r a t e
l i m i t i n g , t h e k i n e t i c s b e c o m e i n d e p e n d e n t o f o x y g e n p r e s s u r e at
t h i s p o i n t . T h i s s i t u a t i o n is s h o w n s c h e m a t i c a l l y in F i g u r e 9 .
O x i d a t i o n of a l l o y s i n t r o d u c e s a h i g h e r d e g r e e of c o m
p l e x i t y . W h e n t w o c o m p o n e n t s a r e p r e s e n t , e a c h is c o m p e t i n g
f o r t h e o x y g e n in a c c o r
o x i d e p e r g r a m a t o m of o x y g e n .
F o r illustrative purposes, con
sider again limiting cases.
T h e f i r s t is a b i n a r y a l l o y , A B ,
w h i c h is v e r y d i l u t e in o n e c o m p o n e n t , e . g . , 9 9 . 5 at $ A a n d
0 . 5 at $ B . F u r t h e r m o r e , a s s u m e t h e r e s p e c t i v e o x i d e s , A O
a n d B O f o r m s o l i d s o l u t i o n s . S u c h a l l o y s of n i c k e l h a v e b e e n
e x t e n s i v e l y s t u d i e d . W h e n n i c k e l c o n t a i n i n g s m a l l a m o u n t s of
c h r o m i u m is o x i d i z e d , t h e o x i d a t i o n k i n e t i c s a r e m o r e r a p i d t h a n
for pure nickel.
T h e r e a s o n is t h a t t h e s m a l l a m o u n t o f c h r o m
i u m e n t e r s the n i c k e l o x i d e c r e a t i n g a g r e a t e r c o n c e n t r a t i o n of
n i c k e l i o n v a c a n c i e s w h i c h in t u r n i n c r e a s e s t h e d i f f u s i o n o f
n i c k e l a n d h e n c e t h e p a r a b o l i c o x i d a t i o n k i n e t i c s (20_).
The
relevant equations a r e as follows.
F o r p u r e n i c k e l o x i d e the
d e v i a t i o n f r o m s t o i c h i o m e t r y is s i m i l a r t o t h a t f o r c o b a l t o u s
o x i d e e x c e p t t h a t d o u b l y i o n i z e d c a t i o n v a c a n c i e s f o r m in p u r e
N i at e l e v a t e d t e m p e r a t u r e s .
2
+ 2h\
(27)
;]*/p f.
(28)
is
(29)
so that
t
(30)
3. W A G N E R
T
or
kp
k
High-Temperature
Corrosion
CUgO
89
Cu 0 + CuO
<
dissoc. pressure
of CuO - Cu^P
log R "
>
CORROSION
90
^ 3 =
N i
increases
N i
CHEMISTRY
a c c o r d i n g l y the o x i d a t i o n r a t e i n c r e a s e s ( 2 ) .
Conversely*
a d d i n g a n a c c e p t o r ( L i ^ ) d e c r e a s e s the c o n c e n t r a t i o n of c a t i o n
v a c a n c i e s and the r a t e d e c r e a s e s ' * ( 2 ) .
S u c h m o d i f i c a t i o n of
o x i d a t i o n k i n e t i c s h a s b e e n t e r m e d the d o p i n g e f f e c t by a l i o v a l e n t a d d i t i o n s . T h e d e f e c t c h e m i s t r y ( n u m b e r a n d t y p e s of
lattice and electronic defects) has been tested u s i n g chiefly e l e c
t r i c a l c o n d u c t i v i t y but a l s o t h e r m o g r a v i m e t r y a n d d i f f u s i o n
studies on doped c r y s t a l s . T h e technique has been used e x t e n
s i v e l y a n d q u a n t i t a t i v e p r e d i c t i o n s c a n be r e a d i l y m a d e p r o v i d e d
t h e d e f e c t s t r u c t u r e o f t h e h o s t l a t t i c e is k n o w n . F o r e x a m p l e ,
in t h e c a s e o f n i c k e l c o n t a i n i n
k (Ni-Cr
p
alloy)
k (pure Ni)
' = const.
m o l e f r a c t i o n C r * . in d o p e d N i O
1
.
m o l e f r a c t i o n h in p u r e N i O
T
<*
(32)
T h i s h o l d s f o r d i l u t e a l l o y s a n d d i l u t e s o l i d s o l u t i o n of o x i d e s .
If h i g h e r c o n c e n t r a t i o n s o f c h r o m i u m a r e u s e d , a s e c o n d p h a s e
of C r 0
m a y f o r m on the outer s u r f a c e .
Consequently,a situa
t i o n s i m i l a r to t h a t d i s c u s s e d a b o v e f o r the f o r m a t i o n of C u O o n
C u 0 obtains. However, transport through C r 0
is v e r y s l o w
and that step b e c o m e s r a t e l i m i t i n g w i t h a c o n c u r r e n t d r a m a t i c
d e c r e a s e in o x i d a t i o n r a t e o f t h e a l l o y .
2
A n o t h e r l i m i t i n g c a s e is t h a t o f a n a l l o y , A B , w h i c h
o x i d i z e s t o f o r m o n l y o n e o x i d e , A O . A n e x a m p l e is t h e n i c k e l platinum alloy.
P l a t i n u m d o e s n ' t o x i d i z e a n d o n l y N i O is
formed.
W h e n a g i v e n a l l o y is o x i d i z e d at h i g h t e m p e r a t u r e s ,
t h e f o r m a t i o n o f N i O u s e s u p t h e n i c k e l in t h e s u r f a c e o f t h e
alloy.
M o r e n i c k e l m u s t d i f f u s e to the s u r f a c e f r o m the b u l k of
t h e a l l o y in o r d e r t o f o r m m o r e N i O . S i m u l t a n e o u s l y , p l a t i n u m
T h e p r e p a r a t i o n o f a L i - N i a l l o y is e x c e e d i n g l y d i f f i c u l t o w i n g
to v a p o r i z a t i o n p r o b l e m s .
The theory has been tested by
v a p o r i z i n g L i onto the g r o w i n g N i O as N i w a s o x i d i z e d .
A
d e c r e a s e in o x i d a t i o n r a t e w a s o b s e r v e d . (2 )
2
3.
WAGNER
High-Temperature
91
Corrosion
Ni (in alloy) + J 0
= NiO
(33)
Ni Of
T h e N i O is p u r e s o i t s a c t i v i t y is s e t e q u a l t o u n i t y in
Eq. (34).
H o w e v e r the a c t i v i t y of n i c k e l , a ^ , d e c r e a s e s w i t h
t i m e ( t h e a l l o y b e c o m e s m o r e c o n c e n t r a t e d in P t ) s o t h e o x y g e n
p a r t i a l p r e s s u r e at t h e a l l o y - N i O i n t e r f a c e i n c r e a s e s w i t h t i m e
(see E q . (34)j T h i s effec
c h a n g e in o x y g e n p r e s s u r
P Q / constant).
F u r t h e r m o r e , different alloys w i l l oxicnze
at d i f f e r e n t r a t e s .
F o r a l l o y s r i c h in n i c k e l , t h e k i n e t i c s w i l l
b e a l m o s t t h o s e o f p u r e n i c k e l . B u t a s t h e c o n c e n t r a t i o n of
p l a t i n u m i n c r e a s e s b e y o n d a c e r t a i n c r i t i c a l v a l u e , d i f f u s i o n of
n i c k e l to the m e t a l - m e t a l o x i d e s u r f a c e w i l l be r a t e - d e t e r
m i n i n g a n d a l a r g e d e c r e a s e in o x i d a t i o n r a t e w i l l o c c u r , i . e . ,
t h e r a t e l i m i t i n g s t e p w i l l b e d i f f u s i o n in t h e a l l o y a n d n o t d i f
f u s i o n t h r o u g h the oxide s c a l e .
a
A n o t h e r l i m i t i n g c a s e is a n a l l o y , A B , n e i t h e r c o m p o n e n t
o f w h i c h i n i t i a l l y r e a c t s t o f o r m a n e x t e r n a l s c a l e a n d in w h i c h
component A has a m u c h greater affinity for oxygen than B . A
c l a s s i c a l e x a m p l e is t h e s i l v e r - i n d i u m a l l o y s y s t e m .
Silver
d o e s n ' t f o r m a n o x i d e at e l e v a t e d t e m p e r a t u r e s , b u t i t d o e s
dissolve oxygen.
S m a l l a m o u n t s of i n d i u m w i l l t h e r e f o r e f o r m
a n o x i d e w i t h i n the a l l o y m e t a l , a s o - c a l l e d s u b s c a l e o r i n t e r n a l
oxide.
T h e p r o c e s s is d i f f u s i o n c o n t r o l l e d a n d t h e d i f f u s i o n o f
o x y g e n in t h e a l l o y is r a t e l i m i t i n g .
Such systems have been
s t u d i e d e x t e n s i v e l y by R . A . R a p p , e s p e c i a l l y the t r a n s i t i o n
f r o m o n l y i n t e r n a l o x i d a t i o n to the f o r m a t i o n of a n e x t e r n a l
scale (22).
W h e n both c o m p o n e n t s of a h o m o g e n e o u s , one p h a s e a l l o y
r e a c t w i t h o x y g e n , t h e r e is a c o m p e t i t i o n f o r t h e o x y g e n in
t e r m s of the a f f i n i t y f o r o x y g e n a s m e n t i o n e d a b o v e .
If t h e
o x i d e s s o f o r m e d a r e i m m i s c i b l e , the n u c l e a t i o n a n d g r o w t h of
e a c h o x i d e p h a s e a s w e l l a s d i s p l a c e m e n t r e a c t i o n s m u s t be
considered.
A s a n e x a m p l e , c o n s i d e r 8 5 - 1 5 b r a s s (85 wt $
c o p p e r = 15 w t $ z i n c ) o x i d i z e d at 7 0 0 C . F o r t h e f i r s t h o u r
t h e k i n e t i c s a r e n e a r l y p a r a b o l i c , i . e. , d i f f u s i o n c o n t r o l l e d .
CORROSION C H E M I S T R Y
92
S0 (g) = SQ(inNiO) +0
2
+e'
(35)
a n d t h a t i f s u l f u r a c t s a s a d o n o r , i t a f f e c t s N i O in t h e s a m e
w a y a s c h r o m i u m d o p i n g , i . e. , c r e a t e s c a t i o n v a c a n c i e s .
H o w e v e r , t h e s o l u b i l i t y o f s u l f u r is v e r y s m a l l [ a b o u t 1 0 / c c
(26_)1 a n d s o m e r e p o r t e d i n c r e a s e s in r a t e s w e r e f a r in e x c e s s
of that w h i c h E q . ( 3 5 ) c o u l d a c c o u n t f o r .
W o r r e l l (27) m a d e a
v e r y important suggestion.
In S 0 - 0
gas m i x t u r e s , it s o m e t i m e s h a p p e n s that two p h a s e ( d u p l e x ) s c a l e s a r e f o r m e d .
T h e s e s c a l e s c o n s i s t of t h i n c h a n n e l s o r s t r i n g e r s of s u l f i d e
e m b e d d e d in a m a t r i x o f o x i d e .
B e c a u s e d i f f u s i o n in t h e s u l f i d e s is s o m u c h f a s t e r t h a n d i f f u s i o n in o x i d e s , t h e r a t e is
d e t e r m i n e d by d i f f u s i o n t h r o u g h the c h a n n e l s . T h i s t y p e of
1 8
3. W A G N E R
High-Temperature
Corrosion
93
Ni-R ALLOY
CONC.
Figure 10. Schematic of the concentration profile of nickelina Ni-Pt alloy being
oxidized to form NiO as a growing oxide scale. The concentration profile of
nickel ion vacancies in NiOisalso shown. In the example discussed, the rate
determining stepisthe diffusion of nickel from the bulk alloy to the surface (and
of platinuminthe opposite direction) rather than diffusion of nickel via vacancies
in NiO (13).
CORROSION C H E M I S T R Y
94
p a r a l l e l d i f f u s i o n p a t h w i l l be v e r y i m p o r t a n t in c o r r o s i o n o f
m u l t i p h a s e a l l o y s that f o r m m u l t i p h a s e s c a l e s .
In t h e f o r e g o i n g , l i m i t i n g c a s e s o f v a r i o u s h i g h t e m p e r a
ture, i s o t h e r m a l corrosion situations have been briefly d i s
cussed.
A c t u a l l y in m o s t e n g i n e e r i n g u s e s , t h e c o r r o s i o n is
n o t i s o t h e r m a l b u t i n v o l v e s c y c l i c t h e r m a l t r e a t m e n t ( e . g . , in
a turbine engine).
In a d d i t i o n to s e e k i n g a n o x i d e s c a l e that
w i l l e x h i b i t s l o w d i f f u s i o n o f t h e r e a c t a n t s , i t is n e c e s s a r y t h a t
the o x i d e r e s i s t t h e r m a l shockM o s t of the p r o t e c t i v e o x i d e s
involve A 1 0 , C r 0
or S i 0 .
E f f o r t s to i n c r e a s e the a d h e r
e n c e of the o x i d e on the m e t a l h a v e b e e n d i r e c t e d to a d d i t i o n s
of c e r t a i n r a r e e a r t h m e t a l o x i d e s to the m e t a l o r a l l o y .
These
o x i d e s a r e u s u a l l y i n s o l u b l e in b o t h t h e m e t a l a n d t h e o x i d e
scale.
S u r p r i s i n g l y , the s m a l l p a r t i c l e s of t h e s e a d d e d o x i d e s
often s e g r e g a t e to the
to p r o j e c t up f r o m th
i n c r e a s e in r e s i s t a n c e t o s p a l l i n g d u r i n g t h e r m a l s h o c k .
One
s u g g e s t i o n is t h a t t h e o x i d e p a r t i c l e s a c t a s " p e g s " t o p i n t h e
oxide scale.
O t h e r e x p l a n a t i o n s i n v o l v e c h a n g e s in t h e i n t e r
face c h e m i s t r y and concurrent adhesion between m e t a l and
s c a l e o r a c h a n g e in p l a s t i c i t y o f t h e o x i d e s c a l e .
In a n y c a s e
t h e e f f e c t is i m p o r t a n t a n d t h e m o s t a p p r o p r i a t e e x p l a n a t i o n
awaits s o m e challenging surface c h e m i s t r y and m i c r o a n a l y s i s .
T h e foregoing s u r v e y was focused on situations where
biilk diffusion p r o c e s s e s were rate determining.
Such systems
a r e a m e n a b l e to a n a l y s i s u s i n g a n e l e c t r o c h e m i c a l
approach.
Other factors such as transport down pores or c r a c k s , v o l a t i
l i z a t i o n o r m e l t i n g of the o x i d e s c a l e m a y o c c u r a n d r e q u i r e
d i f f e r e n t a n a l y s e s but d i f f u s i o n c o n t r o l l e d p r o c e s s e s m a y be
m a t h e m a t i c a l l y m o d e l e d and c o r r e l a t e d w i t h the defect c h e m
i s t r y of the c o r r o s i o n p r o d u c t .
These limiting cases provide
a g u i d e to u n d e r s t a n d i n g t h e m o r e c o m p l e x p h e n o m e n a f r e
quently encountered.
2
Literature Cited
1.
3. WAGNER
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
95
High-Temperature Corrosion
C a r t e r , R. E . a n d Richardson, F. D., J. M e t a l s ( 1 9 5 4 )
6, 1244.
Eror,
N. G. a n d W a g n e r , J. . , Jr., J. P h y s . C h e m .
S o l i d s ( 1 9 6 8 ) , 29, 1597.
Carter,
R. E . a n d Richardson, F. D., J. M e t a l s ( 1 9 5 5 )
7, 336.
P e t t i t , F. S. and W a g n e r , J. . , Jr., Acta Metall. ( 1 9 6 4 )
12, 415.
C h e n , W. K. and J a c k s o n , R. . , J. P h y s . C h e m . S o l i d s
( 1 9 6 9 ) , 30, 1309.
H o l t , J. B., P r o c . Brit. C e r a m . Soc., ( 1 9 6 7 ) , 9, 157.
Krger, F. A . , "The Chemistry of I m p e r f e c t
Crystals,"
N o r t h H o l l a n d P u b l i s h i n g Co., Amsterdam, 1964.
W a g n e r , C., m i m e o g r a p h e d c o u r s e n o t e s , c o u r s e 3.63,
D e p a r t m e n t of Metallurgy,
P e t t i t , F. S., J. E l e c t r o c h e m
F u e k i , K. a n d W a g n e r , J. B., Jr., J. E l e c t r o c h e m . Soc.
( 1 9 6 5 ) , 112, 284.
F u e k i , K. a n d W a g n e r , J. B., Jr., J. Electrochem. Soc.
( 1 9 6 5 ) , 112, 970.
M r o w e c , S., Bull. A c a d . P o l o n . S c i . , S e r , Sci. C h i m .
( 1 9 6 7 ) , XV, 373.
M r o w e c , S., T. W a l e c a n d T. W e r b e r , Corrosion Sci.
( 1 9 6 6 ) , 6, 287.
M r o w e c , S. a n d Stoklosa, Bull. A c a d . P o l o n . Sci., S e r .
Sci. C h i m . ( 1 9 7 0 ) , X V I I I , 5 2 3 ; O x i d a t i o n of Metals, (1971)
3, 291.
P f e i f f e r , H. a n d H a u f f e , ., Z. M e t a l l k . ( 1 9 5 2 ) , 4 3 , 364.
H a u f f e , ., " O x i d a t i o n of Metals," English Translation by
K. Vorres, P l e n u m Press, . ., 1965.
R a p p , R. . , Corrosion, NACE ( 1 9 6 5 ) , 2 1 , 382.
L e v i n , R. L. a n d W a g n e r , J. B., Jr., J. E l e c t r o c h e m .
Soc. ( 1 9 6 1 ) , 108, 954.
A l c o c k , C. . , H o c k i n g , M. G. a n d Z a d o r , S.,
Corrosion
S c i . ( 1 9 6 9 ) , 9, 111.
W a g n e r , J. B., Jr., in " D e f e c t s and Transport in O x i d e s , "
(Seltzer,
M. S. a n d J a f f e e , R. I . , e d s . ) p. 283, P l e n u m
Press, . . , 1974.
C h a n g , D. R., Ph.D. Thesis, N o r t h w e s t e r n U n i v . 1973.
Worrell,
W. L . , lecture "The SO -O o x i d a t i o n of metals,"
p r e s e n t e d at the U. S. J o i n t S e m i n a r , " D e f e c t s and Dif
fusion in Solids," O c t o b e r 4-6, 1976, T o k y o , J a p a n , to be
published.
2
4
Ionic and Electronic Conduction in Nonmetallic Phases
J O H N W.
PATTERSON
PART I
Historical Background
About the turn of the century and shortly thereafter, certain
developments in mathematical physics and in physical chemistry
were realized which wer
and charge transport in
Smoluchowski(2) i n i t i a t e d the modern theory of Brownian motion
by idealizing it as a problem in random f l i g h t s . Then some seventeen years or so later, Joffee(3) proposed that i n t e r s t i t i a l defects could form inside the l a t t i c e of ionic crystals and play a
role in electrical conductivity. The f i r s t tenable model for ionic
conductivity was proposed by Frenkel(4), who recognized that vacancies and i n t e r s t i t i a l s could form internally to account for ion
movement.
Figure 1 is a schematic representation of Frenkel's notion:
an atom or ion can get dislodged from i t s normal site to form an
interstitial-vacancy pair. He further proposed that they do not a l ways recombine but instead may dissociate and thus contribute to
diffusional transport and e l e c t r i c a l conduction. They were free
to wander about in a "random walk" manner essentially equivalent to
that of Brownian motion... this meant they should exhibit a net
d r i f t in an applied f i e l d .
In the fluids considered by Einstein and Smoluchowski, a l l
species large and small are capable of substantial migration at a l l
times. In solid crystals, however, only the i n t e r s t i t i a l atoms and
those next to vacant sites can enjoy any significant amount of motion. Thus, i t was realized that the concentrations of mobile defects are the important things, at least in connection with the ionic
conductivity of crystals. An elaborate analysis of thermodynamic
equilibria of point defects was then developed by Wagner and
Schottky (5z_) in which the laws relating defect concentrations to impurities, ambient p a r t i a l pressures and temperature were worked out
in detail.
Wagner followed this in 1933 by combining v i r t u a l l y a l l the
foregoing concepts to explain the phenomenon of parabolic tarnishing of metal in aggressive environments ^ . He assumed that transport of neutral species was negligible compared to that of ions and
0-8412-0471-3/79/47-089-096$07.50/0
1979 American Chemical Society
4.
PATTERSON
Conduction
in
Nonmetallic
Phases
-
Figure 1.
97
98
CORROSION
CHEMISTRY
I n
4.
PATTERSON
Conduction
in
Nonmetallic
99
Phases
Wagner's tarnishing theory but significant modifications are required for the non-open c i r c u i t c a s e s
) . in short, we can
say that the present understanding of solid-state electrochemistry
is largely due to Wagner's electrochemical theory of tarnishing
and moreover that the concepts as he i n i t i a l l y elucidated them
have remained in tact to the present time. In part II of this paper,
less simple mixed conducting phases and non open c i r c u i t conditions
w i l l be considered
2
100
CORROSION C H E M I S T R Y
II
- u
=
M
M
4.
PATTERSON
Conduction
in
Nonmetallic
Phases
101
ii
h - h y
2
X 2
= -h
l = - V
= - h Ap
X 2
3
/Z F
Since the valences of the cations and anions are always opposite in
sign and the two chemical potential differences are always opposed
in scaling, i t follows that both voltages w i l l have the same sign.
As a matter of fact, i t can be shown that they are also of precisely
the same value for two component scales. This equality, which de
rives from the so called Gibbs-Duhem relation between -^ ^ ^2'
merely means that the two batteries
and V are equivalent to,
and hence may be replaced by, the single one shown dottedin on the
figure.
If voltages
and V simulate the chemical driving forces ac
ting on the ions during scaling, the resistors
and R simulate
the scale's resistance to cation and anion migration. Its imped
ance to electron flow is represented by the single resistor R 3
which is shown connected a l l the way across the scale thickness.
The resistor R 3 simulates the electronic leakage path which weakens
the coulombic fields and allows ion migration to continue indefi
nitely rather than halting. The usual definitional formulas hold
anc
CORROSION C H E M I S T R Y
102
SUBSTRATE
METAL M
AMBIENT ATMOSPHERE
(ACTUAL X GAS,
PARTIAL PRESSURE = p"
SCALING LAYER
ab
EFFECTIVE
CATIONS
X GAS
PARTIAL PRESSURE = Pi
I ANIONS J 2
ELECTRONS J -
Yl
iCATIONS
I !
-
1
CIRCUIT
ANALOG
OF SCALE
*ion
^2jANIONS
R
ELECTRONS
EXTERNAL
LEADS
- oSCALE EMF
Industrial and Engineering Chemistry,
Product Research and Development
Figure 3.
PATTERSON
4.
Conduction
in
Nonmetallic
103
Phases
i -
1 / G
i =
< = 1-2.3
1
Here R is in ohms, the conductance G; is in reciprocal ohms, L
and A are the scale's thickness (cm) and area (cm ), respectively,
and is the so called specific partial conductivity for species
i in the scale.
Keep in mind that we have not really justified the analog
circuit as a v a l i d way to describe the scaling process. As a
matter of fact a rigorous jiustification would probably be a very
d i f f i c u l t task involving some very knotty, philosophical, questions.
What one can do, however, is to proceed with the model on the tentative assumption that i t may work and thus derive as many useful formulas as possible to see how they square with Wagner's more rigorous ones and with experimenta
Hoar and Price did and indee
with relatively l i t t l e effort by using their idea. We w i l l exploit their approach here rather than trying to go through Wagner's
more rigorous development, but i t is worth mentioning that Wagner
himself expressed reservations about the Hoar-Price paper. This
appears in the discussion section that followed their presentation
before the Faraday Society(5).
Let us consider the open c i r c u i t emf that occurs in scaling.
A f a i r l y straightforward dc c i r c u i t analysis of Figure 3 leads to
the following formula
= [t-L + t ]V
6
2
fc
i =
V E1
j =
CORROSION C H E M I S T R Y
104
= 1 - t
10
+ t ]dV
12
13
[t_ + t J d V
Jo
14
-L
x = [Ii + I ] /Z F
2
15
PATTERSON
4.
Conduction
in
Nonmetallic
105
Phases
E / R
"
1 6
x -
/ Z
1 7
then gives
106
CORROSION C H E M I S T R Y
2 2
. = c.V. q./kT
20
where c i is concentration,
is
jump frequency, is temperature
and everything else is constant (jump distance 1, Boltzmann's k
and charge q^) . Thus, the temperature and P
dependences of the
conductivities are due to those of c i and Vj_. In general Vj[ w i l l
always exhibit an Arrhenius dependence on temperature because
hopping is a thermally activated process. This means
will a l
ways go as exp
(-Q/RT) where Q is a constant called the a c t i
vation energy. No dependence of
on partial pressure is ex
pected theoretically and none are found experimentally.
The concentration term has a b i t more f l e x i b i l i t y for d i f f e
rent behaviors: two kinds of temperature dependences (constant or
exponential) and two kinds of p a r t i a l pressure dependences (con
stant or exponential). Beginning with the temperature dependence,
c i w i l l either be independent of temperature or i t w i l l also ex
hibit an Arrhenius dependence, albeit with a different activation
energy than that exhibited by V i . If the carrier concentration is
fixed by extrinsic contaminations, deliberate or not, i t w i l l re
main independent of 2 In some cases however, the concentration
may be small enough that incorporation of atoms from the ambient
2 w i l l cause changes. On the basis of the law of mass-action
arguments, etc., these c i generally vary as Here,
is a constant, usually a ratio of small integers, which ratio is
characteristic of the defect reaction whose equilibrium is in
volved.
The result of a l l of this is that when individual conducti
vities are plotted on a log scale versus log and reciprocal
absolute temperature 1/T, planar sheets generally always result.
This means that only a few measurements (3 minimum) are needed to
x2
PATTERSON
4.
Conduction
in
Nonmetallic
Phases
107
on
x 2
on
108
CORROSION C H E M I S T R Y
4.
PATTERSON
Conduction
in
Nonmetailic
Phases
109
T a b l e I . C a l c u l a t e d and Measured
Rare C o n s t a n t s f o r t h e F o r m a t i o n
of M e t a l - Ncn M e t a l Compounds
NON-METAL
COMPOUND
TC
( r e f . 26, 77)
"RATIONAL RATE CONSTANT"
EQUIVALENT x cm" sec ""
1
Calculated
Ag
(liqj
Ag S
2
220
2.A 1 0 "
Observed
1.6 1 0 "
1 0
61
x^lO"
1 0
3.8 1 0 "
1 1
Cul
195
3.8 X "
AgBr
200
2.7 X 1 0
Cu 0
1000
6.6 X
6.2 1 0 "
Cu 0
1000
4.5 1 0 "
Cu
o;
Cu 0
1000
4.8 X 1 0 "
_q
3.4 X 10
Cu
2
a tin.
atm.
0 ; - 1.6 X 10"
0 ;
Cu 0
1000
2.1 X 1 0 "
Cu
2.2 1 0 "
Cu
Ag
Br
Cu
(Ss)
(gas)
0 ; - 8.3 X 10"
2
- 1 1
3.4
"
9
.9
3.1 x 10
CORROSION C H E M I S T R Y
110
4.
PATTERSON
Conduction
in
Nonmetallic
111
Phases
112
CORROSION
CHEMISTRY
w o u
. = . i + Z.F<j>
1 1
21
4.
PATTERSON
Conduction
in
Nonmetallic
Phases
113
.
i
(or h. )*i
2
+ Z^
22
+ 4e
^3
114
CORROSION C H E M I S T R Y
familiar form
0
+ 2e-* 0
24
IT.
25
Z.n
and
26
and
28
- t
'.' - ! = -Z.F[V. - v j
i l
29
- y!] /Z.F
30
PATTERSON
4.
Conduction
in
Nonmetallic
115
Phases
31
The quantity is of fundamental importance and w i l l be referred
to hereafter as the thermodynamic voltage for species i i t re
presents the chemical driving force or a f f i n i t y which acts to
drive the i-type ions through the medium. Like V ,
has units
of volts.
It is worth digressing here to point out some important points.
F i r s t of a l l i t should be noted that differences or gradients in
are a measure of the overall or combined "driving force" for
migration of the i type p a r t i c l e s . However, in view of Equation 29,
we see that this overall "force" has two parts: A chemical gra
dient part arising from
nent arising from difference
cases of practical interest these two parts oppose each other and
the direction of net migration is determined by whichever is the
stronger force. In electrolysis (charging mode) the V L gradient
is stronger whereas in the discharge mode the chemical driving
forces prevail. And for multicomponent electrolytes i t is most
important to realize that some species may be undergoing electro
l y s i s while others are simultaneously discharging in the same cell.'
This possibility has profound implications for the performance
characteristics and efficiencies of practical electrochemical de
vices of a l l types.
L
i#
116
CORROSION C H E M I S T R Y
I. = G.[V. - V 1
x
i
l
L
33
rf
4.
PATTERSON
Conduction
in
Nonmetallic
117
Phases
CORROSION C H E M I S T R Y
118
Equivalent Circuit Relations
k,e
where indicates that the summation on i goes from 1 through k
(all ion species) with a f i n a l term for the electron contribution
as well. This symbol is used below as well.
If we insert Equation 33 for each I^ in Equation 35, collects
terms, and then notes that the same load voltage
whatever that
turns out to b e i s necessarily impressed on each of the parallel
branches, we can produce the following sequence
k,e
k,e
k,e
L
= Z
G [V.-V ]=S
I
= s
G.V.-V^
3 6
'Vi-V
where
the k,e notation means the same as i t did above. Also, to
simplify notation here, the total conductance G and the transference numbers t ^ have been introduced. They are defined as
follows :
k,e
G
Q ~
G.l
37
T
and
t. = G. / G .
38
T
however the G^ values in 37 and 38 derive from the troublesome
Equation 34.
The transference number t ^ may be regarded as a kind
of normalized conductance or conductivity. These definitions to
gether with Equation 36 yield
=T ^- L
39
4.
PATTERSON
Conduction in Nonmetallic
Phases
119
-,(), ^ )
LEFT
: ^
ELECTRODE ^
: RIGHT
' ELECTRODE
MEDIUM
n (x), a (x)
k
n (x). ( )
e
I-
= 0
Figure 5.
1/R1
CELL
ANALOG
-AW-vw-wv-
EXTERNAL
LOAD
CIRCUIT
LOAD
RESISTOR
Figure 6. Direct current (dc) analog for a multicomponent mixed conducting
electrolytic cell
CORROSION C H E M I S T R Y
120
4.
PATTERSON
Conduction
in
Nonmetallic
Phases
121
122
CORROSION C H E M I S T R Y
- 2
44
Equations 37-44 with Equations 30 and 31 serve as a system of
basic relationships that are quite useful for analyzing the performance of a generalized electrochemical c e l l under various modes
of operation.
It is worth re-emphasizing, however, that the mathematical
simplicity of the equations in this system may be simplicity in
appearance only. Again, the
quantities in G and in the various t-L values which enter into V are not necessarily constant
in general: rather, they are apt to vary somewhat with loading
conditons as was pointed out in the discussion subsequent to Equation 34 above.
T
Capstone Comments
This is one of those tasks that one never really finishes.
Rather one pauses from time to time to sum up, hoping either to
continue again later or perhaps to abandon the effort for good.
In this case I hope to continue, at least for a while, and in
summing up here, l e t me explain why I w i l l only use one example
to do this but many others exist as well.
There have been cases wherein a fluoride ion solid electrolyte,
in particular CaF , has been placed between two different metal
oxide electrodes and behaved as i f i t were an oxide solid electrolyte. That is, the emf was found to be equal to that measured previous with calcia- or y t t r i a - s t a b i l i z e d zirconia! No very suitable explanation has been offered yet, but I think that these
findings have been published or soon w i l l be. But i f my conjectures above are correct, the following explanation could be offered.
The CaF electrolyte is capable of incorporating oxide ions
and transporting them simultaneously with, but perhaps to a much
lesser extent than, fluoride ions. Then CaF could be thought of
as a two-anion electrolyte. Since the electrodes are reversible
and highly "buffered",as i t were, with respect to oxygen, the
thermodynamic voltage V for oxygen acts like a stable battery
in the equivalent c i r c u i t . There is of course a fluoride ion
branch as well, but i t s thermodynamic voltage V behaves not like
a stable battery but rather like a capacitor. This is due to the
fact that fluorine inventories are not fixed by the oxide electrodes and so the trace amounts that do reside therein can change
drastically as the fluoride "counter-ions" get pushed in the d i rection opposite to the flow of oxide ions through the CaF ". In
short, the battery-like voltage V in the oxide-ion branch of the
2
4.
PATTERSON
Conduction
in
Nonmetallic
123
Phases
Acknowlegement
Preparation of this manuscript spanned many months but i t
had to be put into f i n a l form during o f f hours while I was on a
summer leave of absence from ISU. I was working in Dr. Hugh
Isaacs group at Brookhaven National Laboratory at the time and
I am indebted to many of the BNL people for very helpful discussions.
Of utmost importance at that time was the willingness of
Ms. Yvonne Oden to sacrifice many of her off work hours, thus
making i t possible to complete the manuscript just before the
deadline. I shall be forever in her debt for that.
I also wish to acknowledge the Iowa State University, Engineering Research Institute for continued financial support of
research in high temperature electrochemistry and for help
during the i n i t i a l stages of manuscript preparation.
Finally, I wish to thank the Chicago Chapter of the Electrochemical Society and my host, Prof. Bruce Wagner for their kind
invitation and cordial hospitality.
124
CORROSION
CHEMISTRY
Literature Cited
1.
2.
3.
4.
5.
6.
7.
8.
W. Schottky, Z. Phys
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
4.
PATTERSON
Conduction
in
Nonmetallic
125
Phases
22.
23.
24.
25.
T.P. Hoar and L.E. Price Trans. Faraday Soc. 34, 867 (1938)
26.
27.
28.
781 (1966)
(1972)
30.
31.
32.
33.
34.
35.
36.
John Newman, "Advances in Electrochemistry and Electrochemical Engineering", Bol. V, pp. 129-131, C.W. Tobias (editor),
Interscience, New York (1967)
37.
38.
39.
40.
RECEIVED
September 1, 1978.
1540
5
Dissolution of Iron
MORRIS C O H E N
National Research Council of Canada, Division of Chemistry, Ottawa, Canada
0-8412-0471-3/79/47-089-126$06.75/0
1979 A m e r i c a n C h e m i c a l Society
5.
COHEN
Dissolution
of
Iron
127
128
CORROSION C H E M I S T R Y
Figure 1.
Figure 2.
5.
COHEN
Dissolution
of
129
Iron
Solution Composition
The second reactant in the dissolution or corrosion system
is the solution. In most cases this is basically water contain
ing dissolved substances which ionize more or less to cations and
anions. The solution may also contain dissolved gases such as
2> H , or C0 .
These constituents of the aqueous system can
affect the corrosion rate in a variety of ways. 0 in small
amounts may act to increase the rate of corrosion and in s u f f i
ciently high concentrations to inhibit i t . In the presence of
chloride ions, pits tend to go acid with a consequent increased
rate of dissolution of the iron. Some metal ions such as copper,
plate out on the iron surface by an exchange reaction and increase
the corrosion rate by acting as hydrogen depolarizers.
Complexing agents increase the reaction by lowering the effective
concentration of the dissolved ferrous ions. Other constituents,
such as organic amines
the rate of solution. Others
or molybdate, help to form protective oxide films and inhibit the
dissolution reaction. Hence a knowledge of the effects of
various possible constituents of solutions is necessary before
predicting the corrosion behavior of iron.
2
Application of Thermodynamics
With sufficient thermodynamic and equilibrium data i t is
possible to predict whether or not a dissolution reaction w i l l
take place. Also with a knowledge of the free energy change
for a reaction one can calculate the potential at which the
reaction w i l l occur from the relationships
AG = nFE
and = E
nF
+ log Concentration
CORROSION
130
CHEMISTRY
Electrochemistry
and
Corrosion
M a t e r i a l s can d i s s o l v e by d i r e c t d i s s o l u t i o n or by separate
steps.
An example of the former is the d i s s o l u t i o n of sugar in
water and of the l a t t e r the c o r r o s i o n of i r o n in aqueous solutions,.
D i r e c t d i s s o l u t i o n or i r o n may occur in some organic solvents
and in the presence of some c o n s t i t u e n t s in aqueous s o l u t i o n s .
In most aqueous c o r r o s i o n systems i r o n (and most other metals)
goes i n t o s o l u t i o n v i a an e l e c t r o c h e m i c a l process in which there
are separate anodes and cathodes. In a de-aerated s o l u t i o n the
separate r e a c t i o n s are
2+
Fe
*Fe
2H + 2e
+ 2e
->H
2
(anode)
(cathode)
+ 2H 0 + 4e
2
40H~
reaction
is
(cathode)
5.
COHEN
Dissolution
of
Iron
131
132
CORROSION C H E M I S T R Y
Figure S.
pH
10
12
14
16
Figure 4. Evans-type polarization curves: (a) anodic curve intersecting two pos
sible cathode reactions; (b) diffusion control of cathodic reaction; (c) diffusion
control of anodic reaction; and (d) resistance pohrization, measure will depend
on where probe is in relation to anode and cathode
COHEN
5.
Dissolution
Fe + H 0
of
133
Iron
^Fe(H 0) adsorbed
Fe(H 0)ads
^Fe(OH~)ads +
<
>(FeOH)ads + e
Fe(0H~)ads
(FeOH) + H
Fe(OH)ads
+
KFeOH) * + e
+
=^
44
Fe " +
RDS
H0
2
<
Fe(H 0)ads + X~ *
2
ad
FeX~
+ 0H~
FeOH* + X" + 2e
ads
Fe0H+ + H ^=5- Fe " + H 0
o
RDS
44
4 1
H
H
+e
aq
or H 0 + e
2
+
H
Reduction
,
ads
. + OH
ads
aq
+ ads + e
aq
H0 - + e
2
ads
-
1(a)
2
3
+ 0H_.
3(a)
aq
(1) and (3) w i l l predominate over 1(a) and 3(a) at low ph s. The
rate determining step depends in part on the catalytic properties
of the surface. This is particularly true for the recombination
reactions (2) and 3 or 3(a). In some cases the recombination re
action is more d i f f i c u l t than dissolution of atomic hydrogen into
the metal and an alternative reaction
o
(1)
134
CORROSION C H E M I S T R Y
H
metal
ads
(4)
becomes a predominant r e a c t i o n .
I f c a t a l y t i c s i t e s f o r hydrogen
atom recombination e x i s t w i t h i n the metal, such as i n c l u s i o n s ,
then hydrogen gas is formed in the metal and b l i s t e r i n g and rup
ture can occur.
0^ Reduction
0
0
+ e
+ H
0 ads.
2
H0 ads.
2
H0
+ e"
H0
H0
+ H~*
22ads
OH + e
+ 2H
+ 2e
H 0 .
2 2 aq
2H 0.
2H 0.
+ 4H
OH" + 4H+ + 4e
+ 4e~
^2H 0.
2
5.
COHEN
Dissolution
135
of Iron
where * is the A c t i v a t i o n o v e r p o t e n t i a l .
The a p p l i c a t i o n o f
p o t e n t i a l to the e l e c t r o d e w i l l t h e r e f o r e increase the r a t e o f
e i t h e r the forward or reverse r e a c t i o n .
A schematic a c t i v a t i o n energy diagram f o r a r e a c t i o n is
shown in F i g . 5. The dotted l i n e represents the e q u i l i b r i u m
s t a t e and the s o l i d l i n e a p o l a r i z e d s t a t e due to an a p p l i e d
potential.
In the e q u i l i b r i u m s t a t e
_AG*
*a
a ^RT*
_AG*
= i ^= i
a
,
(AGa-nFE)
then = + A e (
-)
a
a
RT
and
A i = A e -(AGc (l-a)nFE)
C
C
RT
'
l
= a + b logi .
(Tafel
equation).
Some o f these r e l a t i o n s h i p s a r e i l l u s t r a t e d g r a p h i c a l l y in
Figure 6. E is the e q u i l i b r i u m p o t e n t i a l . At s u f f i c i e n t l y
high o v e r p o t e n t i a l s a s t r a i g h t l i n e T a f e l region is observed.
At higher a p p l i e d p o t e n t i a l s current may become independent of
p o t e n t i a l because of d i f f u s i o n c o n t r o l or may f a l l o f f the
s t r a i g h t l i n e due to r e s i s t a n c e p o l a r i z a t i o n . In the anodic r e
a c t i o n t h i s is u s u a l l y due to the formation o f f i l m s on the sur
face of the metal.
These concepts can be a p p l i e d q u i t e d i r e c t l y to the c o r r o
s i o n behavior of i r o n . The e f f e c t of d i f f u s i o n c o n t r o l on the
c o r r o s i o n r a t e was shown in the Evans Diagrams (b) and (c) of
F i g u r e 4. In the cathodic case the current becomes constant over
a wide range of p o t e n t i a l s because of c o n t r o l by the r a t e o f
d i f f u s i o n o f the reactant oxygen gas to the cathode. The anodic
r e a c t i o n can be dependent on the r a t e of d i f f u s i o n of e++ from
the anode. The r a t e s in both cases can be increased by s t i r r i n g .
The r a t e of Fe** i o n removal from the anode can a l s o be in
creased by the presence of complexing agents in the s o l u t i o n .
This e f f e c t w i l l be i l l u s t r a t e d l a t e r on with i n h i b i t o r s .
Q
136
CORROSION C H E M I S T R Y
RESISTANCE
POLARIZATION
CONCENTRATION
POLARIZATION
(DIFFUSION CONTROL)
Log i
EXCHANGE CURRENT
CATHODIC Figure 6.
ANODIC
Polarization curves for anodic reactions
Equilib
).
5.
COHEN
Dissolution
of Iron
137
138
CORROSION C H E M I S T R Y
(a)
Figure 7.
(b)
Anodic passivation curves
6.0
20
Figure 8.
40
60
80
CELL POTENTIAL-volts
100
120
acid
5.
COHEN
Dissolution
of
Iron
139
+ FeO (OH)
+ 2H
e.
CORROSION
140
F i l m Breakdown and
CHEMISTRY
Repair
Breakdown of
Films
COHEN
Dissolution
of
Iron
- CHLORIDE-BORATE
SOLUTION
Lu
or
or
10 f-
-800
-400
20
400
800
1200
Figure 10.
142
CORROSION C H E M I S T R Y
1st A r r e s t Y F e ^ + 3H 0 + 2e
2
2Fe
+ +
+ 60H~
(A)
2nd A r r e s t F e 0 , + 4H 0 + 8e
Fe + 80H~
(B)
3 4
2
F e 0 . + 4H 0 + 2e
3Fe** + 80H~
(C)
3 4
2
During the second a r r e s t there is a l s o c o n s i d e r a b l e hydrogen evol u t i o n form the a l t e r n a t e cathodic r e a c t i o n
2H
+ 2e
H .
2
44
F e " + 2e
5.
COHEN
Dissolution
of
143
Iron
I n h i b i t i o n of
tt
it
Corrosion
I n h i b i t o r s can slo
with e i t h e r the anodic
h i b i t o r s can be e f f e c t i v
y
poisoning
hy
drogen e v o l u t i o n and/or oxygen r e d u c t i o n or by i n c r e a s i n g e l e c t r o n i c r e s i s t a n c e . Some examples are z i n c s a l t s which form prec i p i t a t e s , adsorbing organic substances and p o s s i b l y the p o l y phosphates under some c o n d i t i o n s . The d i s c u s s i o n in t h i s l e c t u r e
w i l l deal mainly with anodic i n h i b i t o r s .
These act by i n t e r f e r i n g
with the anodic d i s s o l u t i o n - p r o c e s s by a s s i s t i n g in the formation
and p r e s e r v a t i o n of an oxide f i l m , s i m i l a r to that produced during
anodic o x i d a t i o n of i r o n .
Anodic I n h i b i t o r s
The anodic i n h i b i t o r s are u s u a l l y i n o r g a n i c s a l t s which,
above a minimum c o n c e n t r a t i o n decrease c o r r o s i o n to a n e g l i g i b l e
amount. A t y p i c a l c o n c e n t r a t i o n versus weight-loss curve is shown
f o r sodium n i t r i t e in F i g u r e 12.
In the absence of i n h i b i t o r or
very low concentrations the c o r r o s i o n is l o c a l i z e d and takes the
form of p i t t i n g . At s u f f i c i e n t l y high concentration the i r o n is
completely p r o t e c t e d . T h i s phenomenon of p i t t i n g at concentrations j u s t below that r e q u i r e d f o r i n h i b i t i o n is important and must
be taken i n t o account when anodic i n h i b i t o r s are used. The conc e n t r a t i o n of i n h i b i t o r r e q u i r e d to stop c o r r o s i o n is dependent on
the composition of the s o l u t i o n and is u s u a l l y decreased by the
presence of oxygen and increased by a d d i t i o n s of c h l o r i d e or
complexing agents. The e f f e c t of a strong complexing agent, v e r sene, is shown in Figure 13.
The time required to achieve i n h i b i t i o n , as measured by the attainment of a passive p o t e n t i a l is
increased as the concentration of complexing agent is increased.
The complexing agent keeps the i r o n in s o l u t i o n and prevents the
r e p a i r of pores by d e p o s i t i o n of f e r r i c s a l t s .
The c l o s e connection between p o t e n t i a l and c o r r o s i o n is shown
by the two s e t s of graphs in F i g u r e 14.
Here one can see, that
f o r i r o n in phosphate s o l u t i o n in the presence of oxygen, the time
CORROSION C H E M I S T R Y
-400
CATHODIC REDUCTION CURVE FOR 1 hr
ANODIZED SPECIMEN AT + 6 0 0 mV
500
> -600
-J
C -700 -
_o
',
g - 800
-900HI
-1000
Figure 11.
20
30
40
QUANTITY OF ELECTRICITY mC
50
1200
-GENERAL CORROSION
lOOppmKCI IN AERATED
DISTILLED WATER
, 800
PITTING
400 1
INHIBITION
10
20
30
40
50
60
70
80
90
COHEN
Dissolution
of Iron
TIME-HOURS
Figure 13. Effect of complexing agent, versene, on passivation by sodium
nitrite: (A) etched iron in NaN0 solution; (B) same as in A plus versene
2
146
CORROSION C H E M I S T R Y
COHEN
5.
Dissolution
of
147
Iron
Localized
Corrosion
American Chemical
Society Library
1155
16th
St.
Washington, D. C.
N. W,
20038
Figure 15.
5.
COHEN
Dissolution
of
Iron
149
150
CORROSION C H E M I S T R Y
Corrosion
Figure 16.
(a) 100
5.
COHEN
Dissolution
151
of Iron
O.I2i
TIME, hr
Corrosion
Figure 17. Effect of stirring on potential of iron in chloride and inhibitor solution: (A) started bubbling; (B) stopped bubbling; 1000 ppm NaN0
+ 100
ppm NaCl; 1000 ppm Na HPO^ + 25 ppm NaCl) (17)
2
H^O
N0
ATA
Fe
Fe
FeOOH + 3H* + e
(3)
2Fe 0
(4)
4Fe + 3 0
2H
+2e
Fe 0
2
+ H 0 +e
2
+2e+ 3 H 0
2
Figure 18.
