Corossion & Metal Finishing
Corossion & Metal Finishing
Corossion & Metal Finishing
Module-2
Corrosion
Definition of corrosion:-
Corrosion can be defined as “The destruction (or) deterioration (or) loss of metals by the surrounding
environment through chemical (or) electrochemical reactions”.
Corrosion may occur either in a dry environment (or) in an aqueous medium. The former is called dry corrosion
and the later is called wet corrosion.
It occurs by direct attacks of atmospheoric gasses such as oxygen, hydrogen, sulphide, halogens and sulphur-
dioxide on the metal forming oxide layer.
Ex: - the attack of dry air (or) oxygen on a metal to form an oxide layer over the surface.
Wet corrosion involves reactions in aqueous solution medium. The conducting surface of the metal undergoes an
electrochemical reaction with the moisture and oxygen present in the atmosphere.
According to electrochemical theory, corrosion of metals takes place due to the formation of anodic and cathodic
regions on the same metal surface in the presence of a conducting medium.
O2
Fe2+ Fe2+ OH- OH- H2O
Cathodic region
Anodic region
Electrons
Iron Metal
At the anodic region oxidation reaction takes place and the metal gets converted into its ions, liberating electrons.
Consequently, metal undergoes corrosion at the anodic region.
At the cathodic region, reduction reaction takes place. Since the metal cannot be reduced further, metal atoms at
the cathodic region are unaffected by the cathodic reaction.
At Anode:-
At anode ox ideation takes place in which the metal atoms are converted into their ions liberating electrons.
At Cathode:-
Mg
Mg alloys
ZN
Al
Cd
Al alloys
Base Metal
Mild steel
Cast steel
Pb
Sn
Brass
Cu
Ni
Stainless steel (18% Cr & 8% Ni)
Ag
Metal
Nobel
Ti
Au
PT
Types of Corrosion:-
Ex:-i). When iron is in contact with copper, iron becomes anodic and undergoes corrosion whereas copper becomes
cathodic and remains unaffected.
Fe metal Cu Metal
Anode Cathode
Nacl
Fe2+ + 2OH- Fe (OH) 2 solution
Typical examples of differential aeration corrosion are the Water-line corrosion & Pitting corrosion.
a). Water-line Corrosion:- It is observed in steel (or) Iron water tanks partially filled with water.
Metal just below the water line which is exposed to lower concentration of oxygen becomes anodic and undergoes
corrosion, Part of the tank just above the water line which is exposed to higher concentration of oxygen, becomes cathodic
and remains unaffected.
More
The cell reactions are oxygen, Rust
(Cathode)
At Anode: - M Mn+ +ne-
Less Oxygen
At Cathode: - 2H2O + O2 +4e- 4OH- (Anode) Water
Finally, the ions combine to form corrosion product.
Stress Corrosion:-
Stress corrosion occurs only in the presence of specific corrosive environment and due to stress portion of the
metal.
It occurs when stressed region of the metal are expose to corrosive environment. The stress region act as anode
and undergoes corrosion, the unstressed region act as cathode and is unaffected.
Ex: - Stressed region of mild steel undergoes stress corrosion in the presence of NaOH solution.
Caustic embrittlement:-
Mild steel boilers undergo corrosion at the stressed portion when the operating pressures are b/w 10atm, and
20atm, at high temperature in the presence of alkaline medium called caustic embrittlement.
The boiler water usually contains a certain amount of sodium carbonate which is added to water during softening
process. At high temperature and pressure, the carbonate undergoes hydrolysis to give sodium hydroxide and carbon-di-
oxide and make the boiler water alkaline.
Fine hair line cracks may develop at the stressed portion of the boiler. Boiler water containing alkaline impurities
pass into hair-line cracks and crevices by capillary action. This water evaporates and leaves behind caustic soda (NaOH)
in the cracks, whose concentration increase to 10%, thus a galvanic cell is set up b/w the iron under stress and the iron in
the main body.
Iron under stress acts as the anode and gets corroded resulting in boiler failure. The NaOH reacts with iron and
forms Na2FeO2 [sodium ferrate] which decomposes to give Fe3O4 and NaOH.
NaOH thus formed, further reacts with iron to cause corrosion this leads to the explosion of corrosion.
Several factors affect the rate of corrosion of metals. These are broadly classified as primary factors which are due
to the metal and secondary factors which are due to the environment.
Primary Factors:-
1. Nature of Metal:-
The position of the metal in the galvanic series decides the rate of corrosion and extent of corrosion.
The metals with lower e-de potential values are more reactive and more susceptible for corrosion than the metal
with higher e-de potential values.
The rate of the corrosion depends upon the difference in the position of the metals in the galvanic series, higher
the difference, faster id the corrosion at anode.
Ex: - Li corrodes faster than mg.
Zn corrodes faster than Fe, Cu corrodes faster than Ag.
3. Anodic and Cathodic Areas: - The rate of corrosion is highly influenced by the relative area of anode and cathode.
If the metal has small anodic area and large cathodic area, all the electrons liberated at anode are consumed at the
cathodic region; therefore the rate of anodic reaction is greater and increase the rate of corrosion.
If the metal has, large anodic area and small cathodic area, demand for electrons will be less and this results in
decrease rate of dissolution of metal at anodic region and decrease rate of corrosion.
The rate of corrosion is directly proportional to the ratio of area of cathode to the area of anode.
Area of cathode
Rate of corrosion = Small anodic area
Area of anode
Higher the value of rate of corrosion greater is the rate of corrosion.
Ex: - A broken coating of tin on iron surface, a small
exposed part of iron acts as anode and rest of large Tin-metal
tin coated area act as cathode. Because of small anodic area to [[[cathode]
cathodic area the rate of corrosion is high. Fe-metal
[anode]
Secondary Factors:-
1.PH:-
Acidic media are generally more corrosive than alkaline (or) neutral media. The PH of the solutions decides the
type of cathodic reaction.
The rate of corrosion increases with decreases in PH
Ex: - At PH greater 10, corrosion of iron practically decreases due to the formation of a protective coating of
hydrous oxides of iron. b/w PH 3, iron undergo rapid corrosion due to continuous evolution of hydrogen at the
cathodic region.
The rate of corrosion decreases with increase in PH
Ex: - Zn suffers from corrosion even in the presence of mild acidic medium, where as corrosion is minimum at
PH=11.
This notes was prepared by,
M.PRAVEEN, MSc., M.Tech.,(Ph.D)., according to VTU norms new syllabus -2015
For any doubts contact: saintypraveen@gmail.com
Tuesday, May 05, 2015 8
3.Conductance:-
The rate of corrosion is directly proportional to the conductance medium.
The rate of corrosion increases in the presence of conducting species in the atmosphere because corrosion is in
electrochemical phenomenon.
Ex: - i). The corrosion of metal structures is faster within the clay and mineralized soils than in dry sandy soils.
ii). The rate of corrosion is more in ocean water and less in river water.
The most common method of preventing corrosion is the selection of proper metal (or) alloy for a particular
application.
Ex: - a). Stainless steels-Nitric acid. Ii). Lead-H2SO4
Selecting pure metals to minimize corrosion.
The component should be free from residual stress.
2. Protecting Coating:-
Corrosion is prevented by the application of protective coating on the surface of metal, there by the
metal surface is isolated from the corrosive environment.
Important types of protective coating are,
a. Metal coating. b. Inorganic coating.
a. Metal coating:-
“The process of coating base metal with a layer of protective metal is known as metal coating”.
There are two types of metal coating,
1. Anodic coating. 2. Cathodic coating.
1. Anodic coating;-