^2Fe 0
2
0 + 2 H 0 + 4e
2
+ 6H
(I)
Fe +2H 0
4Fe + N 0 i + 3 H
N0
+ 2e
+ +
Fe 0
+ +
OH"
2Fe + 3 H 0
2
AT C
+ 6e
(2)
+ NH +N (5)
3
(6)
^40H"
(7)
NH
(8)
*~2Fe
(etc)
+ +
+ 60H
(9)
CORROSION
152
CHEMISTRY
13.
14.
15.
16.
17.
Pourbaix, M. " A t l a s of E l e c t r o c h e m i c a l E q u i l i b r i u m in
Aqueous S o l u t i o n s " , Pergamon Press, 1966.
Evans, U.R. "An I n t r o d u c t i o n to M e t a l l i c C o r r o s i o n " ,
Edmund A r n o l d Co. London 1948, Pages 71-3.
T h o r n h i l l , R.S. and Evans, U.R. J. Chem. Soc. (1938) 614.
K e l l y , E . J . J. Electrochem. Soc. (1965) 112, 124.
Lorenz, W.J. C o r r o s i o n Science, (1965) 5, 121.
M u l l e r , W.J. Trans
Edeleanu, C. and Gibson
Nagayama, M. and Cohen, M. J. Electrochem. Soc. (1962),
109, 781.
Nagayama, M. and Cohen M. J. Electrochem. Soc. (1963), 110,
670.
Cohen, M. J. Electrochem. Soc. (1974), 121, 191C.
Evans, U.R. and Stockdale, J. J. Chem. Soc. (1929), 2651.
Staehle, R.W. and Okode, H. E d i t o r s . P a s s i v i t y and I t s
Breakdown on Iron and non-Base A l l o y s , U.S.A.-Japan Seminar.
NACE, Houston, Texas. 1976.
Pryor, M.J. and Cohen, M. J. Electrochem. Soc. (1953), 100,
203.
Hackerman, N. and Cook, E . I . J. Phys. Chem. (1952), 56, 524.
Sato, N. E l e c t r o c h i m i c a A c t a . (1971), 16, 1683.
Bubor, S.F. and Vermilyea, D.A. J. Electrochem. Soc. (1966),
112, 882.
Cohen, ., C o r r o s i o n (1976) 32,
461.
R E C E I V E D September 1, 1 9 7 8 .
6
Ferrous Passivation
V. BRUSIC
Manufacturing Research Laboratory, Systems Products Division,
I B M T. J. Watson Research Center, Yorktown Heights, N Y 10598
The progress o
increasing use of tool
bound to lean increasingly on sophisticated computerized
mechanisms to do his work for him with tighter requirements to
function without decay over years in t e r r e s t r i a l atmosphere.
Materials, mainly metals, used in this fabrication must be
stable.
A piece of metal indeed remains stable for an almost
indefinite period of time provided it is stored in vacuum. In
normal t e r r e s t r i a l environments metals become unstable in
various ways: they develop cracks and break upon strain,
suffer fatigue when subjected to periodic stress, undergo a
process of
embrittlement, as well
as react
with the
surroundings.
With the exception of the expensive noble
metals, metallic surfaces are transformed into oxides and
salts which can peel off
or just dissolve away.
Yet
technology predominantly depends on these thermodynamically
unstable, corrosive, non-noble metals, the most widely used
being aluminum, copper, nickel and, of course, iron. They owe
much of their usefulness to the existence of the passive
state.
The passivation phenomenon was f i r s t observed and
described by the action of n i t r i c acid on iron centuries
ago.(1-3)
From then to the present the complexity of the
problem and the practical importance of i t s application has
earned the attention of many scientists.
The present discussion draws particularly on studies that
have been published recently and are concerned with iron
electrodes under conditions producing passivity which are the
ones that best c l a r i f y the passivation process.
Passivation Phenomena
A) Description, Terminology and Some Practical Aspects of
Anodic Protection.
Passivation can be defined as a constant
0-8412-0471-3/79/47-089-153$08.00/0
1979 American Chemical Society
154
CORROSION C H E M I S T R Y
slowing
down of any a c t i o n , process,
or r e a c t i o n .
More
s p e c i f i c a l l y the term is a p p l i e d to the sometimes observed
transformation
of a corroding and
unstable
surface to a
passive and s t a b l e surface by
superimposing a
double-layer
f i e l d which should a c c e l e r a t e the m e t a l - d i s s o l u t i o n r e a c t i o n
r a t h e r than hinder
i t , i.e., by a s h i f t
of the e l e c t r o d e
p o t e n t i a l in the p o s i t i v e d i r e c t i o n .
T h i s phenomenon of
enforced
passivation(4)
seems unnatural
or
at
least
unexpected.
I t s study
is
f a c i l i t a t e d by
the use
of a
potentiostat.
If i r o n is made a part of the e l e c t r o c h e m i c a l c e l l , a
t e s t e l e c t r o d e , and placed in a p o t e n t i o s t a t i c c i r c u i t , Figure
1,
the
experimental
current-potential
plots
show
an
i n t e r e s t i n g p a t t e r n , Figure 2: with
an increase of p o t e n t i a l ,
s t a r t i n g from the c o r r o s i o n p o t e n t i a l , the d i s s o l u t i o n current
i n c r e a s e s , j u s t as expected. Metal ions go i n t o the s o l u t i o n
M > M
2+
2e~
(1
undergoing h y d r a t i o n .
At s t i l l higher p o t e n t i a l s , however,
the curve deviates from simple l o g a r i t h m i c dependence ( T a f e l
law,
metal d i s s o l u t i o n ) . Apparently
metal d i s s o l u t i o n is
hindered
by another process,
that is, the formation
of a
p r o t e c t i v e film,(4) through a r e a c t i o n of the type
M
+ H 0 -> M - 0 + 2H + 2e~
2
(2)
At the p a s s i v a t i o n p o t e n t i a l V a l i m i t i n g p a s s i v a t i o n current
i
is
reached and
the a c c e l e r a t i o n of metal d i s s o l u t i o n is
efjual to the r e t a r d a t i o n of the process.
Above V
the
c u r r e n t - p o t e n t i a l curve
changes d i r e c t i o n , metal d i s s o l u t i o n
is i n c r e a s i n g l y
hindered
by a process
which e v i d e n t l y
terminates
at
V
, a p o t e n t i a l of "complete p a s s i v i t y . "
Thereafter the HSssolution r a t e , i ,
sometimes orders
of
magnitude smaller than the p a s s i v a t i o n c u r r e n t , is independent
of p o t e n t i a l .
The
increase
in
current at some higher
p o t e n t i a l is determined by
the o v e r p o t e n t i a l of oxygen
discharge,
or the presence of some o x i d a t i o n - r e d u c t i o n
system. The region is termed " t r a n s p a s s i v e . "
The
term "Flade p o t e n t i a l , " V ,
has
a v a r i e t y of
connotations.
Many i n v e s t i g a t o r s , f o l l o w i n g Flade*s o r i g i n a l
d e f i n i t i o n , a s s i g n t h i s name to the p o t e n t i a l at which
an
already passivated metal begins
to l o s e passivity.(5-7) Some
p r e f e r to a s s i g n t h i s term to the p a s s i v a t i o n p o t e n t i a l V (8)
or to V
. (9) According to Tomashov and Charnova(4_) V^, i s t h e
thermodynamic p o t e n t i a l at which the formation
of a metal
oxide becomes p o s s i b l e and i t is lower than V , Figure 2.
As
much as p o s s i b l e the d i s c u s s i o n in t h i s pape? w i l l r e f e r to
P
6.
BRUSic
Ferrous
155
Passivation
PASSIVE
TRANSPASSIVE
'c
POTENTIAL
Figure 2.
156
CORROSION
CHEMISTRY
m e a s u r e (
6.
Ferrous
BRUsic
Passivation
Figure 3.
Figure 4.
157
158
CORROSION C H E M I S T R Y
log i
Figure 5.
log i
Figure 6.
6.
BRUsic
Ferrous
Passivation
159
In c o n t r a s t , i t is suggested
that in the monomolecular
(or l e s s ) oxide l a y e r , metal atoms have l e f t
their regular
p o s i t i o n in the l a t t i c e (at k i n k s i t e s and any other s i t e s ) to
enter together with oxygen atoms i n t o a new,
alternating
arrangement
of
oxygen
and
metal,
which,
even
if
two-dimensional, resembles the arrangement in the formation of
the n u c l e i of the r e s p e c t i v e oxides ( i . e . ,
l a t e r a l bonds are
formed, F i g u r e 8b.(30)
In the three-dimensional oxide there is a
repeated
d i s t a n c e and symmetry r e l a t i o n s h i p in a v e r t i c a l
dimension
a l s o , with e i t h e r a more or l e s s continuous r e l a t i o n s h i p with
the s u b s t r a t e (as in e p i t a x i a l f i l m ) or with a complete
m i s o r i e n t a t i o n , as in n o n e p i t a x i a l , deposited f i l m .
There are a l s o s c i e n t i f i c d i f f i c u l t i e s in approaching the
problem of the e n t i t i e s formed in p a s s i v a t i o n .
T h i s is
p a r t i c u l a r l y so in the r e g i o n of low e l e c t r o d e coverage, where
there is no p o s s i b i l i t y of c l e a r l y d i s t i n g u i s h i n g between the
adsorbed or
(monomolecular or l e s s ) oxide l a y e r without
an
experimental method
that is s u f f i c i e n t l y
sensitive
and
a p p l i c a b l e in s i t u .
Experimental approach to the problem was
for
a long
time connected
with
the measurement
of
e l e c t r o c h e m i c a l parameters, which u s u a l l y measured an average
current d e n s i t y and thus gave l i t t l e
i n f o r m a t i o n about
the
l o c a l current d i s t r i b u t i o n , the nature and t h i c k n e s s of the
p a s s i v e f i l m , and the f i l m d i s t r i b u t i o n over the s u r f a c e . An
CORROSION C H E M I S T R Y
160
log i
Figure^ 7.
-M-O-M-O-M-0-
'
-M-M-M-M-M-M1
Figure 8.
1*1
b)
-0-M-O-M-O-M-
0-M-O-M
c)
-M-M-M-M-M
I I I I I
-M-O-M-O-M-0I
-M-M-M-M-M-M-
-M-M-M-M-M-M-
Characterization of passivating films; (a) adsorbed layer, (b) monomolecular (or less) oxide, and (c) three-dimensional oxide
6.
Ferrous
BRUsic
161
Passivation
l C
where is the t r a n s f e r c o e f f i c i e n t
equal to $z;* when the
valency of the d i s s o l v i n g i o n , z, is u n i t y , is equal to the
symmetry f a c t o r
3,
or to the f r a c t i o n of the
potential
a v a i l a b l e to lower the energy
barrier for dissolution.
The
q u a n t i t y c is the number of atoms per square centimeter, and
i f w i t h the superimposition of a d s o r p t i o n , c' changes w i t h
p o t e n t i a l in such a way that
f
c' = c '
exp
F
(- - 5 7 )
(4)
= k
f
l C
exp
- a)F
RT
1
(5)
162
CORROSION C H E M I S T R Y
as another.
In U h l i g ' s e a r l i e r view, one of the f a c t o r s determining
passivation
(and chemisorption) is the r a t i o of the work
f u n c t i o n to the enthalpy of s u b l i m a t i o n . I f t h i s r a t i o is
less
than u n i t y , c o n d i t i o n s are f a v o r a f l e to p a s s i v a t i o n
because the e l e c t r o n would escape more r e a d i l y than the atom,
f a v o r i n g the chemisorption of substances
like
oxygen.
A
p a s s i v e f i l m is composed, then, from chemisorbed
atomic and
molecular oxygen
(supplemented perhaps by OH and H~0). The
formation of chemical bonds s a t i s f i e s the s u r f a c e a f f i n i t i e s
of the metal without metal atoms l e a v i n g t h e i r l a t t i c e s i t e .
I t is argued (12) that on t y p i c a l t r a n s i t i o n metals (Ni,
W, Cr, T i , Ta) the formation o f such a l a y e r
( i . e . , M-0-0 )
proceeds with more f a v o r a b l e f r e e energy of formation than tne
oxide formation, as they have u n f i l l e d d e l e c t r o n
energy
levels
l e a d i n g t o th
between oxygen and th
typically exhibit passivity
f i l l e d d levels,
such as copper or z i n c , the heats of oxygen
adsorption a r e expected
t o be lower, and the formation of
oxides is l e s s
favorable.
Such
metals do not e x h i b i t
thin-film passivity.
C o r r e l a t i o n of the observed onset of Wagner's p a s s i v i t y
on a l l o y s l i k e Ni-Cu, Ni-Zn-Cu, and Cu-Ni-Al to the occupancy
of the d l e v e l s
of the a l l o y s is given in support of the
theory. According to the theory, the same type of p a s s i v e
film
( i . e . , M-O-O^) is formed in s o l u t i o n s , i n t e r p o s i n g a
s t a b l e b a r r i e r between metal and e l e c t r o l y t e ,
displacing
adsorbed
H^O and i n c r e a s i n g the a c t i v a t i o n energy f o r the
h y d r a t i o n and d i s s o l u t i o n of the metal l a t t i c e .
Such f i l m s
are assumed t o decrease the exchange-current d e n s i t y
i ^ and
thus to i n c r e a s e the p o l a r i z a t i o n of the metal in the noble
d i r e c t i o n , where more oxygen can be adsorbed, which in turn
forms n u c l e a t i o n of metal oxides. Thus in the p a s s i v e s t a t e a
t h i c k e r oxide f i l m may be detected.
B r i e f l y , the theory e x p l a i n s the onset of p a s s i v i t y by
the formation of a t h i n adsorbed
l a y e r that e i t h e r s h i f t s the
e l e c t r o d e p o t e n t i a l in the double
l a y e r or i n f l u e n c e s the
k i n e t i c s of the anodic process; that is, the important
happenings occur in the s m a l l i n t e r f a c e r e g i o n between the
metal and the s o l u t i o n .
K o l o t y r k i n s theory, however, does
not d e f i n e a p h y s i c a l reason f o r the change of a s with
p o t e n t i a l . On the other hand, a chemisorbed f i l m of M-0-0
composition in the e l e c t r o l y t e is not very l i k e l y f o r the
f o l l o w i n g reason: Most metals (Cr, (32) T i , (32) Fe,(28,33)
N i , (34)) can be p a s s i v a t e d in a s o l u t i o n saturated with
deoxygenated argon - that is, with water oxygen - as long as
some water is present in the s o l u t i o n ; ( 3 4 ) in such s o l u t i o n s ,
2
6.
BRUsic
Ferrous
163
Passivation
2.
The Oxide-Film Theory.
The
oxide-film
theory
d e s c r i b e s the s t a t e of improved c o r r o s i o n r e s i s t a n c e through
the formation
o f a p r o t e c t i v e f i l m on the metal substrate;
t h i s c o n s i s t s of the r e a c t i o n products of the metal with i t s
environment. Such a f i l m is a new phase, even i f i t is as
t h i n as a s i n g l e monolayer.(21) E l e c t r o n d i f f r a c t i o n (21) and
ellipsometric(33) studies
give the experimental evidence f o r
the theory.
In t h i s case the physicochemical p r o p e r t i e s of
the metal r e l a t i v e to a c o r r o s i v e medium depend to a l a r g e
degree on the p r o p e r t i e s
of
the p r o t e c t i v e f i l m .
The
p r o p e r t i e s of the f i l m , however, are not uniquely determined.
Adherents t o t h i s
theory have d i f f e r e n t opinions
on the
p o t e n t i a l at which the f i l m
forms, i t s t h i c k n e s s , the
mechanism of formation, and, most important, the "cause" of
passivity.
In the e a r l i e r t h e o r i e s i t was postulated that the
p a s s i v a t i o n follows the formation of a "primary l a y e r " o f
s m a l l c o n d u c t i v i t y , with porous c h a r a c t e r , which is sometimes
due
to
p r e c i p i t a t i o n of metal
salt
on and
near the
electrode.(32)
In the pores the current
i n c r e a s e s , and by
p o l a r i z a t i o n at an "Umschlagspotential"
( V = V , Figure 1) an
a c t u a l passive l a y e r is formed. Thus the e s s e n t i a l concept of
the p a s s i v a t i o n process is connected with the change of the
p r o p e r t i e s (chemical
or p h y s i c a l )
of the primary f i l m at a
certain potential.
The passive f i l m is f r e e from pores and
presents a b a r r i e r between the metal and the environment. I t
is
electronically
conductive and
slowly
corrodes
in
solution.(6,8,24,37)
These general ideas were f u r t h e r developed in d e t a i l by
Sato and Okamoto,(38,39) and B o c k r i s , Reddy, and Rao.(40) Sato
F
164
CORROSION
CHEMISTRY
De Gromoboy and
Shreir(43) argued in t h e i r experimental
study of n i c k e l in s u l f u r i c a c i d that higher oxides may form
d i r e c t l y from the metal and at the metal s u r f a c e ; the observed
"passivation potentials"
(determined
from anodic charging
curves) were found
to correspond c l o s e l y with the p o t e n t i a l s
calculated
for
N i + NiO,
N i ^ N i 0^,
Ni
Ni 0 ,
and
6.
Ferrous
BRusic
Passivation
165
166
CORROSION
CHEMISTRY
(6)
a diagram i n d i c a t e s
that i r o n in deaerated
aqueous s o l u t i o n s
w i l l have a tendency to d i s s o l v e as long as the pH is l e s s
than about 8.5. In order that oxide formation may proceed,
the anode p o t e n t i a l must be at l e a s t as high as that f o r oxide
or
hydroxide
formation from
the metal
and water in the
particular solution.
I f the oxide forms as a "monolayer," there is a question
as to whether or not one can apply with some assurance these
thermodynamic data, obtained on
the bulk oxides.
Some
experimental evidence does i n d i c a t e that f o r many metal-oxide
systems the f r e e energy of formation of the f i r s t monolayer is
indeed
c l o s e to that observed
f o r the bulk phase,(47) and
hence no a p p r e c i a b l e d i f f e r e n c e in the r e v e r s i b l e p o t e n t i a l
should be expected.
On the other hand, Vermilyea(48) has
shown t h a t , i f the f i r s t
monolayer forms by two-dimensional
nucleation,
the
p o t e n t i a l of
the
two-dimensional-film
formation may be lower than that expected from the
6.
BRusic
Ferrous
Passivation
167
168
CORROSION
CHEMISTRY
thickness.(28,44)
2)
Optical.
At least two methods should be
mentioned: ellipsometry and photopotential measurement. Both
can be applied in s i t u while the metal is a part of the
electrochemical
cell
and
under
an
electrochemical
investigation. Ellipsometry is based on the sensitivity with
6.
BRUsic
Ferrous
169
Passivation
4|K
PASSIVE
2_
1
-*T
ACTIVE
C
_ / C
H 0
2
Corrosion Science
Figure 10. Scheme showing anode behavior at various potentials in solutions of
various anion/water concentration ratios CX-/CH OThe processes are as follows:
(1) anodic passivation; () activationthe Flade relation; (2) activation by adding
"corrosive" anions; (3) transpassivation by raising the anode potential; (3') repassivation by lowering the anode potential; (4) breakdown via pitting, leading to
anodic brightening by raising the anode potential; (4') repassivation by lowering
the anode potential; (5) breakdown via pitting, leading to anodic brightening by
adding "corrosive" anions; (5') repassivation by removing "corrosive" anions; (6)
anodic brightening by raising the anode potential in concentrated solution; (6')
anodic etching of brightening metal by lowering potential in concentrated solu
tion; (7) anodic brightening from etching by increasing the anion concentration;
(7 ) anodic etching from brightening by decreasing the anion concentration (14).
2
CORROSION C H E M I S T R Y
170
(hv)
7-
2e
&n J ^n-+r
[M
- C
(7)
6.
BRUsic
Ferrous
Passivation
171
172
CORROSION C H E M I S T R Y
Figure 11. Schematic of ellipsometer: (1) source of light; (2) collimator; (3) pohrizer; (4) compensator; specimen (S) on table (T); (5) filter; (6) analyzer; (7) light
detector
6.
BRUSIC
Ferrous
173
Passivation
r
10
8-l7^A/cm
8
T 6
t 4
-~rt
-400
0
400
800
POTENTIAL [mV,SHE]
Figure 13. Variation of and with potential in steady state and transient (x,
0.8; 0, 0.32; , 0.08 mA cm' ) oxidation; and i-V relation in steady state oxidation.
(,) Galvanostatic oxidation transients; (UO) potentiostatic (steady state) oxi
dation.
2
Figure 14. Charge (Q) as a function of oxidation potential obtained during the
oxidation (O), reduction (X), and reduction corrected for hydrogen (A)
CORROSION C H E M I S T R Y
174
()
i
0
200
400
600
TIME (SEC)
(b)
800
1000
O"
1.0
~ 0.6
0.2
\y
\
0.02
0.06
I
0.14
I 1L
0.10
0.16
TIME (SEC)
(0
<5
_l
r
-2
I
0
LOG
Figure 15.
I
TIME
(a) Increase of film thickness with time; (b) during the initial state;
and (c) with log (t) at490 mV (she)
BRUsic
Ferrous
Passivation
200 400
600
800
176
CORROSION
CHEMISTRY
2. At higher coverages
the growth i n v o l v e s a r a p i d
place-exchange step w i t h a rate-determining Temkin discharge
of OH i n t o s i t e s where the metal is already attached to an OH
group.
This i n i t i a l
OH group is d i s p l a c e d i n t o the f i r s t
l a y e r of
metal
atoms beneath
the s u r f a c e ,
forming
a
two-dimensional oxide
lattice.
Thus
the processes
that
determine the i - V curve are
Fe + OH"
/
dissolution
FeOH
FeOH
+ e
FeOH + e~
\
f i l m formation
(9)
FeOH ^
(8)
HOFe
(10)
FeOH F e
(12)
where Eq. (9) gives i ^ g and Eq.
(12) i ^
. At any time i
i ^
+
The d i s s o l u t i o n mechanism is b a s i c a l l y that of
B o c f r i s et al.,(61) r e c e n t l y confirmed by Epelboin.(52)
Since, according t o the determined
mechanism of f i l m
growth i
f (V,L) at t = const and i ^ 0 at t l a r g e , the
experimental i - V curve is l a r g e l y
determined by
l a t t e r is c a l c u l a t e d
as a f u n c t i o n of i n c r e a s i n g f e ( 0 H )
coverage from 0 to about 0.8 at p o t e n t i a l s in the p r e p e a l
r e g i o n and in the regions of f i n a l covering of the e l e c t r o d e
by the formation of Fe^O^ at p o t e n t i a l s between V
and V .
The f i n a l r a t e s are as f o l l o w s :
at V < V :
1 S
^iss
ff
l 0H~
2 FeOH
( V F / R T
F V
13
>
R T
< / )
< >
e x
<- (
) / )
0-
f(6)
OH,ads
Fe(OH)
and
^ 3(const + FV)
6.
BRUSIC
at V
Ferrous
177
Passivation
< V < V
1PP
i
^(1
diss
- ) exp
(3FV/2RT)
(15)
and
i
= k K ( l - ) exp
(3FV/2RT)
(16)
with
(1 " 0 )
T
= A exp
[-k K
2
t]
(17)
)
o l f Ebersbach,
Schwabe and
R i t t e r (18,19).
It was assumed that the m e t a l - d i s s o l u t i o n r e a c t i o n (rate i ^ )
and
the f i l m - f o r m i n g r e a c t i o n ( i ) at an i n i t i a l l y
bare
9
CORROSION
178
CHEMISTRY
i - ( i + i ) (1 - )
x
(18)
= C i ( l - ) -
- ,
V,a _,a
A
(19)
H +
f i l m degradation, is a f u n c t i o n of
Ht
MO + 2 H
2 +
+ H0
(20)
and K, the r a t e of o x i d e - f i l m
according t o
MO + 2A~ + H 0
2
MA
degradation, is a
f u n c t i o n of
+ 20H~
(21)
Then
- ( i + i ) ( l - (1 + ^ g j - )
x
{1 - e x p [ - ( C i
+ + B)t]})
(22)
where
-*
- i exp
2
< "
V I
fc
H+
(V - V ) ] , i = f c
2
H +
'
(24)
6.
Ferrous
BRUSic
179
Passivation
with
and V being the e q u i l i b r i u m p o t e n t i a l s f o r a c t i v e
d i s s o l u t i o n and f i l m formation.
The d i s s o l u t i o n r a t e i ^ may be determined by the r e a c t i o n
sequence given by Eqs. 8,
9, and
11 ( B o c k r i s
dissolution
mechanism). The mechanism of f i l m formation, r e s u l t i n g in ^
is assumed to be as f o l l o w s :
1
M + H0
2
MOH
+ H0
2
M(OH)
MOH
+ H
+ e
M(OH) + H
2
MO + H 0
(25)
+ e"
(26)
(27)
_
1 + ({Ci
exp[(a F/RT)(V - V ) ] } / ( K + ))
9
(28)
h
i
(29)
exp ( i C t )
According to Eq. (19), the constant C is the covered area
per coulomb and was estimated on the b a s i s of the
area r e q u i r e d f o r oxygen atoms as 10^ cm^/A-sec.
The r a t e constants and
at constant pH
and
2
CORROSION C H E M I S T R Y
180
1 0
6.
BRUsic
Ferrous
181
Passivation
TIME
-250 0
Figure 17.
Effect of time on the current density-potential curves by potentiostatic measurements (calculated for + = 10~ sec' (18))
3
K+B
-250
Figure 18.
182
CORROSION
CHEMISTRY
Final Remarks
Modern passivation theory views the primary passivation
act as a formation of a tightly held layer of monomolecular
dimensions, containing oxide or hydroxide anions and metal
cations; the process involves the formation of a new phase, an
oxide, in steps that include the adsorption as an important
intermediate stage. Thus the long-time controversial aspects
of the adsorption and oxide theories are, in one sense,
combined.
The observed current-potential behavior is a function of
the simultaneous processes of film formation, i t s dissolution,
and metal dissolution.
The latter
seems to be mostly
responsible for
the magnitude of
the current at a l l
potentials.
In the active potential region dissolution is
hindered by a decreas in th
fre electrod
d in th
passive region dissolutio
of the passivating film
LITERATURE CITED
6.
Ferrous
BRUSIC
183
Passivation
(1974).
688
(1963).
(1963).
(29) Sato,
. Electrochem. Soc.,
(1964).
(39) Sato, N.
and Okamoto, M., J. Electrochem. Soc. 111, 197
(1964).
(40) B o c k r i s , J .
O'M.,
Reddy, . . N. and Rao, B.,
J.
Electrochem. Soc., 113, 1133 (1966).
(41) Oshe, A.
I., Oshe, .
K. and Rozenfeld, I. L.,
Elektrokhimiya, 7, 1419 (1971).
( 4 2 ) Kruger, J. and C a l v e r t , J. P., J. Electrochem. Soc., 114,
4 3 (1967).
( 4 3 ) deGromoboy, T. S. and S h r e i r , L. L., E l e c t r o c h i m . Acta,
11, 895 (1966).
(44) F r a k e n t h a l , R. P., J. Electrochem. Soc. 1 1 6 , 580 (1969).
( 4 5 ) F r a k e n t h a l , R. P., E l e c t r o c h i m . Acta 1 6 , 1845 (1971).
( 4 6 ) Hoar, T. P., J. Electrochem. Soc., 117, 17C (1970).
CORROSION C H E M I S T R Y
184
(47)
(48)
(49)
(50)
(51)
(52)
(53)
(54)
(55)
(56)
(57)
(58)
(59)
(60)
(61)
(62)
Brennan, D.,
Hayward, D. O. and T r a p n e l l , B. M.
W.,
Proc. Royal Soc. (London), A256, 81 (1960).
Vermilyea, D. ., in Advances in E l e c t r o c h e m i s t r y
and
E l e c t r o c h e m i c a l Engineering (P. Delahay,
ed.) V o l . 3,
I n t e r s c i e n c e , New York, 1963.
Epelboin, I. and Keddam, M., J. Electrochem. Soc., 117,
1052 (1970).
E p e l b o i n , I . , Gabrielli, C., Keddam, M., Lestrade, J. C.
and Takenouti, H.,
J.
Electrochem. Soc. 119,
1632
(1972).
E p e l b o i n , I., Keddam, M.
and Takenouti, H., J. Appl.
Electrochem., 2, 71 (1972).
E p e l b o i n , I. and Keddam, ., Electrochem. Acta, 17, 177
(1972).
Pigeand, ., J. Electrochem. Soc., 122, 80 (1975).
Oshe, . K. and
(1969).
T r a b a n e l i , G., Zucchi, F. and
Brunoro, G.,
Thin S o l i d
F i l m s , 13, 131 (1972).
Cahan, B.,
Case Western Univ., Cleveland, Ohio, p r i v a t e
communication.
N o v o s e l s k i i , I. M., Andreev, I . N. and Khakimov, M. G.,
Elektrokhimiya, 7, 421 (1970).
M i l i g y , . ., Geana, D. and Lorenz, W. J., Electrochem.
Acta 20, 273 (1975).
Sato, N., Noda, T. and Kudo, ., E l e c t r o c h i m . Acta 19 471
(1974).
B o c k r i s , J. O'M., Genshaw, . ., B r u s i c , V. and Wroblow,
H., E l e c t r o c h i m . Acta 16, 1859 (1971).
Bockris,
J.
O'M.,
D r a z i c , D. and D e s p r i c , A.
R.,
Electrochim. Acta 4, 325 (1961).
Ebersbach, U., Schwabe, K.
and Konig, P., E l e c t r o c h i m .
Acta, 14, 773 (1969).
R E C E I V E D September 1, 1978.
7
Corrosion of Valve Metals
J. E. D R A L E Y
Argonne National Laboratory, Argonne, I L 60439
0-8412-0471-3/79/47-089-185$12.50/0
1979 American Chemical Society
186
CORROSION C H E M I S T R Y
40
MO BOND ENERGY, eV
Journal of the Electrochemical Society
Figure 1.
7.
DRALEY
Corrosion
of Valve
Metals
187
which corresponds
t o a g i v e n s m a l l c u r r e n t in a n o d i z i n g ,
or production of the oxide f i l m .
The f a c t o r s
respons i b l e f o r sound and p r o t e c t i v e f i l m s are not w e l l known.
I t is s t i l l a b i t o f a m y s t e r y w h y s o m e s y s t e m s
form
better films than others.
A number o f i n d i v i d u a l
princ i p l e s have been brought up.
P i l l i n g and Bedworth
y e a r s ago w r o t e t h a t i f t h e volume o f t h e o x i d e
prod u c e d o n a m e t a l s u r f a c e is l e s s t h a n t h e v o l u m e
of
m e t a l f r o m w h i c h i t is p r o d u c e d ,
t h e r e is l i t t l e
chance
t h a t t h e o x i d e f i l m w i l l be p r o t e c t i v e .
As s t a t e d ,
t h a t is a g o o d g u i d i n g p r i n c i p l e , b u t o f a l l t h e
syst e m s f o r w h i c h t h e r e is e x p a n s i o n in t h e f o r m a t i o n o f
oxide, the r u l e doesn t p r e d i c t r e l i a b l y which oxides
w i l l be p r o t e c t i v e and w h i c h w i l l
not.
Strong bonding between the oxide and the m e t a l
s u b s t r a t e seems t o be e s s e n t i a l t o t h e f o r m a t i o n o f
highly protective films
you can h o l d on t o .
The o t h e r s d o n t work v e r y w e l l .
There are a l o t of p e c u l i a r i t i e s ; for example,
in
s o m e c a s e s t h e o x y g e n d i s s o l v e s s i g n i f i c a n t l y in t h e
surface layers of the metal.
This doubtless
reduces
the e f f e c t i v e
s i z e o f the s u r f a c e m e t a l atoms and
allows adjustment of the p o s i t i o n s of the surface
atoms
o f t h e s u b s t r a t e so t h e y w i l l n e a r l y m a t c h the p o s i t i o n s o f t h e c o n t i g u o u s c a t i o n s in t h e
oxides.
F i g u r e 2 shows t h a t w h i c h m e t a l s f o r m g o o d f i l m s
depends not o n l y on the bond energy b u t a l s o on the
s o l u t i o n in w h i c h t h e f i l m s a r e f o r m e d .
These
curves
a r e f o r a n o d i z i n g a l u m i n u m in a s e r i e s o f
solutions.
The top 5 c u r v e s a r e f o r a f a m i l y o f s o l u t i o n s
in
which the f i l m s remain r e s i s t a n t to greater
thicknesses
t h a n f o r t h e two l e s s i d e a l s o l u t i o n s .
In other
solut i o n s , o n e c a n t g e t a n o d i z e d f i l m s at a l l .
The r e s i s t i v i t y o n t h e o r d i n a t e is f o r t h e f i l m i t s e l f .
I
t h i n k i t is i m p o r t a n t t o n o t e t h a t m a n y m e t a l s
form
h y d r a t e d o x i d e s , in w h i c h h y d r o x i d e i o n s a r e
constituents.
As a g e n e r a l r u l e , h i g h l y p r o t e c t i v e f i l m s are
a n h y d r o u s ; i t is o f t e n s p e c u l a t e d t h a t t h e h i g h f i e l d
present during the formation of the f i l m (vide
supra)
contributes to the formation of anhydrous r a t h e r than
the hydrated oxide.
I t is e a s y t o s e e t h a t a h i g h
f i e l d o p e r a t e s when one a n o d i z e s by t h e a p p l i c a t i o n o f
a p o t e n t i a l ; i t is l e s s o b v i o u s , b u t e v i d e n t l y e q u a l l y
true during unassisted thin-film oxidation.
Local
f i e l d s from m u l t i p l y - c h a r g e d cations might
contribute
to the formation of compact,
anhydrous oxides;
the best
films u s u a l l y contain cations w i t h a charge of 3 or
more.
One m o r e t h i n g a b o u t f i l m s :
they often are not
v e r y s t a b l e in w a t e r o r in a q u e o u s s o l u t i o n s .
This
tends to cause confusion about the p r o t e c t i v e n e s s
of
f
CORROSION C H E M I S T R Y
188
100
15
12
60
120
180
240
300
Electrochimica Acta
Figure 2.
7.
DRALEY
Corrosion
of Valve
Metals
189
oxide films.
The f i l m i t s e l f may be a v e r y g o o d o n e ,
b u t i t may d e t e r i o r a t e , s o m e t i m e s s l o w l y ,
sometimes
r a p i d l y ; sometimes r a t h e r g e n e r a l l y and sometimes
locally.
I t s p r o t e c t i v e n e s s t h u s may b e t e m p o r a r y
or i m p e r f e c t as a f u n c t i o n of t i m e .
One o f t h e
conundrums about such p r o t e c t i v e f i l m s has always been
how t o r a t i o n a l i z e t h e d e m o n s t r a t e d r e q u i r e m e n t t o
a d s o r b s p e c i e s o n some m e t a l s u r f a c e s o r o n some m e t a l
oxide surfaces to provide p r o t e c t i o n .
I t appears
to
me t h a t t h e f u n c t i o n o f t h e a d s o r b a t e is t o
protect
the oxide f i l m against the h y d r a t i n g or s o l v a t i n g
effect of the water.
T h e f i l m i t s e l f is g o o d e n o u g h ,
then, to provide p r o t e c t i o n .
I n F i g u r e 3 we s e e t h e r e l a t i o n s h i p b e t w e e n m e t a l oxide bond energy and the T a f e l s l o p e - - t h e r e l a t i o n s h i p
between p o t e n t i a l and l o g c u r r e n t d u r i n g oxide
film
formation.
It says
e r g y is h i g h e r , i t t a k e
g i v e n i n c r e a s e in c u r r e n t , w h i c h is t o s a y t h a t w h e n
t h e b o n d e n e r g y is h i g h e r i t is m o r e d i f f i c u l t t o p a s s
e x t r a c u r r e n t t h r o u g h a more p r o t e c t i v e
film.
C o r r o s i o n and o x i d a t i o n of v a l v e metals g e n e r a l l y
c o n s i s t both of the growth and degradation of
oxide
films.
I ' d l i k e to begin d i s c u s s i o n of the growth of
o x i d e f i l m s w i t h F i g u r e 4, t a k e n f r o m H a u f f e ( 3 ) .
In
t h i s c a s e i r o n is o x i d i z e d d i r e c t l y t o f e r r i c
oxide
in a n i t r i c - n i t r o u s a c i d s o l u t i o n .
The a u t h o r has
suggested the existence of concentration gradients
for cation l a t t i c e defects,
oxygen v a c a n c i e s ,
and
electrons.
D u r i n g t h e g r o w t h o f t h e f i l m , i t is p r o posed that e l e c t r o n s , c a t i o n s , and anions migrate.
A
l a r g e space charge v a r i a t i o n
is
proposed w i t h i n the
o x i d e f i l m a c a s e w h i c h was i g n o r e d f o r y e a r s by
p e o p l e who s t u d i e d o x i d a t i o n .
The r e s u l t a n t c o m b i n a t i o n
of f i e l d s from surface charges and from the space
charge
l e a d s t o n o e l e c t r i c a l f i e l d at t h e i n t e r f a c e
between
Z o n e s I a n d I I at w h i c h l o c a t i o n n e w o x i d e
is
proposed
to form.
O t h e r more c o m p l i c a t e d models have been d e v e l o p e d ; I h a v e c h o s e n t o show t h i s b e c a u s e o f i t s
generally applicable principles.
I n F i g u r e 5 we s e e t h a t s o m e t i m e s s e e m i n g l y
perfect
t h i n f i l m s are f a r from perfect.
T h i s is f o r a n a l u m i n u m a l l o y t h a t , f r o m some t e c h n i q u e s , w o u l d a p p e a r
to
carry a perfect film.
We s e e s o m e o f a s e r i e s
of
p l a t e l e t s g r o w i n g o u t in t h i s t r a n s m i s s i o n e l e c t r o n
micrograph t a k e n across an edge.
F i g u r e 6 shows a c a s e
in w h i c h o x i d e n u c l e i g r o w o n a m e t a l s u r f a c e .
This
o n e is f o r i r o n in l o w p r e s s u r e o x y g e n ; s i m i l a r r e s u l t s
have been observed f o r other m e t a l s .
If
we l o o k
at
this surface during oxidation,
we s e e
only a very
thin
190
CORROSION C H E M I S T R Y
7. D R A L E Y
Corrosion
of Valve
191
Metals
Surface Charge
Plenum Press
Figure 4. Schematic of the concentration of the free electrons and ionr-defect
positions in the homogeneously structured passive layer Fe O with space-charge
inversion (3)
2
Academic Press
Figure 5.
Oxide platelets viewed by electron silhouette on aluminum after corrosion for 30 hoursinsteamat540 C (4)
192
CORROSION C H E M I S T R Y
Oxide nuclei on (111) iron foil after oxidation at 540C and 1.1 X 10~
Ton for 55 minutes (5)
7.
DRALEY
Corrosion
of
Valve
Metals
193
194
CORROSION C H E M I S T R Y
Corrosion
Figure 7.
7. D R A L E Y
Corrosion
of Valve
Metals
CHEMICAL ANALYSIS
24 Cr
14 Cr
-o-
5Cr
20
40
60
40
80
120
196
CORROSION
CHEMISTRY
m e t a l a l s o v a r i e s w i t h i n the f i l m .
That c o r r e s p o n d i n g
t o M 0 is shown. As one g e t s c l o s e r t o the m e t a l ,
shown by s u r f a c e a n a l y s i s a f t e r s p u t t e r i n g some materi a l o f f the s u r f a c e , the o x i d e is seen t o have l e s s
oxygen in i t . I t is a n o t h e r p e c u l a r i t y o f some o f
t h e s e systems t h a t in the t h i n f i l m r a n g e , the s t o i e h i ometry o f the o x i d e is n o t as e x p e c t e d .
As most o f you know, i f n i c k e l is added t o c h r o mium a l l o y s a s e r i e s o f s t a i n l e s s s t e e l s is p r o d u c e d
w i t h q u i t e good c o r r o s i o n r e s i s t a n c e . These a l l o y s
have a w i d e s p r e a d use; t h e s e days t h o s e w i t h composit i o n s c l o s e t o 18 Cr-8 N i are i n c r e a s i n g l y b e i n g u s e d
in many p r a c t i c a l a p p l i c a t i o n s . I guess I s h o u l d
warn you t h a t i f you l i s t e n e d t o Roger S t a e h l e you h e a r d
t h a t t h e r e a r e a number o f i n s t a n c e s when t h e y a l s o
crack unexpectedly a f t e r a p e r i o d of exposure.
So
t h e y a r e by no mean
n e v e r t h e l e s s , they o f f e
in the p e r f o r m a n c e o f low t o moderate c o s t m a t e r i a l s .
F i g u r e 10 is made up t o show t h a t i f aluminum is added
t o 304 s t a i n l e s s s t e e l - - t h a t is r o u g h l y the s i m p l e 18
and 8 s t a i n l e s s s t e e l - - t h e c o r r o s i o n p r o t e c t i o n p r o v i d e d by the o x i d e t o s u p e r h e a t e d steam is v e r y markedly increased.
T h i s element a l s o i n c r e a s e s r e s i s t a n c e
t o o x i d a t i o n by g a s e s - - a i r , oxygen, and c a r b o n d i o x i d e .
Aluminum is an e f f e c t i v e a l l o y i n g c o n s t i t u e n t f o r imp r o v i n g the c o r r o s i o n - o x i d a t i o n r e s i s t a n c e o f i r o n as
w e l l . 'The p r o t e c t i v e f i l m is e n r i c h e d in aluminum
as compared t o the m e t a l c o m p o s i t i o n .
In both systems,
the a l l o y s c o n t a i n i n g aluminum t e n d t o be b r i t t l e .
I have been t a l k i n g t o you about the f o r m a t i o n o f
o x i d e f i l m s ; f o r c o m p l e t e n e s s i t is a p p r o p r i a t e t o
m e n t i o n t h a t f i l m s a l s o grow o r f o r m by r e c r y s t a l l i z a t i o n o f a s u b s t r a t e l a y e r o r by h y d r a t i o n o f a materi a l t h a t was o r i g i n a l l y formed in an anhydrous form.
As a g e n e r a l r u l e , t h o s e c a s e s produce f i l m s t h a t a r e
not h i g h l y p r o t e c t i v e . There is a p r a c t i c a l e x c e p t i o n
t o t h a t : i f one a n o d i z e s aluminum in some e n v i r o n m e n t s ,
n o t a b l y s u l f u r i c a c i d , the c o a t i n g is r e l a t i v e l y p o r o u s ,
w i t h the o x i d e o n l y p a r t i a l l y h y d r a t e d . By b o i l i n g in
w a t e r , h y d r a t i o n is i n c r e a s e d , the volume o f the o x i d e
is i n c r e a s e d and the p o r e s a r e s e a l e d , and an e f f e c t i v e
p r o t e c t i v e l a y e r is formed.
I'd l i k e t o s w i t c h now and t a l k t o you about degr a d a t i o n of f i l m s .
I n o r d e r t o t a l k about c o r r o s i o n
one has t o c o n s i d e r b o t h the growth o f f i l m s and t h e i r
degradation.
As one s t u d i e s them, one f i n d s t h a t the
c o m b i n a t i o n o f the two is the key t o g e t t i n g a measure
o f u n d e r s t a n d i n g o f the b e h a v i o r o f the system. Some
o f the r e s u l t s a r e a l i t t l e u n e x p e c t e d at f i r s t g l a n c e .
2
7.
DRALEY
Corrosion
of Valve
197
Metals
%AI
18.0
17.0
8.8
0.06
20
40
60
80
IOO
120
140
160
TIME, days
Figure 11.
CORROSION
198
CHEMISTRY
I t is w e l l f o r us t o be a l e r t f o r such t h i n g s . L e t me
make an o b v i o u s p l a t i t u d e f o r y o u about a f i l m t h a t
degrades at a c o n s t a n t r a t e .
I t is o b v i o u s t h a t t h e
m e t a l w i l l c o r r o d e at a c o n s t a n t r a t e i f d e g r a d a t i o n
is u n i f o r m a l l o v e r t h e s u r f a c e , a l t h o u g h i t m i g h t
t a k e some time t o r e a c h t h a t s t a g e w h i l e i t b u i l d s up
a film.
That is n o t a r a r e phenomenon. I t is more
common, I t h i n k f o r d e g r a d a t i o n n o t t o be t h a t u n i f o r m
o v e r t h e s u r f a c e . We're g o i n g t o t a l k about some o f
those cases.
F i r s t , l e t us c o n s i d e r a r a t h e r s i m p l e c a s e : t h e
f i l m d i s s o l v e s in w a t e r .
When i r o n o r s t e e l c o r r o d e s
in h i g h t e m p e r a t u r e w a t e r w i t h o u t oxygen, t h e r a t e o f
c o r r o s i o n is d e t e r m i n e d by t h e r a t e at w h i c h t h e f i l m
d i s s o l v e s and is l o s t from t h e s u r f a c e . T h i s r a t e
remains c o n s t a n t i f t h e r e is some p r o c e d u r e t h a t c l e a n s
up t h e w a t e r and keep
such p r o c e d u r e is passag t h r o u g
p
g
l o w e r t e m p e r a t u r e septum in w h i c h some o f t h e m a t e r i a l
p r e c i p i t a t e s . A n o t h e r is passage t h r o u g h an i o n exchange r e s i n o r some o t h e r d e v i c e t h a t w i l l p u r i f y
the w a t e r .
Such l o o p systems o f t e n c o n t a i n suspended
s o l i d c o r r o s i o n p r o d u c t in t h e w a t e r .
This m a t e r i a l
in n u c l e a r r e a c t o r systems is c a l l e d c r u d and i t has
been t h e s o u r c e o f a l o t o f i r r i t a t i o n and money.
These come about because t h e c r u d is r a d i o a c t i v e and
d e p o s i t s in a l l p a r t s o f t h e system, c o m p l i c a t i n g
maintenance.
I f one chooses t h e t e m p e r a t u r e and t h e a l l o y ( e . g .
A288 c o n t a i n i n g 1% N i ) aluminum forms in w a t e r a f i l m
such t h a t t h e r a t e o f f o r m a t i o n is i n v e r s e l y p r o p o r t i o n a l t o t h e t h i c k n e s s . That g i v e s what's c a l l e d t h e
p a r a b o l i c g r o w t h l a w ; d a t a f o r such b e h a v i o r a r e shown
in F i g u r e 11 ( t o g e t h e r w i t h a dashed c u r v e f o r a n o t h e r
experiment).
I n t h e k i n d o f system in riich f r e s h w a t e r
is c o n t i n u o u s l y added and t h e e x c e s s a l l o w e d s i m p l y t o
l e a k o u t , at l e a s t a p a r t i a l l y s a t u r a t e d s o l u t i o n is
l o s t a l l o f t h e t i m e . The two specimens o f F i g u r e 12
have been f i t t e d by c u r v e s t h a t have t h e same growth
r a t e c o n s t a n t as in F i g u r e 11 and d i f f e r e n t d i s s o l u tion rates.
They were in t h e same a u t o c l a v e at s l i g h t l y d i f f e r e n t l o c a t i o n s ; probably the water contained a
l i t t l e more d i s s o l v e d aluminum at one specimen t h a n at
t h e o t h e r . The e x p r e s s i o n f o r t h e p a r a l i n e a r b e h a v i o r
in i t s s i m p l e form is:
dt - L-(g+ft)
'
DRALEY
Corrosion
of Valve
20
Metals
TIME, days
Figure 12.
1
CORROSION OF ALLOY 255 IN
350 WATER
CONTROL SAMPLES
- 0 . 2 MILS OXIDE REMOVED
- 0.4 MILS OXIDE REMOVED
ol0
20
*-
8
I
40
TIME,
60
80
days
Journal of Nuclear Materials
Figure 13.
Al-lNi-
CORROSION
200
CHEMISTRY
where L is t h e amount o f m e t a l l o s s ( d e t e r m i n e d t h r o u g h
t h e use o f a s p e c i a l m e t a l t h i c k n e s s gauge), kp t h e
p a r a b o l i c growth c o n s t a n t , f t h e r a t e o f d i s s o l u t i o n
f o r t h e specimen, and g t h e amount o f d i s s o l u t i o n t h a t
o c c u r s e a r l y in t h e e x p o s u r e . A t l o n g t i m e s t h e c u r v e
f o r L v e r s u s time r e s e m b l e s a s t r a i g h t l i n e w i t h
slope f.
P a r a l i n e a r c o r r o s i o n ( r e l a t e d to d i s s o l u t i o n of
c o r r o s i o n p r o d u c t ) does n o t o c c u r f o r a l l aluminum
a l l o y s in w a t e r at a l l h i g h t e m p e r a t u r e s .
I n F i g u r e 13
are p l o t t e d d a t a f o r an a l l o y ( A l , 1% N i , 0.1% T i )
c o r r o d e d in w a t e r at 350C ( 1 0 ) . The c o r r o s i o n r a t e
was low and c o n s t a n t , as shown b e t t e r in o t h e r f i g u r e s
in the same p u b l i c a t i o n .
F o r some specimens in t h e
f i g u r e 1/3 o r 2/3 o f t h e c o r r o s i o n p r o d u c t was removed
m e c h a n i c a l l y a f t e r t h e f i r s t exposure p e r i o d .
There
was no d i s c e r n i b l e e f f e c
dicating that contro
c l o s e to the m e t a l - o x i d e i n t e r f a c e .
Similar experi
ments f o r t h e a l l o y s and t e m p e r a t u r e s where p a r a l i n e a r
b e h a v i o r o c c u r s showed t h a t removing some o f t h e p r o d u c t caused an i n c r e a s e in subsequent c o r r o s i o n r a t e .
The use o f F i g u r e 14 b e g i n s d i s c u s s i o n about
low t e m p e r a t u r e c o r r o s i o n o f aluminum in w a t e r .
Again
we 11 see t h a t the t o t a l f i l m is n o t c o n t r o l l i n g .
This
is t h e k i n d o f c u r v e o b t a i n e d in a c o u p l e o f t e s t s t h a t
ran f o r a l o n g t i m e in c o n t i n u o u s l y r e f r e s h e d systems.
Note t h a t t h e r a t e is d e c r e a s i n g c o n t i n u a l l y w i t h time
f o r at l e a s t a few y e a r s w e ' l l a n a l y z e t h a t c u r v e
shape s t a r t i n g w i t h F i g u r e 15. A t t h e b e g i n n i n g o f the
t e s t s ( f r o m 0.1 t o n e a r l y 10 h o u r s ) and subsequent t o
about 100 h o u r s , t h e w e i g h t g a i n and t h e m e t a l c o r r o d e d
( d e t e r m i n e d t h r o u g h t h e use o f a s e n s i t i v e m e t a l t h i c k ness gauge) v a r i e d as t h e l o g a r i t h m o f t i m e . The i n i t i a l f i l m is boehmite, t h e same p a r t l y h y d r a t e d o x i d e
t h a t forms at h i g h t e m p e r a t u r e s . When t h i s f i l m b r e a k s
down, b e g i n n i n g in h a l f a day at t h e s e c o n d i t i o n s , we
f i n d t h a t t h e t o t a l amount o f c o r r o s i o n and the t o t a l
o x i d e p r e s e n t q u i c k l y i n c r e a s e s e v e r a l f o l d . The new
p r o d u c t is t h e c o m p l e t e l y h y d r a t e d o x i d e b a y e r i t e . I f
one runs t h e t e s t a l o n g time ( F i g u r e 16) one f i n d s
t h a t t h e l o g a r i t h m i c r a t e law h o l d s . The dashed l i n e s
i n d i c a t e t h a t the i n i t i a l p e r i o d is s e n s i t i v e t o t h e
t e s t p r o c e d u r e t h a t is used.
The h e i g h t o f t h e p l a t e a u v a r i e s i n v e r s e l y w i t h how w e l l r e f r e s h e d t h e s o l u tion
is.
The r e q u i r e m e n t f o r measurement s e n s i t i v i t y
in t h i s t e s t was s u b s t a n t i a l . As a m a t t e r o f f a c t ,
t h e (eddy c u r r e n t ) gauge l i m i t a t i o n came n o t by i t s
s e n s i t i v i t y (about t e n Angstrom u n i t s in d i a m e t e r f o r
1
DRALEY
Corrosion
of Valve
200
Metals
201
40
TIME , days
U.S. Atomic Energy Commission and
European Atomic Energy Society
Figure 14.
0.1
10
100
TIME, hours
1000
CORROSION
CHEMISTRY
7.
DRALEY
Corrosion
of
Valve
Metals
203
CORROSION C H E M I S T R Y
204
7.
DRALEY
Corrosion
of Valve
Metals
205
206
CORROSION C H E M I S T R Y
10,000 F
Il
- I -'
5
10
'
- - 1 '
.
SO 100
500 1000
Exponrt URN, Dtys
. . I 11
5000 10,000
. . Ii
DRALEY
Figure 21.
Corrosion
of Valve
Metals
207
CORROSION
208
CHEMISTRY
d e a l i n g w i t h continuous f i l m s c o n t i n u o u s l y growing
w i t h o u t breakdown. I t happens t h a t t h e breakdown
on t h e s e specimens o c c u r r e d o v e r much o f t h e s u r f a c e
at the same t i m e so t h e phenomenon is v i s i b l e .
If i t
o c c u r r e d l o c a l l y and n o t at t h e same t i m e at d i f f e r e n t
p l a c e s , y o u w o u l d n o t see t h e " h i l l s " , b u t odd k i n e t i c s
such as r o u g h l y c u b i c .
The phenomenon is by no means
unique t o zirconium.
F i g u r e 21 shows s i m i l a r b e h a v i o r
by aluminum, a l s o in h i g h t e m p e r a t u r e w a t e r .
The o x i d a t i o n o f z i r c o n i u m in oxygen at e l e v a t e d
temperatures f o l l o w s near-cubic k i n e t i c s f o r awhile,
t h e n p a r a b o l i c k i n e t i c s . A number o f e f f o r t s have been
made t o e x p l a i n t h e n e a r - c u b i c b e h a v i o r , w i t h l o c a l i z e d
o r l i n e d i f f u s i o n as t h a t perhaps g e n e r a l l y p r e f e r r e d .
I ' l l describe f o r yo
and p r e s e n t e d i n f o r m a l l
l i s h e d . A t 700C t h e r e is an i n i t i a l l a y e r o f z i r c o nium o x i d e t h a t is r e l a t i v e l y p e r f e c t , t h a t forms
i n t e r f e r e n c e c o l o r s on s m a l l a r e a s ; t h e r a t e o f growth
o f f i l m and t h e r a t e o f d i f f u s i o n o f oxygen i n t o t h e
m e t a l depend on t h e o r i e n t a t i o n o f t h e m e t a l c r y s t a l .
P o l y c r y s t a l l i n e specimens p r e p a r e d m e t a l l u r g i c a l l y
in d i f f e r e n t ways, always p u r e and e q u a l l y c a r e f u l l y
t r e a t e d , o x i d i z e at q u i t e d i f f e r e n t r a t e s i n i t i a l l y ,
but t h e r a t e s become e q u a l at l o n g ( p a r a b o l i c ) t i m e s .
To f i t t h e s e f a c t s I d e v e l o p e d t h e f o l l o w i n g model.
The f i l m t h a t grows on t h e m e t a l s u r f a c e is p r o p o s e d
t o be u n i q u e and d i f f e r e n t from t h e one t h a t is s t a b l e
on t h e o u t s i d e and at l o n g t i m e s .
X-ray d i f f r a c t i o n
p a t t e r n s t a k e n in o u r l a b o r a t o r y d u r i n g i n i t i a l o x i d a t i o n ( s p e c i a l apparatus) suggest t h a t t h i s oxide
is t e t r a g o n a l r a t h e r t h a n t h e commonly found monoc l i n i c oxide.
A t any s m a l l a r e a on t h e s u r f a c e , f o r
example f o r a s i n g l e c r y s t a l s u r f a c e , t h e i n i t i a l f i l m
grows and oxygen d i f f u s e s i n t o t h e m e t a l at r a t e s t h a t
are unique f o r that area u n t i l the f i l m t h i c k n e s s
r e a c h e s t h e v a l u e s ( p r o p o s e d t o be t h e same f o r a l l
areas).
The i n i t i a l f i l m t h e n t r a n s f o r m s n e a r l y ins t a n t a n e o u s l y i n t o f i n a l f i l m , and t h e i n i t i a l f i l m
t h e n grows a g a i n in a second c y c l e , a g a i n t o t h e
l i m i t i n g t h i c k n e s s s. The p a r a b o l i c r a t e c o n s t a n t
f o r t h e f i n a l f i l m is c ( v a l u e t h e same f o r a l l a r e a s )
and t h e r a t e o f growth o f i n i t i a l f i l m at a p a r t i c u l a r
a r e a is
t
7.
DRALEY
Corrosion
of Valve
Metals
209
210
CORROSION C H E M I S T R Y
DRALEY
Corrosion
of Valve
Metals
212
CORROSION
CHEMISTRY
> MOH
+ H
+ e" ,
+ e"
1/2
1/2
+ OH"
+ 2H 0 + 2e~ + 20H"
2
and
,
so c a t h o d i c r e a c t i o n s produce a l k a l i .
Directly related
t o the pH a r e the s t a b i l i t i e s o f the v a r i o u s s p e c i e s
f o r the c o r r o d i n g m e t a l
P o u r b a i x Diagram f o
pH zones in w h i c h F e 0 o r F e ( 0 H ) a r e s t a b l e and thus
in w h i c h p r o t e c t i v e f i l m s o f t h e s e s u b s t a n c e s m i g h t
f o r m at a t o t a l i o n i c c o n c e n t r a t i o n o f 10"
M.
When a f i l m is p r e s e n t , the hydrogen p r o d u c e d from
the second r e a c t i o n above is n o t n e c e s s a r i l y a l l l i b e r ated d i r e c t l y i n t o the water or s o l u t i o n .
Some o f i t
may be l i b e r a t e d b e n e a t h the f i l m as shown in F i g u r e 26.
The r e s u l t may be l o c a l r u p t u r i n g o f the o x i d e f i l m - a form of degradation--the f o r m a t i o n of metal h y d r i d e ,
o r the e n t r y o f hydrogen i n t o t h e m e t a l , depending on
w h i c h a r e f e a s i b l e o r most f a v o r a b l e . I b e l i e v e t h e r e
a r e a number o f cases where f i l m r u p t u r e o c c u r s , a l though they a r e o f t e n n o t easy t o i d e n t i f y . We have
d e c l a r e d the b e l i e f t h a t i t is i m p o r t a n t in the c o r r o s i o n o f aluminum a l l o y s below the b o i l i n g p o i n t o f
w a t e r ( 1 9 ) . To p r o v i d e e v i d e n c e o f t h i s , we r a n a ser i e s o f e x p e r i m e n t s t o d e t e r m i n e the l o g a r i t h m i c c o r r o s i o n r a t e c o n s t a n t f o r 1100 aluminum at 70C w i t h pot e n t i a l s c o n t r o l l e d by a s p e c i a l i n t e r r u p t i n g p o t e n t i o s t a t ( 2 0 ) . The r e s u l t s , in F i g u r e 27, show t h a t
a n o d i c p o l a r i z a t i o n (diamond-shaped p o i n t s ) caused
lower c o r r o s i o n r a t e s t h a n the u n p o l a r i z e d runs ( c i r c u l a r p o i n t s ) . A r e d u c t i o n in c a t h o d i c damage t o the
f i l m is s u g g e s t e d . The p o t e n t i a l above w h i c h hydrogen
s h o u l d n o t be l i b e r a t e d cannot be i d e n t i f i e d because
the l o c a l pH d u r i n g a n o d i c p o l a r i z a t i o n c o u l d be cons i d e r a b l y below the 6+ o f the d i s t i l l e d w a t e r at 70.
We s h a l l see something e n l i g h t e n i n g on t h i s l a t e r .
For
pH 6 the p o t e n t i a l f o r the r e v e r s i b l e hydrogen l i b e r a t i o n r e a c t i o n is c a l c u l a t e d t o be about -0.4 v o l t .
Most n o t a b l y at h i g h e r t e m p e r a t u r e s , aluminum a l s o
s u f f e r s f r o m the e n t r y o f c o r r o s i o n p r o d u c t i n t o the
2
7.
DRALEY
Corrosion
of Valve
213
Metals
214
CORROSION
CHEMISTRY
-0.5
-0.4
-0.3
-0.2
SOLUTION POTENTIAL,
Figure 27.
V.
-0.1
Vs.
STD.
0
HYD.
+0.1
ELEC.
7.
DRALEY
Corrosion
of
Valve
215
Metals
metal.
I n F i g u r e 28 a r e shown specimens o f commerc i a l l y p u r e aluminum a f t e r two weeks exposure t o w a t e r
at 275C. The b l i s t e r i n g p r o g r e s s e s w i t h more s e v e r e
exposure c o n d i t i o n s , as shown in F i g u r e 29 (66 h o u r s ,
300C). Some o f the b l i s t e r s a r e h o l l o w b e f o r e the
w a t e r g a i n s a c c e s s and b e f o r e they become o x i d e - m e t a l
m i x t u r e s . F i g u r e 30 shows what happens at a h i g h e r
t e m p e r a t u r e ; t h i s exposure was f o u r hours at 315C.
I f v a r y i n g amounts o f m a t e r i a l a r e e t c h e d from t h e
s u r f a c e o f a s e r i e s o f samples c o r r o d e d f o r a b r i e f
p e r i o d , and each r e m a i n i n g sample is a n a l y z e d f o r hydrogen c o n t e n t , the hydrogen in t h e e t c h e d - o f f l a y e r s
can be c a l c u l a t e d .
The r e s u l t s in F i g u r e 31 show t h a t
the hydrogen c o n t e n t o f the s u r f a c e l a y e r s i n c r e a s e d
q u i t e a b i t , and demonstrate t h a t the gas t h a t formed
t h e b l i s t e r s was hydrogen
I f t h e h y d r o g e n is produced
l a r g e l y at a p o s i t i o
as in F i g u r e 32, th
t h i s c a s e , the aluminum is s i m p l y b o l t e d t o a p i e c e
of s t a i n l e s s s t e e l .
Exposure c o n d i t i o n s were as in
F i g u r e 30.
I f t o the w a t e r a r e added i o n s t h a t a r e
r e d u c i b l e t o m e t a l ( l a r g e l y at a c t i v e cathode s p o t s )
m e t a l d e n d r i t e s a r e formed. The n i c k e l d e n d r i t e s in
F i g u r e 33 were formed in t h i s way; no s e v e r e c o r r o s i o n
o f 1100 aluminum was o b s e r v e d d u r i n g c o r r o s i o n exposure
t o s t a b l e n i c k e l s a l t s o l u t i o n s at e l e v a t e d temperat u r e s . F i g u r e 34 s u g g e s t s t h a t i f d e p o s i t s o f somet h i n g l i k e t h e n i c k e l - a l u m i n u m compound N i A l were
u s e d t h e y w o u l d a c t as v e r y e f f e c t i v e cathodes f o r
hydrogen l i b e r a t i o n .
F o r t h i s r e a s o n , we made aluminumn i c k e l a l l o y s in w h i c h N i A l p r e c i p a t e d . As i n d i c a t e d
in F i g u r e 35, some 1% n i c k e l a l l o y s showed e x c e l l e n t
c o r r o s i o n r e s i s t a n c e in h i g h t e m p e r a t u r e w a t e r .
I won't
d i s c u s s d e t a i l s o f c o m p o s i t i o n and m e t a l l u r g i c a l p r e p a r a t i o n ; t h e y were f o u n d t o be i m p o r t a n t .
Uranium c o r r o d e s in o x y g e n - f r e e w a t e r at a c o n s t a n t
r a t e t o f o r m U0 in t h e form o f a r e l a t i v e l y u n p r o t e c t i v e l a y e r ; F i g u r e 36 shows such c o r r o s i o n r a t e s on an
A r r h e n i u s p l o t . When the r a t e g e t s v e r y l a r g e at e l e v a t e d t e m p e r a t u r e s , u r a n i u m h y d r i d e can be found m i x e d
in w i t h t h e o x i d e powder. I f oxygen is p r e s e n t in the
water, f o r a long p e r i o d p r o t e c t i v e oxide f i l m s are
formed; t h e s e e v e n t u a l l y break down l o c a l l y and s p r e a d .
F i g u r e 37 shows t h a t t h e whole s u r f a c e e v e n t u a l l y becomes bad.
We b e l i e v e t h a t some h y d r i d e was r e g u l a r l y
formed b e n e a t h t h e o x i d e b o t h in d e a e r a t e d and a e r a t e d
w a t e r , and t h a t the h y d r i d e s u b s e q u e n t l y was c o n v e r t e d
t o t h e more s t a b l e o x i d e . T h i s is b e l i e v e d t o be a
case o f f i l m d e g r a d a t i o n by t h e f o r m a t i o n o f h y d r i d e
beneath i t .
For s e l e c t e d uranium a l l o y s , oxide f i l m s
3
216
CORROSION C H E M I S T R Y
Corrosion
Figure 29.
DRALEY
Corrosion
of Valve
217
Metals
Corrosion
Figure 30.
TREATMENT
AVERAGE
METAL
ETCHED
AWAY
HYDROGEN
CONTENT
(WHOLE
SAMPLE)
12.1 ppm
AS CORRODED
ESTIMATE OF
HYDROGEN IN
INCREMENTS OF
ETCHED METAL
7.3
0.016 mm
3.0
ETCHED
0.041
1.8
59
ETCHED
0.071
1.1
24
ETCHED
0.120
1.0
ETCHED
0.22
0.8
0.6
AS STRIPPED
280 ppm
218
CORROSION C H E M I S T R Y
Corrosion
Figure 32.
1100 aluminum,
distilled waterat315C (22)
Figure 33.
Corrosion
4
solution
++
Corrosion
DRALEY
219
of Valve Metals
FeAI
C
J -.5
--^^
FeNiAI,
-.4
1 *
Ni AU
^ -
>
-.2
'
.-
A - " ^
A
TEMPERATURE: 2 9 0 C
MEDIUM: DISTILLED WATER
20
CATHODIC
Figure 34.
30
I
10
40
50
60
70
80
'X800l-350C
LINE REPRESENTS
~b DATA OF DILLON
'
U\A/_CIQ/1Q >
Aqueous corrosion of aluminum alloy A288 (Al -\- 1% Ni, 0.5% Fe,
0.1% Ti) (24)
220
CORROSION C H E M I S T R Y
7. D R A L E Y
Corrosion
of Valve
Metals
221
CORROSION C H E M I S T R Y
222
,f
DRALEY
Corrosion
of Valve
223
Metals
1000/T ( K)
e
Figure 38.
Effect of gas removal on corrosion in water at 290C of U-5% Zr1V Nb alloy (28)
2
CORROSION CHEMISTRY
224
1.6,
(30)
Variation in oxidation rate with applied potential: zirconium in oxygen at 700C (30)
7.
DRALEY
Corrosion
of
Valve
Metals
225
CORROSION C H E M I S T R Y
226
7.
DRALEY
Corrosion
Figure 44.
of Valve
Metals
227
228
CORROSION C H E M I S T R Y
been r e p o r t e d .
Even exposed t o the b u l k w a t e r , we
found, t h r o u g h the use o f a s p e c i a l e l e c t r o d e a few
t e n t h s o f a m i l l i m e t e r in t i p d i a m e t e r ( F i g u r e 4 5 ) ,
t h a t the pH n e a r specimens o f 1100 aluminum c o r r o d i n g
in d i s t i l l e d w a t e r r e a c h e d v a l u e s below 3 ( F i g u r e 4 6 ) .
Some y e a r s ago, Howard F r a n c i s , t h e n o f the A r mour R e s e a r c h F o u n d a t i o n , made some t i m e - l a p s e m o t i o n
p i c t u r e s o f a p o t e n t i a l map o f the s o l u t i o n n e x t t o
s t e e l and aluminum a l l o y s p i t t i n g in s a l t w a t e r .
The
p r e s e n c e o f some a c t i v e cathode p o i n t s and g r o w i n g p i t s
was r e a d i l y d i s p l a y e d .
I s u g g e s t e d t h a t he r u n the
f i l m backwards t o see whether the a c t i v e p i t s had been
a c t i v e cathodes j u s t b e f o r e t h e i r i n i t i a t i o n .
He
l a t e r t o l d me he had done so, and t h e y had been w i t h
few o r no e x c e p t i o n s .
A t h i g h t e m p e r a t u r e s f i l m breakdown and p i t t i n g
f o r aluminum a l l o y s t a k e
when t h e r e is a h i g
metal surface.
I f t h e r e a r e a l o t o f specimens in the
system, the c o r r o s i o n r a t e is l o w e r t h a n i f the a r e a
is s m a l l . The r e s u l t s o f some e x p l o r a t o r y e x p e r i m e n t s
a r e shown in F i g u r e 47.
A l a r g e a r e a ( f a c t o r o f 20)
o f aluminum a l l o y i n h i b i t e d c o r r o s i o n w h i l e a l a r g e
area of s t a i n l e s s s t e e l d i d not.
The e f f e c t is c e r t a i n l y r e l a t e d t o c o r r o s i o n p r o d u c t in the system
somehow. I n F i g u r e 48 one can see t h a t the c o r r o s i o n
p r o d u c t l o s t f r o m the specimen was s u b s t a n t i a l l y in
e x c e s s o f t h a t w h i c h d i s s o l v e d in the system.
We
t h o u g h t t h a t , at l o c a l p i t s and b r e a k s the c o r r o s i o n
p r o d u c t , as i t r e a c h e d the o x i d e s u r f a c e and was app r o x i m a t e l y n e u t r a l i z e d , was swept away as p a r t i c l e s o f
oxide.
We t h o u g h t t h a t the a d d i t i o n o f c o l l o i d a l
p a r t i c l e s t o the s o l u t i o n w o u l d t e n d t o " p l u g " openi n g s , r e d u c e the l o s s o f o x i d e , and lower the c o r r o s i o n r a t e . F i g u r e 49 shows t h a t when a h y d r a t e d c o l l o i d was i n j e c t e d in t o the system, a v e r y low c o r r o s i o n r a t e was o b t a i n e d .
I n the same system the
e f f e c t s o f p o l a r i z i n g c u r r e n t on c o r r o s i o n r a t e ( F i g u r e 50) a r e s i m i l a r t o what I showed you at low temp e r a t u r e in F i g u r e 27:
a n o d i c p r o t e c t i o n and c a t h o d i c
stimulation.
The message I'd l i k e t o l e a v e w i t h you t o n i g h t
is t h a t some f i l m s a r e v e r y good and v e r y p r o t e c t i v e ,
some f i l m s have b r e a k s in them, and they a r e moderatel y good, some become bad by some o f the s t r a n g e s t
mechanisms: an u n d e r s t a n d i n g o r c o r r o s i o n phenomena
sometimes r e q u i r e s q u i t e a b i t o f i n g e n u i t y .
7.
DRALEY
Corrosion
of Valve
229
Metals
Corrosion
Figure 45.
230
CORROSION
CHEMISTRY
7
2 mm FROM
UPSTREAM
EDGE
--"
2mm FROM
DO\ VNSTREAM
ED(SE
w
^^CENTER
5
X
CL
40
80
TIME, mins
120
160
200
Corrosion
Figure 46.
Figure 47.
7. D R A L E Y
Corrosion
of Valve
231
Metals
Large Area,
28-day Exposure
13.1 mg/cm
4.60 mg/cm
29.1 mg/cm
10.2 mg/cm
11.6 mg/cm
7.9 mg/cm
17.5 mg/cm
2.3 mg/cm
40%
78%
Aluminum Area
70 cm
1470 cm
2.4 mg/cm
0.14 mg/cm
0.14
0.06
20
18
16
METAL CORRODED, m
^ 14
_
DISTILLED
^i.JV'' *
00
j*"^
WATER
UPSTREAM
_ -
DOWNSTREAM Q
-
1
"
1
15
^
1
20
25
TIME, days
30
35
40
232
CORROSION
CHEMISTRY
18
14
DISTILLED WATER
150
50
100
CATHODE
50
100
ANODE
150
/cm
7.
DRALEY
Corrosion
of Valve
Metals
233
L i t e r a t u r e Cited
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
CORROSION CHEMISTRY
234
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
1978.
8
The Role of Water in Atmospheric Corrosion
P. B. P. PHIPPS and D . W . R I C E
I B M Corporation, San Jose, CA 95193
0-8412-0471-3/79/47-089-235$06.75/0
1979 American C h e m i c a l Society
CORROSION CHEMISTRY
236
1.0
O b s e r v a t i o n s on A t m o s p h e r i c
Corrosion
PHipps
A N D RICE
Water in Atmospheric
Table I
Accelerated reference
atmosphere t e s t
TC
H0
25
N0
480 p p b
8055 RH
so
310 p p b
170 p p b
3
HS
15 p p b
C 1
4.0
Corrosion
3 ppb
r-
Figure 1.
Weight gain of Ni Fe o
80
foil as a function of
pS0
and RH
238
CORROSION C H E M I S T R Y
8.
PHiPPS A N D R I C E
Water in Atmospheric
239
Corrosion
1.3
Summary of Atmospheric Corrosion Data. From
the raw data we can draw several s a l i e n t points.
(i) D i f f e r e n t measurements reveal d i f f e r e n t
aspects of a complex phenomenon.
( i i ) Several phases are often (3_) present in
r e a l atmospheric corrosion products.
A wide
range of oxidation l e v e l s is present on samples
at one time.
(iii)
Structures containing d i s s i m i l a r metals
show galvanic i n t e r a c t i o n s q u a l i t a t i v e l y l i k e
those seen in aqueous environments.
(iv) The rough morphology and the mixed products suggest that models of the r a t e - l i m i t i n g
steps w i l l be more complex than those of, e.g.,
t a r n i s h i n g . As is true of low temperature
oxidation (2^) , k i n e t i c studies of the rate of
growth contain too few parameters to resolve these
complexities.
Mechanistic experiments must be designed.
240
CORROSION
CHEMISTRY
Time Days
Figure S.
p f f l p p s A N D RICE
Water in Atmospheric
Corrosion
i50r
/bb%
75% RH /
/
/
/
RH
//
/
Cr Thin Films on AI
//
//
. 75% RH
...*.*.*-"-'- 55% RH
^^S.CS.iS^.'-i:^
2
Figure 4.
- 3 0 % RH
3
Time Days
242
CORROSION CHEMISTRY
Unexposed
A 2 0 % R.H. 72 h.
4 5 % R.H.
6 2 % R.H.
7 5 % R.H.
150
200
Depth
Figure 5.
Wavelength (Microns)
4
_J
3500
3000
2500
10
II
II
2000
1800
1600
1400
1200
1000
12
L_
L_
800
Frequency (cm )
-1
Figure 6.
8.
PHiPPS A N D R I C E
Water in Atmospheric
Corrosion
243
(v)
RH is a dominant v a r i a b l e in many examples.
A l t h o u g h t h e RH at w h i c h c o r r o s i o n is n e g l i g i b l e
depends on t h e o t h e r gases p r e s e n t and t h e s e n s i t i v i t y o f d e t e c t i o n , d i f f e r e n t a u t h o r s see t h e
same t r e n d . T h i s is b r o u g h t o u t by comparing some
p u b l i s h e d " c r i t i c a l r e l a t i v e h u m i d i t i e s " at w h i c h
c o r r o s i o n is m i n i m a l , see T a b l e I I . S e v e r a l
workers
5_, 6_) have r e p o r t e d e l e c t r o c h e m i c a l
c u r r e n t s between d i s s i m i l a r m e t a l s , c o n n e c t e d by
a d s o r b e d w a t e r . I t is i n f e r r e d t h a t t h i s c u r r e n t
between p a r t i c u l a r m e t a l s at a s i n g l e p o t e n t i a l
r e p r e s e n t s "the t i m e o f w e t n e s s " , and t h i s w i l l be
a measure o f t h e amount o f ( g a l v a n i c ) c o r r o s i o n
between h e t e r o g e n i t i e s on a s i n g l e m e t a l o r between any d i s s i m i l a r m e t a l s exposed t o t h e same
atmosphere.
Thi constitute
extrem
statement
of the g e n e r a l i t
a b l e in many c a s e
atmospheri
We m i g h t summarize t h a t t h e RH is g e n e r a l l y
a dominant f a c t o r . F o r most m e t a l s , o x i d e s a r e
a l w a y s p r e s e n t on t h e s u r f a c e e x c e p t at t h e b o t tom o f g r o w i n g p i t s , b u t no c o r r o s i o n is d e t e c t e d
below 15% RH and most m e t a l s do c o r r o d e above 75%
RH r e g a r d l e s s o f t h e p a r t i c u l a r gas c o n c e n t r a t i o n s
o r s p e c i e s in t h e a c c e l e r a t e d a t m o s p h e r i c t e s t .
T h i s f o c u s e s a t t e n t i o n on t h e q u e s t i o n o f what is
c h a n g i n g so d r a m a t i c a l l y when t h e RH changes a f a c t o r
of f i v e t i m e s . C l e a r l y , t h e q u a n t i t y o f a d s o r b e d w a t e r
is i n c r e a s i n g b u t , at 15% RH (3 mm at 25C), a s i g n i f i c a n t q u a n t i t y o f a d s o r b e d w a t e r is p r e s e n t and at 75%
RH, by d e f i n i t i o n , t h e e q u i l i b r i u m q u a n t i t y cannot be
so g r e a t t h a t t h e w a t e r has b u l k p r o p e r t i e s .
(Bulk
w a t e r would e v a p o r a t e i f t h e RH is l e s s t h a n 100%!)
Water a d s o r p t i o n d a t a on c o r r o d i n g m e t a l s s u r f a c e s is
n o t a v a i l a b l e . Water a d s o r p t i o n c o m p r i s e s t h e s u b j e c t
of t h e second s e c t i o n o f t h i s p a p e r .
2.0
Water A d s o r p t i o n
Two t y p e s o f w a t e r a d s o r p t i o n e x p e r i m e n t s a r e
r e l e v a n t t o o u r a n a l y s i s . As an example o f one t y p e ,
t h i n f i l m s o f h i g h p u r i t y aluminum, d e p o s i t e d at 10"^
t o r r , show changes in work f u n c t i o n (1) and LEED p a t t e r n (S) and g a i n mass (9J due t o e x p o s u r e t o 10~ t o r r
sec o f H 0. T h i s d a t a can be a t t r i b u t e d t o M-OH d i p o l e s
o
244
CORROSION
CHEMISTRY
Table II C r i t i c a l r e l a t i v e humidities
ppm
Metal
by Volume
C r i t . RH %
Ref.
Fe
3000
100
200
100
0.5
0.1
75
65
85
>90
80
80
1
3
5
5
6
7
Cu
3000
100
0.5
0.1
0.1
60
63
1
2,3
90
Ni
1000
70
Al
3000
1000
300
0
0.1
82
52
52
85
90
1
4
4
4
7
References
1. Sanyal, B. and Bhadwer, D., J. S c i . Ind. Res. India
(1957) 18A, 69.
2. Vernon, W.H., Trans. Far. Soc. (1935) 3J_, 1668.
3. Vernon, W.H., J. Electrochem. Soc. (1933) 64, 31.
4. A z i z , P.M. and Godard, H.P., Corrosion (1957) 1_5, 39.
5. S k o r c h e l l e t t i , V.V. and Tukachinsky, S.E., J. App.
Chem. USSR (1953) 26, 27, (1955) 28, 65.
6. Clarke, S.G. and Longhurst, E.E., J. Applied Chem.
(1961)
435.
7. Sydberger, T. and Vanneberg, N.G., Corr. S c i . (1972)
12:, 775.
8. Duncan, J.R. and Spedding, D.J., Corr. S c i . (1973)
1J3, 993.
8.
PHIPPS A N D RICE
Water in Atmospheric
Corrosion
245
o r i e n t e d on t h e s u r f a c e . S i m i l a r r e s u l t s have been
r e p o r t e d f o r c l e a v e d A l (10J , f o r Au (U) , W (V2) , P t
(13) , Fe (14) . I n a second type o f e x p e r i m e n t , powdered
m e t a l o x i d e s w i t h h i g h c a t a l y t i c a c t i v i t y and s u r f a c e
a r e a a r e exposed t o w a t e r and an a d s o r p t i o n i s o t h e r m is
plotted.
I n f r a r e d a d s o r p t i o n (15,1_), NMR r e l a x a t i o n
(IZ'UL'ID
d i e l e c t r i c d i s p e r s i o n (2,2J_,^2) may be
measured on t h e s u b s t a n t i a l number o f m o l e c u l e s a d sorbed on t h e s u b s t a n t i a l a r e a . U n f o r t u n a t e l y , i t is
d i f f i c u l t t o apply e i t h e r o f these f i e l d s o f i n v e s t i g a t i o n t o the e l u c i d a t i o n o f atmospheric c o r r o s i o n .
The d e f i n i t i o n o f t h e powder s u r f a c e s t e n d s t o be
in terms o f t h e p r o c e s s f o r making i t (23). T h i s
l i m i t s i n t e r p r e t a t i o n o f t h e i r p r o p e r t i e s and a l s o
l i m i t s g e n e r a l i z a t i o n o f p r o p e r t i e s t o surfaces which
have been g e n e r a t e d
v e r s e l y , t h e LEED, Auge
e m i s s i o n p r o p e r t i e s , e t c . , a r e measured on f r e s h s u r f a c e s in UHV, because i n t e r p r e t a t i o n is d i f f i c u l t f o r
more complex c o n f i g u r a t i o n s and because e x p e r i e n c e has
shown t h a t t h e s e p r o p e r t i e s a r e p r o f o u n d l y m o d i f i e d and
c o n f u s e d by e x p o s u r e t o r e a l atmospheres.
The e x p e r i m e n t d e s c r i b e d below is an a t t e m p t t o
b r i d g e t h i s gap. S u r f a c e s r e l e v a n t t o a t m o s p h e r i c
c o r r o s i o n were p r e p a r e d from c l e a n m e t a l s u r f a c e s by
r e a c t i o n w i t h w a t e r v a p o r under c l e a n c o n d i t i o n s . The
p r o p e r t i e s measured w a t e r a d s o r p t i o n c h a r a c t e r i s t i c s
were s e l e c t e d because t h e y were b e l i e v e d t o have a
major and d i r e c t a p p l i c a t i o n t o a t m o s p h e r i c c o r r o s i o n .
o
CORROSION C H E M I S T R Y
246
Figure 7.
8. PHiPPS A N D R I C E
Water in Atmospheric
Corrosion
(ii)
The c l e a n m e t a l s u r f a c e was now exposed t o
water vapor.
T h i s was o b t a i n e d from w a t e r w h i c h
had been r e p e a t e d l y d i s t i l l e d in t h e UHV chamber.
The p r e s s u r e o f w a t e r was d e t e r m i n e d by t h e temp e r a t u r e o f t h i s w a t e r . T o t a l gas p r e s s u r eint h e
vacuum system was m o n i t o r e d by an i o n gauge, w h i c h
was n o t o p e r a t e d c o n t i n u o u s l y , and by a t h e r m a l
c o n d u c t i v i t y gauge. A f t e r a b r i e f t r a n s i e n t t h e
frequency o f o s c i l l a t i o n s decreased t o a steady
v a l u e i n d i c a t i n g t h a t t h e mass o f t h e q u a r t z
c r y s t a l s u r f a c e had i n c r e a s e d due t o r e a c t i o n w i t h
t h e w a t e r . An e f f e c t was o b s e r v e d f o r p r e s s u r e a s
low as 10"" t o r r .
(iii)
The m e t a l f i l m was now exposed t o a
g r e a t e r w a t e r v a p o r p r e s s u r e and t h e a d s o r p t i o n
was m o n i t o r e d u n t i l
p r o c e d u r e was r e p e a t e
w a t e r v a p o r p r e s s u r e up t o ~ 10 t o r r .
_
(iv)
The w a t e r v a p o r was pumped o f f t o 10""
t o r r , b u t t h e mass o f t h e f i l m d i d n o t r e v e r t t o
i t s o r i g i n a l v a l u e , p a r t o f the adsorbed water
c o u l d n o t be d e s o r b e d at t h i s t e m p e r a t u r e and
pressure.
(v) The w a t e r p r e s s u r e was r e t u r n e d t o 11 t o r r ,
and t h e f i l m was l e f t t o e q u i l i b r a t e f o r 72 h o u r s .
(vi)
The w a t e r a d s o r p t i o n c u r v e was now meas u r e d on t h e r e a c t e d s u r f a c e . The i r r e v e r s i b l e
a d s o r p t i o n w h i c h o c c u r r e d d u r i n g measurement was
n e g l i g i b l e compared w i t h t h a t w h i c h had t a k e n
p l a c e d u r i n g t h i s 72 h o u r s o f c o n d i t i o n i n gatt h e
highest pressure.
R e v e r s i b l e a d s o r p t i o n d a t a was
recorded.
( I tisi n t e r e s t i n g t o n o t e t h a t a h i g h
p r e s s u r e o f hydrogen d e v e l o p e d in t h e vacuum
chamber a s i n d i c a t e d b y a gas w i t h a v e r y h i g h
t h e r m a l c o n d u c t i v i t y w h i c h c o u l d n o t be s o r p t i o n
pumped o r cryopumped b u t c o u l d be removed by
t i t a n i u m s u b l i m a t i o n . T h i s was p r o b a b l y formed
by r e a c t i o n o f t h e w a t e r v a p o r w i t h t h e f r e s h l y
d e p o s i t e d m e t a l on t h e w a l l s o f t h e chamber.)
(vii)
The f i l m was now exposed t o w a t e r v a p o r at
11 t o r r f o r a f u r t h e r 100 h o u r s .
The f r e q u e n c y
decreased monotonically.
(viii)
Dry a i r was i n t r o d u c e d . The c r y s t a l
showed t h a t no mass change o c c u r r e d .
The a d s o r p t i o n c u r v e was remeasured.
The e q u i l i b r i u m mass changes a r e p l o t t e d in
F i g u r e 8 on t h e b a s i s o f t h e BET model (25) . From
t h i s model we d e r i v e t h e a c t i v a t i o n e n t h a l p y f o r
w a t e r a d s o r p t i o n and t h e a r e a o f t h e f i l m .
These
6
American Chemical
Society Ubory
1155 16th st a w.
In Corrosion
Chemistry;0.
Brubaker,
G., et al.;
Washington,
C. 20036
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
CORROSION C H E M I S T R Y
248
Figure 8.
8.
PHipps
A N D RICE
Water
in
Atmospheric
Corrosion
249
p a r a m e t e r s a r e shown in T a b l e I I I as t h e r a t i o o f
t h e t r u e a r e a t o t h e a p p a r e n t a r e a . The q u a n t i t y
o f w a t e r a d s o r b e d is p l o t t e d in u n i t s o f l a y e r s o f
w a t e r on t h e b a s i s o f t h i s a n a l y s i s ( F i g u r e 9).
(ix)
F i n a l l y , s e v e r a l m e t a l s were d e p o s i t e d on
q u a r t z c r y s t a l s and t h e w a t e r a d s o r p t i o n was meas u r e d in l a b o r a t o r y a i r at c o n t r o l l e d r e l a t i v e
humidities.
These measurements were i n t e n d e d t o r e p r e s e n t
a more p r a c t i c a l degree o f c l e a n l i n e s s t h a n t h e
s u r f a c e p r e p a r e d in UHV.
Some d a t a a r e shown in
F i g u r e 10 and 11, and r e s u l t s f o r a number o f
m e t a l s a r e shown in T a b l e I I I w h i c h compares t h e s e
measurements w i t h p u b l i s h e d d a t a .
2.2
D i s c u s s i o n . The c l e a n f i l m o f m e t a l ^ r e a c t s
r a p i d l y w i t h water vapo
produce H2 and in t h
hydroxide/oxide.
(Evidence on t h e n a t u r e o f t h i s r e a c t i o n p r o d u c t is r e v i e w e d below.) The r e a c t i o n is
effectively irreversible.
The f r e q u e n c y t r a n s i e n t
o b s e r v e d when t h e m e t a l was f i r s t exposed is p r o b a b l y
due t o a t h e r m a l g r a d i e n t in t h e q u a r t z a s s o c i a t e d w i t h
t h i s r a p i d h i g h l y e x o t h e r m i c r e a c t i o n . Subsequent
w a t e r is a d s o r b e d ( r e v e r s i b l y ) on t o p o f t h i s r e a c t i o n
p r o d u c t (~ 1 monolayer at 25% RH, ~ 2 at 50% RH). T h i s
a d s o r b e d w a t e r r e a c t s w i t h t h e s u b s t r a t e m e t a l much
more s l o w l y t h a n t h e i n i t i a l r e a c t i o n . (One monolayer
r e a c t i n g in s i x t y h o u r s at 10 t o r r H2O.)
Dramatic
r e d u c t i o n s in r e a c t i o n r a t e a f t e r t h e f o r m a t i o n o f t h e
f i r s t monolayer have been r e p o r t e d from o b s e r v a t i o n s in
s o l u t i o n (2j5) . I t has been shown t h a t w a t e r is n e c e s s a r y f o r t h i s p a s s i v a t i o n o f n i c k e l (27). As t h e compound f i l m grows t h i c k e r , t h e a c t i v a t i o n energy f o r
w a t e r a d s o r p t i o n does n o t change b u t t h e a r e a i n c r e a s e s .
T h i s is c o n s i s t e n t w i t h t h e l i n e a r growth r a t e .
Rea c t i o n r a t e is l i m i t e d by t h e monolayer. T h i s is
c o n s t a n t l y , but s l o w l y , regenerated.
The q u a n t i t y o f
a d s o r b e d w a t e r i n c r e a s e s w i t h t i m e at c o n s t a n t RH
because t h e a r e a o f porous h y d r o x i d e i n c r e a s e s w i t h
time.
The newly g e n e r a t e d s u r f a c e is a r e l a t i v e l y
c l e a n ( r e p r o d u c i b l e ) a d s o r b e n t (perhaps t h i s a c c o u n t s
f o r t h e s i m i l a r a d s o r p t i o n on f i l m s exposed t o l a b o r a t o r y a i r and t h o s e p r e p a r e d in an u l t r a c l e a n e n v i r o n ment) .
We t u r n now t o d i s c u s s f i r s t t h e e v i d e n c e on t h e
nature of the adsorbing s u r f a c e , then the p r o p e r t i e s
o f t h e a d s o r b e d aqueous phase. T h i s l e a d s us t o a
model f o r a t m o s p h e r i c c o r r o s i o n .
CORROSION C H E M I S T R Y
250
Table III
I s o s t e r i c : Heat
kcals/mole
Roughness
BET
Ref.
Au
11 .84
12
11.5
2.5
6
10
1,2
3
4
Fe
11 .72
13.2
3
2
2
3
Co
11.7
2.9
Ni
11.7
Pt
10.1-12.4
11.3 13.3
6
8
Sn
Ni Fe
12.75
12.44
11.74
3.5
5.4
1
2
Fe Cr
11.73
Fe Ni Cr
13
2.2
A l 0 OH
12+0.5
F e
12.8
11.6
10
23
Ni 0
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
pmpps A N D R I C E
Water in Atmospheric
0.1
0.2
0.3
0.4
Corrosion
0.5
0.6
0.7
CORROSION C H E M I S T R Y
252
Figure 10.
prapps A N D RICE
Water in Atmospheric
Corrosion
10 r
CORROSION C H E M I S T R Y
254
2.3 The S o l i d A d s o r b e n t . I n c h a r a c t e r i z i n g t h e
l a y e r on w h i c h a d s o r p t i o n o f w a t e r t a k e s p l a c e r e v e r s i b l y , we make t h e f o l l o w i n g o b s e r v a t i o n s :
(i)
The i n i t i a l r e a c t i o n o f a c l e a n m e t a l
s u r f a c e w i t h w a t e r v a p o r is e f f e c t i v e l y i r r e v e r
s i b l e and i n v o l v e s d i s s o c i a t i o n . T h i s is s u p p o r t e d
by t h e e v o l u t i o n o f h y d r o g e n , t h e r a p i d e x o t h e r m i c
r e a c t i o n , and p u b l i s h e d d a t a f o r s e v e r a l m e t a l s
(7,8,9,10, .
(ii)
The i n i t i a l r e a c t i o n p r o d u c t p r o t e c t s t h e
m e t a l from f u r t h e r a t t a c k .
By t h e word " p r o t e c t , "
we a r e c o n t r a s t i n g t h e r a t e o f f o r m a t i o n o f t h e
i n i t i a l i r r e v e r s i b l y bound l a y e r w i t h i n ~ 10"** t o r r
seconds w i t h t h e subsequent growth o f an i r r e v e r
s i b l y bound monolayer in ~ 60 h o u r s at 11 t o r r =
2.5 10*> t o r r seconds
Auger e l e c t r o n s p e c t r o
scopy
and 1_6
be r e p r e s e n t e d a
further attack.
D i r e c t e v i d e n c e o f hydrogen b e i n g
p r e s e n t in t h e p a s s i v e f i l m formed in s o l u t i o n was
p r o v i d e d by mass s p e c t r o m e t r y (30).
Electron
d i f f r a c t i o n (2,29) has shown t h a t t h e p r o t e c t i v e
s u r f a c e l a y e r s w h i c h form on i r o n w h i c h has been
a i r o x i d i z e d o r p a s s i v a t e d in s o l u t i o n a r e s u r p r i s
i n g l y s i m i l a r . The s t r u c t u r e s a r e based on t h e
c u b i c c l o s e packed oxygen l a t t i c e o f Fe 0 OH, o r
Fe304.
The d e t a i l s o f t h e p r o c e s s e s by w h i c h p a r t i
c u l a r monolayers l i m i t t h e k i n e t i c s o f metal
o x i d a t i o n and " o x i d e " d i s s o l u t i o n have been t h e
s u b j e c t o f much i n v e s t i g a t i o n . (See r e f e r e n c e s
on p a s s i v i t y , (2_6 and 28) .) We do n o t a t t e m p t t o
review o r develop these studies here, but t o
e s t a b l i s h a c o n n e c t i o n between a t m o s p h e r i c c o r r o
s i o n and t h e r e l a t i v e l y w e l l s t u d i e d d i s c i p l i n e s
o f aqueous c o r r o s i o n and d r y o x i d a t i o n .
(iii)
The c r i t i c a l i n t e r a c t i o n w h i c h b i n d s
w a t e r when i t is o s t e n s i b l y a d s o r b e d on a
c o r r o d i n g m e t a l is t h e i n t e r a c t i o n w i t h an o x i d e
o r o x y h y d r o x i d e . The o b s e r v a t i o n o f s i m i l a r
a d s o r p t i o n c h a r a c t e r i s t i c s on many m e t a l s ( a f t e r
t h e i n i t i a l i r r e v e r s i b l e r e a c t i o n ) and on many
o x i d e s s u p p o r t s t h i s (Table I I I ) . The g e n e r a l i
z a t i o n is r e i n f o r c e d by t h e o b s e r v a t i o n t h a t i f
e x p o s u r e is p r o l o n g e d t h e a r e a o f t h e a d s o r b e n t
i n c r e a s e s , b u t t h e o t h e r c h a r a c t e r i s t i c s do n o t
change. A p p a r e n t l y , t h e g r o w i n g l a y e r is r e l a
t i v e l y p o r o u s , and t h e p r o t e c t i o n is p r o b a b l y due
to the l a y e r adjacent t o the metal. This follows
from t h e i n c r e a s i n g a r e a and t h e c o n s t a n t r a t e o f
growth.
8.
PHipps A N D RICE
Water in Atmospheric
255
Corrosion
HCl
Gases
H0
N0
373
294
263
188
Heat of evap.
kcal/mole
10.5
5.1
3.8
Pressure in a i r
atmospheres
"
io-
so
IO"
IO"
90
3.2
IO"
256
CORROSION
CHEMISTRY
8.
PHIPPS A N D RICE
Water in Atmospheric
Corrosion
257
CORROSION C H E M I S T R Y
258
We now r e t u r n t o d i s c u s s t h e o b s e r v a t i o n s o f atm o s p h e r i c c o r r o s i o n d e s c r i b e d in t h e f i r s t p a r t o f t h i s
paper in terms o f t h e models o f t h e a d s o r b e d phase dev e l o p e d in t h e p r e v i o u s s e c t i o n s .
3.0
The
R o l e o f Water in A t m o s p h e r i c
Corrosion
3.1
General Observations.
We saw t h a t w a t e r v a p o r
p l a y s a dominant r o l e in many a t m o s p h e r i c c o r r o s i o n
p r o c e s s e s ( F i g u r e s 1,3,5), and t h a t t h e r e is an i m p l i c i t
o b s e r v a t i o n t h a t t h e t e m p e r a t u r e dependence o f c o r r o s i o n
may be a p p r o x i m a t e l y r e p r e s e n t e d by t h e v a p o r p r e s s u r e
o f w a t e r ( i . e . , RH is t h e r a t i o n a l v a r i a b l e ) .
These
o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e a d s o r p t i o n model
w i t h h i g h r a t e s o f g a l v a n i c c o r r o s i o n o c c u r i n g in t h i c k
l a y e r s o f w a t e r and o n l y s l o w e r o x i d a t i o n o c c u r i n g in
t h e regime o f RH at w h i c
The s i m i l a r dependenc
s e v e r a l r e a c t a n t s is a l s o c o n s i s t e n t w i t h a d s o r p t i o n on
s i m i l a r o x y h y d r o x i d e l a y e r s even though t h e a r e a t e n d s
t o i n c r e a s e . The s i m u l t a n e o u s p r e s e n c e o f s e v e r a l o x i d a t i o n p r o d u c t s in p o r o u s l a y e r s ( F i g u r e s 5 and 6) is
c o n s i s t e n t w i t h t h e inhomogeneous g a l v a n i c r e a c t i o n s .
The p r o c e s s is d i f f e r e n t from t h e g r a d a t i o n o f s t o i c h i ometry n e c e s s a r i l y p r e s e n t in a d h e r e n t p r o t e c t i v e l a y e r s .
D e l i q u e s c e n c e o f t h e p r o d u c t s is e x p e c t e d t o a f f e c t c o r r o s i o n r a t e s and t h e i r dependence on RH c o n s i d e r a b l y
(4_,5^, 1_0/iLi) # t h e w a t e r t h i c k n e s s b e i n g g r e a t l y i n c r e a s e d .
3.2
I n t e r m e d i a t e RH.
A t i n t e r m e d i a t e RH, ~ 40%,
w e i g h t changes a r e m i n o r but r o u g h e n i n g is seen ( F i g u r e
3) and s e n s i t i v e s u r f a c e c h e m i s t r y shows t h a t t h i n l a y e r s
o f c o r r o s i o n p r o d u c t s can be d e t e c t e d ( F i g u r e 4 and 5 ) .
This
r o u g h e n i n g may be a s s o c i a t e d w i t h t h e f a c t t h a t a d s o r p t i o n shows c l u s t e r i n g even on homogeneous s u r f a c e s , and
p r a c t i c a l s u r f a c e s a r e c e r t a i n l y inhomogeneous w i t h
r e s p e c t t o a d s o r p t i o n . The p r e f e r r e d s i t e s f o r a t t a c k
may be r e l a t e d t o t h e s p e c i f i c a d s o r p t i o n s i t e s des c r i b e d f o r SO2 (45,46j and H S (7); t h e y may be s i t e s
where t h e w a t e r a b s o r p t i o n is f a v o r e d ; o r t h e y may be
s i t e s at w h i c h t h e o x i d e is weak. Once c o r r o s i o n is
i n i t i a t e d , d e l i q u e s c e n t c o r r o s i o n p r o d u c t s may a g g r a v a t e
l o c a l a t t a c k (see b e l o w ) .
2
3.3
Low RH.
A t t h o s e s i t e s , and f o r t h o s e c o n d i t i o n s , in w h i c h t h e a v e r a g e t h i c k n e s s is in t h e o r d e r
o f one m o l e c u l e 3 t h e d i e l e c t r i c c o n s t a n t is low and
h y d r a t i o n o f i o n s H *, OH", O2, M ,
SO2-, e t c . , is
4
n+
8.
PHipps A N D R I C E
Water in Atmospheric
Corrosion
259
260
CORROSION C H E M I S T R Y
L i t e r a t u r e Cited
1.
8.
PHIPPSANDR I C E
Water in Atmospheric
Corrosion
261
9
Corrosion Inhibition and Inhibitors
RUDOLF H. HAUSLER
Gordon Lab, Inc., 925 Patton Rd., P.O. Box 605, Great Bend, KS 67580
An Educational Lecture or Paper on Corrosion I n h i b i t i o n could e a s i l y develop into an ambitious undertaking if it were intende
ture concerned wit
i t o r s and the multitude of mechanisms proposed as explanations of t h e i r a c t i o n .
The past approaches aimed
at understanding Corrosion I n h i b i t i o n have been many,
ranging from phenomenological screening of chemical
compounds in a given environment to detailed e l e c t r o chemical adsorption studies.
Let me state that the
ultimate purpose of i n h i b i t o r studies ought to be the
development of p r e d i c t i v e criteria for i n h i b i t o r e f f e c tiveness rather than the mere explanation of e x p e r i mental Results or speculation about possible mechanisms.
While such p r e d i c t i v e criteria have been developed in a few cases it appears that most mechanistic
studies have merely contributed to speculative guide
l i n e s h e l p f u l in the unraveling process of i n h i b i t o r
a c t i o n , but have been far from successful in providing
a complete understanding of p r a c t i c a l i n h i b i t i o n phenomena l e t alone p r e d i c t i v e criteria for more e f f e c t i v e
molecules.
The reason for t h i s state of a f f a i r s may be seen
in past emphasis on surface phenomenological studies
which attempted to model the metal surface as an array
of surface atoms with some valences saturated by subsurface metal atoms and other valences saturated by
ions or molecules making up the environment.
This
model led to the d e s c r i p t i o n of the interface in terms
of the Helmholz and Guy-Chapman double layer t h e o r i e s ,
and i n h i b i t o r s were v i s u a l i z e d as i n t e r f e r i n g with the
double layer structure through adsorption on the surface atoms of the metal, thereby a l t e r i n g the e l e c t r o chemical reaction rates which are governed by the energetics of the double l a y e r . While t h i s model has been
0-8412-0471-3/79/47-089-262$13.85/0
1979 American Chemical Society
9.
HAUSLER
Corrosion
Inhibition
263
Corrosion
Mechanism
C o r r o s i o n i n h i b i t i o n is g e n e r a l l y d e s c r i b e d as the
i n t e r f e r e n c e o f a substance f o r e i g n t o the c o r r o s i v e
medium w i t h the c o r r o s i o n r e a c t i o n o r r e a c t i o n s , and i t
is v i s u a l i z e d t h a t such i n t e r f e r e n c e t a k e s p l a c e
t h r o u g h the a d s o r p t i o n o f the i n h i b i t o r on the m e t a l
surface.
While t h i s concept has been s u c c e s s f u l in
many i d e a l i z e d s i t u a t i o n s , such as the c o r r o s i o n o f
i r o n in h y d r o c h l o r i c a c i d CD i t s a p p l i c a t i o n has been
f a r t o o g e n e r a l and too p r o l i f i c in o r d e r t o advance
the u n d e r s t a n d i n g o f c o r r o s i o n i n h i b i t i o n in any s i g n ificant
way.
I t may t h e r e f o r e be h e l p f u l t o d i s c u s s b r i e f l y a
k i n e t i c model o f the c o r r o s i o n p r o c e s s i t s e l f and t h e n
d e r i v e from i t t h e v a r i o u s ways in which i n h i b i t i o n can
CORROSION C H E M I S T R Y
264
take place.
T h i s appears a l l the more n e c e s s a r y as c o r r o s i o n a f t e r more t h a n 5 0 y e a r s o f i n t e n s i v e r e s e a r c h
is s t i l l c o n s i d e r e d in many t e x t b o o k s t o be a p u r e l y
chemical process, while only s p e c i a l l i m i t i n g cases
such as g a l v a n i c c o r r o s i o n and perhaps p i t t i n g are r e c o g n i z e d as e l e c t r o c h e m i c a l in n a t u r e .
Let us emphasize from the b e g i n n i n g t h a t a l l c o r r o s i o n (the o x i d a t i v e c o n v e r s i o n o f a m e t a l t o i t s meta l i o n s ) is e l e c t r o c h e m i c a l in n a t u r e .
This implies
the e x i s t e n c e o f s i m u l t a n e o u s a n o d i c and c a t h o d i c c u r r e n t s o f e q u a l magnitude a c r o s s the i n t e r f a c e o f the
metal.
I t is by no means n e c e s s a r y
( a l t h o u g h somet i m e s u s e f u l ) , t o p o s t u l a t e permanent l o c a l i z e d anodes
and cathodes as a m i c r o s c o p i c concept w i t h f i x e d space
c o o r d i n a t e s in o r d e r t o d e v e l o p a k i n e t i c model a p p l i c a b l e t o the r a t e s o f the c o r r o s i o n p r o c e s s e s .
The c o r rosion reactions, tha
m e t a l and t h e c a t h o d i
may indeed t a k e p l a c e w i t h s t a t i s t i c a l d i s t r i b u t i o n in
time and space on the s u r f a c e o f the m e t a l .
The p r o o f
o f t h i s t h e s i s was g i v e n by Wagner and Traud (JL) in
1938 by means o f t h e d i s c u s s i o n o f the l i m i t i n g case
o f z i n c amalgam d i s s o l u t i o n . L i q u i d z i n c amalgam must
be c o n s i d e r e d a c o m p l e t e l y homogeneous phase where i t
is i m p o s s i b l e t o d e f i n e , in a r i g o r o u s thermodynamic
sense, l o c a l g a l v a n i c elements on the s u r f a c e .
The
d i s s o l u t i o n o f z i n c amalgam in d i l u t e h y d r o c h l o r i c a c i d
c o u l d t h e r e f o r e proceed by a c h e m i c a l mechanism as
shown in e q u a t i o n s 1 and 2:
Zn + 2H+
{Zn H21
**{zn H }
2 +
(1)
Zn
+ 2
+ H
(2)
9.
HAUSLER
Corrosion
Inhibition
265
Zn
* Zn
+
2H *H
+ 2
+ 2e
- 2e
(3)
(4)
f h i s v e r i f i c a t i o n o f the e l e c t r o c h e m i c a l n a t u r e o f the
c o r r o s i o n p r o c e s s a l s o l e a d s t o the r e a l i z a t i o n o f f o u r
d i s t i n c t and s e p a r a t e b a s i c p r o c e s s e s i n v o l v e d in
c o r r o s i o n , which are l i n k e d t o g e t h e r m e r e l y by the
n e c e s s i t y o f p r e s e r v i n g e l e c t r o n e u t r a l i t y in the o v e r a l l reaction.
These are
a) the a n o d i c or o x i d a t i v e p r o c e s s ;
b) the c a t h o d i c o r r e d u c t i v e p r o c e s s ;
c) the e l e c t r o n i c charge t r a n s f e r p r o c e s s in b o t h
d i r e c t i o n s a c r o s s the i n t e r f a c e ;
d) the i o n i c charge t r a n s f e r p r o c e s s which is r e q u i r e d t o m a i n t a i n e l e c t r o n e u t r a l i t y on the
e l e c t r o l y t e s i d e due t o the d i s a p p e a r a n c e o f
i o n i c charges in the c a t h o d i c p r o c e s s and the
f o r m a t i o n o f i o n i c charges in the a n o d i c p r o cess .
One o f t h e s e f o u r r e a c t i o n s is u s u a l l y the s l o w e s t , and
r a t e d e t e r m i n i n g f o r the o v e r a l l p r o c e s s .
Corrosion
i n h i b i t i o n t a k e s advantage o f t h i s c o m p l e x i t y o f r e a c t i o n s by a t t e m p t i n g t o i n t e r f e r e w i t h any o f them
i n d i v i d u a l l y or j o i n t l y .
Thus a c o r r o s i o n i n h i b i t o r
may f u r t h e r slow the r a t e o f the s l o w e s t r e a c t i o n or
may b r i n g about a r a t e l i m i t a t i o n o f one or the o t h e r
o f the r e m a i n i n g t h r e e p r o c e s s e s .
While i t is g e n e r a l l y known t h a t c o r r o s i o n i n h i b i t o r s may a f f e c t the an-
CORROSION C H E M I S T R Y
266
o d i c o r the c a t h o d i c r e a c t i o n , i t is l e s s w e l l known
t h a t i n h i b i t i v e means may a f f e c t e i t h e r the e l e c t r o l y t i c c o n d u c t i o n or even the e l e c t r o n i c c o n d u c t i o n p r o cess .
Systematic C l a s s i f i c a t i o n
Of C o r r o s i o n
Inhibitors
9.
HAUSLER
Corrosion
267
Inhibition
Table I
(A)
Causes o f I n t e r f a c e - I n h i b i t i o n and
I n h i b i t ion
Electrolyte-Layer-
1.
Interface-Inhibition.
1.1 caused by the
s q u e e z i n g out
effect.
1.2 caused by a d s o r p t i o n .
1.3 caused by e l e c t r o s o r p t i o n .
1.4 caused by coverage w i t h a p o l y m o l e c u l a r o r
polymerous l a y e r .
2.
Electrolyte-Layer-Inhibition.
2.1 m e c h a n i c a l E. caused by
2.11 c o l l o i d s o r s u s p e n s i o n s
2.12 v i s c o u
2.13 pores in
ers .
2.2 c h e m i c a l E. caused by s u b s t a n c e s r e a c t i n g w i t h
components o f homogeneous p a r t i a l r e a c t i o n s o f
the e l e c t r o d e r e a c t i o n .
2.3 e l e c t r o c h e m i c a l E. caused by changes o f t h e
p o t e n t i a l in the d i f f u s e double l a y e r dependent
on a coverage o f the i n t e r f a c e w i t h i o n s
M o s t l y however, t h e s e and o t h e r m o l e c u l e s as w e l l
as i o n s or i o n p a i r s coming from the e l e c t r o l y t e may
be adsorbed on the m e t a l l i c ( o r s e m i c o n d u c t i v e ) p a r t
o f the i n t e r f a c e .
D i s r e g a r d i n g the s p e c i a l case o f the
p o t e n t i a l o f z e r o charge at the m e t a l l i c s u r f a c e , the
conductor w i l l u s u a l l y c a r r y a p o s i t i v e or negative
charge which may f a v o r i t s coverage w i t h charged o r
p o l a r s u b s t a n c e s o c c u r i n g near t h e i n t e r f a c e .
I t is
v e r y d i f f i c u l t , however, t o d i s t i n g u i s h between pure
a d s o r p t i o n o f n e u t r a l m o l e c u l e s on the s u r f a c e o f t h e
c o r r o d i n g m e t a l by means o f Van d e r Waals or a chemi c a l f o r c e s , and e l e c t r o s o r p t i o n which t a k e s p l a c e by
p o t e n t i a l dependent e l e c t r o s t a t i c f o r c e s .
O f t e n the
two mechanims o p e r a t e in c o n j u n c t i o n . I t has been
shown f o r i n s t a n c e , t h a t o r g a n i c amines become much
more e f f e c t i v e c o r r o s i o n i n h i b i t o r s in a c i d medium i f
a h a l i d e is p r e s e n t . H a l i d e i o n s , p a r t i c u l a r l y i o d i d e ,
a r e s t r o n g l y adsorbed on m e t a l s u r f a c e s thus f o r m i n g a
n e g a t i v e l y charged l a y e r on the m e t a l s u r f a c e onto
which p r o t o n a t e d o r g a n i c amines adsorb s u b s e q u e n t l y ,
with a r e s u l t i n g i n h i b i t i o n of corrosion reactions.
F i n a l l y , one can t a l k about an i n t e r f a c e e f f e c t
which is caused by coverage o f the m e t a l s u r f a c e w i t h
a polymerous l a y e r .
Some a u t h o r s t h i n k t h a t c e r t a i n
268
CORROSION C H E M I S T R Y
9.
Corrosion
HAUSLER
269
Inhibition
^Me
n +
+ ne
(5)
Ox + n e ^ R e d -
(6)
These p a r t i a l r e a c t i o n s proceed w i t h e q u a l r e a c
t i o n r a t e s when t h e m e t a l is f r e e l y c o r r o d i n g . I n o r
der t o e x p r e s s t h e r e a c t i o n r a t e s in terms o f a c u r
r e n t , t h e c o n v e r s i o n p e r time is m u l t i p l i e d by t h e
Faraday c o n s t a n t a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n :
J
(7)
= Q' F
Where J e q u a l s c u r r e n t , number o f e l e c t r o n s t r a n s
f e r r e d p e r molecule, F e q u a l s Faraday c o n s t a n t and Q" is
the r a t e o f t h e r e a c t i o n e x p r e s s e d in terms o f moles
per u n i t t i m e .
I f t h e above e q u a t i o n is d i v i d e d by t h e
s u r f a c e a r e a t h e c u r r e n t d e n s i t y can be e x p r e s s e d as
follows :
J
- _ .
F.n
(8)
The p a r t i a l a n o d i c ( i ) and c a t h o d i c ( i ) c u r r e n t
d e n s i t i e s a r e then e x p r e s s e d as e x p o n e n t i a l f u n c t i o n s
of the o v e r - p o t e n t i a l ( n
n ) which is t h e d i f f e r e n c e
between t h e o p e r a t i n g p o t e n t i a l () and t h e e q u i l i b r i u m
p o t e n t i a l ( E E ) of the p a r t i c u l a r p a r t i a l r e a c t i o n .
These r e l a t i o n s h i p s a r e shown g r a p h i c a l l y and e x p l i c i t
l y in F i g . 1 . Thus by d e f i n i n g t h e a n o d i c and c a t h o d i c
c u r r e n t p o t e n t i a l r e l a t i o n s h i p s f o r a c o r r o d i n g system
a
270
CORROSION C H E M I S T R Y
9.
HAUSLER
Corrosion
271
Inhibition
constant.
T h i s is o f t e n not the case s i n c e the consumption o f an o x i d a n t (oxygen o r p r o t o n s ) d u r i n g c a t h odic p o l a r i z a t i o n r a p i d l y leads to d i f f u s i o n l i m i t a t i o n
( d i f f u s i o n o v e r - p o t e n t i a l ) w h i l e secondary r e a c t i o n s
d u r i n g p o l a r i z a t i o n in the a n o d i c d i r e c t i o n o f t e n l e a d
t o p r e c i p i t a t i o n o f m e t a l h y d r o x i d e s and c o n s e q u e n t l y
p a s s i v a t i o n phenomena.
In s p i t e o f such e x p e r i m e n t a l d i f f i c u l t i e s the
Evans diagram has been e x t r e m e l y u s e f u l in d e t e r m i n ing c e r t a i n c h a r a c t e r i s t i c s of i n h i b i t o r s .
In a c i d sol u t i o n s f o r example, the e l e c t r o d e r e a c t i o n s f o l l o w a
b e h a v i o r which is q u i t e p r e d i c t a b l e on the b a s i s o f the
e l e c t r o n t r a n s f e r b e i n g the r a t e d e t e r m i n i n g s t e p .
Thus Hackerman (JJ and Nobe (JL) have s t u d i e d such s y s tems e x t e n s i v e l y and found t h a t the e f f e c t i v e n e s s o f
c e r t a i n amine i n h i b i t o r s can be e x p l a i n e d by t h e i r adsorption behavior.
Th
proved t o be p r e d i c t a b l
s t r u c t u r e and c o n f i g u r a t i o n .
In n e u t r a l or a l k a l i n e s o l u t i o n , or s o l u t i o n s o f
low c o n d u c t i v i t y , however, one f i n d s v e r y s m a l l T a f a e l
r e g i o n s , or the T a f a e l r e g i o n s are e s s e n t i a l l y n o n e x i s tent.
The r e a s o n s f o r such b e h a v i o r are many:
a) S t e r n (JL) f o r example has shown t h a t the c a t h o d i c p o l a r i z a t i o n c u r v e s f o r i r o n c o r r o d i n g in oxygen
f r e e sodium c h l o r i d e s o l u t i o n show hydrogen i o n d i f f u s i o n l i m i t a t i o n at a c u r r e n t o f 1 0 "
a/cm at a pH o f
2.
Thus in such systems above a pH 1.5 e s s e n t i a l l y no
T a f e l r e g i o n is o b s e r v e d due t o a d i f f u s i o n o v e r - p o tential.
b) R e s i s t a n c e s or o v e r - p o t e n t i a l s o t h e r than t h o s e
d e t e r m i n i n g the r a t e o f the e l e c t r o n t r a n s f e r r e a c t i o n
are o f t e n i n c l u d e d in the c u r r e n t p o t e n t i a l measurement. The most f r e q u e n t s i t u a t i o n is the i n c l u s i o n
o f an ohmic r e s i s t a n c e which o c c u r s between the worki n g e l e c t r o d e and the r e f e r e n c e e l e c t r o d e .
Such ohmic
e l e c t r o l y t e r e s i s t a n c e s can e a s i l y be determined by
p o s i t i o n i n g the r e f e r e n c e e l e c t r o d e at v a r y i n g d i s t a n c e s
from
the
working e l e c t r o d e , or by v a r i o u s mathematic a l p r o c e d u r e s i n c l u d i n g a u t o m a t i c i R - d r o p compensating
electronic devices.
M a n s f e l d has r e c e n t l y d i s c u s s e d
the e f f e c t o f such r e s i s t a n c e on c u r r e n t p o t e n t i a l
c u r v e s (10).
A more s e r i o u s r e s i s t a n c e o f t e n i n f l u e n c i n g the p o t e n t i a l d e t e r m i n a t i o n in p o l a r i z a t i o n s t u d i e s
is caused by the f o r m a t i o n o f . a c o r r o s i o n p r o d u c t l a y e r
on the s u r f a c e o f the c o r r o d i n g t e s t e l e c t r o d e .
This
s i t u a t i o n is c o n s i d e r a b l y more complex because, f i r s t ,
t h i s a d d i t i o n a l r e s i s t a n c e is most l i k e l y not ohmic in
n a t u r e due t o the s e m i c o n d u c t i n g p r o p e r t i e s o f the c o r r o s i o n p r o d u c t l a y e r and second, a n o d i c and c a t h o d i c
4
272
CORROSION
CHEMISTRY
Of
I n h i b i t o r s On
P o l a r i z a t i o n Behavior
9. HAUSLER
Corrosion
273
Inhibition
't
an F
7Q
= l
a
V
e x
'
^RT
(l-a)n F
\_i =-i -exp(-
V '- ' c
e E
Wt
'ext
an F
'corr L Pl" RT
-(l-)n F
c
/-T^a
RT ^corr'J
ex
rehtionships in a corroding
+ blog i + i R
Q
//
//
//
./
//
/
^ ^ 7
\^77
,0
ext =
!
= a + blog i
= a'+ b'log i
' + b'log i + i R
c
Wr
W ^ P ^ ^ o f r ^ a ^ ' ^ p f ^ ^ H o r r " ^
Figure 2.
CORROSION
CHEMISTRY
HAUSLER
Corrosion
Inhibition
CORROSION
276
CHEMISTRY
Table I I
WEIGHT LOSS MEASUREMENTS OF CARBON STEEL IN 10% SULFUR
IC ACID AS A FUNCTION OF TYPE AND CONCENTRATION OF IN
HIBITOR AT 2 5 C.
Inhibitor
C o n c e n t r a t i o n Weight l o s s C o r r o s i o n
of I n h i b i t o r
mg/2 5cm .
Potential
(mol/1)
day
mV v s . H
2
Blank
0
Aniline
0.0063
Diethylaniline
^
0.0063
p-Phenylene diamine
0.0 06 3
3-Naphthyl-amine
sat.sol
Phenyl--naphthyl amin
Pyridine
0.0063
Quinoline
0. 063
Quinoline
0.0063
^a-Naphthoquinoline
0.063
a-Naphthoquinoline
0063
-Naphthoquinoline
063
-Naphthoquinoline
0063
2,4,-Dimethylquinoline
063
2,4,-Dimethylquinoline
, 0063
2,6-Dimethylquinoline
, 063
2,6-Dimethylquinoline
E t h y l q u i n o l i n i u m bromide0.0063
Acridine
sat.sol.
A c r i d i n e orange
sat.sol.
Acridine red
sat.sol.
Acriflavine
sat.sol.
Water s o l u b l e P e t r o l e u m
sat.sol.
sulfonate
sat.sol
Sulfonated o i l
sat.sol.
Commercial I n h i b i t o r A
sat.sol.
C sat.sol.
D sat.sol.
956
943
513
886
374
235
233
209
225
210
370
122
78
59
73
58
321
178
175
64
63
404
24
61
112
201
181
177
172
178
172
192
176
192
170
184
227
201
184
196
170
23
23
44
85
51
212
227
227
216
218
223
9.
HAUSLER
Corrosion
Inhibition
277
lustrtes t h e s e p o i n t s in a s u r v e y o f p o t e n t i a l s h i f t s
and c o r r o s i o n r a t e s f o r v a r i o u s i n h i b i t o r s on carbon
s t e e l in s u l f u r i c a c i d .
Such r e l a t i v e l y simple e l e c t r o c h e m i c a l t e c h n i q u e s t h e r e f o r e can be c o n s i d e r e d u s e f u l t o o l s f o r the r a p i d assessment o f i n h i b i t o r a c t i v i t y and f o r the purpose o f o b t a i n i n g p r e l i m i n a r y inf o r m a t i o n on a p o s s i b l e i n h i b i t i o n mechanism. Howe v e r , p o l a r i z a t i o n measurements, e i t h e r in the form o f
" L i n e a r p o l a r i z a t i o n measurements" o r T a f e l - s l o p e det e r m i n a t i o n s are f r a u g h t w i t h p e r i l ( c . f . H a u s l e r 11).
Even i f a system is r e l a t i v l y w e l l behaved, the i n f o r m a t i o n a l v a l u e o b t a i n e d from p o l a r i z a t i o n curves is
r e l a t i v e l y s m a l l and s h o u l d be combined w i t h more ext e n s i v e d e t e r m i n a t i o n o f the e l e c t r o d e k i n e t i c parameters,
a d s o r p t i o n s t u d i e s and mass t r a n s f e r s t u d i e s
in o r d e r t o e l u c i d a t e the mechanism o f a p a r t i c u l a r
corrosion inhibitor
inhibitive effects
t i o n o f the i n t e r p h a s e c h e m i s t r y w i l l e v e n t u a l l y l e a d
t o the p r e d i c t i v e c r i t e r i a f o r i n h i b i t o r b e h a v i o r .
Very few such i n v e s t i g a t i o n s have been c a r r i e d out
in the p a s t .
I t appears t h a t i n v e s t i g a t o r s were
m o s t l y s a t i s f i e d w i t h d e m o n s t r a t i n g the i n h i b i t o r y
e f f e c t o f c h e m i c a l s u b s t a n c e s and s u b s e q u e n t l y f o r c i n g
t h o s e s u b s t a n c e s i n t o one o r the o t h e r s i m p l e mechani s t i c concepts.
The few i n v e s t i g a t i o n s which have
p i n p o i n t e d v a s t l y more complex b e h a v i o r , have m o s t l y
been o v e r l o o k e d .
In the i n t e r e s t o f s t i m u l a t i n g more
r e s e a r c h in the f i e l d o f i n h i b i t o r chemistry, t h i s r e view s h a l l s t r e s s the more e x o t i c i n v e s t i g a t i o n s .
In
p a r t i c u l a r i t w i l l be s u g g e s t e d t h a t c h e m i c a l r e a c t i o n s
o f i n h i b i t o r s o c c u r i n g in the i n t e r p h a s e between c o r r o s i o n p r o d u c t s and the i n h i b i t o r are a more g e n e r a l
and w i d e s p r e a d phenomenon t h a n has g e n e r a l l y been bel i e v e d in the p a s t .
I n h i b i t i o n By T h i o u r e a And
Quinoline
Derivatives
A l a r g e number o f i n v e s t i g a t i o n s in a c i d media
have l e d t o the c o n c l u s i o n t h a t the i n h i b i t i o n e f f e c t
caused by r e l a t i v e l y s m a l l and s i m p l e m o l e c u l e s is due
t o t h e i r a d s o r p t i o n on the m e t a l s u r f a c e .
Compounds o f
t h i s n a t u r e u s u a l l y c o n t a i n s u l f u r and n i t r o g e n , o r are
o f the groups o f h i g h e r a l k y l - a l c o h o l s and f a t t y a c i d s .
T y p i c a l compounds t o be d i s c u s s e d h e r e in more d e t a i l
are q u i n o l i n e and t h i o u r e a d e r i v a t i v e s . F i g . 5 shows
a comparison of the e f f e c t i v e n e s s o f s e v e r a l such
compounds determined by means o f weight l o s s measurements on carbon s t e e l in 5% s u l f u r i c a c i d at 40 C.
as a f u n c t i o n o f the i n h i b i t o r c o n c e n t r a t i o n .
A cur-
10
10
10
-4
10
10
Springer-Verlag
0.25
0.50
0.75
1.00
Jf
10
Ethyl-thiourea
m-tolyl- thiourea
o- S p-tolylthiourea ~"
Methyl-thioure
Thiourea
Quinoline
2,6-Dimethyl-quinoline
N-ethyl-quinoline
- & -naphthoquinoline
9.
HAUSLER
Corrosion
Inhibition
279
s o r y e x a m i n a t i o n o f F i g . 5 might l e a d t o t h e c o n c l u s i o n
t h a t the r e l a t i o n s h i p s o f c o r r o s i o n r a t e v s . i n h i b i t o r
c o n c e n t r a t i o n are b a s i c a l l y q u i t e s i m i l a r f o r t h e two
c l a s s e s o f compounds and d i f f e r o n l y q u a n t i t a t i v e l y
w i t h r e s p e c t t o the e f f i c i e n c y .
I t w i l l be shown how
e v e r , t h a t t h e r e are indeed p r o f o u n d d i f f e r e n c e s be
tween the two c l a s s e s o f i n h i b i t o r s .
Hoar and H o l i d a y
(12) found r e l a t i v e l y simple c o n d i t i o n s as a r e s u l t o f
t h e i r i n v e s t i g a t i o n o f the c o r r o s i o n i n h i b i t i o n o f i r o n
in 5% s u l f u r i c a c i d by 2 , 6 - d i m e t h y l q u i n o l i n e .
These
a u t h o r s determined f i r s t the e x t e r n a l c u r r e n t - p o t e n t i a l
c u r v e s in the u n i h i b i t e d s o l u t i o n and t h e n the a n o d i c
p a r t i a l c u r r e n t - p o t e n t i a l c u r v e s at d i f f e r e n t i n h i b i
t o r c o n c e n t r a t i o n s and the q u a n t i t y o f d i s s o l v e d i r o n
in t h e e l e c t r o l y t e at c e r t a i n e l e c t r o d e p o t e n t i a l s .
The d i f f e r e n c e between t h e a n o d i c e x t e r n a l c u r r e n t and
the i n d e p e n d e n t l y determine
( d i s s o l v e d Fe) is th
The r e s u l t s as o b t a i n e d by Hoar and H o l i d a y are shown
in F i g . 6 .
The dashed curve r e p r e s e n t s the e x t e r n a l
p o l a r i z a t i o n b e h a v i o r in the absence o f i n h i b i t o r and
the b l a c k l i n e s are the T a f e l s l o p e s f o r the a n o d i c
p a r t i a l c u r r e n t d e n s i t y (the m e t a l d i s s o l u t i o n ) f o r
different i n h i b i t o r concentrations.
The c a t h o d i c p a r
t i a l c u r r e n t d e n s i t y (hydrogen e v e o l u t i o n ) is found f o r
a l l v a l u e s o f the i n h i b i t o r c o n c e n t r a t i o n s in the shad
ed a r e a .
T h e r e f o r e , i t is o b v i o u s t h a t the i n h i b i t o r
in t h i s case a c t s e x c l u s i v e l y by r e d u c i n g the a n o d i c
r e a c t i o n r a t e but not the c a t h o d i c one.
Kaesche and Hackerman (13) have i n v e s t i g a t e d the
i n h i b i t i o n o f s e v e r a l a l i p h a t i c and a r o m a t i c amines on
pure i r o n c o r r o d i n g in IN
hydrochloric acid.
These
a u t h o r s o b s e r v e d in t h i r t e e n out o f f o u r t e e n cases t h a t
the i n h i b i t i o n was b o t h a n o d i c and c a t h o d i c , a l b e i t
predominantly anodic.
The e x c e p t i o n was methylamine
which a c t e d o n l y c a t h o d i c a l l y . In the case o f the c o r
r o s i o n i n h i b i t i o n on pure i r o n by -naphthoquinoline in
sodium s u l f a t e / s u l f u r i c a c i d s o l u t i o n
one o b s e r v e s
a simple p a r a l l e l s h i f t o f the a n o d i c and c a t h o d i c
T a f e l l i n e s towards s m a l l e r v a l u e s o f c u r r e n t d e n s i t y .
Here the e f f e c t is almost s y m e t r i c a l , i n d i c a t i n g t h a t
t h i s i n h i b i t o r a c t s t o t h e same e x t e n t upon a n o d i c and
c a t h o d i c r e a c t i o n r a t e s . T h e r e f o r e , the e f f e c t o f
3 - n a p h t h o q u i n o l i n e can be e x p l a i n e d on t h e b a s i s t h a t
i t s a d s o r p t i o n b l o c k s a f r a c t i o n o f the m e t a l s u r f a c e
f o r a l l electrode r e a c t i o n s . I f equation 9 describes
the e x t e r n a l p o l a r i z a t i o n b e h a v i o r in terms o f a f u n c
t i o n o f the p a r t i a l c u r r e n t p o t e n t i a l r e l a t i o n s h i p f o r
the a n o d i c and c a t h o d i c r e a c t i o n s in the u s u a l terms:
280
CORROSION
-024
CHEMISTRY
Springer-Verlag
Figure 6.
9.
HAUSLER
( i
Corrosion
281
Inhibition
( i
<>
where: i = exchange c u r r e n t d e n s i t y
= electrode potential
B
= anodic T a f e l slope
cath
cathodic T a f e l slope
t h e n e q u a t i o n 10 d e s c r i b e s the p o l a r i z a t i o n
i n h i b i t e d e l e c t r o d e s in the same terms:
a n
( i e x t >I=(i >I
n
e x
behavior
^-{^^cath^f
I f the i n h i b i t o r doe
-naphthoquinoline)
(B
of
an>o = < B
a n
affect
(B
i ;
th
Tafel
= (B
c a
slope
th)l
(se
<H>
the i n h i b i t e d a n o d i c c u r r e n t d e n s i t y is a simple f r a c
t i o n o f the u n i n h i b i t e d a n o d i c c u r r e n t d e n s i t y as shown
in e q u a t i o n 12.
( i an ) ! = ( ian ) (1-)
a n
and
f o r the
(12)
cathodic
partial
current
<icath>I = <icath>
( 1
"
( 1 3 )
I t f o l l o w s t h a t the i n h i b i t e d e x t e r n a l c u r r e n t is a
s i m i l a r f r a c t i o n o f the u n i n h i b i t e d e x t e r n a l c u r r e n t
as i n d i c a t e d in e q u a t i o n 14
( i
e x t > I = <iext>o
<1">
I t a l s o f o l l o w s t h a t the degree of i n h i b i t i o n in
case is d i r e c t l y p r o p o r t i o n a l t o the f r a c t i o n o f
s u r f a c e c o v e r e d w i t h adsorbed i n h i b i t o r :
^orr
P=
ligh
this
the
=9
(15)
T O
1
corr
In g e n e r a l , however, t h e s e e q u a t i o n s are not a p p l i c a b l e
s i n c e s y m m e t r i c a l a n o d i c and c a t h o d i c i n h i b i t i o n is a
r a r e case.
In the case o f d i m e t h y l q u i n o l i n e w h i c h , as
shown in F i g . 6 , does not a f f e c t the c a t h o d i c hydrogen
e v o l u t i o n , Hoare and H o l i d a y (1?)
have proposed a
d i f f e r e n t approach.
One can assume t h a t two k i n d s o f
CORROSION
282
CHEMISTRY
(iextl = <*2>
( 1
" 1
e x
pf
/ ( B
(l-9 ) exp|-e/(B
2
c a t h
) |
0
(16)
Since dimethylquinolin
i n h i b i t o r i t is p r o b a b l
sorbed.
One can f u r t h e r s p e c u l a t e t h a t the a d s o r p t i o n
o c c u r s o n l y on the s i t e s S-, , t h a t is the l a t t i c e d i s
s o l u t i o n s i t e s which w i l l f a v o r the a d s o r p t i o n o f
f o r e i g n m o l e c u l e s from an e n e r g e t i c p o i n t o f view.
T h e r e f o r e , 9 w i l l be a p p r o x i m a t l y z e r o .
E q u a t i o n 16
then reduces t o e q u a t i o n 17:
2
(i
ext>I = (iaVo
( 1
- 1
e x
PI
e 1 (B
an>ci "
'igathV
expj-e/(B
c a t h
) j
0
(17)
9.
HAUSLER
Corrosion
Inhibition
283
p h e n y l t h i o u r e a . S p e c i f i c a l l y , i t appears the e l e c t r o
c h e m i c a l d e s o r p t i o n o f hydrogen is g r e a t l y h i n d e r e d .
T h i s mechanism may p o s s i b l y f i n d c o n f i r m a t i o n in the
i n d e p e n d e n t l y o b s e r v e d f a c t t h a t in the p r e s e n c e of
p h e n y l t h i o u r e a c o n s i d e r a b l y more hydrogen d i f f u s e s i n t o
the m e t a l . However, the i n h i b i t i o n mechanism o f p h e n y l t h i o u r e a is p r o b a b l y c o n s i d e r a b l y more c o m p l i c a t e d t h a n
t h a t , as can be seen from the v e r y complex r e l a t i o n
s h i p o f the c o r r o s i o n p o t e n t i a l w i t h the i n h i b i t o r con
centration.
T h i s is shown in F i g . 8 , in comparison
with a s i m i l a r r e l a t i o n s h i p f o r
g-naphthoquinoline.
Hoar (16) found t h a t in the c o r r o s i o n i n h i b i t i o n o f
i r o n in h y d r o c h l o r i c a c i d by -naphthoquinoline, the
c o r r o s i o n p o t e n t i a l i n c r e a s e s m o n o t o n i c a l l y w i t h in
c r e a s i n g i n h i b i t o r c o n c e n t r a t i o n , w h i l e in the case of
o - t o l y l t h i o u r e a one o b s e r v e s f i r s t a d e c r e a s e of the
corrosion potentia
i n h i b i t o r concentrations
h i b i t i o n o f the c a t h o d i c p a r t i a l r e a c t i o n at s m a l l in
h i b i t o r c o n c e n t r a t i o n s is e x h i b i t e d a l s o by
phenylt h i o u r e a a c c o r d i n g t o Kaesche.
F u r t h e r m o r e , in the
s e r i e s o f the t h i o u r e a d e r i v a t i v e s one o f t e n f i n d s c o r
r o s i o n a c c e l e r a t i o n at s m a l l c o n c e n t r a t i o n s , as f o r in
s t a n c e in the case o f p h e n y l t h i o u r e a at c o n c e n t r a t i o n s
o f 10moles per l i t e r .
T h i s appears t o be due t o a
s m a l l c a t h o d i c d e c o m p o s i t i o n of t h i o u r e a and i t s de
r i v a t i v e s in the course of which hydrogen s u l f i d e is
formed. As is w e l l known, hydrogen s u l f i d e tends t o
a c c e l e r a t e c o r r o s i o n , in p a r t i c u l a r the a n o d i c p a r t i a l
r e a c t i o n o f d i s s o l u t i o n o f i r o n , which has been demon
s t r a t e d i n d e p e n d e n t l y by o t h e r a u t h o r s (17).
I t is g e n e r a l l y assumed t h a t i o n s which can a c c e l
e r a t e e i t h e r o r both p a r t i a l r e a c t i o n s in a c o r r o s i o n
p r o c e s s are c a p a b l e of b e i n g adsorbed on the i r o n s u r
face.
Thus i t is known t h a t hydrogen s u l f i d e i o n s
which a c c e l e r a t e both p a r t i a l r e a c t i o n s o f a c i d c o r
r o s i o n ( a l t h o u g h p r e d o m i n a n t l y the a n o d i c o n e ) , and
f o r m i c a c i d m o l e c u l e s which c a t a l y z e the c a t h o d i c p a r
t i a l r e a c t i o n but i n h i b i t the a n o d i c one, as w e l l as
commercial i n h i b i t o r s which r e d u c e both p a r t i a l r e
a c t i o n s , are in f a c t adsorbed on the i r o n s u r f a c e .
As
a consequence the mere f a c t t h a t a d s o r p t i o n t a k e s p l a c e
cannot be used t o p r e d i c t an e x p e c t e d change in c o r
r o s i o n r a t e as i t is a l s o known t h a t h a l i d e i o n s c a t a l i z e the a n o d i c d i s s o l u t i o n o f i n d i u m , w h i l e hydroxy1
a d s o r p t i o n c a t a l y z e s the a n o d i c d i s s o l u t i o n of i r o n .
F u r t h e r m o r e , i t is a l s o known t h a t c e r t a i n i o n s can
act e i t h e r as a c a t a l y s t o r an i n h i b i t o r when a d s o r b
ed on the m e t a l s u r f a c e depending on the type of m e t a l
considered.
K o l o t y r k i n (IB) o b s e r v e d t h a t the a d s o r p -
CORROSION CHEMISTRY
284
4000
-0.45
/3-Naphthoqulnollne
o-tolyl-thlourea
Springer-Verlag
Figure 8. Weight loss and rest potential
of mild steelin10% H SO as a function
of the concentration of -naphthoquinoline and o-tolylthiourea (3)
z
9.
HAUSLER
Corrosion
Inhibition
285
t i o n o f i o d i n e i o n s c o n s i s t e n t l y i n c r e a s e s the hydrogen
o v e r - v o l t a g e on s i l v e r but d e c r e a s e s i t on mercury.
On
l e a d one o b s e r v e s t h a t s m a l l amounts o f adsorbed i o d i n e
i n c r e a s e the hydrogen o v e r - p o t e n t i a l , l a r g e r amounts,
however, d e c r e a s e i t . In the case o f the hydrogen evo l u t i o n i t is p r o b a b l y the h e a t o f a d s o r p t i o n o f the
atomic hydrogen on a p a r t i c u l a r m e t a l which c o n t r o l s
the a c c e l e r a t i o n or i n h i b i t i o n o f t h i s r e a c t i o n upon
a d s o r p t i o n o f a f o r e i g n i o n . With r e s p e c t t o the
k i n e t i c s o f the a n o d i c d i s s o l u t i o n o f m e t a l , however,
the i m p o r t a n t parameter is most l i k e l y the s t r e n g t h o f
the complex f o r m a t i o n between s u r f a c e m e t a l atoms and
the adsorbed p a r t i c l e .
I f the complex f o r m a t i o n is
weak, t h e r e f o r e h a r d l y a f f e c t i n g the bonding f o r c e s
h o l d i n g the s u r f a c e atoms in the m e t a l l a t t i c e , one
would e x p e c t i n h i b i t i o n t h r o u g h simple b l o c k i n g o f the
dissolution sites.
Conversely
a c t between the s u r f a c
p a r t i c l e , one would e x p e c t c a t a l y s i s o f the m e t a l d i s solution.
A l o n g t h e s e l i n e s one a l s o has t o c o n s i d e r
the p o s s i b i l i t y t h a t one k i n d o f adsorbed p a r t i c l e
can
be d i s p l a c e d by a d i f f e r e n t k i n d .
One would t h e r e f o r e
suggest t h a t i n c r e a s i n g the c o n c e n t r a t i o n o f t h i o u r e a
d e r i v a t i v e s e v e n t u a l l y leads to c o r r o s i o n i n h i b i t i o n
because of c o m p e t i t i v e a d s o r p t i o n w i t h the hydrogen
s u l f i d e i o n which causes a c c e l e r a t i o n o f c o r r o s i o n .
A c c e l e r a t i o n of c o r r o s i o n has been demonstrated
w i t h s t r o n g complexing agents such as EDTA s a l t s .
Howe v e r , as EDTA is s u b s t i t u t e d w i t h l o n g e r a l k y l c h a i n s ,
the c a t a l y t i c e f f e c t is g r a d u a l l y l o s t and i n h i b i t i o n
is o b t a i n e d .
T h i s s t r o n g l y s u g g e s t s t h a t the a d s o r p t i o n o f the i n h i b i t o r p a r t i c l e on the m e t a l s u r f a c e ,
a p u r e l y i n t e r f a c i a l phenomenon, is not the predominant
f e a t u r e t o be s t u d i e d , but t h a t in f a c t the c h e m i s t r y
in the i n t e r p h a s e , in p a r t i c u l a r , the f o r m a t i o n o f
c o r r o s i o n p r o d u c t s and c o r r o s i o n p r o d u c t l a y e r s has t o
be g i v e n more c o n s i d e r a t i o n .
A case in p o i n t is a study made by Ross (JJ) on the
d i s s o l u t i o n o f i r o n in 0.5 m o l a r s u l f u r i c a c i d in the
p r e s e n c e o f t h i o u r e a at 40 C. The r e s u l t s o f t h i s
s t u d y , which was conducted as a f u n c t i o n o f the f l o w
r a t e , are shown in F i g . 9 .
I t appears t h a t the u n i n h i b i t e d d i s s o l u t i o n o f i r o n f o l l o w s e x p e c t e d mass t r a n s f e r b e h a v i o r b o t h in the l a m i n a r and t u r b u l e n t r e g i o n s .
However, at two i n h i b i t o r c o n c e n t r a t i o n s marked d e v i a t i o n s from the e x p e c t e d mass t r a n s f e r b e h a v i o r are observed.
Ross attempted t o e x p l a i n t h e s e r e s u l t s on
the b a s i s t h a t d i f f e r e n t i n h i b i t o r c o n c e n t r a t i o n s a f f e c t the a n o d i c and c a t h o d i c p o l a r i z a t i o n in d i f f e r e n t
ways, t a k i n g a l s o i n t o c o n s i d e r a t i o n t h a t at s m a l l
286
CORROSION
CHEMISTRY
9.
HAUSLER
Corrosion
Inhibition
287
t h i o u r e a c o n c e n t r a t i o n s hydrogen s u l f i d e is formed on
the s u r f a c e o f the m e t a l c a t a l i z i n g the a n o d i c r e a c t i o n , t h e r e b y a c h i e v i n g a c o r r o s i o n r a t e which is
h i g h e r t h a n the b l a n k c o r r o s i o n r a t e .
However, R o s s s
e x p l a n a t i o n l e a v e s many ends u n t i e d .
One has t o exp l a i n f i r s t o f a l l the mass t r a n s f e r b e h a v i o r f o r the
u n i n h i b i t e d d i s s o l u t i o n o f i r o n in s u l f u r i c a c i d .
At
a p r o t o n c o n c e n t r a t i o n o f one normal and at c o r r o s i o n
r a t e s as measured, p r o t o n d i f f u s i o n cannot be r a t e controlling.
Second,the i r o n d i f f u s i o n away from the meta l s u r f a c e is not e x p e c t e d t o be r a t e l i m i t i n g , s i n c e
i r o n can in f a c t d i f f u s e away from the s u r f a c e as f a s t
as i t is formed, a f a c t t h a t has been e x p e r i m e n t a l l y
v e r i f i e d by t h i s a u t h o r (see b e l o w ) .
One must t h e n
assume t h a t the o b s e r v e d mass t r a n s f e r b e h a v i o r is
caused by a secondary r e a c t i o n most l i k e l y the d i s s o l u t i o n of a c o r r o s i o
t a i n i r o n s u l f a t e s ar
as the monohydrate o f the f e r r o u s s u l f a t e , and f e r r i c
sulfate.
In R o s s s experiment oxygen was
present
s i n c e the a u t h o r does not mention any p r e c a u t i o n s f o r
k e e p i n g oxygen out o f the e x p e r i m e n t a l system.
At low t h i o u r e a c o n c e n t r a t i o n s ( 2 1 0 ~ m o l a r ) i t
is o b s e r v e d t h a t the c o r r o s i o n r a t e v a r i e s approximatel y w i t h the 1/5 power o f the f l o w r a t e .
This author
has o b s e r v e d t h a t , in hydrogen s u l f i d e s a t u r a t e d s o l u t i o n s at 70 C in the p r e s e n c e o f s m a l l amounts o f
ammonium c h l o r i d e * the c a t h o d i c p a r t i a l r e a c t i o n f o r
i r o n c o r r o s i o n v a r i e s w i t h the l / 6 t h power o f the f l o w
rate.
T h i s maybe a c o i n c i d e n c e ; however, s i n c e the
f o r m a t i o n o f hydrogen s u l f i d e and i r o n s u l f i d e on
c o r r o d i n g i r o n s u r f a c e s in a c i d t h i o u r e a c o n t a i n i n g
media has been o b s e r v e d i n d e p e n d e n t l y by o t h e r a u t h o r s ,
i t is q u i t e l i k e l y t h a t the abnormal mass t r a n s f e r beh a v i o r o b s e r v e d by Ross f o r the s m a l l t h i o u r e a concent r a t i o n is in f a c t caused by an i r o n s u l f i d e l a y e r on
the s u r f a c e o f the m e t a l .
At h i g h e r t h i o u r e a concent r a t i o n s t h e r e may w e l l be c o m p e t i t i o n between the
t h i o u r e a m o l e c u l e and the hydrogen s u l f i d e in the
comp l e x i n g r e a c t i o n w i t h i r o n . I t s h o u l d be n o t e d , t h a t
the next h i g h e r t h i o u r e a c o n c e n t r a t i o n is t h r e e o r d e r s
o f magnitude l a r g e r but a c h i e v e s o n l y a m a r g i n a l inhibition effect.
One would t h e r e f o r e assume t h a t
t h i o u r e a is in c o m p e t i t i o n w i t h s u l f a t e or water f o r
l i g a n d p o s i t i o n s around the i r o n . S i n c e t h i o u r e a is a
n e u t r a l molecule, i t is q u i t e u n d e r s t a n d a b l e t h a t i t
w i l l reduce the d i s s o l u t i o n r a t e o f i r o n s u l f a t e merely
by b l o c k i n g a c c e s s o f water m o l e c u l e s t o the i r o n s u l fate surface.
A complete e x p l a n a t i o n o f Ross's s c a n t y
d a t a is c o m p l i c a t e d by the p r e s e n c e of oxygen which
1
CORROSION
288
CHEMISTRY
T h i s f o r m a l i s m i m p l i e s t h a t the p o l a r i z a t i o n o f
the t r i p l e bond can be s t a b i l i z e d f i r s t by a n o n c l a s s i c a l carbonium i o n and f u r t h e r by an -keto-double bond
c o n f i r g u r a t i o n which is known t o complex s t r o n g l y w i t h
t r a n s i t i o n metal ions.
I t is n o t e d t h a t the h y d r o x y l
group has t o be l o c a t e d not o n l y in -position but on
a secondary carbon atom f o r s t r o n g c o r r o s i o n i n h i b i t i o n
to r e s u l t .
T h i s is t o be e x p e c t e d , s i n c e p r o t o n s are
much more apt t o form n o n c l a s s i c a l carbonium i o n s t h a n
m e t h y l groups.
However, i f the t e r m i n a l p r o t o n on the
t r i p l e bond is s u b s t i t u t e d w i t h a s t r o n g e l e c t r o p h i l i c
group,the p o l a r i z a t i o n o f the t r i p l e bond becomes
s t r o n g enough t o i n v o l v e the above i n d i c a t e d
tauto
merism.
One can f u r t h e r r a t i o n a l i z e t h a t a compound w i t h
a n o n - t e r m i n a l t r i p l e bond e x p e r i e n c e s some s t e r i c
h i n d r a n c e in the f o r m a t i o n of complexes w i t h t r a n s i
t i o n metal ions.
Thus, the e f f e c t i n d i c a t e d in T a b l e 3
which T e d e s c h i a s c r i b e d t o s t e r i c h i n d r a n c e is r a t h e r
a k i n e t i c e f f e c t c o n c e r n i n g the i n t r a - m o l e c u l a r s h i f t s
Corrosion
9. H A U S L E R
289
Inhibition
Table I I I
Decreasing
1. L o c a t i o n o f T r i p l e
R
R]_
Rl
= CH ^C H
3
OH
R]_ = H or CH
R
Bond:
r
2
Inhibition
1 1
2. S t e r i c h i n d r a n c e
H
H6C=CH>
OH
"
A_
c = c
OH "
OH
_ =c_c_
c
"
H
CH 6C=CH>>
H
3
R
~2
OH
CH C=CH
3
OH
3. A l k y l Chain Length:
H
R <jjC=CH
2
CH
N : H - > C H > CH
CH ^
o f T r i p l e Bond and H y d r o x y l F u n c t i o n :
4. P o s i t i o n
CH C H C H C H C = C H
3
CH C H C = C C H C H 0 H
3
OH
INHIBITOR
NO INHIBITOR
290
CORROSION
CHEMISTRY
T a b l e IV
Type C h a i n
Formula
OH
Length C o r r o s i o n Rate
OHCH C===CH
CH
OHCHC=CH
I
CH
CH CH CH CHC=CH
0. 026
>1.6
0. 002
OH
CHq
CH CHCH C=C
CHo
OH
3
Table V
Formula
H
lo
3
Temperature
C o n c e n t r a t i o n C o r r o s i o n Rate
C-
200
0.2
6H
175
0.3
0.221
>1.8
>1.8
<f 3
CH C=CCI
3
OH
CH
20 0
0. 2
175
0.3
0.243
200
175
0. 4
0.4
0. 004
0.002
17 5
0 .1
Some
CHbC=CI
6H
CH
3
C H -(C = C H
3
0[CH CH 0] H
2
Activity
9.
HAUSLER
Corrosion
291
Inhibition
n e c e s s a r y f o r complex f o r m a t i o n . I t i s a l s o i n t e r e s t
i n g t o note t h a t s u b s t i t u t e d p r o p a r g y l a l c o h o l s become
more e f f e c t i v e as the a l k y l c h a i n i n c r e a s e s .
This
e f f e c t most l i k e l y has t o do w i t h the n a t u r e o f t h e
"adsorbed l a y e r " i n t h a t i n c r e a s i n g l e n g t h o f the a l k y l
c h a i n i m p a r t s g r e a t e r h y d r o p h o b i c i t y t o the i n t e r f a c i a l
l a y e r , thus s q u e e z i n g out water m o l e c u l e s from the
interphase.
Q u a n t i t a t i v e r e s u l t s f o r t h i s e f f e c t are
shown i n T a b l e 4.
The temperature e f f e c t o f some a c e t y l e n i c d e r i v
atives
i s quite surprising. Dimethyl-propargyl
a l c o h o l l o s e s i t s e f f i c i e n c y as the temperature r i s e s
from 175 t o 200 F.
The f a c t t h a t t h i s i s no l o n g e r
t r u e f o r the i o d i n e - s u b s t i t u t e d compound i s i n s u p p o r t
of t h e above mechanism
Furthermore
i f the h y d r o x y l
group on the d i m e t h y
a p o l y o l group, th
l o s t , u n d e r l i n i n g the importance o f the h y d r o x y l group
as an i n t e g r a l p a r t o f t h e i n h i b i t i o n mechanism. I t
s h o u l d f u r t h e r be n o t e d , t h a t t h e e q u i v a l e n t compounds
c o n t a i n i n g double bonds i n s t e a d o f the t r i p l e bond
show no c o r r o s i o n i n h i b i t i o n whatsoever.
Tedeschi s
attempt t o f o r m a l i z e the i n h i b i t i o n mechanism o f
a c e t y l e n i c compounds has r e c e n t l y been p u b l i s h e d , (22)
and i s shown i n T a b l e 6.
Here the i n t e r a c t i o n o f p r o t o n a t e d a l k y n o l s such as m e t h y l - b u t y n o l and h e x y n o l
w i t h themselves and the m e t a l s u r f a c e i s i l l u s t r a t e d
i n the b u i l d i n g up o f a complex i n h i b i t o r m u l t i l a y e r .
Such a charged m o l e c u l a r b a r r i e r i s analogous t o a
t h r e e - d i m e n s i o n a l polymer i n which a l a r g e excess o f
p r o t o n s i s k e p t from t h e m e t a l by r e p u l s i o n o f l i k e
charges o r by i n t e r a c t i o n w i t h the b a s i c - f i e l d o f
the t r i p l e bond.
A c c o r d i n g t o P o l i n g (23) IR s t u d i e s have i n d i c a t e d
t h a t such f i l m s may be up t o 200 t h i c k .
I f the a v e r age m o l e c u l e i s e s t i m a t e d a t about 4 i n d i a m e t e r , t h e n
a b a r r i e r o f up t o f i f t y m o l e c u l e s t h i c k i s p o s s i b l e .
The a b i l i t y o f t h e t r i p l e bond t o f u n c t i o n as a Broens t e d base i n hydrogen bonding w i t h e i t h e r a c i d i c
e t h y n y l e p r o t o n s o r h y d r o x y l g r o u p s has a l s o been proved
by IR s t u d i e s C23).
The f o r m a t i o n o f such a space charge l a y e r i s i n t u i t i v e l y a p p e a l i n g , a l t h o u g h i t i s u n l i k e l y t o extend
v e r y f a r . I f p r o t o n s were t o p l a y a r o l e i n the b u i l d up o f a l a r g e t h r e e - d i m e n s i o n a l polymerous network o f
the k i n d shown i n T a b l e 6, t h e n n e c e s s a r i l y some nega t i v e c o u n t e r i o n s would have t o be b u i l t i n t o such a
layer.
However, a more s e r i o u s f l a w o f t h i s model i s
t h a t i t cannot account f o r t h e m i g r a t i o n o f two o r
three v a l e n t metal ions through
such a polymer l a y e r .
!
292
CORROSION
CHEMISTRY
T a b l e VI
Methyl Butynol
(MB) - H e x y n o l
( C H ) C G s = CH
3
(H)
CH (CH )2HC=CH
H+
OH
(MB)
H
C
?3 7
H
H0:H
HC==CC
C H2 )
, (3
3
H 0
2
(CH)
" H:9H
(CHo) GC^CH
HCaeCH
//
HC===C
""H : 0 H
C3
o H7
7
(C3
H ) 2- C, ~ C s C H
q
H:0H +
9.
HAUSLER
Corrosion
Inhibition
293
In h y d r o c h l o r i c a c i d f o r i n s t a n c e a t room temperature
the c o r r o s i o n r a t e a t 9 5% i n h i b i t i o n i s s t i l l s e v e r a l
hundred mpy s.
T h e r e f o r e , u n d e r steady s t a t e con
d i t i o n s t h e r e i s a f l u x o f i r o n i o n s a c r o s s the i n t e r
phase which must be accommodated i n the i n h i b i t i o n mech
anism.
T h i s c o u l d be done more e a s i l y by assuming a
c o r r o s i o n p r o d u c t l a y e r o f d i s c r e t e t h i c k n e s s composed
o f a complex formed from m e t a l i o n s and i n h i b i t o r .
De
t a i l s o f t h i s model w i l l be d i s c u s s e d below.
Thus, i t i s i m p o r t a n t t o p o i n t out t h a t any model
o f the i n h i b i t i o n mechanism has t o i n c l u d e not o n l y an
e x p l a n a t i o n o f the i n t e r f e r e n c e w i t h the s u r f a c e
k i n e t i c s but a l s o w i t h the charge t r a n s f e r p r o c e s s e s
a c r o s s the boundry l a y e r .
The study o f such t r a n s f e r p r o c e s s e s was one o f
the purposes o f a f a i r l y e x t e n s i v e i n v e s t i g a t i o n o f the
i n h i b i t i o n of c o r r o s i o
acetylenic corrosio
t h i s i n v e s t i g a t i o n were: the c o n c e n t r a t i o n o f the
a c i d , the c o n c e n t r a t i o n o f the i n h i b i t o r , the f l o w r a t e
and the oxygen c o n c e n t r a t i o n i n the c o r r o s i v e medium.
While most o f the e x p e r i m e n t a l d a t a were o b t a i n e d by
means o f the so c a l l e d r e s i s t a n c e p r o b e , p o l a r i z a t i o n
measurements were c a r r i e d out i n o r d e r t o e l u c i d a t e
some o f the more p e c u l i a r r e s u l t s . The e x p e r i m e n t a l
arrangement i s more f u l l y e x p l a i n e d i n ( 2 4 ) .
O r i g i n a l l y i t had been i n t e n d e d t o f i n d a c o r
r e l a t i o n between i n h i b i t o r c o n s t i t u t i o n and i n h i b i t o r
activity.
The approach t a k e n i n e v a l u a t i n g the d a t a
e s s e n t i a l l y f o l l o w e d the t h i n k i n g o f o t h e r i n v e s t i g a
t o r s i n the f i e l d .
I t was assumed t h a t the parameters
o f an a d s o r p t i o n i s o t h e r m would be a means t o c o r r e
l a t e c h e m i c a l s t r u c t u r e w i t h i n h i b i t o r performance
f
(15.)
294
CORROSION
Q =
1 +
C H E M I S T R Y
(19)
KC
i n h
.
i
^ =
(20)
(1-P)
log = k - l o g . C
i n h
+k
(21)
A s t i l l f u r t h e r i s o t h e r m ( e q u a t i o n 22) i s a t t r i
b u t e d t o Tempkin and assumes a l i n e a r dependence o f t h e
a d s o r p t i o n energy on f r a c t i o n a l coverage.
= I
l o g C i h + const
n
(22)
9.
HAUSLER
Corrosion
Inhibition
295
T h i s assumption i s s u p p o r t e d i f a p l o t o f vs
l o g C ^ ^ yields a straight line.
I n t h i s c a s e , as i n
the case o f t h e F r e u n d l i c h i s o t h e r m , c o m p e t i t i v e ad
s o r p t i o n between i n h i b i t o r m o l e c u l e s and e i t h e r water
o r o t h e r c o n s t i t u e n t s o f t h e c o r r o s i v e medium i s p a r t
o f t h e assumptions.
Thus, except i n t h e case o f t h e
Langmuir i s o t h e r m , c o m p e t i t i v e a d s o r p t i o n i s r e c o g n i z e d
as i m p o r t a n t , hence t h e Langmuir i s o t h e r m seems some
what i d e a l , n o t w i t h s t a n d i n g t h e f a c t d i s c u s s e d e a r
l i e r , t h a t t h e a d s o r p t i o n may be h i g h l y s e l e c t i v e w i t h
r e s p e c t t o t h e e l e c t r o d e r e a c t i o n and may f u r t h e r m o r e
be p o t e n t i a l dependent.
Hence, i s o t h e r m p l o t t i n g i s
a t b e s t o n l y a means t o c o r r e l a t e d a t a and t o r e g i s t e r
p o s s i b l e c h e m i c a l e f f e c t s o f t h e system on a p u r e l y
comparative b a s i s . F o r a more d e t a i l e d d i s c u s s i o n o f
a d s o r p t i o n i s o t h e r m s i n e l e c t r o l y t e systems, t h e r e a d e r
i s r e f e r r e d to the
Parsons (26) Delaha
In t h e c o u r s e o f s t u d y i n g some a c e t y l e n i c i n
h i b i t o r s i n h y d r o c h l o r i c a c i d , a number o f d i f f e r e n t
i s o t h e r m type c o r r e l a t i o n were found.
Thus Fig.10
shows a Langmuir p l o t f o r 2 - b u t y n e - l , 4 - d i o l
i n 6N
h y d r o c h l o r i c a c i d under a e r a t e d c o n d i t i o n s . Two d i f
f e r e n t commercial p r o d u c t s o f t h i s compound were used.
I t i s shown t h a t t h e s t r a i g h t l i n e r e l a t i o n s h i p i s
o b t a i n e d w i t h a good r e p r o d u c i b i l i t y over t h r e e o r d e r s
o f magnitude o f i n h i b i t o r c o n c e n t r a t i o n .
In the l i g h t
o f what was s a i d above, t h i s was a s u r p r i s i n g r e s u l t ,
however, i t corresponds t o o t h e r s i m i l a r o b s e r v a t i o n s
reported i n the l i t e r a t u r e .
F i g . 1 1 .shows t h e same type o f p l o t f o r t h e i d e n t
i c a l i n h i b i t o r i n h y d r o c h l o r i c a c i d o f d i f f e r e n t con
centrations.
I t can now be seen t h a t i n weaker h y d r o
c h l o r i c a c i d the l i n e a r r e l a t i o n s h i p i s not r e t a i n e d .
At t h e h i g h e r i n h i b i t o r c o n c e n t r a t i o n s i n h i b i t i o n i s
reduced w i t h r e s p e c t t o 6N h y d r o c h l o r i c a c i d , w h i l e a t
the lower c o n c e n t r a t i o n s t h e i n h i b i t o r seems t o be
more e f f e c t i v e i n 4N than i n 6N h y d r o c h l o r i c a c i d .
The
decrease of i n h i b i t o r e f f i c i e n c y with decreasing a c i d
c o n c e n t r a t i o n i s unexpected.
No s i m i l a r r e s u l t s seem
t o have been r e p o r t e d e l s e w h e r e .
An i n t e r e s t i n g e f f e c t i s o b s e r v e d , F i g , 1 2 , when
the s i x normal h y d r o c h l o r i c a c i d s o l u t i o n i s d e a e r a t e d
with nitrogen.
I t i s seen t h a t t h e absence o f oxygen
reduces the e f f i c i e n c y o f the i n h i b i t o r c o n s i d e r a b l y .
F i n a l l y F i g . 13, shows a comparison o f 2 - b u t y n e - l ,
4 - d i o l , i n M-N h y d r o c h l o r i c a c i d under a e r a t e d and dea e r a t e d c o n d i t i o n s . While a t lower c o n c e n t r a t i o n s t h e
p r e s e n c e o f oxygen does n o t seem t o be i m p o r t a n t a t
higher concentrations better i n h i b i t o r e f f i c i e n c y i s
CORROSION CHEMISTRY
296
100
in 6N H CI aerated
HAUSLER
Corrosion
Inhibition
CORROSION C H E M I S T R Y
in 6N HCl
9.
HAUSLER
Corrosion
Inhibition
299
100
2-butyne-l,4-diol
300
CORROSION
CHEMISTRY
observed i n d e a e r a t e d vs a e r a t e d h y d r o c h l o r i c a c i d , o r
i n o t h e r words, the r e v e r s e o f the e f f e c t observed i n
6N h y d r o c h l o r i c a c i d .
S i m i l a r experiments w i t h p r o p a r g y l a l c o h o l are
shown i n F i g . 1 4 .
T h i s s e r i e s o f experiments was
c a r r i e d out i n 4N h y d r o c h l o r i c a c i d under a e r a t e d and
d e a e r a t e d c o n d i t i o n s . A p p a r e n t l y a d i f f e r e n t mechanism
a p p l i e s f o r p r o p a r g y l a l c o h o l because the c u r v a t u r e of
the r e l a t i o n s h i p between p r o t e c t i o n and i n h i b i t o r conc e n t r a t i o n p l o t t e d i n Langmuir type f a s h i o n i s r e v e r s e d
from what was observed f o r b u t y n e - d i o l .
While the oxygen e f f e c t w i t h b u t y n e - d i o l i n 4N h y d r o c h l o r i c a c i d
was s i g n i f i c a n t o n l y a t h i g h e r i n h i b i t o r c o n c e n t r a t i o n s , i t becomes e v i d e n t t h a t f o r p r o p a r g y l a l c o h o l
oxygen r e d u c e s the i n h i b i t o r e f f i c i e n c y o v e r a l a r g e
c o n c e n t r a t i o n range.
Most i m p o r t a n t l y p r o p a r g y l
a l c o h o l i s more e f f e c t i v
In summary, i n d i r e c
d i c a t e some s u r p r i z i n g r e s u l t s .
F i r s t a rather large
e f f e c t o f s m a l l changes i n a c i d c o n c e n t r a t i o n i s obs e r v e d f o r the b u t y n e - d i o l .
I t would be r a t h e r d i f f i c u l t t o e x p l a i n t h i s i n terms o f a d s o r p t i o n t h e o r y
because the a d s o r p t i o n i s o t h e r m i n 4N h y d r o c h l o r i c
a c i d c r o s s e s the one o b t a i n e d i n 6N h y d r o c h l o r i c a c i d .
The pronounced and complex e f f e c t o f oxygen i s
even more unexpected i n terms o f a d s o r p t i o n t h e o r y ,
because oxygen i s not t h e r m o d y n a m i c a l l y s t a b l e under
the c o n d i t i o n s o f i r o n c o r r o s i o n i n h y d r o c h l o r i c a c i d .
Note a l s o , i n t h i s c o n t e x t , the oxygen e f f e c t i n 6N
h y d r o c h l o r i c a c i d i s e x a c t l y o p p o s i t e from 4N h y d r o chloric acid.
A n d , f i n a l l y , i n 4N a c i d we f i n d t h a t the
oxygen e f f e c t i s i n the same d i r e c t i o n f o r p r o p a r g y l
a l c o h o l and b u t y n e - d i o l , w i t h p r o p a r g y l a l c o h o l b e i n g
much s t r o n g e r a f f e c t e d . In terms o f a d s o r p t i o n t h e o r y
one would expect the b e t t e r i n h i b i t o r t o be adsorbed
more s t r o n g l y and hence l e s s a f f e c t e d by c o m p e t i t i v e
adsorption.
The e x p e r i m e n t a l r e s u l t s i n d i c a t e the
opposite.
A complete and e x t e n s i v e r a t i o n a l f o r the r e j e c t i o n o f a d s o r p t i o n t h e r o y i n the p r e s e n t case would
e a s i l y go beyond the frame work o f t h i s paper. Howe v e r , s i n c e i n the a r e a o f a c e t y l e n i c i n h i b i t o r s the
b u i l d - u p o f s u r f a c e f i l m s up t o 20 0 A has a l r e a d y been
o b s e r v e d (25), i t maybe more f r u i t f u l t o f o l l o w t h i s
l i n e of reasoning.
Further experimental e f f o r t , of
course, ought t o attempt t o c l a r i f y the n a t u r e o f t h i s
surface f i l m .
The above d e s c r i b e d e f f e c t s such as
the a c i d and the oxygen e f f e c t are not immediately
obv i o u s i f such a s u r f a c e f i l m i s o f the n a t u r e shown i n
T a b l e 6.
However, some o b s e r v a t i o n s i n v o l v i n g the de-
9.
HAUSLER
Corrosion
Inhibition
301
pendence o f i n h i b i t e d c o r r o s i o n r a t e s on f l o w r a t e may
shed f u r t h e r l i g h t on t h i s problem.
F i g . 1 5 , shows the c o r r o s i o n r a t e o f i r o n i n 4N
h y d r o c h l o r i c a c i d under a e r a t e d c o n d i t i o n s as a f u n c t i o n o f f l o w r a t e and d i f f e r e n t i n h i b i t o r c o n c e n t r a tions.
I t i s noted t h a t the b l a n k c o r r o s i o n r a t e i n c r e a s e s s l i g h t l y w i t h f l o w r a t e w h i l e a somewhat^
s t r o n g e r dependence i s o b s e r v e d f o r 0.01 and 0.02
per
cent i n h i b i t o r s o l u t i o n s . At the s t r o n g e r i n h i b i t o r
c o n c e n t r a t i o n (0.1%) the f l o w dependence becomes complex.
S i n c e t h e s e experiments were not c a r r i e d out under
i d e a l l y d e f i n e d f l o w c o n d i t i o n s the dependence o f c o r r o s i o n r a t e on f l o w r a t e w i l l be d i s c u s s e d o n l y i n a
q u a l i t a t i v e manner. Under l a m i n a r f l o w c o n d i t i o n s and
mass t r a n s f e r c o n t r o l one would have expected the c o r rosion rate to increas
v e l o c i t y w h i l e unde
a l i t y would p r e v a i l .
However, i n F i g . 1 5 one f i n d s t h a t
the c o r r o s i o n r a t e v a r i e s a p p r o x i m a t e l y w i t h the
0.2
t o 0.3 power o f the f l o w r a t e .
I t appears t h e r e f o r e
t h a t the o b s e r v e d dependence on the f l o w r a t e does not
obey c o n v e n t i o n a l mass t r a n s f e r t h e o r y .
A flow e f f e c t
might be e x p e c t e d i n u n i n h i b i t e d h y d r o c h l o r i c a c i d because hydrogen b u b b l e s , formed on the s u r f a c e o f t h e
m e t a l , are f a s t e r and more e a s i l y removed a t h i g h e r
flow r a t e s .
While t h i s argument c o u l d be a p p l i e d i n
d i s c u s s i n g F i g . 1 5 , we f i n d i n Fig.16 t h a t the f l o w
e f f e c t a t s i m i l a r c o r r o s i o n r a t e s i s much l e s s p r o nounced under d e a e r a t e d c o n d i t i o n s .
We t h e r e f o r e have
t o c o n c l u d e t h a t the o b s e r v e d f l o w e f f e c t i s not
mec h a n i c a l and cannot be r e l a t e d t o pure mass t r a n s f e r
c o n t r o l e i t h e r . In F i g . 1 7 , the f l o w dependence o f the
c o r r o s i o n r a t e i s shown f o r 2 - b u t y n e - l , 4 - d i o l i n dea e r a t e d 4N h y d r o c h l o r i c a c i d .
Note t h a t the c o r r o s i o n
r a t e appears t o be n o t i c e a b l y a f f e c t e d o n l y at t h e
higher flow r a t e s .
F i n a l l y , i n F i g . 1 8 , we observe t h a t
i n c r e a s e d f l o w r a t e can e i t h e r i n c r e a s e or d e c r e a s e the
c o r r o s i o n r a t e i n the p r e s e n c e o f an i n h i b i t o r .
This
e f f e c t was o b s e r v e d r e p r o d u c i b l y o n l y i n 6N h y d r o c h l o r i c a c i d w i t h 2 - b u t y n e - l , 4 - d i o l under d e a e r a t e d
c o n d i t i o n s f o r 0.2% and 0.1% i n h i b i t o r c o n c e n t r a t i o n .
T h i s b e h a v i o r i n d i c a t e s t h a t t h e c o r r o s i o n r a t e i s cont r o l l e d by the s u p e r p o s i t i o n o f two p a r t i a l r e a c t i o n
r a t e s each o f which i s mass t r a n s f e r dependent t o a
c e r t a i n extent.
In terms o f the model d e l i n e a t e d i n
T a b l e 6, i t i s suggested t h a t t h e
three-dimensional
p o l y m e r i c l a y e r made up by i n h i b i t o r m o l e c u l e s i s i n
fact a three-dimensional
c h e l a t e made up o f i r o n i o n s
and i n h i b i t o r m o l e c u l e s .
The c o r r o s i o n r a t e i s t h e n
CORROSION C H E M I S T R Y
150
4N HCI
100
50
10
1,4-Butyne diol
(deaerated)
_
\
CL
I
0O00I
0.0005
0.001
0.005
CONCENTRATION
Figure 14.
0.05 0.1
0.01
(%)
1500
J 1000
<
or
0.01
%^vy-
500
c/)
^ ^ ^ - ^ C ) . 0 2
or
or
100
50
0.01
J7-
/ o
*^0J%
1
0.05
0.1
1
0.5
1
1.0
FLOWRATE (gal/min)
Figure 15.
9.
HAUSLER
Corrosion
Inhibition
303
1500
10
0.01
0.05
0.1
0.5
1.0
FLOWRATE (gal/min)
Figure 16.
304
CORROSION C H E M I S T R Y
5000
FLOWRATE (gal/min)
Figure 17.
0.0
02
0.4
0.6
0.8
deaerated
1.0
FLOWRATE (gal/min)
Figure 18.
2-Butyne-l,4-diol,
9.
HAUSLER
Corrosion
Inhibition
305
c o n t r o l l e d by the t h i c k n e s s of t h i s l a y e r which, i n t u r n ,
i s dependent on the f o r m a t i o n r a t e o f the c h e l a t e as
w e l l as i t s d i s s o l u t i o n r a t e . T h i s suggested model
i s n a t u r a l l y based on a number o f assumptions, such as
a p a r t i a l l y s o l u b l e c h e l a t e between the a c e t y l e n i c
i n h i b i t o r and i r o n , and a d i f f e r e n t c h e m i s t r y o f the
2 - v a l e n t from the 3 - v a l e n t c h e l a t e , i n o r d e r t o e x p l a i n
the oxygen e f f e c t .
S i n c e t h i s c h e l a t e has a f i n i t e
s o l u b i l i t y and i s c o n s t a n t l y formed d u r i n g the c o r r o s i o n p r o c e s s , one can u n d e r s t a n d t h a t i n one i n s t a n c e
i t s d i s s o l u t i o n r a t e i s r a t e d e t e r m i n i n g , w h i l e i n the
o t h e r i n s t a n c e i t s f o r m a t i o n r a t e maybe r a t e d e t e r m i n i n g , the l a t t e r b e i n g c o n t r o l l e d by the mass t r a n s f e r
of the i n h i b i t o r from t h e b u l k of the s o l u t i o n t o the
s u r f a c e of the m e t a l .
While many q u e s t i o n s i n t h i s
model have t o be l e f t open at t h i s stage and await
further investigation
an i n h i b i t e d i r o n e l e c t r o d
the n a t u r e o f the s u r f a c e c o r r o s i o n p r o d u c t l a y e r and
hence the i n t e r p h a s e c h e m i s t r y d e f i n e d above.
Fig.19
shows t y p i c a l p o l a r i z a t i o n curves o b t a i n e d i n 4N h y d r o c h l o r i c a c i d under a e r a t e d and d e a e r a t e d c o n d i t i o n s and
w i t h 0.5% p r o p a r g y l a l c o h o l (2M).
The r e s u l t s i n d i c a t e the time dependence of t h e p o l a r i z a t i o n curves
as w e l l as the dependence on scan r a t e . The b l a n k
c o r r o s i o n r a t e s , which can be seen t o i n c r e a s e w i t h
t i m e , are an i n d i c a t i o n o f the p r o g r e s s i v e l y l a r g e r
surface area.
The a n o d i c T a f e l s l o p e s a r e q u i t e i n dependent o f scan r a t e w h i l e the c a t h o d i c T a f e l s l o p e s
appear t o i n c r e a s e s l i g h t l y w i t h f a s t e r scan r a t e s .
T h i s l a s t e f f e c t may be an i n d i c a t i o n o f a d s o r p t i o n o f
a small impurity
i n the e l e c t r o l y t e .
The i n h i b i t e d
c o r r o s i o n r a t e s d e c r e a s e w i t h time and become e s s e n t i a l l y c o n s t a n t a f t e r about two h o u r s .
These s l o p e s are
not dependent on scan r a t e or on c o r r o s i o n r a t e .
The
most i n t e r e s t i n g e f f e c t i s o b s e r v e d when the i n h i b i t e d
hydrochloric a c i d s o l u t i o n i s aerated:
the anodic
T a f e l s l o p e i n c r e a s e s w h i l e the c a t h o d i c T a f e l s l o p e
d e c r e a s e s d r a m a t i c a l l y . As would have been e x p e c t e d
from the r e s i s t a n c e probe measurement the c o r r o s i o n
r a t e i n the a e r a t e d i n h i b i t o r s o l u t i o n i n c r e a s e s .
These o b s e r v a t i o n s have been found t o be o f g e n e r a l
n a t u r e i n d i f f e r e n t h y d r o c h l o r i c a c i d as w e l l as v a r y ing propargyl alcohol concentrations.
While most o f
the o b s e r v e d e f f e c t s i n Fig.19 c o u l d be e x p l a i n e d i n
terms o f c o n v e n t i o n a l e l e c t r o c h e m i c a l k i n e t i c s , the
f a c t i s t h a t i n the p r e s e n c e of oxygen and p r o p a r g y l
a l c o h o l the c a t h o d i c curve shows a l i m i t i n g c u r r e n t beh a v i o r and i s r a t h e r d i f f i c u l t t o u n d e r s t a n d .
However,
i f one were t o assume an i r o n - i n h i b i t o r c h e l a t e f i l m
CORROSION C H E M I S T R Y
306
10,000
<
s^co^"
2 9 0
^ ^ < <^Blank
^ 6 0 min Slow (I)
^ 150 min Fast (I)
180 min Slow (H)
/80 min Fast
Aerated, Slow
^ry^
120 min Slow
1,000
Inhibited
20 min Slow
10
Anodi
I
80
1
120
40
40
80
120
Polarization 77 (mV)
Universal Oil Products
Figure 19.
Table V I
PROTECTIVE
COMPLEX
METAL
I
F e F e ^ + 2e
HYDRODYNAMIC
BOUNDARY LAYER
BULK
FLUID
,1
2H + 2e
D AC
Fe
Fe
c
Inh
X ' e
e
A C
Inh
r
F e
Inh
o
F e + Inh + +
{Felnh}
9.
HAUSLER
Corrosion
307
Inhibition
I n h i b i t i o n In The
P r e s e n c e Of Hydrogen S u l -
I n t r o d u c t i o n . The need f o r c o r r o s i o n p r o t e c t i o n
i n the p r e s e n c e o f hydrogen s u l f i d e a r i s e s most p r o m i n e n t l y i n the p e t r o l e u m i n d u s t r y , s p e c i f i c a l l y p r o -
308
CORROSION
CHEMISTRY
9.
HAUSLER
Corrosion
309
Inhibition
and H ) .
However, i f one l o o k s a t a s u b s u r f a c e s t r u c t u r e which was p u l l e d from an o i l w e l l , one f i n d s t h a t
the s u r f a c e i s not o n l y h y d r o p h o b i c but l o o k s b l a c k i n s t e a d o f m e t a l l i c , as one might expect from l a b o r a t o r y
experiments.
T h i s has s u b s e q u e n t l y
l e d t o the i d e a
t h a t the " f i l m i n g amine" i n h i b i t o r i s not adsorbed on
the m e t a l s u r f a c e but on an i r o n s u l f i d e f i l m c o v e r i n g
the m e t a l s u r f a c e .
While the r e s u l t o f a h y d r o p h o b i c
s u r f a c e i s the same, the l a t t e r concept l e a d s t o a
more r i g o r o u s d i s c u s s i o n o f the i n h i b i t i o n mechanism
and a v o i d s some o f the c o n f u s i o n which has been gene r a t e d by d i a g n o s t i c t e s t s which were based on s u r f a c e
a c t i v i t y o r w e t t i n g a n g l e measurements performed on a
c l e a n metal s u r f a c e .
The p r o o f o f the importance o f i r o n s u l f i d e i n
the i n t e r a c t i o n between the c o r r o s i o n i n h i b i t o r and
the m e t a l s u r f a c e wa
shown (lu)
that corrosio
enhanced and p r o l o n g e d when the c o r r o s i o n i n h i b i t o r
was adsorbed on a p r e s u l f i d e d specimen r a t h e r than on
the n o n s u l f i d e d s u r f a c e .
These measurements were made
e l e c t r o c h e m i c a l l y and gave s u p p o r t t o the p r a c t i c a l
well-known f a c t t h a t p e r i o d i c " f i l m i n g " c o u l d i n h i b i t
the c o r r o s i o n f o r r e l a t i v e l y l o n g p e r i o d s o f time.
In o r d e r t o u n d e r s t a n d the i r o n s u l f i d e - i n h i b i t o r
i n t e r a c t i o n , i t w i l l be n e c e s s a r y t o r e v i e w b r i e f l y
the c o r r o s i o n mechanism o f i r o n i n the p r e s e n c e o f
hydrogen s u l f i d e i n a two-phase medium.
C o r r o s i o n Of
I r o n In The
Presence Of Hydrogen S u l f i d e
310
CORROSION C H E M I S T R Y
not be l i n e a r ( c o n s t a n t c o r r o s i o n r a t e ) but p a r a b o l i c
( l i n e a r i t y between the c o r r o s i o n r a t e and the square
root of time).
I t was shown, however, t h a t the p a r a b o l i c i t y o f
s u l f i d e c o r r o s i o n i s e x t r e m e l y s e n s i t i v e t o the p r e sence o f minute q u a n t i t i e s o f oxygen.
It i s believed
t h a t many o f the e a r l i e r r e s u l t s may have been a f f e c t e d
by the i n c o m p l e t e e l i m i n a t i o n o f a i r from the c o r r o s i v e medium o r i t s i n a d v e r t e n t p r e s e n c e .
C a r e f u l measurements o f the c o r r o s i o n r a t e i n the
p r e s e n c e o f hydrogen s u l f i d e under r i g o r o u s e x c l u s i o n
o f a i r showed almost p e r f e c t p a r a b o l i c c o r r o s i o n k i n e t i c s , i n d i c a t i n g the r a t e c o n t r o l l i n g f a c t o r o f an
iron sulfide film (30.
I t was s u b s e q u e n t l y a l s o shown
i n (37) t h a t minute q u a n t i t i e s o f oxygen, as l i t t l e as
400 ppm i n the gas phase i n e q u i l i b r i u m w i t h the c o r r o s i v e media, a c c e l e r a t
10 times r e s u l t i n g i
kinetics.
F u r t h e r s t u d i e s (38) i n d i c a t e d t h a t the
t h i c k n e s s o f the r a t e c o n t r o l l i n g i r o n s u l f i d e f i l m
becomes c o n s t a n t a l t h o u g h the t o t a l i r o n s u l f i d e s c a l e
maybe much l a r g e r than t h a t .
T h i s was c o n f i r m e d by
e l e c t r o c h e m i c a l measurements o v e r l o n g p e r i o d s o f t i m e .
By a c t u a l l y w e i g h i n g the amount o f i r o n s u l f i d e b u i l t
up on the c o r r o d i n g specimen as a f u n c t i o n o f pH and
oxygen c o n c e n t r a t i o n (3JL)> i t was shown t h a t the "protectivness" o f the r a t e c o n t r o l l i n g i r o n s u l f i d e f i l m
i n c r e a s e s w i t h i n c r e a s i n g pH and d e c r e a s e s w i t h i n c r e a s i n g oxygen c o n c e n t r a t i o n .
However, the concent r a t i o n o f d i s s o l v e d s u l f i d i c s u l f u r has e s s e n t i a l l y
no e f f e c t on i r o n s u l f i d e f i l m i n the r e g i o n o f 1500
t o 25,000 ppm
(1Z).
A most s u r p r i s i n g b e h a v i o r was found when the pol a r i z a t i o n c u r r e n t o f a c o r r o d i n g specimen was o b s e r v e d
as a f u n c t i o n o f flow r a t e .
Fig.20
shows the r e s u l t s
which were o b t a i n e d .
A n o d i c and c a t h o d i c p o l a r i z a t i o n
c u r r e n t s i n m i l l i a m p s a t p o t e n t i a l s of +_ 50 m i l l i v o l t s
from the c o r r o s i o n p o t e n t i a l are p l o t t e d a g a i n s t f l o w
rate.
In s e v e r a l runs r e p r o d u c i b l e b e h a v i o r was
observed.
The c a t h o d i c c u r r e n t v a r i e s w i t h flow r a t e t o
the -g- t h power, w h i l e the a n o d i c c u r r e n t v a r i e s w i t h
f l o w r a t e t o the ^_ t h power. T h i s r e l a t i o n s h i p h o l d s
t r u e o v e r more t h a n one decade o f f l o w r a t e s .
It is
v e r y d i f f i c u l t t o e x p l a i n such r e s u l t s i n terms o f
c o n v e n t i o n a l mass t r a n s f e r l i m i t a t i o n s . I t i s w e l l
known t h a t the d i f f u s i o n l i m i t e d c u r r e n t v a r i e s app r o x i m a t e l y p r o p o r t i o n a l l y t o the f l o w r a t e i n the t u r b u l e n t r e g i o n and w i t h the square r o o t o f f l o w r a t e i n
the l a m i n a r r e g i o n .
I f on the o t h e r hand the p o l a r i z a t i o n c u r r e n t was t r a n s p o r t l i m i t e d a c r o s s the c o r r o s i o n
HAUSLER
Corrosion
Inhibition
Polarization Current at - E
C Q | T
= 5 0 mV
2nd Run
1st Run
2nd Run
Anodic
0.05
J ~V
0.1
0.2
0.4
FLOWRATE (gal/min)
Figure 20.
Table VIII
METAL
LIQUID
SCALE
Corrosion: Fe Fe + 2e
H S - FeJ + 2 + FeS + H
2
2H = H + 2
+
{H S + 1/2 0 = H 0 + S + 2}
=
2e + 2 -
Fe + FeJ -
++
Liquid Tronsfer
dC +
ti
D +
u
"
s
312
CORROSION
CHEMISTRY
p r o d u c t l a y e r , n o dependency on f l o w r a t e s h o u l d be
found.
I t i s t h e r e f o r e n e c e s s a r y t o p o s t u l a t e a more com
p l e x mechanism, such as i s suggested i n T a b l e 8.
Since
the c o r r o d i n g m e t a l i s covered by a c o r r o s i o n p r o d u c t
l a y e r , the f o l l o w i n g r e a c t i o n s have t o t a k e p l a c e a t
the m e t a l / s c a l e i n t e r f a c e : a. o x i d a t i o n o f i r o n ; b. the
consumption o f e l e c t r o n h o l e s by e l e c t r o n s ( o r p o s s i b l y
d i s c h a r g e o f p r o t o n s w i t h subsequent s o l u t i o n o f h y d r o
gen i n the m e t a l ) ; c. the combinatio n o f i r o n i o n v a
c a n c i e s w i t h newly formed i r o n i o n s . The l a t t e r two
p r o c e s s e s take p l a c e w i t h the r e l e a s e o f energy.
These
r e a c t i o n s n e c e s s i t a t e a c o n c e n t r a t i o n g r a d i e n t through
the c o r r o s i o n p r o d u c t l a y e r o f i r o n i o n v a c a n c i e s and
electron holes.
( I t i s u n d e r s t o o d t h a t the mechanism
c o u l d be w r i t t e n i n terms o f i r o n i o n s moving i n t e r s t i t i a l l y , i n which
t r o n conductor.
Thi
ever).
S i n c e the c o r r o s i o n p r o c e s s t a k e s p l a c e w i t h
f o r m a t i o n o f i r o n s u l f i d e the f o l l o w i n g r e a c t i o n s have
t o take p l a c e a t the s c a l e l i q u i d i n t e r f a c e : a. f o r
mation o f i r o n s u l f i d e and i r o n i o n v a c a n c i e s ; b. d i s
charge of p r o t o n s t o form hydrogen and e l e c t r o n h o l e s ;
c. i f oxygen i s p r e s e n t , the e l e c t r o n h o l e s can be
formed by r e d u c t i o n o f oxygen. The f o r m a t i o n o f hy
drogen from p r o t o n s a l s o n e c e s s i t a t e s a p r o t o n con
c e n t r a t i o n g r a d i e n t i n the l i q u i d phase ( i f oxygen i s
p r e s e n t , the c o r r e s p o n d i n g oxygen c o n c e n t r a t i o n g r a d
i e n t would have t o e x i s t as w e l l ) .
The r e a c t i o n s t a k
i n g p l a c e a t the s c a l e l i q u i d i n t e r p h a s e can be v i s
u a l i z e d as e q u i l i b r i u m r e a c t i o n s . T h i s means t h a t the
s u l f i d e c o n c e n t r a t i o n i n f l u e n c e s the i r o n i o n vacancy
c o n c e n t r a t i o n i n the s c a l e , and the p r o t o n concen
t r a t i o n i n the l i q u i d a f f e c t s the e l e c t r o n h o l e con
c e n t r a t i o n i n the s c a l e .
S i n c e the n a t u r e of the
s c a l e determines the r a t e o f the s o l i d s t a t e t r a n s f e r
phenomena, and the f l o w r a t e determines the l i q u i d
boundary l a y e r c o n c e n t r a t i o n g r a d i e n t , hence the
l i q u i d - s o l i d i n t e r f a c i a l p r o t o n , H S and oxygen concen
t r a t i o n s , i t f o l l o w s t h a t l i q u i d and s o l i d s t a t e mass
and charge t r a n s f e r a r e l i n k e d t o g e t h e r by the c h e m i c a l
e q u i l i b r i a e s t a b l i s h e d a t the s c a l e - l i q u i d i n t e r f a c e .
An attempt was made t o f o r m u l a t e t h e s e r e l a t i o n
s h i p s a n a l y t i c a l l y , i n o r d e r t o c o n f i r m the dependence
o f c o r r o s i o n r a t e on f l o w r a t e observed i n F i g . 2 0 .
Thus e q u a t i o n 23 r e s t a t e s the f o r m a t i o n o f i r o n i o n
v a c a n c i e s and e l e c t r o n h o l e s a t the s c a l e / l i q u i d i n t e r
face.
HS
l-*Fe + 2 + FeS + H
(23)
2
9.
HAUSLER
Corrosion
313
Inhibition
2-**2H
2H+
2(FeS)
(24)
+ Fe"
= (H*) +
(25)
()
Kr
(H+)
(26)
()*
and
(27)
()'
A +
(H )
i n d i c a t i n g t h e cube o f t h e e l e c t r o n h o l e c o n c e n t a t i o n
t o be i n v e r s e l y p r o p o r t i o n a l t o t h e p r o t o n c o n c e n
tration.
A c c o r d i n g t o B e n n e t t and Meyers (40.) t h e mass
f l u x a c c r o s s t h e l i q u i d boundary l a y e r can be f o r m u l a t
ed as f o l l o w s :
1/3
Re}- (Sc)
K"3 ( CH
N
H+
cath
(28)
= K
(C^+-C +) (Urn) *
0
H +
c )
(29)
cath
S u b s t i t u t i n g e q u a t i o n 27
into equation
29
314
CORROSION C H E M I S T R Y
( i
and
cath
catn
) 3
=*
:
A+B(H )
f i n a l l y with equation
(i
)3 cath
"
Q l
28
K
cu
U
1/2
m
"
4
t a
o^2 ^" ^
n s
(31)
B
icath
(32)
where :
= e l e c t r o n h o l e ; p o s i t i v e charge
In o r d e r t o stop t h e c o r r o s i o n p r o c e s s t h i s r e
a c t i o n has t o be impeded.
T h i s can be a c h i e v e d i f t h e
p r o t o n adsorbed on t h e s u r f a c e c o n t a i n i n g t h e i r o n s u l
f i d e s p e c i e s i s r e p l a c e d by a c a t i o n , t h e r e d u c t i o n o f
which i s n o t as e a s i l y a c c o m p l i s h e d as t h e r e d u c t i o n
of t h e p r o t o n .
I t i s s u g g e s t e d t h a t t h e a l k y l ammonium
9.
HAUSLER
Corrosion
315
Inhibition
RNH^
^=^5JFeS-RNH | + H
(33)
= surface
complex
(H )
' C
(34)
where :
(H )
formation constant
d i s s o c i a t i o n constant of surface
d i s s o c i a t i o n c o n s t a n t o f amine
proton concentration
c o n c e n t r a t i o n of amine
c o n c e n t r a t i o n of s u r f a c e - s u l f i d e
JFe-S"|
FeS-H
JFe-SH|
The two main f a c t o r s which a f f e c t the concent r a t i o n o f the s u r f a c e complex are i t s f o r m a t i o n cons t a n t K4 and the pH.
The l a t t e r has been shown t o be
true experimentally (Table
9).
C h e m i s t r y Of C o r r o s i o n I n h i b i t o r s . There are two
q u e s t i o n s which a r i s e w i t h r e s p e c t t o
. These are
1. How can the f o r m a t i o n c o n s t a n t o f the s u r f a c e comp l e x be i n c r e a s e d ?
2. Why i s n ' t the a d s o r p t i o n p r o c e s s r e v e r s i b l e ?
That i s t o say, why do s m a l l c o n c e n t r a t i o n s o f amine
(0.25 ppm) l e a d t o l a r g e s u r f a c e coverage and consequently large i n h i b i t o r e f f i c i e n c y ?
A l t h o u g h water m o l e c u l e s do not appear i n Equat i o n 34, i t i s n e v e r t h e l e s s r e a s o n a b l e t o assume t h a t
316
CORROSION C H E M I S T R Y
T a b l e IX
Concentration
Inhibitor
Armand
i n ppm f o r 90-95% P r o t e c t i o n
pH
4.2-4. 5
2.5
pH
6-6.5
pH
7-7.2
10-15
20
10
20
3-4
7-10
20
P e t e r E.
0.2
n. a.
n. a.
Conny E.
0.25-0. 5
Teddy E.
0.2-0. 3
2-3
Kathy
2.5-3
Norman
Dora
Talbot
the i r o n s u l f i d e s u r f a c e i s c o v e r e d w i t h a l a y e r o r
more o f water m o l e c u l e s , because t h e p r o t o n adsorbed
on t h e .surface i r o n s u l f i d e has a tendency t o d i s s o c i a t e and, t h e r e f o r e , needs t o be s o l v a t e d .
The f r e e
a l k y l ammonium i o n and i t s c o u n t e r i o n , e.g.., a b i s u l f i d e i o n , a r e both s o l v a t e d by water.
In the adsorpt i o n s t e p an e l e c t r o n i c a l l y n e u t r a l s u r f a c e complex
i s formed. Thus, a p r o t o n s o l v a t e d by water and i t s
c o u n t e r i o n , t h e b i s u l f i d e i o n , a r e s e t f r e e . The
e s s e n t i a l l y n e u t r a l s u r f a c e complex, does n o t r e q u i r e
s o l v a t i o n and because o f i t s h y d r o p h o b i c n a t u r e w i l l
have a tendency t o d i s p l a c e water m o l e c u l e s from t h e
iron s u l f i d e surface.
Since the desorption process
would a g a i n r e q u i r e s o l v a t i o n o f t h e i r o n s u l f i d e s u r f a c e , i t can no l o n g e r t a k e p l a c e because water molec u l e s a r e permanently d i s p l a c e d from t h e s u r f a c e by
the h y d r o p h o b i c n a t u r e o f t h e s u r f a c e complex. As a
consequence i t can be p r e d i c t e d t h a t an i n h i b i t o r w i t h
a h i g h e r l i p o p h i l i c c h a r a c t e r w i l l be more permanently
adsorbed.
I t i s known t h a t amines become more l i p o p h i l i c ,
or water i n s o l u b l e , w i t h i n c r e a s i n g a l k y l c h a i n l e n g t h .
Thus water s o l u b l e amines such as m o r p h o l i n e ( T a b l e l O )
show no i n h i b i t i o n e f f i c i e n c y a t t h e s e s m a l l concent r a t i o n s because t h e i r a d s o r p t i o n i s as easy as t h e i r
desorption.
Cll
Alkyl_
0
0
5
10
0
81
97
0.5
2
5
30
100
0.5
1
56
97
99
100
0.125
0.25
0.5
0.75
Morpholine
Concen P r o t e c
tion
tration
%
ppm
ft/sec
0.062
Propylenediamine
Concen P r o t e c
tion
tration
%
ppm
HS
Protec
tion
%
1 5
C
Alkyl^
Propylenediamine
Concen P r o t e c
tion
tration
%
ppm
Toluene-Water, S a t u r a t e d w i t h
2 00 ppm C I "
70C
Carbon S t e e l , l i n e a r v e l o c i t y
Concen
tration
ppm
Tallow
Propylenediamine
Conditions :
E f f e c t i v e n e s s of Various A l k y l Propylenediamines
and
Water S o l u b l e Amines as P r o c e s s C o r r o s i o n I n h i b i t o r s
CO
h-*
g-
cl
>
318
CORROSION CHEMISTRY
9. HAUSLER
Corrosion Inhibition
319
320
CORROSION CHEMISTRY
10
Stress-Corrosion Cracking
J. C. SCULLY
Department of Metallurgy, University of Leeds, Leeds LS2 9JT, England
S t r e s s c o r r o s i o n c r a c k i n g i s t h e phenomenon by
which
alloys
fail
by c r a c k i n g when s i m u l t a n e o u s l y
s t r e s s e d and expose
occur at stress l e v e l
cause
failure
in
air.
While the a p p l i c a t i o n o f the
s t r e s s may be
multi-axial,
it
is
n e c e s s a r y f o r it t o
have a
tensile
component and c r a c k i n g will u s u a l l y
occur p e r p e n d i c u l a r l y to
it.
Stress corrosion cracking
r e p r e s e n t s the most h i g h l y
localized
form o f c o r r o s i o n
t h a t is e v e r e n c o u n t e r e d .
W h i l e t h e d i s c u s s i o n in this c h a p t e r i s c o n f i n e d
to
metallic
alloys,
s t r e s s c o r r o s i o n c r a c k i n g is p a r t
o f a l a r g e r range o f phenomena, s i n c e s i m i l a r
failures
o c c u r in n o n m e t a l l i c m a t e r i a l s ,
e.g.,
g l a s s in H2O,
o r g a n i c polymers in p o l a r s o l v e n t s , a l u m i n a in H2O.
F u r t h e r m o r e , w h i l e most o f the d i s c u s s i o n is c o n f i n e d
t o a l l o y s c r a c k i n g in aqueous e n v i r o n m e n t s , s i m i l a r
c r a c k i n g in some alloys o c c u r s in o r g a n i c
liquids,
steam, d r y gases and in b o t h liquid and solid m e t a l s .
The amount o f c o r r o s i v e may be q u i t e s m a l l .
Failures
have been c a u s e d , f o r example, by t h e
perspiration
residue of a single
fingerprint.
Within the confines
o f a s h o r t c h a p t e r , it is n o t p o s s i b l e t o d i s c u s s
e v e r y example o f such
failures.
The d e s c r i p t i o n s b e low a r e c o n f i n e d m a i n l y t o s t r e s s c o r r o s i o n c r a c k i n g i n
aqueous m e d i a .
As an
industrial
problem s t r e s s c o r r o s i o n c r a c k i n g
is o f c o n s i d e r a b l e i m p o r t a n c e .
There is a l o n g h i s t o r y
o f major and minor
failures,
particularly
i n the c h e m i c a l i n d u s t r y and in t h e t r a n s p o r t i n d u s t r y ,
particularly
o f components i n s h i p s and p l a n e s . It is a major
potential
s o u r c e o f failure in t h e n u c l e a r power
i n d u s t r y in w h i c h , f o r example, a u s t e n i t i c s t a i n l e s s
s t e e l s may fail i n h i g h purity water c o n t a i n i n g oxygen
and c h l o r i d e i o n s a t t h e level o f p p b .
0-8412-0471-3/79/47-089-321$07.50/0
1979 American Chemical Society
CORROSION C H E M I S T R Y
322
General Features
of Stress Corrosion
Cracking
10.
SCULLY
Stress-Corrosion
Cracking
323
324
CORROSION
CHEMISTRY
Cracking
10.
SCULLY
Table I
Stress-Corrosion
Cracking
325
A l l o y / E n v i r o n m e n t Systems E x h i b i t i n g
Corrosion Cracking
Stress
Alloy
Environment
Mild steel
Ho
High s t r e n g t h s t e e l s
Aqueous e l e c t r o l y t e s , p a r t i c u
l a r l y when c o n t a i n i n g ^ S
Austenitic
steels
H o t , cone, c h l o r i d e s o l u t i o n s ,
c h l o r i d e - c o n t a m i n a t e d steam
stainless
High N i a l l o y s
H i g h p u r i t y steam
-brass
Ammoniacal
Al alloys
Ti alloys
Aqueous C I , B r and I s o l u
t i o n s ; organic l i q u i d s , N 0^
solutions
solu
Mg a l l o y s
Aqueous C I /CrO^
solutions
Zr a l l o y s
Aqueous C l ~ s o l u t i o n s ;
l i q u i d s , I S) 350C
organic
CORROSION
326
CHEMISTRY
= t
(1)
SCC
10.
SCULLY
Figure 1.
Stress-Corrosion
Cracking
327
Klc
CO
LU
sec
CO
CO
LU
CC
TIME TO FAILURE
328
CORROSION C H E M I S T R Y
10.
SCULLY
Stress-Corrosion
Cracking
329
<
oc
II
CO
oc
DC !=
Co
CO LU
LU
>
DC
hco
Klc
STRESS INTENSITY,
Figure 3.
330
CORROSION C H E M I S T R Y
0
"
icf
100
20
40
60
80
1
1 1
1
1 1
ALLOY 7079-T651
2.5 cm THICK PLATE
CRACK ORIENTATION TL
2 MOLAR KI SOLUTION (HoO+GLYCEROL)
TEMPERATURE 23 *C
POTENTIAL-450 mVvsE +
u
/u
VISCOSITY (CENTIPOISE)
10"
10"
7.2
/
"
24
H^QOOOOOOOO
68
417
/Zdoooo
10"
10"
icf
I
J
1
10
1 1
10
15
20 25
30
N.A.T.O.
Figure 4.
10.
SCULLY
Stress-Corrosion
331
Cracking
STRESS INTENSITY
(kg-mm
20
40
60
80
J
1
J
1
1
3/2
1 0
, 0
-7i
10"
13
I
0
)
100
1
A L L O Y 7178 - T651 + O V E R A G E D AT 1 6 0 C
2.5 cm THICK PLATE
CRACK ORIENTATION T L
SATURATED AQUEOUS NaCI SOLUTION
OPEN CIRCUIT
TEMPERATURE 2 3 C
I
5
I
I
I
I
10
15
20
25
STRESS INTENSITY (MN/m )
I
30
3/2
N.A.T.O.
Figure 5.
CORROSION C H E M I S T R Y
ACIDIC
>
INCREASING pH
POTENTIAL
N.A.T.O.
Figure 6.
SCULLY
Stress-Corrosion
-3
10
20
333
Cracking
-3/2,
100
10
8mNaI
5mKI
10
0.5 m
tz 10
0.2 m
0.1 m
0.05 m
r7
10
0.02 m
0.002m and
DISTILLED WATER
10
10
10
ALLOY 7079-T651
2.5 cm THICK PLATE
CRACK ORIENTATION TL
AQUEOUS IODIDE SOLUTIONS
POTENTIAL- 450 mV vs ,+
TEMPERATURE 23 'C
2
pH* 6
10
10
15
20
25
30
3/2
STRESS INTENSITY (MN/m)
4
N.A.T.O.
Figure 7.
CORROSION C H E M I S T R Y
334
The
Importance o f
Repassivation
SCULLY
Stress-Corrosion
Cracking
335
CORROSION
336
CHEMISTRY
The i m p o r t a n t f a c t o r i s t h e r e p a s s i v a t i o n time s i n c e i t
d e t e r m i n e s f o r how l o n g t h e m e t a l and e n v i r o n m e n t c a n
react together.
F i g u r e 8 c a n be c o n s i d e r e d i n a more g e n e r a l sense.
The p l a s t i c d e f o r m a t i o n t h a t c r e a t e s t h e s l i p s t e p c a n
be c o n s i d e r e d as a s t r a i n - r a t e t h a t , i n t h e most gene r a l way, i s c r e a t i n g new, u n f i l m e d , m e t a l s u r f a c e . I t
w i l l be governed by m e c h a n i c a l and m e t a l l u r g i c a l
f a c t o r s . The p r o c e s s t h a t c a u s e s f i l m f o r m a t i o n i s
e l e c t r o c h e m i c a l and i t w i l l be dependent upon t h e
p o t e n t i a l and a l l o t h e r e l e c t r o c h e m i c a l f a c t o r s .
It is
possible to consider that stress corrosion cracking
o c c u r s when t h e r e i s a c r i t i c a l imbalance between t h e s e
two r a t e p r o c e s s e s one c r e a t i n g f r e s h m e t a l a r e a ,
t h e o t h e r f i l m i n g t h a t a r e a . Thus, t h e s i t u a t i o n dep i c t e d s c h e m a t i c a l l y i n F i g u r e 8 i s n o t merely t h a t o f
a f i l m b r o k e n by a
be e x p e c t e d , and i
a l l o y s and m e t a l s i n e n v i r o n m e n t s t h a t a r e known n o t t o
cause s t r e s s c o r r o s i o n c r a c k i n g . What i s e n v i s a g e d i n
F i g u r e 8 i s a p a r t i c u l a r s e t o f circumstances i n which
p a s s i v a t i o n i s d e l a y e d f o r a narrow range o f t i m e i n t e r vals.
A good example o f t h e i m p o r t a n c e o f r e p a s s i v a t i o n
i s shown i n F i g u r e 9 (8). A t i t a n i u m a l l o y , T i - 5 A l 2.5 Sn, i n t h e form o f t e n s i l e specimens i s s t r a i n e d
d y n a m i c a l l y i n two d i f f e r e n t s o l u t i o n s .
I n aqueous
NaCl s o l u t i o n T i does n o t c o r r o d e and r e p a s s i v a t i o n c a n
be e x p e c t e d t o o c c u r . A t h i g h c r o s s h e a d speeds t h e
t e s t ' i s o v e r i n a v e r y s h o r t t i m e and t h e r e i s no t i m e
for crack i n i t i a t i o n .
A f r a c t u r e i s observed f r a c t o graphically with the elongation t o fracture
and t h e
t e x t u r e t h e same as i n a i r . A t l o w e r c r o s s h e a d speeds
s t r e s s c o r r o s i o n c r a c k propagation occurs w i t h a consequent r e d u c t i o n i n f and c h a r a c t e r i s t i c c l e a v a g e
f r a c t o g r a p h y . A t t h e l o w e s t c r o s s h e a d speeds t h e
r e l a t i v e l y s l o w s t r a i n - r a t e i n d u c e d on t h e s u r f a c e i s
such t h a t r e p a s s i v a t i o n p r e d o m i n a t e s o v e r c r a c k i n g and
no c r a c k p r o p a g a t i o n o c c u r s . F r a c t o g r a p h i c a l l y , a i r
f r a c t u r e i s seen and e i s h i g h . Thus, i n t h e aqueous
s o l u t i o n c r a c k i n g i s c o n f i n e d t o a narrow range o f
speeds. T h i s b e h a v i o r c a n be c o n t r a s t e d w i t h t h a t obs e r v e d f o r t h e same a l l o y exposed t o a CH3OH + 1 v o l . %
cone HC1 m i x t u r e w h i c h i s c o r r o s i v e t o T i and i n w h i c h ,
t h e r e f o r e , no r e p a s s i v a t i o n m i g h t be e x p e c t e d .
At high
c r o s s h e a d speeds t h e same b e h a v i o r i s seen as t h a t
o b s e r v e d i n t h e aqueous s o l u t i o n . A t t h e l o w e s t c r o s s head s p e e d s , however, because no r e p a s s i v a t i o n i s
p o s s i b l e , c r a c k i n g i s o b s e r v e d and because t h e t e s t
t a k e s i n c r e a s i n g l y l o n g t i m e s as t h e c r o s s h e a d speed i s
f
10.
SCULLY
Stress-Corrosion
337
Cracking
16h
Corrosion Science
Figure 9. The rehtionship between elongation-to-failure (e ) and crosshead speed
for a Ti-5Al-2.5Sn
alloy exposed to (1) aqueous NaCl, and (2) a
CH OH/HCl
mixture (8)
f
CORROSION
338
CHEMISTRY
l o w e r e d , the f f a l l s c o n t i n u a l l y . I t can be e x p e c t e d
t h a t t h e c r a c k v e l o c i t y would f a l l u n t i l i t became
s i m i l a r t o the c o r r o s i o n r a t e o f the u n s t r e s s e d a l l o y
w h i c h , i n t h i s example, i s s e l e c t i v e l y i n t e r g r a n u l a r .
I n t h e CH3OH s o l u t i o n no K j
o r any o t h e r t h r e s h o l d
can be a n t i c i p a t e d and i t i s not found. I n the aqueous
solution a K i
can be e x p e c t e d , c o r r e s p o n d i n g t o
t h a t v a l u e o f t h a t r e s u l t s i n so low a s u r f a c e s t r a i n r a t e t h a t r e p a s s i v a t i o n can o c c u r .
This s t r a i n - r a t e
has been c h a r a c t e r i z e d
(SO . K j
i s commonly
o b s e r v e d i n T i a l l o y s exposed t o aqueous c h l o r i d e s o l u
t i o n s . From F i g u r e 9 and a l l t h a t has been d e s c r i b e d ,
i t can be a p p r e c i a t e d t h a t K i
i s not a m a t e r i a l
constant.
F o r a g i v e n a l l o y i t v a r i e s a c c o r d i n g t o the
e n v i r o n m e n t i n w h i c h i t i s measured. I t i s not an
a l l o y t h a t has a t h r e s h o l d ; i t i s the a l l o y + e n v i r o n
ment w h i c h may e x h i b i
The t y p e o f e x p e r i m e n encompasse
Figur
become i n c r e a s i n g l y i m p o r t a n t as a method o f t e s t i n g .
The c o n d i t i o n s a r e s e v e r e , t h e t e s t s a r e r a p i d and t h e
imposed c o n d i t i o n s o f a s l o w s t r a i n - r a t e a r e s i m i l a r t o
those o c c u r r i n g at a crack t i p . For reasons d i s c u s s e d
below, e x p e r i m e n t s s h o u l d be done p o t e n t i o s t a t i c a l l y . A
r e c e n t c o n f e r e n c e (1_0) was d e v o t e d t o t h e c o n s t a n t
e x t e n s i o n r a t e t e s t , o r g a n i z e d by A.S.T.M.
The i m p o r t a n c e o f r e p a s s i v a t i o n and t h e i n t e r a c
t i o n of t h i s process with a s t r a i n i n g metal surface
p r o b a b l y c o n s t i t u t e s t h e e s s e n s e o f many s t r e s s c o r r o
s i o n c r a c k i n g mechanisms. S i n c e r e p a s s i v a t i o n i s an
e l e c t r o c h e m i c a l phenomenon, i t m i g h t be e x p e c t e d t h a t
the necessary imbalance to achieve s t r e s s c o r r o s i o n ,
r e f e r r e d t o above, w i l l o c c u r o n l y o v e r a narrow range
o f p o t e n t i a l , c o r r e s p o n d i n g t o a narrow range o f r e
p a s s i v a t i o n r a t e s . Such r a t e s can be e x p e c t e d t o
change m a r k e d l y i n t h o s e r e g i o n s o f p o t e n t i a l where t h e
p a s s i v e range changes t o a p i t t i n g range o r t o an
a c t i v e r a n g e , i . e . , where the f i l m i s c h a n g i n g from
b e i n g t h e s t a b l e s u r f a c e c o n f i g u r a t i o n t o where i t i s
an u n s t a b l e c o n f i g u r a t i o n . The zones o f p o t e n t i a l
where c r a c k i n g m i g h t be e x p e c t e d a r e shown i n F i g u r e 10.
There a r e examples i n t h e l i t e r a t u r e o f c r a c k i n g
o c c u r r i n g i n such r e g i o n s (2^) . The s i t e o f t h e s e
r e g i o n s depends upon a number o f e l e c t r o c h e m i c a l
f a c t o r s . S t r e s s c o r r o s i o n f a i l u r e o c c u r s under openc i r c u i t c o n d i t i o n s when t h e c o r r o s i o n p o t e n t i a l o f an
a l l o y l i e s w i t h i n the p o t e n t i a l range f o r c r a c k i n g o f
t h a t a l l o y i n the p a r t i c u l a r e n v i r o n m e n t . Systems de
s c r i b e d i n Table I f a l l i n t o t h i s category.
I t must be
emphasized t h a t under c o n t r o l l e d p o t e n t i a l c o n d i t i o n s
s t r e s s c o r r o s i o n f a i l u r e may o c c u r i n a l l o y s exposed t o
s
10.
SCULLY
Stress-Corrosion
Cracking
339
i - i
(i - i )t~
1
(2)
i = i
max
exp(-gt)
(3)
340
CORROSION C H E M I S T R Y
where 3 i s a c o n s t a n t , and i
i s t h e i n i t i a l maximum
value of the c u r r e n t a f t e r d e s t r u c t i o n of the f i l m .
When a f i l m i s b r o k e n , t h e i n i t i a l monolayer a d s o r b s
w i t h i n 20-50 ms ( 1 2 ) . The measured c u r r e n t i s made up
p a r t l y o f d i s s o l u t i o n and p a r t l y o f f i l m f o r m a t i o n ,
t h e r a t i o between them f a l l i n g w i t h t i m e .
m
S t r e s s C o r r o s i o n Mechanisms
The c i r c u m s t a n c e s under w h i c h t h e f i l m i s b r o k e n
and r e p a i r e d have been d e s c r i b e d a t l e n g t h s i n c e t h e y
c o n t r o l the subsequent r e a c t i o n t i m e d u r i n g w h i c h t h e
c r a c k a c t u a l l y grows. I t has been s t r o n g l y emphasized
s i n c e u n l e s s t h i s p o i n t i s u n d e r s t o o d c o n f u s i o n can
a r i s e because t h e a l t e r a t i o n o f an e x p e r i m e n t a l v a r i a b l e , e.g., p o t e n t i a l o r pH may have a g r e a t e r e f f e c t
on r e p a s s i v a t i o n t h e
less that p o s s i b i l i t
i n t e r p r e t a t i o n i s l i k e l y t o a r i s e on o c c a s i o n .
An
example i s g i v e n below w i t h r e s p e c t t o t h e e f f e c t o f
c a t h o d i c p o l a r i z a t i o n on s t r e s s c o r r o s i o n c r a c k i n g o f
T i a l l o y s i n aqueous c h l o r i d e s o l u t i o n s . I t i s a
g e n e r a l p o i n t and i s not o f t e n emphasized.
The Mechanism o f
Cracking
10.
SCULLY
Stress-Corrosion
341
Cracking
o c c u r r e d by p r e f e r e n t i a l d i s s o l u t i o n o v e r a narrow p r e f e r r e d f r o n t a l o n g a p a t h t h a t was p r e - e x i s t i n g ( 1 3 ) .
T h i s was p r o b a b l y t r u e f o r t h e p a r t i c u l a r system Eing
i n v e s t i g a t e d , but s i n c e c r a c k i n g occurs i n a l l o y s which
most c e r t a i n l y do n o t have a p r e - e x i s t i n g p a t h , more
r e c e n t i d e a s have a t t e m p t e d t o e x p l a i n t h e l o c a l i z e d
c o r r o s i o n by a p r o c e s s i n v o l v i n g r e p a s s i v a t i o n o f the
crack s i d e s , thus c o n c e n t r a t i n g the c u r r e n t a t the t i p
where v a r i o u s forms o f d i r e c t i o n a l l o c a l i z e d d i s s o l u t i o n occurs.
The p o s s i b l e p r e f e r e n t i a l c o r r o s i o n o f
d i s l o c a t i o n s p i l e d up a t t h e c r a c k t i p as a r e s u l t o f
t h e a c t i n g s t r e s s has been c o n s i d e r e d a t l e n g t h o v e r
many y e a r s .
The s t r a i n energy a s s o c i a t e d w i t h such
pile-ups of d i s l o c a t i o n s provides l i t t l e a d d i t i o n a l
driving force for dissolution
As a r e s u l t a t t e n t i o n has f o c u s e d on m i n u t e c o m p o s i t i o n a l changes i n a
metal l a t t i c e o c c u r r i n
w h i c h may cause s i g n i f i c a n
d i s s o l u t i o n on an a t o m i s t i c s c a l e (1_5) These i d e a s
were o r i g i n a l l y a s s o c i a t e d w i t h e l e c t r o n m i c r o s c o p e
o b s e r v a t i o n s o f t h e edges o f e l e c t r o p o l i s h e d t h i n
f o i l s where r e g i o n s o f d i s l o c a t i o n p i l e - u p s showed,
v e r y d i r e c t i o n a l d i s s o l u t i o n ( 1 6 ) . Such p r o c e s s e s
g i v e r i s e t o u n u s u a l m o r p h o l o g i c a l e f f e c t s ( V7) and
may c a u s e t u n n e l c o r r o s i o n (1_8) .
A t some s t a g e i t i s n e c e s s a r y t o know whether t h e
c u r r e n t f l o w i n g i s s u f f i c i e n t to account f o r the maximun v e l o c i t y o f t h e c r a c k , a r e q u i r e m e n t sometimes
r e f e r r e d t o as f a r a d a i c e q u i v a l e n c e .
From t h e a p p l i c a t i o n o f F a r a d a y ' s laws and assuming t h a t d i s s o l u t i o n
a c c o u n t s f o r 10095 o f t h e c r a c k p r o p a g a t i o n t h e n :
f
where = c r a c k v e l o c i t y , J - c h e m i c a l e q u i v a l e n t , F =
the F a r a d a y , = a l l o y d e n s i t y and i = d i s s o l u t i o n
current density.
F o r a number o f s t r e s s c o r r o s i o n c r a c k i n g s y s t e m s ,
e.g., m i l d s t e e l , a u s t e n i t i c s t a i n l e s s s t e e l s , measured
b a r e s u r f a c e c u r r e n t d e n s i t i e s appear t o be o f t h e
r i g h t o r d e r o f magnitude (_3) F o r o t h e r a l l o y s , e.g.,
T i a l l o y s , i n w h i c h t h e c r a c k v e l o c i t i e s a r e v e r y much
h i g h e r t h a n i n s t e e l s , much h i g h e r c u r r e n t d e n s i t i e s
are r e q u i r e d , of the order r e q u i r e d f o r e l e c t r o c h e m i c a l
m a c h i n i n g . T h e i r e x i s t e n c e has been c l a i m e d (2).
R e f e r e n c e t o F i g u r e 8, t o g e t h e r w i t h t h e c o n c e p t
of r e p a s s i v a t i o n , i n d i c a t e s d u r i n g the propagation of
c r a c k s t h e c u r r e n t s h o u l d c o n s i s t o f a number o f s u c c e s
s i v e t r a n s i e n t s . One i s shown i n F i g u r e 11, w h i c h
m i g h t be t a k e n t o c o r r e s p o n d t o t h e sequence o f e v e n t s
CORROSION C H E M I S T R Y
342
t
Figure 11. A general schematic of the current transient during the sequence of
events drawn in Figure 8. The hatched area represents the total charge flowing.
10.
SCULLY
Stress-Corrosion
Cracking
343
d e s c r i b e d i n F i g u r e 8. What i s i m p o r t a n t i s t h e t o t a l
amount o f c o r r o s i o n , r e p r e s e n t e d by t h e c h a r g e , w h i c h
i s h a t c h e d i n F i g u r e 11.
I t has been argued (9) t h a t
s t r e s s c o r r o s i o n c r a c k p r o p a g a t i o n o c c u r s as a r e s u l t
o f t h e passage o f a c o n s t a n t c h a r g e Q i / w h i c h t h e n
i n i t i a t e s t h e n e x t i n c r e m e n t o f c r a c k growth.
Crack
a r r e s t occurs i f r e p a s s i v a t i o n occurs before Q i
has
passed.
From such c o n s i d e r a t i o n s , w i t h i n t e g r a t i o n o f
the a p p r o p r i a t e r e p a s s i v a t i o n r a t e equation over the
t i m e l i m i t s between s u c c e s s i v e s l i p s t e p e v e n t s , i t
has been p o s s i b l e t o d e r i v e t h e shape o f t h e c o r r o s i o n
c u r r e n t : t i m e c u r v e s o f Stages I and I I shown i n F i g ure 3 (9) .
m
Hydrogen E m b r i t t l e m e n t .
A t t h e c r a c k t i p i n many
a l l o y s l o c a l a c i d i t y and low p o t e n t i a l s e n s u r e t h a t
hydrogen i o n d i s c h a r g
T i , Zr a l l o y s and f o
p l a i n c a r b o n s t e e l s and a l s o f o r Mg a l l o y s i n w h i c h
the c r a c k t i p s o l u t i o n i s a l k a l i n e .
The q u e s t i o n i s
whether any o f t h e s e a l l o y s c r a c k as a r e s u l t o f H
a d s o r p t i o n w h i c h o c c u r s a f t e r n e u t r a l i z a t o n has
occurred
H +
( 5 )
= ads
and b e f o r e d e s o r p t i o n has o c c u r r e d by
H
ads
H +
ads
either
= 2
H
( 6 )
or
ads - 2
H
( 7 )
i n a c i d i c s o l u t i o n s and t h e e q u i v a l e n t r e a c t i o n s a p p r o p r i a t e f o r a l k a l i n e s o l u t i o n s i n t h e c a s e o f Mg a l l o y s .
Some p r o p o r t i o n o f Hads w i l l be absorbed by t h e a l l o y
i n any s i t u a t i o n . An i m p o r t a n t r a t i o i s H (absorbed/
H (evolved) and t h i s i s l i k e l y t o be v e r y s e n s i t i v e t o
c a t h o d i c p o i s o n s i n t h e s o l u t i o n and t o c e r t a i n e l e ments w i t h i n t h e a l l o y .
C r a c k i n g i n A l and T i a l l o y s
and i n h i g h s t r e n g t h s t e e l s i s a c c e l e r a t e d by t h e
presence of c a t h o d i c poisons. T h i s i s very s t r o n g
evidence t h a t H p l a y s a r o l e i n the c r a c k i n g process.
A f t e r e n t e r i n g t h e m e t a l i t may c o l l e c t i n s i d e
t r a p s and cause d e c o h e s i o n .
I t may combine around i n c l u s i o n s and cause l o c a l r e g i o n s o f v e r y h i g h p r e s s u r e ,
r e s u l t i n g i n b l i s t e r i n g and p o s s i b l y even f i s s i o n i n g .
I n T i and Zr a l l o y s i t may form h y d r i d e s w h i c h i n t e r a c t w i t h t h e l a t t i c e , p r o m o t i n g c l e a v a g e by impeding
344
CORROSION CHEMISTRY
t h e movement o f d i s l o c a t i o n s .
The e f f e c t s o f H on t h e
m e c h a n i c a l p r o p e r t i e s o f m e t a l s i s a v e r y complex subj e c t and no a t t e m p t has been made o t h e r t h a n t o i n d i cate s e v e r a l p o s s i b l e g e n e r a l e f f e c t s . Another area
o f some d i f f i c u l t y i s t h a t t h e d i f f u s i o n r a t e s o f H
o f t e n appear t o be t o o s l o w t o a c c o u n t f o r t h e obs e r v e d c r a c k p r o p a g a t i o n r a t e s . The r e c o n c i l i n g o f
these c o n t r a r y observations c a l l s f o r c o n s i d e r a b l e
care.
S t r e s s S o r p t i o n . T h i s mechanism supposes t h a t
t h e r e a c t i o n between a s p e c i e s i n t h e environment and
t h e m e t a l atoms a t t h e c r a c k t i p can cause a r e d i s t r i b u t i o n o f e l e c t r o n s i n t h e o r b i t s o f t h e atoms so t h a t
t h e bond between them i s weakened ( 1 9 ) . I t i s n o t
p o s s i b l e t o c i t e e x p e r i m e n t a l d a t t K a t would
t
t h i s concept f o r th
absence may m e r e l y r e f l e c t t h e d i f f i c u l t i e s o f o b t a i n
i n g such d a t a .
C o r r o s i o n F i l m F r a c t u r e . A t v a r i o u s t i m e s i t has
been s u g g e s t e d t h a t t h e f r a c t u r e o f c o r r o s i o n p r o d u c t
f i l m s a t t h e c r a c k t i p w i t h t h e i r subsequent r e f o r m a t i o n c o n s t i t u t e s t h e main f o r w a r d movement o f t h e
c r a c k . The e v i d e n c e f o r such a mechanism i s n o t t e r r i b l y c l e a r . T h i c k f i l m formed on s t e e l s and b r a s s e s ,
f o r example, may form by p r e c i p i t a t i o n from s o l u t i o n
r a t h e r t h a n by d i r e c t c o r r o s i o n o f t h e m e t a l t o a
s o l i d compound. Where t h i s o c c u r s , t h e i n i t i a l d i s s o l u t i o n t h a t p r e c e d e s the p r e c i p i t a t i o n would appear
to f i t i n t o the a c t i v e path category. I f f r a c t u r e of
t h e p r e c i p i t a t e d f i l m i s n e c e s s a r y , t h e n i t becomes a
m a t t e r o f c h o i c e whether t h i s c o n s t i t u t e s a s e p a r a t e
c a t e g o r y o r a s p e c i a l c a s e o f t h e a c t i v e p a t h mechanism.
F o r m i l d s t e e l and b r a s s e s t h i s t y p e o f e x p l a n a t i o n i s
commonly a s s o c i a t e d w i t h i n t e r g r a n u l a r c r a c k i n g . What
needs t o be e x p l a i n e d i s why p r e f e r e n t i a l c o r r o s i o n o f
t h e g r a i n boundary o c c u r s . O f t e n t h i s i s a t t r i b u t e d
t o t h e p r e s e n c e w i t h i n t h e g r a i n boundary o f an u n i d e n t i f i e d element i n s o l i d s o l u t i o n w h i c h a l t e r s t h e
dissolution kinetics.
I t i s a requirement t h a t the
f i l m i s s u f f i c i e n t l y p r o t e c t i v e t o reduce t h e r e a c t i o n
r a t e t o an i n s i g n i f i c a n t v a l u e so t h a t i t s f r a c t u r e i s
n e c e s s a r y f o r t h e c r a c k p r o p a g a t i n g r e a c t i o n t o be r e initiated.
On b r a s s , f o r example, i n ammoniacal
s o l u t i o n s t h i c k f i l m s form o v e r a wide range o f pH b u t
i t i s o n l y o v e r a narrow range t h a t t h i s t y p e o f mechanism may a p p l y (2), c o r r e s p o n d i n g t o t h e f o r m a t i o n o f
a relatively protective film.
10.
SCULLY
Stress-Corrosion
345
Cracking
346
CORROSION
CHEMISTRY
w h i c h t h e r o l e o f t h e p o t e n t i a l on t h e r e p a s s i v a t i o n
p r o c e s s i s more i m p o r t a n t t h e n i t s r o l e on t h e mecha n i s t i c r e a c t i o n o c c u r r i n g on t h e u n f i l m e d m e t a l s u r face.
I n very strong a c i d s o l u t i o n s , i n which t h e
f i l m i s u n s t a b l e and r e p a s s i v a t i o n w i l l n o t t h e r e f o r e
o c c u r , c a t h o d i c p o l a r i z a t i o n has no e f f e c t on c r a c k
v e l o c i t y (2J_, 2) . Where an a c t i v e p a t h mechanism i s
o p e r a t i v e , c a t h o d i c p o l a r i z a t i o n w i l l always lengthen
t f and l o w e r t h e c r a c k v e l o c i t y , e v e n t u a l l y c a u s i n g
crack a r r e s t .
Some a l l o y s e x h i b i t r e v e r s i b l e e m b r i t t l e m e n t ,
e.g., Mg (22), A l (22), T i (24) and Zr (25) a l l o y s .
F o r each a l T o y , e x p e r i m e n t s H v e been done i n w h i c h
specimens were exposed u n s t r e s s e d , t o s o l u t i o n s t h a t
can cause c r a c k i n g , under a v a r i e t y o f c o n d i t i o n s . I f
b r o k e n i n a i r i m m e d i a t e l y a f t e r removal from t h e s o l u t i o n , specimens e x h i b i t e
f
fracture surfaces c h a r a c t e r i s t i
cracking.
I f l a p s e o f t i m e o c c u r s between removal
from s o l u t i o n and s t r e s s i n g , specimens e x h i b i t e d
i n c r e a s e d v a l u e s o f Zf and d e c r e a s e d amounts o f s t r e s s
corrosion-type fracture with increasing length of
lapse o f time.
T h i s b e h a v i o r i s c h a r a c t e r i s t i c o f hydrogen e m b r i t t l e m e n t f r a c t u r e and has been i n t e r p r e t e d as
such f o r t h e f o u r t y p e s o f a l l o y s d e s c r i b e d .
These
are simple b u t very c l e a r experiments.
Preventative
Measures
I t i s c l e a r l y i m p o r t a n t t o know how t o a v o i d , o r
at l e a s t minimize the incidence o f the occurrence o f
such a w i d e s p r e a d t y p e o f f a i l u r e .
The c h o i c e s a r e r e l a t i v e l y few i n number and c a n be c o n v e n i e n t l y c o n s i d e r e d under t h e h e a d i n g s o f t h e words t h a t make up
name o f t h e problem.
S t r e s s . From F i g u r e s 1 and 2 i t has a l r e a d y been
s t a t e d t h a t s t r e s s c o r r o s i o n c r a c k i n g c a n be r e d u c e d
o r a b o l i s h e d a l t o g e t h e r by r e d u c i n g t h e s t r e s s l e v e l ,
d e p e n d i n g upon whether t h e system e x h i b i t s a t h r e s h o l d
stress or value.
I n p r a c t i c e t h i s may mean e n s u r i n g
t h a t components a r e s t r e s s r e l i e f a n n e a l e d , e.g.,
brass tubes a f t e r drawing, o r t h a t welds a r e given
post-weld heat treatements, since i t i s r e s i d u a l
s t r e s s t h a t i s o f t e n t h e p r o b l e m . To m i n i m i z e f a i l u r e s
i n s t e e l tube assemblies h a n d l i n g sour o i l w e l l s , f o r
example, i t i s recommended t h a t a l l w e l d s be k e p t be
low a c e r t a i n h a r d n e s s l e v e l (26). The h a r d n e s s c a n
be measured i n s i t u .
Design can help i n reducing
o p e r a t i n g s t r e s s l e v e l s a l s o . C r i t i c a l f l a w d e p t h has
10.
SCULLY
Stress-Corrosion
Cracking
347
a l s o been m e n t i o n e d . C o n t r o l l i n g f l a w d e p t h i s e a s i e r
t o d e s c r i b e t h e n t o a c h i e v e b u t c a r e s h o u l d be t a k e n
t o p r e v e n t l a r g e f l a w d e p t h s , p a r t i c u l a r l y where a
K
l e v e l exists.
There a r e examples o f t a n k s b e i n g
operated c o n t a i n i n g l i q u i d s t h a t cause s t r e s s c o r r o
s i o n c r a c k i n g w i t h r e s i d u a l and o p e r a t i n g s t r e s s e s
c o n t r o l l e d so t h a t K j
i s n o t exceeded. F o r h i g h
strength a l l o y s overtempering o r overaging w i l l often
g i v e a much more a c c e p t a b l e l i f e t i m e a t t h e c o s t o f a
lower a l l o y s t r e n g t h .
Such c o n s i d e r a t i o n s c a l l f o r a
b a l a n c e between t h e r e q u i r e m e n t s o f t h e p l a n t o p e r a t o r
and t h e demands o f t h e p l a n t d e s i g n e r .
I
Corrosion.
R e d u c i n g c o r r o s i o n by t h e u s e o f i n h i b i t o r s w i l l commonly r e d u c e o r even e l i m i n a t e s t r e s s
c o r r o s i o n c r a c k i n g , p o s s i b l y by moving t h e c o r r o s i o n
p o t e n t i a l outside th
these are r e f e r r e d t
i f t h i s i s t h e i r sole function, cracking w i l l occur
i f t h e c o r r o s i o n p o t e n t i a l moves back i n t o t h e c r a c k
i n g r a n g e . The c o n t r a s t i s made w i t h s a f e i n h i b i t o r s
w h i c h reduce o r p r e v e n t c r a c k i n g even i f t h e c o r r o s i o n
p o t e n t i a l i s i n the c r a c k i n g range. I n p r a c t i c e t h e
use o f i n h i b i t o r s o f e i t h e r c a t e g o r y may be r e s t r i c t e d
by (1) s o l u b i l i t y p r o b l e m s , (2) economic a s p e c t s and
(3) p r a c t i c a l i t y l i m i t s .
Many f a i l u r e s o c c u r i n steam
o r under c o n d e n s a t i o n c o n d i t i o n s .
I n both o f these
cases the t r a n s p o r t o f i n h i b i t o r s t o s i t e s o f crack
i n i t i a t i o n i s n o t f e a s i b l e . W i t h o t h e r s y s t e m s , how
e v e r , q u i t e s m a l l a d d i t i o n s o r changes may e l i m i n a t e
p r o b l e m s , e.g., 1-2% H 0 t o CH3OH/HCI m i x t u r e s , u s i n g
impure 0/| r a t h e r t h a n pure N2O4, b o t h f o r T i a l l o y s .
C a t h o d i c p r o t e c t i o n may a l s o a c h i e v e t h e same e f f e c t
o f a l o w e r e d c o r r o s i o n r a t e . As w i t h i n h i b i t o r s i t s
use i s l i m i t e d and f o r much t h e same r e a s o n s . M e n t i o n
must be made o f c u r r e n t s t r e s s c o r r o s i o n c r a c k i n g
problems i n gas t r u n k t r a n s m i s s i o n l i n e s i n t h e U.S.A.
and e l s e w h e r e . A number o f f a i l u r e s have o c c u r r e d i n
these l i n e s as a r e s u l t o f c e r t a i n carbonate/bicarbo
nate mixtures being generated i n the a l k a l i n e l i q u i d
a d j a c e n t t o t h e p i p e by t h e a p p l i c a t i o n o f c a t h o d i c
p r o t e c t i o n (*0 . T h i s i s i n no way an argument a g a i n s t
the u s e o f s u c h a p r o t e c t i o n method. I t i s m e r e l y a
w a r n i n g about p o s s i b l e e f f e c t s . These f a i l u r e s a l s o
u n d e r l i n e some o f t h e d i f f i c u l t
technical/economic
d e c i s i o n s a t t e n d a n t upon such f a i l u r e s .
There a r e two
q u i t e d i f f e r e n t q u e s t i o n s t o be s e t t l e d :
(1) what c a n
be done t o r e d u c e t h e i n c i d e n c e o f c r a c k i n g i n e x i s t
i n g p i p e l i n e s ? and (2) what c a n be done t o r e d u c e t h e
p o s s i b i l i t y o f c r a c k i n g i n p i p e l i n e s t o be i n s t a l l e d
i n the future?
B o t h t e c h n i c a l l y and e c o n o m i c a l l y t h e
2
American Chemical
Society Library
1155 16th St. N. W.
Washington,
D. C. 20036
In Corrosion Chemistry; Brubaker, G., et al.;
348
CORROSION C H E M I S T R Y
q u e s t i o n s pose a number o f d i f f e r e n t p r o b l e m s . T h i s i s
i n some ways t y p i c a l o f a l l p r e v e n t a t i v e measures.
There a r e many f a c t o r s t o be c o n s i d e r e d b e f o r e s e t t l i n g
upon an a c c e p t a b l e s o l u t i o n .
The use o f p a i n t i n g s and c o a t i n g s must be ment i o n e d . They a r e u s u a l l y a p p l i e d f o r o t h e r r e a s o n s
t o r e d u c e the r a t e o f g e n e r a l c o r r o s i o n and the c o s t o f
c a t h o d i c p r o t e c t i o n . To the e x t e n t t h e y a r e s u c c e s s f u l ,
they w i l l f r e q u e n t l y minimize the i n c i d e n c e of s t r e s s
c o r r o s i o n f a i l u r e s . C o a t i n g s a l w a y s have f a u l t s i n
them and depending upon c o a t i n g s a l o n e i s u s u a l l y unw i s e . I t i s i n t h e i r use w i t h systems o f c a t h o d i c
p r o t e c t i o n t h a t t h e y a r e t o be j u d g e d , t o g e t h e r w i t h
a b i l i t y t o w i t h s t a n d s e r v i c e c o n d i t i o n s , e.g., c o n t i n ual temperature f l u c t u a t i o n s .
I n c r e a s i n g the c o r r o s i o n r a t e might appear t o be
a rather d r a s t i c counte
sion cracking.
Sinc
c o r r o s i o n , e x t e n d i n g c o r r o s i o n o v e r t h e whole o f the
s u r f a c e w i l l u s u a l l y l e s s e n t h e p r o b a b i l i t y o f such
failures.
T h i s a p p r o a c h i s u n l i k e l y e v e r t o be a
permanent remedy. I t i s employed i n making up m i x t u r e s
c o n t a i n i n g HC1 t o c l e a n a u s t e n i t i c s t a i n l e s s s t e e l
p a r t s i n c h e m i c a l p l a n t : t h e c o r r o s i o n r a t e i s maint a i n e d > 10 mpy
(27).
Cracking.
P o s s i b l e e f f e c t s of plane s t r e s s /
p l a n e s t r a i n t r a n s i t i o n s have a l r e a d y been i n d i c a t e d .
The e f f e c t i v e n e s s o f t h e s t r a i n - r a t e i n p r o m o t i n g
c r a c k i n g can be i m p o r t a n t where e n g i n e e r i n g s t r u c t u r e s
a r e s u b j e c t t o p e r i o d i c s t r a i n i n g , e.g., p i p e l i n e s . I t
i s p o s s i b l e t o have i n t e r r u p t e d l o a d i n g s t r e s s c o r r o s i o n c r a c k i n g i n w h i c h c a s e the s t r e s s combines t o
produce a s t r a i n t r a n s i e n t t h a t g e n e r a t e s a c r a c k
i n c r e m e n t . T h i s i s d i f f e r e n t from c o r r o s i o n f a t i g u e
and t h e i n t e r f a c e between the two has been d i s c u s s e d
(28). I n t e r r u p t e d l o a d i n g s t r e s s c o r r o s i o n c r a c k i n g
can be o f p a r t i c u l a r i m p o r t a n c e where t h e system exh i b i t s a t h r e s h o l d under c o n s t a n t l o a d l a b o r a t o r y
t e s t i n g conditions. A small f l u c t u a t i o n i n load
(1-2%) can reduce t h e t h r e s h o l d by 50% (3).
Unless
laboratory t e s t s simulate accurately service condit i o n s , c o n f u s i o n and a l a c k o f c o n f i d e n c e a r e l i k e l y
t o ensue.
S i n c e a t e n s i l e component o f s t r e s s i s r e q u i r e d ,
s t r e s s c o r r o s i o n c r a c k i n g can be p r e v e n t e d by p u t t i n g
t h e s u r f a c e o f a component i n t o c o m p r e s s i o n , e.g., by
short-peening.
Where t h i s i s p r a c t i c a b l e , i t i s
d e s i r a b l e . The t r e a t m e n t needs t o be a p p l i e d u n i f o r m ly.
I t w i l l n o t be e f f e c t i v e i f p i t t i n g o c c u r s on t h e
compressed l a y e r .
10. SCULLY
Stress-Corrosion Cracking
349
350
CORROSION CHEMISTRY
4.
11
0-8412-0471-3/79/47-089-351$10.00/0
1979 American Chemical Society
CORROSION
352
CHEMISTRY
WILKES
Industrial
Figure 1.
Problems
N a t u r a l Draft or C h i m n e y T o w e r
Figure 2.
Figure 3.
GO
ce
I*
CORROSION C H E M I S T R Y
356
DIE. S C L
PC
ENGINE
CYCUE.
COOLING TOWER
1
CLOSED
^ HEAT
EXCHANGER
WATER
CYCLE.
R E C I R CWLATION
Figure 4.
OIL.
COOL.K.K
OPftN
D I E S E L E-MGirSEl
PUMP
CYCLE
COOLING
TOWER
Z.
152
RECIRCULATING
CUOStO
Figure 5.
r*UMP
CYCLE
11.
WILKES
Industrial
Problems
357
358
CORROSION
CHEMISTRY
11.
WILKES
Industrial
Problems
359
w a t e r c o m p l e t e l y s t a b l e a n d n o n - c o r r o s i v e a t 70 d e g r e e s
F . m i g h t become e i t h e r s c a l i n g or c o r r o s i v e , when p a s s ed t h r o u g h an e x c h a n g e r where t e m p e r a t u r e was r a i s e d t o
130 d e g r e e s F .
T h e r e f o r e , c o r r o s i o n c o n t r o l by S a t u r a t i o n Index adjustment alone r a r e l y o f f e r e d a p r a c t i c a l
solution.
The n e x t a p p r o a c h t o s c a l e c o n t r o l was r e d u c t i o n
o f p o t e n t i a l c a l c i u m c a r b o n a t e by a c i d i f i c a t i o n o f
makeup w a t e r .
S u l f u r i c a c i d i s added to reduce
alkal i n i t y and c o n v e r t most of the c a l c i u m b i c a r b o n a t e
to calcium s u l f a t e .
C a l c i u m s u l f a t e i s s o l u b l e up
to about 1,700 p a r t s per m i l l i o n i n c o o l i n g w a t e r s a t
o r d i n a r y temperatures, whereas c a l c i u m carbonate
solub i l i t y i s l e s s t h a n 30 p p m .
So by s l i g h t l y a c i d i f y i n g
m a k e u p w a t e r we g r e a t l y r e d u c e t e n d e n c y f o r c a l c i u m
carbonate deposition.
The c a l c i u m s u l f a t e l e v e l i n
concentrated cooling
adjustmentmanually
O b v i o u s l y w h e n we d e l i b e r a t e l y a d d s u l f u r i c a c i d
t o c o o l i n g w a t e r , we r e d u c e a l k a l i n i t y a n d a l s o d e p r e s s
pH.
T h i s increases c o r r o s i v i t y of the c i r c u l a t i n g
water, which i s saturated with dissolved oxygen,
cont a c t s many d i s s i m i l a r m e t a l s , a n d i s e l e v a t e d i n t e m p e r ature.
In the process of a c i d i f i c a t i o n to prevent
s c a l e d e p o s i t i o n so heat t r a n s f e r equipment w i l l
funct i o n e f f i c i e n t l y , we k n o w i n g l y b u i l d i n a d d e d
corrosion
f a c t o r s and i n c r e a s e c o r r o s i o n c o n t r o l d i f f i c u l t y .
Now
we n e e d t o f i n d c o r r o s i o n i n h i b i t o r c o m b i n a t i o n s w h i c h
w i l l be p r a c t i c a l f o r use i n i n d u s t r i a l s y s t e m s ;
can
be t o l e r a t e d f r o m t h e v i e w p o i n t s o f t o x i c i t y and p o l l u t i o n c o n t r o l s and w i l l e f f e c t i v e l y p r o t e c t these
multim e t a l c i r c u i t s d u r i n g t h e i r n o r m a l s e r v i c e l i f e o f 20 t o
30 y e a r s .
One e a r l y a p p r o a c h f o r c o o l i n g w a t e r c o r r o s i o n i n h i b i t i o n was t h e u s e o f i n o r g a n i c p o l y p h o s p h a t e s .
These
c o m p l e x m o l e c u l a r l y d e h y d r a t e d p h o s p h a t e s came i n t o
widespread i n d u s t r i a l use b e g i n n i n g i n the
1930's.
T h e r e i s some d i s a g r e e m e n t a b o u t how p o l y p h o s p h a t e s
f u n c t i o n as c o r r o s i o n i n h i b i t o r s .
Generally accepted
t h e o r y i s t h a t , i n an a e r a t e d s y s t e m , they cause f o r m a t i o n of a p r o t e c t i v e surface f i l m which contains both
i r o n o x i d e and p h o s p h o r u s , perhaps an i r o n
phosphate.
P o l y p h o s p h a t e s w i l l not work i n a system t h a t i s d e v o i d
of oxygen, nor i n a stagnant system.
Polyphosphate corr o s i o n i n h i b i t i o n required flow, to replace the
ironphosphate f i l m as f a s t as i t i s removed or
depleted.
P o l y p h o s p h a t e s are u n s t a b l e and s u b j e c t to problems i n
h i g h t e m p e r a t u r e c i r c u i t s , o r where pH f l u c t u a t i o n
occurs.
E c o l o g i c a l considerations also are involved,
because of p o s s i b l e p o l l u t i o n c o n t r i b u t i o n s of r e s i d u a l
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CORROSION
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phosphates i n c o o l i n g tower d i s c h a r g e s to
surface
waterslakes, streams,
etc.
In the e a r l y 1950's, combinations of a l k a l i
chromate (an a n o d i c i n h i b i t o r ) and p o l y p h o s p h a t e
(generally
a c c e p t e d as c a t h o d i c )
came i n t o p r o m i n e n c e f o r
cooling
system c o r r o s i o n i n h i b i t i o n .
The c o m b i n a t i o n o f
chromate w i t h phosphates proved h i g h l y e f f i c i e n t i n c o m p a r i son w i t h s t r a i g h t phosphate or s t r a i g h t chromate, and
c o u l d be u s e d a t s u b s t a n t i a l l y l o w e r
concentrations.
For example, a c o o l i n g system t h a t had been t r e a t e d
w i t h 4 0 0 ppm a l k a l i c h r o m a t e , w i t h c o o l i n g w a t e r p H
a d j u s t e d t o t h e n e u t r a l r a n g e o f pH 7 t o 8, c o u l d be
e q u a l l y w e l l p r o t e c t e d by a c o m b i n a t i o n o f c h r o m a t e and
p o l y p h o s p h a t e , w i t h c h r o m a t e c o n c e n t r a t i o n o f 30 t o 40
p p m , a n d p h o s p h a t e a t 10 t o 20 p p m .
In the l a t e 1950's, chromate-phosphate
systems
incorporating zinc a
introduced, followe
out phosphate.
Using c h r o m a t e - z i n c , or
polyphosphatec h r o m a t e - z i n c i n h i b i t o r s i t was p o s s i b l e t o c u t w o r k i n g
concentrations s t i l l further.
I t was n e c e s s a r y
to
c o n t r o l pH t o make t h i s i n h i b i t o r s y s t e m f u n c t i o n
effectively.
A t i n c r e a s e d pH ( a b o v e pH 7 . 5 ) t e n d e n c y
for
z i n c l o s s by p r e c i p i t a t i o n i n c r e a s e s .
F u r t h e r m o r e , pH
r i s e may c a u s e h e a t e x c h a n g e r s t o b e c o m e f o u l e d by z i n c
hydroxide s l i m e s or z i n c phosphate.
We h a v e m e n t i o n e d r e l a t i o n s h i p o f s c a l i n g a n d d e p o s i t i o n to corrosion c o n t r o l .
When c o n s i d e r i n g
deposit i o n causes, we're concerned not only with calcium c a r bonate and c a l c i u m p h o s p h a t e , but a l s o w i t h d u s t and
o t h e r i m p u r i t i e s . The c o o l i n g tower a c t s as an e f f i c i e n t
s c r u b b e r for such s o l i d s , and f o r c o m b u s t i o n gases
such as s u l p h u r d i o x i d e which are p o t e n t r e d u c i n g
agents
for chromtes.
We m u s t a l s o c o n s i d e r
microbiological
g r o w t h s w h i c h form s l i m y d e p o s i t s and i n t e r f e r e w i t h
heat t r a n s f e r .
I n s h o r t , we m u s t b e c o n c e r n e d
about
a l l causes of f o u l i n g i n c l u d i n g d e p o s i t i o n of i n h i b i t o r
r e a c t i o n p r o d u c t s , p r o c e s s l e a k a g e s and c o r r o s i o n
products.
A l l o f t h e p r e c e d i n g i m p u r i t i e s m u s t be d e a l t
w i t h e f f e c t i v e l y so t h a t c o o l i n g water c o r r o s i o n
inhibitors can f u n c t i o n .
Unless the metal surfaces i n the
system are kept c l e a n and a c c e s s i b l e to the
inhibitor,
t h e n t h e c o r r o s i o n i n h i b i t o r c a n n o t be e x p e c t e d
to
function.
I f we p e r m i t g r o w t h o f m i c r o b i o l o g i c a l l a y e r s
on a s u r f a c e , both heat t r a n s f e r and c o r r o s i o n
inhibition will
suffer.
The need t o keep m e t a l s u r f a c e s c l e a n l e d t o the
development of a d d i t i v e s c a l l e d a n t i f o u l a n t s and d i s persants.
Many n a t u r a l o r g a n i c s s u c h as t a n n i n s and
l i g n i n d e r i v a t i v e s have been used as d i s p e r s a n t s .
As an
11.
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Industrial
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361
CORROSION
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CHEMISTRY
t i o n a l c o r r o s i o n s u p p r e s s i o n i s p r o v i d e d by i n c r e a s e d
a l k a l i n i t y and pH i n n o n - a c i d i f i e d systems, p l u s i n h i b i t o r p r o p e r t i e s o f the phosphonate squestrants. I t has
a l s o been d i s c o v e r e d t h a t o t h e r i n h i b i t o r s such as
chromate can be used i n c o m b i n a t i o n w i t h s p e c i a l l y
s e l e c t e d squestrants t o o b t a i n improved c o r r o s i o n
inhibition.
T h i s p r e s e n t a t i o n w i l l not attempt d e t a i l e d d i s c u s s i o n o f m i c r o b i c i d e s , which a r e e s s e n t i a l t o maintenance
o f c l e a n m e t a l s u r f a c e s needed f o r e f f e c t i v e c o r r o s i o n
i n h i b i t i o n . A l l o f the p r e v i o u s l y d i s c u s s e d needs a l s o
must be met, i n c l u d i n g d e p o s i t c o n t r o l , a n t i f o u l a n t s t o
p r e v e n t l o c a l i z e d a c c u m u l a t i o n s o f suspended, i n s o l u b l e
c o n t a m i n a n t s , and a d d i t i o n o f c o r r o s i o n i n h i b i t o r s .
The
c o n t r i b u t i o n s o f m i c r o b i i c i d e s and a s s o c i a t e d a n t i f o u l a n t s
to c o r r o s i o n p r e v e n t i o n i n c o o l i n g systems cannot be
m i n i m i z e d . We a r e c o n c e r n e
b i o l o g i c a l growths ( a l g a
l y and i n t e r f e r e w i t h u n i f o r m water d i s t r i b u t i o n t h r o u g h
the tower, but a l s o w i t h s l i m e f o r m e r s t h a t d e p o s i t i n s i d e heat exchanger t u b e s , and which reduce e f f i c i e n c y
o f h e a t t r a n s f e r j u s t as would s c a l e d e p o s i t s . B i o c i d e s
a l s o p l a y a key r o l e i n p r o t e c t i o n o f wood s t r u c t u r a l
members and wood f i l l s e c t i o n s a g a i n s t d e s t r u c t i o n by
r o t or f u n g a l m i c r o o r g a n i s m s .
The way i n which a q u a t i c growths and m i c r o o r g a n i s m s
can i n t e r f e r e w i t h u n i f o r m c i r c u l a t i o n , or s c r e e n o f f
p a r t o f heat exchange s u r f a c e s from u n i f o r m c o n t a c t w i t h
c o o l i n g water and c o r r o s i o n i n h i b i t o r s can be v i s u a l i z e d
from F i g u r e 6. T h i s shows a s i z e a b l e a c c u m u l a t i o n o f
a q u a t i c weeds on the i n l e t t o a l a r g e heat exchanger.
Some tubes a r e t o t a l l y b l o c k e d .
A c t i v e c o r r o s i o n may be
e x p e c t e d beneath d e p o s i t s where m e t a l s u r f a c e s a r e
s c r e e n e d from f r e e c o n t a c t w i t h i n h i b i t e d c o o l i n g w a t e r .
C l o s e d R e c i r c u l a t i n g C o o l i n g Systems
I n open r e c i r c u l a t i n g systems e v a p o r a t i o n i s the
major f a c t o r i n heat d i s p o s a l . I n t h e s e e v a p o r a t i v e
systems c i r c u l a t i n g water i s c o n t i n u o u s l y scrubbed w i t h
a i r , t h e r e f o r e s a t u r a t e d w i t h d i s s o l v e d oxygen.
In
c o n t r a s t , water i n c l o s e d r e c i r c u l a t i n g c o o l i n g systems
u s u a l l y w i l l c o n t a i n minimum d i s s o l v e d oxygen, even
though the systems may i n c l u d e v e n t e d e x p a n s i o n t a n k s .
C l o s e d c o o l i n g svstems o p e r a t e a t a p p r e c i a b l e p r e s s u r e , o f t e n 30 pounds per square i n c h or h i g h e r .
They
are e s s e n t i a l f o r t e m p e r a t u r e c o n t r o l o f i n t e r n a l comb u s t i o n e n g i n e s used i n a u t o m o b i l e s , t r a c t o r s , t r u c k s ,
buses and r a i l w a y l o c o m o t i v e s .
D i e s e l engines a l s o
p r o v i d e m o t i v e power f o r r i v e r and l a k e v e s s e l s and a r e
WILKES
Figure 6.
Industrial
Problems
363
364
CORROSION
CHEMISTRY
used t o d r i v e e l e c t r i c g e n e r a t o r s i n power p l a n t s , b o t h
f o r f u l l time and emergency stand-by u s e .
I n t y p i c a l r a i l w a y d i e s e l e n g i n e s , c o o l i n g water
i s pumped t h r o u g h water j a c k e t s o r c o r e d l i n e r s i n
which t h e p i s t o n s work. I t then p a s s e s t h r o u g h t h e
c y l i n d e r heads which must d i s s i p a t e tremendous heat
c r e a t e d by c o m b u s t i o n o f d i e s e l f u e l under p r e s s u r e ;
f i n a l l y i t r e t u r n s t o an e x p a n s i o n tank v i a f a n - c o o l e d
r a d i a t o r s e c t i o n s . A p a r a l l e l c o o l i n g flow passes
t h r o u g h l u b r i c a t i n g o i l heat exchangers t o remove a d d i t i o n a l heat.
In the design o f r a i l w a y D i e s e l engines the b u i l d e r s a p p a r e n t l y , d i d n o t r e c o g n i z e t h a t t h e r e c o u l d be
c o r r o s i o n problems w i t h i n t h e c o o l i n g c i r c u i t s .
This
may e x p l a i n why some c o o l i n g systems i n c l u d e d as many
as t w e l v e d i s s i m i l a r m e t a l s , a l l e l e c t r i c a l l y c o u p l e d
t o g e t h e r . M e t a l s use
b r a s s , A d m i r a l t y , phospho
c a s t and wrought aluminum, t i n - p l a t e d A d m i r a l t y , s o l d e r
and o t h e r s i n a v a r i e t y o f dangerous c o u p l e s .
The r a i l r o a d s were t o l d by l o c o m o t i v e b u i l d e r s t h a t
when they bought D i e s e l s t o r e p l a c e steam l o c o m o t i v e s ,
they c o u l d f o r g e t about water p r o b l e m s ,
unfortunately
the d e s i g n e r s were n o t c o r r o s i o n e n g i n e e r s , and d i d n ' t
d e s i g n c o o l i n g systems t o w i t h s t a n d c o n t i n u o u s c o n t a c t
w i t h h i g h t e m p e r a t u r e c o o l i n g w a t e r . There was an
added need f o r b o i l e r f e e d w a t e r , s i n c e D i e s e l - p o w e r e d
t r a i n s s t i l l r e q u i r e d steam f o r h e a t i n g c a r s , f o r l a v a t o r i e s and f o r d i n i n g c a r s . T h i s r e q u i r e d compact steam
g e n e r a t o r s o f u n i q u e d e s i g n , s m a l l enough t o be i n s t a l l e d aboard D i e s e l l o c o m o t i v e s , y e t c a p a b l e o f g e n e r a t i n g
3 , 0 0 0 - 5 , 0 0 0 l b s . steam/hour.
These b o i l e r s r e q u i r e d
m i n e r a l - f r e e feedwater supplemented by c o r r o s i o n i n h i b i t o r s . C o o l i n g systems a l s o needed h i g h - q u a l i t y makeup
water, p l u s s p e c i a l c o r r o s i o n i n h i b i t o r s t o p r o t e c t the
multimetal c i r c u i t s .
A t some l o c a t i o n s where D i e s e l e n g i n e s were used t o
d r i v e e l e c t r i c g e n e r a t o r s , a t t e m p t s were made t o r e c o v e r
p a r t o f t h e heat i n r e c i r c u l a t i n g c o o l i n g w a t e r . E n g i n e
c o o l a n t c i r c u l a t i n g a t 3 0 pounds p r e s s u r e and temperat u r e s o f 2 4 0 t o 2 5 0 d e g r e e s F. was a l l o w e d t o pass
t h r o u g h an o r i f i c e i n t o an e x p a n s i o n chamber. P a r t o f
the water would f l a s h i n t o low p r e s s u r e steam, which
c o u l d be used f o r b u i l d i n g h e a t i n g . T h e o r e t i c a l l y , a l l
t h e steam condensed i n space h e a t e r s and r a d i a t o r s
would be r e c o v e r e d as c o n d e n s a t e and r e t u r n e d t o t h e
e n g i n e c o o l i n g system. U n f o r t u n a t e l y , h e a t i n g systems
a r e never c o m p l e t e l y t i g h t , so l o s s e s o f steam and c o n d e n s a t e o c c u r r e d . T h i s c r e a t e d a need f o r c o n t i n u o u s
makeup t o t h e c o o l i n g s y s t e m w h i c h i n e f f e c t had become
11.
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Industrial
Problems
365
a steam g e n e r a t o r .
S i n c e i t was o p e r a t i n g as a s t e a m
generator,
t h e c o o l i n g s y s t e m now r e q u i r e d b l o w d o w n
(to c o n t r o l b u i l d - u p of d i s s o l v e d s o l i d s ) , p l u s v o l a t i l e
c o r r o s i o n i n h i b i t o r s capable of p r o t e c t i n g
steam-condens i n g and condensate
r e t u r n c i r c u i t s p l u s the i n h i b i t o r s
a l r e a d y needed for c o o l i n g system c o r r o s i o n
control.
Most i n t e r n a l combusion engines include
fan-cooled
radiators in their cooling circuits.
Figure 7 is part
of a t y p i c a l r a d i a t o r s e c t i o n .
The c o m p l e t e
radiator
s e c t i o n had 212 r e c t a n g u l a r t u b e s a b o u t one i n c h by
1/8
i n c h i n s i d e d i a m e t e r , w i t h tube metal t h i c k n e s s
about 0.005 i n c h .
The t u b e m e t a l g e n e r a l l y was A d m i r a l t y b r a s s , a 7 0 / 3 0 b r a s s w i t h 1% t i n .
The header a l s o
is
A d m i r a l t y , c o v e r e d w i t h a 1/8
inch l a y e r 60/40 l e a d - t i n
solder.
The t u b e s f i r s t a r e l o c k e d i n p l a c e w i t h
90/10
solder before a p p l y i n g the s e a l i n g l a y e r of 60/40
solder.
The r a d i a t o r t u b e
tin plated.
This combinatio
rosion couple of A d m i r a l t y brass with t i n p l a t i n g i n
c o m b i n a t i o n w i t h two s o l d e r s and s u b j e c t t o
intense
s t r e s s e s i n the header
areas.
Some d i e s e l l o c o m o t i v e s m u s t w o r k u n d e r
extreme
c o l d weather c o n d i t i o n s with a l t e r n a t e periods of
idling
and maximum l o a d i n g .
T h i s can c r e a t e extrenie r a d i a t o r
stresses which contribute to c o r r o s i o n .
One s u c h
locat i o n i s deep p i t open c u t i r o n mines of N o r t h e r n M i n n e sota.
When e n g i n e s w e r e i d l i n g , r a d i a t o r s e c t i o n s
were
bypassed and would r e a c h ambient t e m p e r a t u r e , as low as
-40 degrees F .
U n d e r h e a v y l o a d , r a d i a t o r s w o u l d be
h i t by sudden f l o w s of c o o l i n g w a t e r a t
temperatures
of 220-240 degrees F .
The r e s u l t i n g s t r e s s l o a d s
created c o r r o s i o n f a i l u r e s of the t i n - c o a t e d
tube-solder
j o i n t at the header.
Water l e a k s and r a d i a t o r f a i l u r e s
resulted.
F i g u r e 8 i s a c l o s e u p o f one t u b e end from
a failed radiator section.
I n d i c a t i o n s of
separation
o f t h e t u b e f r o m i t s s u r r o u n d i n g s o l d e r l a y e r c a n be
seen a t the r i g h t and l e f t tube
sides.
Many y e a r s a g o , c e r t a i n A r c t i c e x p l o r e r s c a r r i e d
food i n cans sealed with pure t i n s o l d e r .
Under A r c t i c
t e m p e r a t u r e s t h i s t i n s o l d e r bond f a i l e d
completely.
A p p a r e n t l y the t i n changed from a c r y s t a l l i n e form to
an amorphous, powdered s o l i d which d e s t r o y e d the s e a l
integrity.
R e s u l t i n g f o o d s p o i l a g e c a u s e d d e a t h s by
poisoning.
T h i s type c o r r o s i o n f a i l u r e of pure t i n ,
c a l l e d " t i n p e s t , " or " t i n d i s e a s e , " a l s o had been
o b s e r v e d many y e a r s a g o i n R u s s i a .
Russian cathedrals
u s u a l l y were not h e a t e d , and i n the w i n t e r would r e a c h
extremely low temperatures.
P i p e o r g a n s i n some c a t h e d r a l s h a d p i p e s made o f p u r e b l o c k t i n .
Under extreme
c o l d t e m p e r a t u r e s v i b r a t i n g o r g a n p i p e s were known
366
CORROSION C H E M I S T R Y
Figure 7.
WILKES
Figure 8.
Industrial
Problems
367
CORROSION
368
CHEMISTRY
to
WILKES
Figure 9.
Industrial
Problems
369
CORROSION C H E M I S T R Y
370
Figure 10.
11.
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Industrial
371
Problems
Two
forms o f
dezincification
CORROSION
372
Figure 11.
Figure 12.
CHEMISTRY
WILKES
Figure 13.
Industrial
Problems
373
374
CORROSION CHEMISTRY
Figure 14.
WILKES
Industrial
Figure 15.
Problems
CORROSION C H E M I S T R Y
376
Figure 16.
11.
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Industrial
Problems
377
Aluminum F a i l u r e s .
Many D i e s e l e n g i n e c o o l i n g
systems
were f i r s t c o n s t r u c t e d w i t h aluminum headers and o t h e r
components d i r e c t l y coupled to c a s t i r o n b l o c k s ,
with
copper connections,
bronze s c r e e n s and o t h e r
dangerous
multi-metal couples.
These dangerous
combinations
proved almost impossible to p r o t e c t , even with high
concentrations of chromate c o r r o s i o n i n h i b i t o r s .
In
t h e s e aluminum-copper and a l u m i n u m - i r o n c o u p l e s ,
failure
by c o m p l e t e p e n e t r a t i o n o f 5/8 i n c h aluminum header
p l a t e s o c c u r r e d i n t h r e e months of
service.
CORROSION C H E M I S T R Y
378
Figure 17.
Figure 18. Thin cross-section of admiralty brass tube wall penetrated by plugtype dezincification. (Upper section) specimen polished, unetched. (Lower section) same specimen polished and etched. Magnification about 170 diameters.
380
CORROSION C H E M I S T R Y
F i g u r e 19 i s a n e x a m p l e o f a n a l u m i n u m h e a d e r
p l a t e damaged by c o n t a c t w i t h h i g h a l k a l i n i t y ,
high
pH c o o l i n g w a t e r .
F i g u r e 20 i s a p h o t o m i c r o g r a p h o f
one s e c t i o n of the p l a t e , i l l u s t r a t i n g the c r a c k i n g and
e x f o l i a t i o n which occurred.
Hydrogen released during
corrosion reactions causes i n t e r n a l pressure i n f i s s u r e s , a c c e l e r a t i n g s w e l l i n g and s e p a r a t i o n of
this
rolled plate.
U n d e r m a g n i f i c a t i o n o f 250 d i a m e t e r s ,
the c r a c k i n g and s w e l l i n g are c l e a r l y v i s i b l e .
A few
s m a l l n o d u l e s of copper were found s u g g e s t i n g t h a t p r e s e n c e o f c o p p e r a l s o was i n v o l v e d i n t h i s c o r r o s i o n
failure.
I n C a n a d a , a number o f mine sweepers were b u i l t
using a l l non-magnetic m a t e r i a l s .
The e n t i r e e n g i n e
b l o c k was c o n s t r u c t e d o f a c l a d a l u m i n u m a l l o y .
For
c o r r o s i o n i n h i b i t i o n i n the c l o s e d c o o l i n g system, an
inhibitor containin
applied.
The p h o s p h a t e - b a s e
c o r r o s i o n of the engine b l o c k s , r e s u l t i n g i n p e n e t r a tion failures.
Deep p i t t i n g l e a d i n g t o p e n e t r a t i o n o f
t h e c a s t a l u m i n u m b l o c k i s shown i n F i g u r e 2 1 .
Many o f
the p i t s s e r v e d as s t r e s s r a i s e r s , c a u s i n g c r a c k i n g .
A t the base of the p i t s where r e d u c i n g a c t i o n accompan i e s c o r r o s i o n , c o p p e r d e p o s i t i o n c a n be s e e n .
Copper
comes from c o r r o s i o n of b r o n z e c i r c u l a t i n g pumps, p h o s p h o r - b r o n z e f i t t i n g s and copper a l l o y l i n e s .
These
c o o l i n g s y s t e m s c o u l d have been w e l l p r o t e c t e d by a
c h r o m a t e i n h i b i t o r , o r by a b o r a t e - n i t r i t e - M B T p r o d u c t ,
b u t e x p e r i e n c e d a c c e l e r a t e d a t t a c k when u s i n g t h e i m proper phosphate-silicate
inhibitor.
Fretting Attack.
This is. a type of c o r r o s i v e a c t i o n
i n w h i c h m e t a l t r a n s f e r s from one b e a r i n g s u r f a c e
to
a n o t h e r , when v i b r a t i o n o r o t h e r m e c h a n i c a l f o r c e s
cause s l i g h t r e l a t i v e movement.
I n F i g u r e .22 we a r e
l o o k i n g down i n t o t h e o p e n i n g o f a D i e s e l e n g i n e
cylinder block i n t o which a ribbed c y l i n d e r l i n e r i s seated.
Where the r i b s a r e i n c o n t a c t w i t h the e n g i n e
block,
t r a n s f e r o f m e t a l by f r e t t i n g has o c c u r r e d .
Cooling
water c i r c u l a t e s between the b l o c k and the l i n e r .
To
p r e v e n t c r e v i c e a t t a c k between l i n e r r i b s and e n g i n e
b l o c k , and a v o i d i n i t i a t i o n of f r e t t i n g damage,
both
e n g i n e b l o c k and l i n e r s u r f a c e s s h o u l d have been p r e treated ("pickled")
with a strong concentration of
a l k a l i n e c h r o m a t e i n h i b i t o r (1% t o 5% s t r e n g t h ) , b e f o r e
attempting to seat the l i n e r .
F i g u r e 2_3 s h o w s t h e
ribbed area at top of the l i n e r .
Absence of
adequate
c o r r o s i o n c o n t r o l o f t h i s c o o l i n g s y s t e m i s shown by
s e v e r e c r e v i c e c o r r o s i o n and f r e t t i n g damage on t h e
c o n t a c t f a c e s o f l i n e r r i b s , and a l s o by t h e e x t e n s i v e
CO
oo
CORROSION C H E M I S T R Y
382
Figure 20.
WILKES
Figure 21.
Industrial
Problems
383
CORROSION C H E M I S T R Y
384
Figure 22.
11.
WILKES
Figure 23.
Industrial
Problems
385
Cylinder-liner ribs show metal loss by fretting attack and crevice corrosion plus "worm holing" attack on liner at base of ribs
CORROSION
386
'worm h o l e ' c o r r o s i o n i n l i n e r
s t r e s s e d region at the base of
w a l l s , i n the
liner
ribs.
CHEMISTRY
severely
Cavitation-Corrosion.
T h i s form of c o r r o s i o n a t t a c k ,
which causes l o c a l i z e d f a i l u r e s of c y l i n d e r l i n e r s
i n D i e s e l e n g i n e s , has been r e s p o n s i b l e for
serious
m a i n t e n a n c e p r o b l e m s i n power u n i t s of t r u c k s , b u s e s ,
r a i l w a y l o c o m o t i v e s and m a r i n e t r a n s p o r t .
The
concent r a t e d a r e a of a t t a c k , and s e v e r i t y of c o r r o s i o n
are
c l e a r l y shown i n F i g u r e 2.
The a r e a s of
greatest
a t t a c k o c c u r 90 d e g r e e s f r o m t h e c r a n k s h a f t c e n t e r
line
o n t h e t h r u s t o r c o m p r e s s i o n s i d e ; a l i g h t e r a t t a c k may
appear on the o p p o s i t e s i d e .
Other c h a r a c t e r i s t i c s
s h o w n i n F i g u r e 2_5 i n c l u d e : 1) H o n e y c o m b e d
appearance
of corroded metal.
2) C o r r o d e d s u r f a c e s s u b s t a n t i a l l y
free of c o r r o s i o n products.
3) A t t a c k o c c u r s i n s h a r p l y
defined areas, irregula
boundaries between a f f e c t e
C a v i t a t i o n c o r r o s i o n of c y l i n d e r l i n e r s i s not
c a u s e d by h i g h v e l o c i t y w a t e r f l o w , nor by i m p i n g e m e n t
of coolant streams.
Rather, i t appears that v i b r a t o r y
effects are p r i m a r i l y responsible.
Under the tremendous s t r e s s e s of f u e l c o m p r e s s i o n and c o m b u s t i o n i n the
D i e s e l c y c l e , t h e s e 3/4 i n c h t h i c k c y l i n d e r l i n e r s
v i b r a t e o r " r i n g " a t f r e q u e n c i e s e s t i m a t e d t o be i n
the range of 7,000 to 8,000 c y c l e s per second,
and
higher.
In v i b r a t i n g at such high frequencies,
the
liner
m e t a l i s " j e r k e d " away f r o m c o o l i n g w a t e r c o n t a c t i n g
the s u r f a c e so r a p i d l y as to reduce l o c a l c o o l a n t
pressure below the vapor pressure of the coolant
fluid,
forming minute v a p o r - f i l l e d c a v i t i e s , or
"bubbles."
When t h e r e v e r s e v i b r a t i o n c y c l e o c c u r s , t h e v a p o r f i l l e d c a v i t i e s i n s t a n t l y implode or c o l l a p s e .
The r e c u r r i n g c o l l a p s e and r e f o r m a t i o n of v a p o r - f i l l e d c a v i ties occurs i n microseconds.
Because the
"bubbles"
form and c o l l a p s e so r a p i d l y , the energy they r e l e a s e
is
measured i n hundreds of tons per square i n c h .
From
study of photomicrographs of f a i l e d m e t a l , i t a l s o
a p p e a r s t h a t when t h e s e v a p o r c a v i t i e s i m p l o d e o r
coll a p s e , they d r i v e water and water vapor r i g h t i n t o
the g r a i n s t r u c t u r e .
When t h e v a p o r c a v i t i e s f o r m
a g a i n , and p r e s s u r e i s r e l i e v e d t r a p p e d water vapor
explodes upward.
P h o t o m i c r o g r a p h s show t h a t c r y s t a l l i n e s t r u c t u r e h a s been d i s t o r t e d and r e a r r a n g e d by t h e
tension e f f e c t s of escaping water vapor.
The unwanted e n g i n e v i b r a t i o n i s r e l a t e d to c r a n k s h a f t r o t a t i o n a l s p e e d , b u t i s c a u s e d by e n g i n e i m b a l a n c e , c r a n k s h a f t m i s a l i g n m e n t , or e x c e s s i v e wear of
^ " l i n d e r l i n e r s and p i s t o n s on the t h r u s t s i d e .
In
WILKES
Industrial
Problems
Corrosion
Figure 24.
388
CORROSION C H E M I S T R Y
Figure 25. Concentrated zone of loss, absence of corrosion products, and sharply
defined boundaries identify cavitation-corrosion attack on cylinder liner
11.
WILKES
Industrial
Problems
389
discussing mechanical/physical f a c t o r s c o n t r i b u t i n g to
c a v i t a t i o n - c o r r o s i o n o f c y l i n d e r l i n e r s , S p e l l e r and
LaQue (J) made the f o l l o w i n g s u g g e s t i o n s :
1)
Reduce l i n e r v i b r a t i o n by any p r a c t i c a b l e means;
i . e . , r e d e s i g n i n g l i n e r s u p p o r t s t o i n t r o d u c e dampening
e f f e c t s , or p r o v i d i n g
cushioning.
2)
Develop and use h a r d e r , more r e s i s t a n t l i n e r m e t a l s .
(Hard a l l o y c a s t i r o n l i n e r s respond b e t t e r t o c o r r o s i o n i n h i b i t i o n than do s o f t c a s t i r o n l i n e r s . )
3)
Reduce o p p o r t u n i t y f o r development o f vapor c a v i t i e s by o p e r a t i n g a t h i g h e r p r e s s u r e s and t e m p e r a t u r e s
i n c o o l i n g s y s t e m s , and p o s s i b l y by i n t r o d u c t i o n o f a i r
b u b b l e s as c u s h i o n s a g a i n s t v a p o r - c a v i t y c o l l a p s e .
4) M a i n t a i n chromate c o r r o s i o n i n h i b i t o r s i n c o o l i n g
water a t c o n c e n t r a t i o n s above 2,000 mg/1
(as C r O ^ ) ,
and h o l d c o o l a n t pH w i t h i n the range o f 8.5-9.5.
5)
Include s u i t a b l
between c o r r o s i o n i n h i b i t o r
F i e l d e x p e r i e n c e showed t h a t c y l i n d e r l i n e r s c o u l d
not be p r o t e c t e d a g a i n s t c a v i t a t i o n - c o r r o s i o n by p l a t i n g
w i t h chromium or h a r d e r m e t a l s , or by p l a s t i c c o a t i n g s .
Both would e x p l o d e o f f a t the same r a t e as d i d the
o r i g i n a l l i n e r m e t a l . But t e s t s showed t h a t s i n c e
c o r r o s i o n f a c t o r s were i n v o l v e d , as w e l l as m e c h a n i c a l /
p h y s i c a l f a c t o r s , new c y l i n d e r l i n e r s c o u l d be p r o t e c t e d
a g a i n s t c a v i t a t i o n c o r r o s i o n by p r o p e r
concentrations
o f a l k a l i n e chromate i n h i b i t o r s . C a v i t a t i o n a t t a c k i n
p r o g r e s s might not be a r r e s t e d on o l d e r l i n e r s o f eng i n e s w i t h s e v e r e v i b r a t i o n a l p r o b l e m s , even when u s i n g
chromtes. But on newer e n g i n e s , p r o p e r l y i n h i b i t e d
w i t h chromate from the f i r s t day o f s e r v i c e , c a v i t a t i o n
c o r r o s i o n c o u l d be c o m p l e t e l y p r e v e n t e d .
One o t h e r i n h i b i t o r type t h a t w i l l p r e v e n t c a v i t a t i o n - c o r r o s i o n i s the s o l u b l e o i l t y p e , which i n c o r p o r a t e s a l i g h t m i n e r a l o i l p l u s e m u l s i f i e r s and a d s o r p t i o n - t y p e i n h i b i t o r s , such as o r g a n i c amines. U n f o r t u nately, although e f f e c t i v e i n c o n t r o l l i n g c a v i t a t i o n ,
( p o s s i b l y by c u s h i o n i n g e f f e c t s o f the adsorbed o i l
r e i n f o r c e d f i l m ) they s o f t e n and damage rubber connect o r s and s e a l s , cause l e a k a g e and water l o s s . When
e m u l s i f i e r s are e x h a u s t e d , the o i l e m u l s i o n b r e a k s , and
a l l o w s o i l y f i l m s t o form on heat t r a n s f e r s u r f a c e s .
Some of the amine type e m u l s i f i e r s and i n h i b i t o r s are
p o t e n t i a l l y d e s t r u c t i v e t o copper a l l o y s i n the c i r c u l a t i n g system, a l s o . One o t h e r type i n h i b i t o r t h a t has
been used e f f e c t i v e l y f o r many y e a r s i n c l o s e d c o o l i n g
systems i s g e n e r i c a l l y i d e n t i f i e d as " b o r a t e - n i t r a t e . "
These c o m b i n a t i o n s use sodium n i t r i t e , b u f f e r e d by
a l k a l i n e b o r a t e s , and s i l i c a t e s w i t h s u p p l e m e n t a l i n h i b i t o r s such as m e r c a p t o b e n z o t h i a z o l e , t h i a z o l e or benzo-
390
CORROSION CHEMISTRY
RECEIVED
September 1, 1978.
12
High-Temperature Corrosion in Coal Gasification Plants
V. L. HILL, D. YATES, and B. A. HUMPHREYS
IIT Research Institute, 10 West 35 Street, Chicago, IL 60616
0-8412-0471-3/79/47-089-391$05.75/0
1979 American Chemical Society
392
CORROSION
CHEMISTRY
f o r h i g h oxygen and l o w s u l f u r a c t i v i t y e n v i r o n m e n t s
are subject t o o x i d a t i o n - c o r r o s i o n or e r o s i o n - c o r r o s i o n
i n t h e g a s i f i e r environment.
Background
P r i o r t o 1972, no i n f o r m a t i o n e x i s t e d on t h e behavior o f high-temperature m a t e r i a l s i n g a s i f i c a t i o n
e n v i r o n m e n t s . M a t e r i a l s d a t a were a v a i l a b l e f o r h i g h p r e s s u r e equipment a t low o p e r a t i n g t e m p e r a t u r e s , o r
f o r l o w - p r e s s u r e equipment a t h i g h o p e r a t i n g temperat u r e s . The emerging c o a l g a s i f i c a t i o n p r o c e s s e s ,
t h e r e f o r e , r e p r e s e n t e d a new environment f o r h i g h temperature m a t e r i a l s . Behavior o f m a t e r i a l s , both
m e t a l l i c and r e f r a c t o r y , c o u l d n o t be p r e d i c t e d b a s e d
on t h e i r performance on t h e n - e x i s t i n g equipment.
D u r i n g 1972 t h
p o r t e d by t h e America
f o r t s i n coal g a s i f i c a t i o n materials research. Since
1975 t h e m a j o r s u p p o r t f o r t h e program has been supp l i e d by t h e Department o f Energy i n c o o p e r a t i o n w i t h
AGA. F i v e phases o f e f f o r t have been d e f i n e d , w i t h work
i n p r o g r e s s . These a r e a s a r e : Phase I - L a b o r a t o r y
H i g h Temperature O x i d a t i o n - C o r r o s i o n ; Phase I I - P i l o t
P l a n t C o r r o s i o n S t u d i e s ; Phase I I I - Quench System
Aqueous C o r r o s i o n ; Phase I V - H i g h Temperature E r o s i o n C o r r o s i o n ; and Phase V - M e c h a n i c a l P r o p e r t y Measurements, The f i r s t f o u r phases a r e b e i n g c o n d u c t e d a t
the I I T Research I n s t i t u t e .
A major p o t e n t i a l problem f o r c o a l g a s i f i c a t i o n
m a t e r i a l s due t o h i g h - s u l f u r c o a l s i s i n d i c a t e d by
T a b l e I , The t a b l e compares t h e m e l t i n g p o i n t s o f s u l f i d e s and e u t e c t i c t e m p e r a t u r e s o f t h e m e t a l - m e t a l
s u l f i d e systems f o r i r o n , n i c k e l , c o b a l t , and c h r o m i um ( 1 ) . I t may be seen t h a t t h e m e t a l - m e t a l s u l f i d e
e u t e c t i c t e m p e r a t u r e s v a r y from 1193F f o r n i c k e l t o
2462F f o r chromium. A l l t h r e e e l e m e n t a l bases o f
h i g h - t e m p e r a t u r e a l l o y s , i r o n , n i c k e l , and c o b a l t , exh i b i t e u t e c t i c t e m p e r a t u r e s o f 1810F o r l e s s . The
n i c k e l - n i c k e l s u l f i d e e u t e c t i c i s 1193F.
The s i g n i f i c a n c e o f t h e e u t e c t i c t e m p e r a t u r e s
shown i n T a b l e I i s t h a t i n h i g h - s u l f u r gases m e l t i n g
of c o r r o s i o n products o f high-temperature a l l o y s can
occur w i t h i n t h e i r normal o p e r a t i n g range. Molten corr o s i o n products a r e not developed d u r i n g a i r o x i d a t i o n .
F u r t h e r m o r e , n i c k e l - c h r o m i u m a l l o y s a r e g e n e r a l l y emp l o y e d i n a i r e n v i r o n m e n t s a t h i g h e r o p e r a t i n g tempera t u r e s because o f t h e i r h i g h e r s t r e n g t h and o x i d a t i o n
resistance.
I n s u l f u r - c o n t a i n i n g atmospheres t h e s e
a l l o y s a r e most s u s c e p t i b l e t o m e l t i n g .
Iron-base
12.
HILL ET AL.
Coal-Gasiftcation
Plants
393
a l l o y s w o u l d be e x p e c t e d t o have t h e h i g h e s t m e l t i n g
t e m p e r a t u r e s , but a r e g e n e r a l l y l e s s o x i d a t i o n r e s i s t a n t and/or have l o w e r s t r e n g t h .
Thus, a l l o y s e l e c t i o n
f o r h i g h - t e m p e r a t u r e c o a l g a s i f i c a t i o n s e r v i c e tends t o
be t h e i n v e r s e o f t h a t a p p l i c a b l e t o h i g h - t e m p e r a t u r e
air oxidation.
O x i d a t i o n - c o r r o s i o n data i n c l u d e d i n t h i s paper
were g e n e r a t e d i n Phases I and I I o f the program.
Other phases o f the I1TRI work have been d e s c r i b e d i n
current l i t e r a t u r e (2-9).
M e t a l l o s s i n t E i s program has g e n e r a l l y r e s u l t e d
f r o m t h e combined e f f e c t s o f oxygen and s u l f u r .
Thus,
the term o x i d a t i o n - c o r r o s i o n i s u s e d i n t h i s paper t o
d e f i n e m e t a l l o s s , No a t t e m p t i s made t o s e p a r a t e the
i n d i v i d u a l e f f e c t s o f t h e c o r r o d e n t s p e c i e s s i n c e chemi c a l a n a l y s i s o f the c o r r o s i o n p r o d u c t s was n o t conducted.
Results
Phase 1 - L a b o r a t o r y High-Temperature C o r r o s i o n
Testing.
The Phase I c o r r o s i o n program began i n 1973
w i t h d e s i g n and f a b r i c a t i o n o f two r e a c t o r s c a p a b l e o f
o p e r a t i n g t o 2000F a t 1000 p s i . T e s t i n g i n a t y p i c a l
c o a l g a s i f i c a t i o n atmosphere began i n 1973.
To d a t e ,
o v e r 40,000 h r s of t e s t i n g have been completed i n the 2
r e a c t o r s i n v o l v i n g 57 c o m m e r c i a l and d e v e l o p m e n t a l
a l l o y s and c o a t i n g s .
D e t a i l s o f the t e s t equipment
have been r e p o r t e d e l s e w h e r e (2) and w i l l , t h e r e f o r e ,
n o t be d i s c u s s e d i n t h i s p a p e r .
The g a s i f i e r atmosphere u s e d f o r most Phase I c o r r o s i o n t e s t s i s given i n Table I I . During t e s t i n g ,
o n l y the i n l e t gas c o m p o s i t i o n shown was c o n t r o l l e d .
The t e m p e r a t u r e - p r e s s u r e dependent e q u i l i b r i u m gas comp o s i t i o n s shown r e p r e s e n t the a c t u a l t e s t gas composit i o n a t each t e s t temperature.
G e n e r a l l y , the e q u i l i b r i u m gas c o m p o s i t i o n was o b t a i n e d i n the r e a c t o r by
i n t e r a c t i o n o f the i n l e t gas s p e c i e s .
T h i s was v e r i f i e d by a n a l y s e s o f t h e r e a c t o r e x i t gas i n a gas
chromato graph.
The c o m p o s i t i o n s o f the a l l o y s e v a l u a t e d i n Phase
I are summarized i n T a b l e I I I . These a l l o y s r e p r e s e n t
most c l a s s e s o f h i g h - t e m p e r a t u r e i r o n - , n i c k e l - , and
c o b a l t - b a s e a l l o y s t h a t c o u l d be c o n s i d e r e d f o r c o a l
g a s i f i c a t i o n s e r v i c e . Pack a l u m i n i z e d and c h r o m i z e d
c o a t i n g s on A I S I 310 and IN-800 were a l s o e v a l u a t e d i n
the t e s t program.
C o r r o s i o n data contained i n t h i s paper are not i n t e n d e d t o summarize the r e s u l t s f o r the 57 a l l o y s and
c o a t i n g s exposed f o r p e r i o d s o f up t o 5000 h r .
Rather,
394
CORROSION
CHEMISTRY
Table I
EUTECTIC TEMPERATURES OF SELECTED
METAL-METAL SULFIDE SYSTEMS
3
MP o f
Sulfide
Eutectic
Ni-Ni S2
3
C0-C04S3
810
932
Sulfur
Concentration
at E u t e c t i c
w/o
a/o
Eutectic
Temperature
""C
1490
645
1193
33.4
21.5
1710
877
1611
40
26.6
Fe-FeS
1190
2174
988
1810
44
31
Cr-CrS
1565
2849
1350
2462
43.9
32.5
Reference 1
b Formed p e r i t e c t i c a l l y .
a
Table I I
INLET AND EQUILIBRIUM GAS COMPOSITION
OF PHASE I CORROSION TESTS
Gas C o m p o s i t i o n , v/o
Gasifier
Component
H
CO
co
CH
NH
Equilibrium
1500F
23
24
900 F
4
18
11
17
12
25
19
15
19
9
1
Inlet
1800F
31
H2S
0-1.0
0-1.0
0-1.0
0-1.0
H 0
Bal
Bal
Bal
Bal
I n l e t gas c o m p o s i t i o n c o n s t a n t f o r a l l t e s t s i n
g a s i f i e r gas.
A t 1000 p s i and i n d i c a t e d
temperature.
12.
HILL
Coal-Gasification
E T A L .
395
Plants
Table III
CHEMICAL COMPOSITION OF ALLOYS SELECTED
FOR TESTING IN PHASE I
C o m p o s i t i o n , w/o
Co
Cr
~NT
Alloy
Series
"AT"
202
0. 05
0. 64
0. 55
71 .02
9. 24
18. 18
304
0. 05
1. 45
0. 54
70. 04
9. 10
18. 76
?16
0. 05
1. 65
0. 43
65. 23
13. ?8
17. 14
309
0. 11
0. 54
0. 74
60. 46
14 70
22. 97
314
0. 06
1. 90
2. 21
51. 64
20. 00
24. 00
310
0. 06
1. 71
0. 68
52. 16
20. 20
25. 00
446
0. 10
0. 45
IN-600
0. 05
0. 15
IN-601
0. 04
0. 24
IN-800
0..03
0. 80
0..33
47. ,08
30. 84
20. 60
0.,10
32
0,.03
0.,72
0..40
43..22
32..22
21..40
0,.02
.76
0..03
0..03
0..03
0..30
48, 40
50..00
0,.01
HC 250
3,.03
0..5
0,.7
68,.1
0,.1
27,.5
0.1
HD 45
0,.48
0,.7
1 .5
62,.1
5,.2
29,.9
0.1
HL 40
0 .47
0,.6
1 .4
47 .1
19,.4
30 .9
0.1
HL
0 .42
0 .7
2 ,4
45 .8
19 3
31 .4
RA-333
0 .05
1 .5
1 .4
15 .5
47 .5
26 .2
C r u t e m p 25
0 .07
1 .5
0 .6
47 .2
24 .8
25 .4
M u l t i m e t N155
0 .11
1 .4
0 .7
29 .1
19 .8
21 .8
19 .5
H a y n e s 150
0 .06
0. S
0 .2
16 .5
1 .7
27 .9
49 .6
H a y n e s 188
0 .08
0 .7
0 .4
1 .4
23 .3
23 .4
35 .7
S t e l l i t e 6B
1 .0
1 .4
0 .6
2 .0
2 .4
28 .5
56 .4
VE 441
0 .03
0 .1
0 .1
SI .5
0 .004
0 .2
2 .3
0 .04
8 .2
0 .7
62 .9
312
0.15
0.5
Bal
30
329
0.05
0.4
Bal
4.,3
27.1
29
310(A1)
310(Cr)
0.42Ti
IN-800(A1)
IN-SOO(Cr)
IN-793
IN-671
(50/50)
Ser i e s
40-3Si
Nl
Armco
21-6-9
0 .20
0 .01
0.1
0.4
0 .01
90 .0
2 .8
6 .8
20 .7
2.7W
3.8
3 .0
1.1 Cb+Ta,
3.9W
3.0
0.1
0 .22
0.6
1.1
6.5W
15 .1
3.2
0.04Zr
4 .4
0.1
0.22
S e r i e s 3a
AL
29-4-4
0.005
Bal
4.,0
AL
EX-20
1.0
Bal
20
1.4
5
30
12W
Bal
Co-Cr-W No. 1
2.5
Thermalloy
0.4
Bal
35
26
0.55
Bal
47
27
12.5
22.0
18.5
Bal
21.8
2.5
Bal
21.5
0.2
31
21
0.35
41
21.5
0.10
Wiscalloy
63WC
30/50N
A r m c o 18SR
0.05
Armco
0.06
22-13-5
1.0
0.15
1.0
1.0
I n c o n e l 625
0.05
0.25
0.25
S a n i c r o 32X
0.08
I n c o l o y 825
0.03
Hastelloy
Bal
0.50
0.25
30
4W
0.40Ti
18.0
Bal
Bal
5.0
5W
15
2.5
2.0
0.20Cb,
0.20V, 0.3N
9.0
0.6W
9.0
0.2Ti,3.65Cb
0.35Ti,
3.0
3W
2.25Cu,0.9Ti
396
CORROSION
CHEMISTRY
Tabl
_C
Alloy
si
> w/ V
Fe
Co
Series
0.25
1.,0
1.,0
2.0
10.5
29.5
HK
40
0.4
2.,0
2..0
Bal
20
28
HK
40-3S
0.35/
0.45
2.,0
3.,0
Bal
18/22
24/28
Thermalloy
63
0.4
Bal
35
26
Thermalloy
63W
0.40
Bal
35
26
35.0
19.0
50
48
0.05
1..5
1,.25
43
IN-657
IN-738
0.17
0..2
0..3
0.5
556
0.1
1..5
0..4
Bal
20
54
0..2
0,.2
Bal
Bal
Ho
Other
3b
FSX-414
RA-330
Al "
16.0
Bal
8.5
3.4
7.0W
0.5
0.5
1.75
5W
1.5Cb
2.6W,0.9Cb
617
0.07
AL-16-5-Y
0.006
22
20
22
12.5
15.8
0.3
3.0
1.0
5.4
2.5W,0.02La,
1.0Cb+Ta
0.41Y
12.
HILL ET AL.
Cocil-Gasification
Plants
397
CORROSION CHEMISTRY
398
60
50
125
Legend
IN-800
IN-800(A1)
A I S I 310
125
310(A1)
AISI
A I S I 309
40
30
20
10
1500
1800
1650
Temperature, F
Figure 1.
12.
HILL ET AL.
Coal-Gasification
Plants
399
CORROSION C H E M I S T R Y
400
+20
-20
-40
Legend
-60
-80
AISI 309
AISI 314
AISI 446
IN-800(A1)
IN-800
IN-671
1000
2000
3000
4000
J L
5000
Time, hr
Figure 3.
12.
HILL ET AL.
Coal-Gasification
Plants
401
t r a n s i t i o n s to higher o x i d a t i o n - c o r r o s i o n (weight l o s s )
a f t e r 1000 h r . A l t h o u g h not shown, s i m i l a r b e h a v i o r
was o b s e r v e d f o r A I S I 310 s t a i n l e s s s t e e l .
A I S I 309
d e m o n s t r a t e d a t r a n s i t i o n t o w e i g h t g a i n a f t e r 4000 h r .
R a p i d t r a n s i t i o n i n the w e i g h t change c u r v e s g e n e r a l l y
was c o i n c i d e n t w i t h the development o f l o c a l i z e d and/or
general m e l t i n g of the c o r r o s i o n products.
These r e s u l t s s u g g e s t the e r r o r s t h a t c o u l d o c c u r i f 1000 h r
d a t a were e x t r a p o l a t e d t o y e a r l y o x i d a t i o n - c o r r o s i o n
r a t e s . To d a t e , t e s t i n g f o r 5000 h r has been c o m p l e t e d
o n l y a t 1800F i n t h e CGA atmosphere c o n t a i n i n g 0.5 v/o
H2S.
The c u r r e n t r e s u l t s o f 5000 h r t e s t s a t 1800F i n
t h e CGA gas c o n t a i n i n g 0.5 v/o H2S are summarized i n
T a b l e IV. Here t h e 1000, 3000, and 5000 h r t o t a l c o r r o s i o n d a t a have been l i n e a r l y e x t r a p o l a t e d t o mpy c o r rosion rates. Severa
A I S I 310, and A I S I 3 1 4 - - t h a
r a t e s o f 20-40 mpy a f t e r 1000 h r i n d i c a t e d t r a n s i t i o n s
t o h i g h e r r a t e s i n 1000-2000 h r . As a r e s u l t , the l i n e a r l y e x t r a p o l a t e d r a t e s a t 3000 h r were g r e a t e r t h a n
80 mpy.
Other a l l o y s , such as HK-40 and IN-617, exh i b i t e d i n t e r n a l p e n e t r a t i o n a t 1000 h r t h a t d i d not
i n c r e a s e s i g n i f i c a n t l y f o r the longer exposures.
For
t h e s e a l l o y s , c o r r o s i o n d a t a o b t a i n e d by l i n e a r e x t r a p o l a t i o n o f 1000 h r d a t a was m a i n t a i n e d f o r 3000 and
5000 h r , r e s p e c t i v e l y . The d a t a shown i n T a b l e IV i l l u s t r a t e t h a t 1000 h r c o r r o s i o n d a t a c o u l d not be l i n e a r l y e x t r a p o l a t e d t o mpy c o r r o s i o n r a t e s f o r a l l
alloys.
I t was n e c e s s a r y to c o n d u c t 5000 h r t e s t s t o
v e r i f y t h e e x i s t e n c e o f i n c u b a t i o n t i m e s o f 1000-4000
h r f o r t r a n s i t i o n s to r a p i d c o r r o s i o n .
M i c r o s t r u c t u r e s o f two a l l o y s exposed i n the CGA
atmosphere a r e p r e s e n t e d i n F i g s . 4 and 5.
The m i c r o s t r u c t u r e o f A I S I 314 exposed 1000 h r a t 1800F i n the
CGA gas c o n t a i n i n g 0 v/o H2S i s shown i n F i g . 4.
The
a d h e r e n t , l a y e r e d s c a l e on t h i s a l l o y c o n s i s t e d o f f o u r t e e n i n d i v i d u a l m e t a l - o x i d e l a y e r s . M e t a l phase v i s i b l e
i n t h e s c a l e was n i c k e l - r i c h c o n t a i n i n g some i r o n , but
was f r e e o f chromium. O x i d e phase was c h r o m i u m - r i c h
t e n d i n g towards a c h r o m i u m - i r o n s p i n e l a t t h e o x i d e m e t a l i n t e r f a c e . T h i s u n u s u a l m i c r o s t r u c t u r e was a l s o
o b s e r v e d on A I S I 309 and 310 exposed under t h e same
c o n d i t i o n s , a l t h o u g h fewer l a y e r s were p r e s e n t .
I n c o n t r a s t , t h e m i c r o s t r u c t u r e o f IN-671 exposed
5000 h r i n the CGA a t 1800F i s shown i n F i g . 5.
Here,
a t h i n , dense t w o - l a y e r e d s c a l e was o b s e r v e d w i t h m i n o r
g r a i n boundary i n t e r n a l c o r r o s i o n . T o t a l m e t a l l o s s i n
5000 h r was about 2 m i l s . The IN-671 ( 5 0 N i - 5 0 C r ) a l l o y
a l o n g w i t h a l u m i n i z e d A I S I 310 and IN-800 g e n e r a l l y
402
CORROSION C H E M I S T R Y
T a b l e IV
LINEARLY EXTRAPOLATED CORROSION RATES OF PHASE I ALLOYS
EXPOSED 1000-5000 HR AT 1800F IN CGA ENVIRONMENT
(0.5 v/o H S )
2
<20 mpy
20-40 mpy
40-80 .mpy
IN-671, 3 1 0 ( A 1 ) ,
8 0 0 ( A 1 ) , 188, 6B,
T63WC, FSX-414,
A l l o y X, Co-Cr-WNo. 1, N155, 150,
HL40, RA-333,
Crutemp 25
309, 310,
446, HK40,
IN-800,
1N-617
IN-738,
556, 314
>80 mpy
3000 h r
3 1 0 ( A 1 ) , HL40,
IN-617, 1N-657,
FSX-414, Co-Cr-W
No. 1, 1 5 0
IN-738
32X, 446
3 1 4 * 309,
310, 556*
5000 h r
IN-800, IN-671
8 0 0 ( A l ) , N155,
A l l o y X, 188, 6B
a
HK40,
Crutemp
25, T63WC
RA-333
2000 hr.
12.
HILL E T A L .
Figure 4.
Coal-Gasification
Plants
Figure 5.
403
404
CORROSION C H E M I S T R Y
Start-up
42
24
_2
12
53
273
43
10
60
12
_6_
58
racks.
46
12
158
43
55
2
1
111
--8
12
30
--
12
4
1.2
45
25
2
23
11
30
28
16
31
1.3
2,3
Specimen Racks
Installed
Analyzed
Exposure Metals Refr, Metals Refi
31
58
31
Shipped
Total
Battelle
Steam-Iron*
BI-GAS
Synthane
CONOCO COAL
HYGAS
Plant
Total
Locations
Total
Metals Refr. Required
Table V
406
CORROSION C H E M I S T R Y
A t y p i c a l r a c k employed f o r i n s t a l l a t i o n o f spec
imens i n p i l o t p l a n t s i s shown i n F i g . 6. B o t h c o r r o
s i o n coupons, 2 1 0.35 i n . t h i c k , and bend s p e c i
mens i n t e n d e d t o d e t e r m i n e s t r e s s - c o r r o s i o n c r a c k i n g
s u s c e p t i b i l i t y , are included i nthe i n s t a l l a t i o n f o r
aqueous c o r r o s i o n t e s t i n g . Specimens a r e s e p a r a t e d by
high density alumina spacers t o e l i m i n a t e electrochemi
c a l e f f e c t s . During exposure, t h e racks are welded t o
e x i s t i n g components i n t h e p i l o t p l a n t equipment.
H i g h - t e m p e r a t u r e gas phase o x i d a t i o n - c o r r o s i o n
d a t a have been o b t a i n e d f o r two e x p o s u r e s i n t h e CONOCO
COAL p l a n t and one e x p o s u r e i n t h e HYGAS p l a n t .
Table
VI summarizes t h e o p e r a t i n g e n v i r o n m e n t s and i n - p l a n t
t i m e s f o r t h e s e e x p o s u r e s . S i n c e t h e p i l o t p l a n t s op
e r a t e a t v a r i a b l e t e m p e r a t u r e s , p r e s s u r e s , and gas com
p o s i t i o n s , w e i g h t e d average v a l u e s a r e g i v e n f o r t h e
p l a n t exposures.
Linearly extrapolate
t h e 1150 h r f i r s t exposure i n t h e HYGAS g a s i f i e r o f f gas a r e p l o t t e d i n F i g . 7. I n t h i s l o c a t i o n a t 580F
(average) c a r b o n s t e e l , A I S I 410, A I S I 304, IN-800, and
titanium e x h i b i t e d very l i m i t e d corrosion. A l l o y IN600 and Monel 400 h a d c o r r o s i o n r a t e s o f 42 and 124 mpy,
respectively.
T e s t exposure o x i d a t i o n - c o r r o s i o n d a t a f o r s e l e c t e d
a l l o y s i n t h e f l u i d i z e d b e d o f t h e HYGAS g a s i f i e r a r e
shown i n F i g . 8. A l l o y s exposed i n t h i s l o c a t i o n a r e
d i f f e r e n t from t h o s e i n t h e g a s i f i e r o f f - g a s because o f
t h e h i g h e r o p e r a t i n g t e m p e r a t u r e . The f l u i d i z e d b e d
represents t h e highest operating temperature o f t h e four
t e s t l o c a t i o n s i n t h e HYGAS g a s i f i e r .
First-exposure
d a t a , however, i n d i c a t e r e l a t i v e l y m i n o r c o r r o s i o n o f
A I S I 430, A I S I 309, IN-600, A l l o y X, and RA-333 o f 4 t o
18 mpy. A g a i n , IN-600 showed e x t e n s i v e a t t a c k ( c o m p l e t e
c o r r o s i o n o f 0.250 i n . t h i c k specimens) i n 1720 h r o f
p l a n t o p e r a t i o n d u r i n g t h e f i r s t e x p o s u r e . Thus, I N 600 ( N i - 1 6 C r ) e x h i b i t e d s e v e r e c o r r o s i o n o v e r t h e e n t i r e
o p e r a t i n g t e m p e r a t u r e range o f t h e HYGAS g a s i f i e r . A l
though n o t shown i n F i g . 8, IN-601 ( N i - 2 3 C r - l A l ) h a d a
c o r r o s i o n r a t e o f 12 mpy i n t h e HYGAS f l u i d i z e d b e d .
Two e x p o s u r e s have been c o m p l e t e d i n b o t h t h e
CONOCO COAL g a s i f i e r and r e g e n e r a t o r .
L i n e a r l y extrap
o l a t e d c o r r o s i o n r a t e s f o r s e l e c t e d a l l o y s exposed i n
t h e s e CONOCO COAL t e s t l o c a t i o n s a r e p r e s e n t e d i n F i g s .
9 a n d 10. The d u r a t i o n o f e x p o s u r e i n t h e s e t e s t l o c a
t i o n s was about 800 and 1600 h r i n t h e f i r s t and second
exposures, r e s p e c t i v e l y .
F i g u r e 9 shows t h a t t h e c o r r o s i o n r a t e s i n t h e
CONOCO COAL g a s i f i e r were r e l a t i v e l y l o w f o r b o t h expo
sures.
The a p p a r e n t r e d u c t i o n i n c o r r o s i o n r a t e f o r
HILL E T AL.
Figure 6.
Coal-Gasification
Plants
407
CORROSION
408
CHEMISTRY
Table V I
CORROSION CONDITIONS IN CONOCO COAL
AND HYGAS PLANT EXPOSURES
Temp. ,
F
Exposure
Location
Press.,
psi
Time,
hr
CONOCO COAL P l a n t
Gasifier,
off-gas
a
Regenerator^
1425
150
1600
1850
150
800
1850
150
1600
HYGAS P l a n t
Gasifier,
fluidized bed
Gasifier,
off-gas^
a
Gas
1340
980
1718
580
980
1150
c o m p o s i t i o n :i
^Gas c o m p o s i t i o n !r
c
Not
^Gas
7 0 N , 2 5 C 0 , 5C0, t r a c e H2 S (v/o)
2
analyzed,
composition :
0
2
12.
Coal-Gasification
HILL ET AL.
C Steel
Figure 7.
410
304
409
Plants
600
800
Monel
400
c.c.
430
Figure 8.
Titanium
309
800
RA-333
Alloy X
600
HYGAS
410
CORROSION C H E M I S T R Y
ce
IN-800
Figure 9.
310
310(A1)
304
ce
IN-671
Alloy X
IN-800
Figure 10.
IN-800(A1)
ce
IN-800(A1)
310
310(A1)
304
IN-671
CONOCO
Alloy X
CONOCO
12.
HILL E T A L .
Coal-Gasification
Plants
411
CORROSION C H E M I S T R Y
412
s i g n i f i c a n t l y to the e x i s t i n g p i l o t p l a n t i n f o r m a t i o n .
Environmental conditions i n these plants d i f f e r
signifi c a n t l y f r o m t h o s e o f t h e H Y G A S a n d CONOCO C O A L p l a n t s .
A c c o r d i n g l y , t h e new p i l o t p l a n t i n f o r m a t i o n w i l l
provide o x i d a t i o n - c o r r o s i o n data for a wider range of
gas
compositions
and o p e r a t i n g
temperatures.
Summary
of
Results
S i n c e the e f f o r t d e s c r i b e d h e r e i n i s an ongoing
p r o g r a m , w e l l - d e f i n e d c o n c l u s i o n s a r e , as y e t ,
inappropriate.
Some t r e n d s , h o w e v e r ,
are r e a d i l y evident i n
o x i d a t i o n - c o r r o s i o n obtained to date i n coal
gasificat i o n atmosphere.
It is clear that coal gasification
e n v i r o n m e n t s a r e much m o r e s e v e r e t h a n a i r a t t h e same
temperatures.
F u r t h e r m o r e , a chromium content of
20
weight percent,
an
high-temperature alloys is required for long-term res i s t a n c e t o CGA e n v i r o n m e n t s .
The r o l e of
secondary
a d d i t i o n s , aluminum, t i t a n i u m , s i l i c o n , molybdenum,
tungsten, etc.,
and r e s i d u a l s such as manganese,
has
not been c l e a r l y e s t a b l i s h e d .
O x i d a t i o n - c o r r o s i o n data obtained from the p i l o t
p l a n t s g e n e r a l l y compare w e l l w i t h l a b o r a t o r y d a t a i n
ranking of high-temperature a l l o y s .
Pilot plant res u l t s , however,
i n d i c a t e more severe c o r r o s i o n than
laboratory oxidation-corrosion data.
T h i s s h o u l d be
expected because of c y c l i c operation of p i l o t plants
and a d d i t i o n a l v a r i a b l e s c o m p r i s i n g the p i l o t p l a n t environments.
The c o n t r i b u t i o n of e r o s i o n and e r o s i o n c o r r o s i o n by c o a l a s h , c h a r , and s u l f u r sorbents to
the
c o r r o s i o n p r o c e s s i n the p i l o t p l a n t s has not been
defined.
Laboratory oxidation-corrosion data indicate that
e x t r a p o l a t i o n of short-term o x i d a t i o n - c o r r o s i o n
data
to yearly rates is d i f f i c u l t .
These e x t r a p o l a t i o n s
are
necessary to p r o v i d e a b a s i s f o r comparing o x i d a t i o n c o r r o s i o n d a t a o b t a i n e d f r o m v a r i a b l e CGA e x p o s u r e
times.
E x t r a p o l a t e d data, p a r t i c u l a r l y at h i g h
H2S
c o n c e n t r a t i o n s i n t h e CGA a t m o s p h e r e ,
s h o u l d be
employed
with caution.
Long-term k i n e t i c s of the o x i d a t i o n c o r r o s i o n process can r e s u l t i n t r a n s i t i o n s i n c o r r o s i o n b e h a v i o r to h i g h r a t e s not p r e d i c t a b l e by
short
exposures.
S i m i l a r behavior, breakaway o x i d a t i o n ,
o c c u r s i n a i r p r i m a r i l y a t temperatures above 2000F.
A c k n o w l e dgment s
to
A.
The
0.
appreciation
Council,
Coal-Gasification Plants
413
September 1, 1978.
INDEX
A
Acetylenic compounds, formation of
polymerous layers with
292f
Acetylenic inhibitors
288, 302/
Acquisition rate, molar
105
Activation
control, anodic partial process
under
65
controlled partial processes
59
energy diagrams
136/
energy for hydration
162
overpotential
135
Active state, characterization of
166
Activity (ies) of
cupric ion
45
ferrous and ferric ions
46/
water
43/
Adsorption
182
of corrosion products formed on
NiFe, IR
242/
isotherms
248/
measurements, indirect
300
mechanism involving an ion
exchange step
318
theory
159,161,300
Air-cooled radiator section
366/
Airfoil-stress-corrosion cracking
2
Airplane turbine engine applications ..
92
Alkyl propylenediamines and water
soluble amines as process corrosion inhibitors, effectiveness of .... 317/
Alloy-oxide interface
91
Alloys
chemical composition of
395f
corrosion rates of
402i, 409/, 410/
oxidation of
88
passivity on
162
weight change of selected
400/
Aluminum
alloy(s)
aqueous corrosion of
219/
corrosion of
207/
high-temperature film breakdown
for
228
high-temperature pitting for
228
parabolic corrosion of
197/
paralinear corrosion of
199/
corroded surface of "commercially
pure"
227/
Aluminum (continued)
corrosion of
early stages
corrosion rate constant for
failures
galvanic corrosion of
intermetallic compounds, cathodic
polarization curves for
long-time corrosion of
low-temperature corrosion of
oxide platelets on
resistance for growing barrier
films on
Amines, lipophilic
197/
201/
214/
377
241/
219/
202/
200
191/
188/
316
414
415
INDEX
Auger of cobalt
Auger electron spectroscopy
242/
32, 239
Battery
anodic-cathodic reactions
electrode
mixed potential
lead-acid
storage
Behavior, solid material
B E T model
Blisters
Blowdown
Boehmite
Boiler-feed-water stabilizations
Bond energy
metal-oxygen
Brass, dezincification of
Buffer solutions
Bulk diffusion
controlled process
processes
14
15/
18/
19
18/
14
7
255
215
365
200
2
187
190/
37
141
76
82
94
C
Calcium carbonate scale control
357
Cathodic
-anodic equilibrium
14
-anodic reactions, battery
15/
inhibition
272,274/
partial process
53, 55/
under transport control
65
partial reaction
57
polarization
345
curves for aluminum intermetallic compounds
219/
process
16
protection
156
reactions
50
reduction
142,144/
Tafel slopes
305
Cavitation corrosion
376/, 386
attack
388/
damage
375/
prevention of, soluble oil type
389
protection against
389
vibratory
387/
Cell-battery applications, fuel
98
C G A (coal gasification atmosphere) .. 401
Characterization of active and
passive states
166
Characterization of passivating films .. 160/
Charge flowing, total
342/
Charge transfer
21
processes
293
Chelate-metal inhibitor structure
307
Chemical
composition of alloys
potential of oxygen
process of scaling, electrical
formulation of
Chemisorption
Chemistry of corrosion inhibitors
Circuit
analog, dc
conditions, open
equivalent
approach
description
models, kinds of
potentiostatic
theory, closed
theory, open
of parabolic oxidation
Clean system concept
395i
87/
101
162
315
43/
Ill
117
118
101
100
Ill
155/
116
96
107
361
circuit theory
116
cooling systems
351
recirculating
362
system radiator section
367/
Coal gasification
atmosphere ( C G A )
401
materials research
392
plants
92,391
Coals, high-sulfur
392
Coatings
348
Cobalt, Auger of
242/
Coefficients, diffusion
84
Colloid, hydrated
228
Complexing agent, versene, effect
on passivation
145/
Compound-electrolyte interface
26/
Compounds, rate constants for forma
tion of metal-nonmetal
109i
Concentration cell(s)
368,373/
corrosion damage
374/
metal ion
372/
oxygen
372/
Conduction, mixed
medium
112
transport in
112
processes
105
in solids
100
Conductivity (ies)
dependence of total
108/
electronic
83/
-emf-transference number data
98
independent ionic
107
partial
dependence of
198/
partial pressure dependence of .... 105
temperature dependences of
105
product, transference number-total 116
416
CORROSION C H E M I S T R Y
Corrosion (continued)
electrochemistry of
209
engineer
3,4/, 6/, 7,31
fatigue
339,348
-fracture processes
36
high-temperature
76, 391
"immune" to
47
inhibiting paints
2
inhibition
126,143,144/, 262
cooling system
360
definition of
263
formation of oxide
films
126
phenomena
263
in presence of hydrogen sulfide .. 307
inhibitors
chemistry of
315
classification of
266
effectiveness of alkyl propylenediamines and water soluble
polyphosphates
359
of iron
126, 306/
isothermal
94
kinetics
35
lecture classifications
13/
localized
147
mechanism
263
mitigation
47
mixed potential process
63/
oxide removal, influence of
199/
paralinear
200
and potential, relationship between 146/
potentials
2,14,66
process (es)
214/
electrochemical kinetics of
58
kinetics of
263
metallic
58
products formed on NiFe, IR
adsorption of
242/
rate(s)
60,66,129,139,198
of alloys
402*, 409/, 410/
constant for aluminum
214/
vs. inhibitor concentration
279
of iron
160/
of parabolic
92
of steel
286/
of uranium
220/
reactions, anodic oxidation
264
reactions, cathodic depolarization .. 264
resistance
193,196,215
response
236
specimen rack
407/
studies, electrochemical techniques
in
35
system(s)
15/
multiple partial process
67
terminology
3,5/
INDEX
417
Corrosion (continued)
testing, laboratory hightemperature
tests
of valve metals
"worm hole"
Corrosive gas
Coulometry
Crack
path of, intergranular
path of, transgranular
propagation
velocity
Cracking
airfoil-stress-corrosion
mechanism of
stress-corrosion
393
231/,394i
185
386
76
175/
323
323
334
328, 345
127, 381/, 382/
2
340
36,127,149,
321,327/, 406
in aqueous media
321
mechanisms
33
systems for
32
systems exhibiting
325i
Creep strain-rate
349
Crevice corrosion
385/
Croloy, corrosion of
194/
Croloy-5
193
Crystal lattice
97/
Crystalline salt phase
27
Current
48
density
52
corrosion
67
instantaneous
flux
71
-potential curves
181/
lines, potential-log
16
-potential curves
180
potential diagram
280/
Curves, anodic passivation
138/
D
Data
on atmospheric corrosion
235
galvanostatic
72/
potentiostatic
72/
Decohesion
343
Defect chemistry
90
Degradation, rate of oxide-film
178
Density, corrosion current
60
Dependence of partial conductivities 108/
Dependence of total conductivities .... 108/
Deposition
360
Dezincification
378/
of brass
371
layer type
377
plug type
377, 379/
Diagram ( s )
activation energy
136/
Evans
135, 269-271
Diagram(s) (continued)
polarization
63/, 68/
Pourbaix (see Pourbaix diagrams)
ternary
121/
Diesel engines
364
Diffraction, electron
141/
Diffusion
coefficients
84
controlled
91
kinetics
76
process, bulk
82
data, radiotracer
86
processes, bulk
94
tracer
83/
Diffusivity
116
Dipole orientation
21
Direct current (dc) analog
119/
Disorder
anti-Frenkel
81
Frenkel
79
Schottky
79
Displacement reaction
92,93/
Dissociation pressure of oxide
87/
Dissociation reactions, local
equilibrium of
114
Dissolution
203
anodic
128/, 138/
definition of
126
direct
130
of iron
126
kinetics
134
metal
136/
of metal lattice
162
of oxides, reductive
142
rate
179
reaction, predict
129
by separate steps
130
Tafel slope of free
177
Double recirculating cooling system .. 356/
Dry oxidation
235
E d d y current
Electric field, at interface
Electrical formulation of chemical
process of scaling
Electrochemical
behavior, effect of oxidizer
concentration on
behavior, effect of velocity on
cell
thermodynamic voltage of
condition, definition of
kinetics
of corrosion processes
200
39
101
157/
'. 158/
119/
121/
113
48
58
418
CORROSION C H E M I S T R Y
Electrochemical ( continued )
oxidation
47, 50
polarization systems
69, 70/
reactions
14,16, 41
techniques in corrosion studies
35
thermodynamics
38
Electrochemically active chemical
species
42
Electrochemistry of corrosion
209
Electrochemistry, solid-state
99
Electrode
conditions, ion blocking
117
exchange kinetics
Ill
polarizable
21,22/
potential ( s ) ( see also Potentiostatic)
14,38,39,43/
at 2 5 C
40f
-current relationships
14
equilibrium
40, 44
measurements
6
reactions
49/
control of
57
properties of
48
reference
69, 271
standard hydrogen ( S H E )
39
surface
11,21
test
43/, 69,154
working
271
Electrodedeposition of iron
142
Electrolysis ( charging mode )
115
Electrolyte
applications, high-temperature
solid
110
emf sensors, solid
110
fuel cells, solid
105
resistance, effect of
273/
two-anion
122
Electrolytic cell, multicomponent
mixed conducting
119/
Electrolytic plating
27, 28/
Electron
density
23
diffraction
141/
low energy ( L E E D )
32
microscope, scanning
32
spectroscopy for chemical
analysis ( E S C A )
32
Electroneutrality, laws of
265
Electronic(s)
conduction
96
conductivity
83/
semiconductor
23
transference numbers
98
Electropolishing
127,137
Electrosorption, van der Waals
267
Ellipsometer
172/
Ellipsometric parameters
171
Ellipsometry
164,236
Embrittlement
127
hydrogen
340,343
metal
156
emf sensors, solid electrolyte
110
emfs, thermocouple
114
Engines, internal compustion
365
Equilibrium
approach, local
112
electrode potential
44
of dissociation reactions, local
114
reaction, metal-metal ion
44
Equivalent circuit
118
approach
101
description
100
Erosion-corrosion
391
E S C A ( electron spectroscopy for
chemical analysis )
32
Evans diagram
135,269-271
Evans polarization curves
132/
Evaporative systems
362
Exfoliation
381/, 382/
F
Failure, stress-corrosion
Faradaic processes
Faraday
constants
Ferric ion reduction
Ferrous passivation
Film(s)
on aluminum, resistance for
growing barrier
breakdown
for aluminum alloys, hightemperature
localized
characterization of passivating
degradation of
formation of oxide
-forming metals
-forming reaction
fracture, corrosion
growth
and degradation of oxide
Tafel slope for
with pore, reaction on
reduction
repair
thicknesses
unstable in aqueous solution
Filming amines
Fitting
Flade potential, definition of
Flade relation
Formation of metal-nonmetal compounds, rate constants for
338
21
113,159
40, 269
67
153
188/
139,140
228
149
160/
196
196
185
177
340, 344
171
189
190/
151/
171
140
187
187
308
383/
154
169/
109f
419
INDEX
Formation, solid phase
27, 28/, 30/
Fracture toughness
326
Free
-energy conditions, zero
114
-energy data, thermodynamic
14
enthalpy
35
standard
36
Frenkel
defect
97/
Disorder
79
anti81
notion
96
Fretting attack
380, 384/, 385/
Freundlich isotherm
295
Fuel cell-battery applications
98
Fuels cells, solid electrolyte
105
G
Galvanostatic ( constant curren
data
72
Gas
corrosive
76
permeability studies
109
phase
11
Gasification environments, behavior
of high-temperature materials in 392
Gibbs-Duhem relation
101
Gold, water adsorption on
252/
Guy-Chapman double layer theory .... 262
H
Heat exchanger, cooling systems,
clogged
363/
Helmholz double layer theory
262
High coolant velocity
369/
High-temperature
-cooling water
364
corrosion
391
testing, laboratory
393
materials in gasification environments, behavior of
392
solid electrolyte applications
110
Hydrated oxides
187
Hydration, activation energy for
162
Hydrogen sulfide
concentration on corrosion, effect of 399/
corrosion inhibition in presence of 307
corrosion of iron in presence of
309
Hydrolysis systems
105
I
Ideal solution
"Immune" to corrosion
Immunity
256
47
129
causes of
267t
of iron corrosion
296/
mechanism, importance of hydroxyl
group
291
by thiourea and quinoline
derivatives
277
Inhibitor(s)
276/, 278/
acetylenic
288
activity and inhibitor constitution,
correlation between
293
amine
271
anodic
143
concentration vs. corrosion rate
279
concentrations, man-failures in
maintenance of
371
constitution and inhibitor activity,
correlation between
293
dangerous
347
definition of
126
interaction with iron sulfide surface 314
nonoxidizing
147,148/
oxidizing
147,148/
propargyl alcohol
306/
safe
347
Interface
alloy-oxide
91
compound-electrolyte
26/
metalcompound
24
metal oxide
80/
oxide
77
solution
39,48
oxide-gas
77, 80/
solid-electrolyte
23
Ion
blocking electrode conditions
exchange resin
117
198
CORROSION C H E M I S T R Y
420
Ion (continued)
exchange step, adsorption
mechanism involving
318
movements
21
Ionic
absorption
21
compound scale
99
conduction
96
conductivity, independent
107
migrations
101
transference numbers
98
Iron
-chromium alloys
193,195/
corrosion
126, 302/, 306/
in H S
311/
inhibition of
282, 296/
rate of
160/
dissolution of
126
electrodeposition of
142
-ferrous ion reaction
4
passivation
13
Pourbaix diagram
44,47,49/, 132/, 212
in presence of hydrogen sulfide,
corrosion of
309
sulfide surface, inhibitor interaction
with
314
Isothermal corrosion
94
Isotherms, adsorption
248/
water
245
i - V curve
155/, 170,180
0
Kinetics
diffusion controlled
electrochemical
of corrosion processes
electrode exchange
of metal oxidation
oxidation
parabolic oxidation
parabolic tarnishing
76
48
58
Ill
254
88
78/
96
L
Langelier Saturation Index
Langmuir adsorption constant
Langmuir models
Lattice, crystal
Le Chatelier's Principle
Lead-acid battery
Lead-acid storage battery
Lead oxide
Linear polarization measurements
Liquid boundary layer, mass flux
across
Liquid phase
Liquid-solid systems
361
294
255
97/
209
18/
14
16
277
313
11
19
Localized corrosion
Localized film breakdown
Low energy electron diffraction
(LEED)
Luggin capillary
147
149
32
69, 70/
M
Man-failures in maintenance of
inhibitor concentrations
371
Marker movement
77
Mass
flux across liquid boundary layer ... 313
transfer coefficient
56, 67
transport control
57
transport-controlled data
66
Materials research, coal gasification .. 392
Materials science
1
Mechanims, definition of
126
cracking
Melt, solidification of
Metal
-compound interface
dissolution
reaction
-electrolyte system
embrittlement
film-forming
hydride, formation of
inhibitor-chelate structure
ion concentration cell
lattice, dissolution of
-metal ion equilibrium reaction
-metal oxide interface
-metal sulfide systems
-nonmetal compounds, rate con
stants for formation of
oxidation, kinetics of
-oxide interface
-oxygen bond energy
passivation of
scaling systems
-solution interface
-solution potential difference
surface of
Metallic corrosion processes
Metallic systems
Microbicides
Microelectrode, glass, silver/silver
iodide, p H
Migration fluxes, steady state
Migrations, ionic
Mitigation, corrosion
Mixed conduction
medium
transport in
processes
338
28/
24
136/
177
11
156
185
212
307
372/
162
44
80/
394i
109i
254
77
190/
71
98
39
166
127
58
42
362
229/
116
101
47
112
112
105
421
INDEX
Mixed conduction (continued)
solids
theory of
Mixed (ionic and electronic)
conductors
Mixed potentials)
battery electrode
systems
Mobility, absolute
Mobility, electric
Models, kinds of circuit
Molar acquisition rate
Monolayer-oxide theories
Morphology of scaling layers
Multicomponent mixed conducting
electrolytic cells
Multicomponent mixed conductors
under closed circuit conditions,
motivating factors
100
110
100
14
19
65
116
116
Ill
105
165
107
119/
110
354/
209
164, 249
242/
73, 74
170
96
147,148/
198
p
Ohms law
Open circuit conditions
Open circuit theory
of parabolic oxidation
Open-recirculating towers
Optical
ellipsometry
photopotential measurement
Overaging, effect of
Oxidation
of alloys
-corrosion data
test exposure
corrosion rates, yearly
cubic
electrochemical
iron foil
kinetics
metal-oxide interface
open circuit theory of parabolic
potential
potentiostatic
rate
120
117
96
107
351
168
168
331/
88
393
406
401
203
47, 50
192/
88
222
107
173/
172/
87/
Oxidation (continued)
steady state
173/
thin-film
187
transient
173/
Oxide(s)
-film
degradation, rate of
178
structure of
140
theory
163
of passivity
159
-gas interface
77, 80/
hydrated
187
platelets on aluminum
191/
protective
94
Oxidizer concentration on electro
chemical behavior, effect of
157/
Oxidizing inhibitors
147,148/
Oxygen
chemical potential of
87/
reduction
Oxyhydroxide surface
11
259
Parabolic
growth law
198
oxidation, open circuit theory of
107
rate constant
89/, 208
scaling, physical processes
99
scaling rate constants
98
tarnishing kinetics
96
Parabolicyit of sulfide corrosion,
minute quantities of oxygen
310
Parameters, water adsorption
250i
Partial
conductivities, dependence of
108/
pressure dependences
106
constant
106
exponential
106
of partial conductivities
105
process ( es)
activation controlled
59
anodic
58
cathodic
53, 55/, 57
corrosion systems, multiple
67
Passivating film, characterization of .... 160/
Passivation
129
anodic
137
"cause" of
156
curves
138/
effect of complexing agent,
versene, on
145/
enforced
154
ferrous
153
iron
137
of nickel
164
422
CORROSION C H E M I S T R Y
Passivation ( continued )
phenomena
153
surface
19
theories
159
of titanium, spontaneous
158/
Passive
films
14
state, characterization of
166
state, definition of
156
Passivity
342/
on alloys
162
definition of
47,126
oxide-film theory of
159
Permalloy, water adsorption in
monolayers on
251/
Permeability studies, gas
109
Petroleum industry, corrosion
inhibitors in
308
p H on stress corrosion crack velocity,
influence of
332/
Phase
crystalline salt
2
equilibria
14
gas
11
liquid
11
nonmetallic
96
Physical arrangement, scaling
102/
Pilot plant testing
404
program
405i
Pitting
149,150/, 225,
243, 334, 342/, 371
for aluminum alloys, high
temperature
228
corrosion
339
Plants, coal gasification
92
Plating, electrolytic
27, 28/
Plating, vapor
28/
technique
27
Polarizable electrode
21,22/
Polarization
anodic
141/, 345
cathodic
345
curves
for anodic reactions
136/
Evans
132/
"superposition" of
58
diagram
63/, 68/
intermittent resistance
137
systems, electrochemical
69, 70/
Pollutants cause corrosion
236
Pollution controls
359
Polymerous layers with acetylenic
compounds, formation of
292f
Polyphosphates as corrosion inhibitors 359
Pore, reaction on film with
151/
Potential(s)
-current density curves
181/
difference, metal-solution
166
Potential ( s ) ( continued )
electrode
38, 39, 43/
measurements
69
equilibrium electrode
40
- l o g current lines
16
of oxygen, chemical
87/
systems, mixed
65
Potentiostatic (electrode potential) ..
71
circuit
155/
data
72/
oxidation
172/
Pourbaix diagrams
41, 42, 43/, 44,
45/, 46/, 130,213/
corrosion behavior
130
iron
44, 47,49/, 132/, 212
for F e - ,
167/
regions; corrosion, immunity, and
passivity
47
Powder surfaces
245
Predictive criteria for inhibitor effec
Pressure, oxygen partial
Preventative measures, stress-corro
sion cracking
Problem solving
Protective oxides
Pulse technique
87/
346
3
94
170
277
R
Radiator section, air-cooled
366/
Radiator section, closed-system
367/
Radiotracer diffusion data
86
Raoult's law
256
Rate
constants for the formation of
metal-nonmetal compounds .... 109f
constants, parabolic scaling
98
determining
94
step
93/
film thicknesses
177
limiting
91
oxidation
87/
of oxide-film degradation
178
of parabolic corrosion
92
Reaction mechanisms
131
Redox
50
definition of
38
systems
14
Reduction
cathodic
142,144/
ferric ion
67
oxygen
11
INDEX
423
Reference electrodes
69 Stable species
42
Refractive index
170 Stainless steels
196
Relative humidities, critical
243, 244t Standard
Remote sensor applications
98
electrode potential
41
Repassivation
334, 335/
free enthalpy
36
time
336
hydrogen electrode ( S H E )
39,44
Resistance for growing barrier films
Steady state migration
fluxes
116
on aluminum
188/
Steady state oxidation
173/
Resistant to atmospheric corrosion .... 239 Steel, corrosion rate of
286/
Steel, rest potential of
284/
Stirring
151/
S
Stoichiometry, state of
79
14
Saturation Index, Langelier
361 Storage battery, lead-acid
348
Scale control, calcium carbonate
357 Strain-rate
creep
349
Scale, ionic compound
99
surface
338
Scaling
360
Stress
electrical formulation of chemical
-corrosion
process of
101
cracking
36,127,149,
layers, morphology of
107
physical arrangement
102
rate
99,10
in aqueous media
321
simulated
102/
electrochemistry
322, 323
systems, metal
98
fracture mechanics
322, 323
Scanning electron microscope
32
mechanisms
338
Scattering, atmospheric corrosion
physical metallurgy
322
measured by light
240/
propagation of
335/
Schottky Disorder
79
anti79
systems of
324
Self-diffusion data
86
systems exhibiting
325i
Semiconductor electronics
23
velocity, effect of solution conSemiconductor solid
23
centration upon
333/
Sensor applications, remote
98
velocity, influence of p H on .... 332/
Shock, thermal
94
failure
338
Solid
mechanisms
340
adsorbent
254
susceptibility, testing for
324
electrolyte fuel cells
105
intensity
327/
-electrolyte interface
23
factor
326
material behavior
7
level
346
mixed conduction in
100
sorption
340,344
phase formation
27, 28/, 30/
Structural disorder, anti81
semiconductor
23 Sulfide corrosion, parabolocity of
310
state chemistry
16, 19
minute quantities of oxygen
310
-state electrochemistry
99 Sulfur concentration, high
391
surfaces, imperfections
128/
Sulfuric acid
16
Solidification
29 Surface
of melt
28/
chemistry
16
Solution
electrode
11, 21
composition
129
impurities
127
kinetics
293
concentration upon stress-corrosion
passivation
19
crack velocity, effect of
333/
phenomena
7
precipitation
28/
phenomenological studies
262
viscosity, effect of
330/
states
23
Sound metal loss
397
strain-rate
338
Spectroscopy, Auger electron
32
73,74
Spectroscopy, x-ray photoelectron
32 Symbols
Stability Index
358 Systems exhibiting stress-corrosion
Stabilizations, boiler-feed-water
2
cracking
325f
424
CORROSION C H E M I S T R Y
Tafel
behavior
66
constant or slope, cathodic
52
constants (slopes)
51
law
154
line
53
region(s)
61,135,270,271
data
62
slope(s)
14, 65, 189, 270, 277, 279-282
anodic
305
cathodic
305
for film growth
190/
of free dissolution
177
Tarnishing
259
Temkin discharge
176
Temperature on corrosion, effect of .... 398/
Temperature dependences
106
constant
106
exponential
10
of partial conductivities
10
Terminology, corrosion
3,5/
Ternary diagram
121/
Test atmosphere
236
Test electrodes
43/, 69,154, 270
Theory of mixed conduction
110
Theory of passivity, oxide-filmadsorption
165
Thermal shock
94
Thermocouple emf s
114
Thermodynamic ( s )
of active-passive transition
166
electrochemical
38
free energy data
14
voltage
115
of electrochemical cell
121/
Thin-film oxidation
187
Thiourea derivatives, inhibition by .... 277
Time-to-failure
326
Tin pest
365
Titanium, spontaneous passivation of 158/
Total conductivities, dependence of .. 108/
Toxicity
359
Tracer diffusion
83/
Transference number(s)
98,104
-total conductivity product
116
Transient oxidation
173/
Transition, thermodynamics of
active-passive
166
Transport
control, cathodic partial process
under
65
in mixed conduction medium
112
electron migration
112
ion migration
112
numbers
84
processes
87/
92
122
U
Uniform corrosion, definition of
Uranium
alloys, corrosion of
corrosion
rate
59
215
223/
221/
220/
V
Valve metals, corrosion of
185
van der Waals electrosorption
267
van der Waals force
267
Vapor plating
28/
technique
27
Velocity on electrochemical behavior,
effect of
158/
Versene on passivation, effect of
complexing agent
Voltage
of electrochemical cell, thermo
dynamic
-measuring device
thermodynamic
145/
121/
39
115
W
Water
adsorption
on gold
isotherms
in monolayers on Permalloy
parameters
summary of
in atmospheric corrosion
role of
Weight change of selected alloys
"Worm hole corrosion"
"Worm holing" attack
42
235,243
252/
245
251/
250i
257
258
235
400/
386
385/
X
X-ray photoelectron spectroscopy
32
2
Zero-free energy conditions
114
Zinc amalgan, dissolution of
264
Zircaloy-2, corrosion of
206/, 207/
Zirconium
crystal, oxidation of
211/
kinetic behavior
203
oxidation of
208,210/, 224/
poly crystalline
210/
oxide, recrystallization
203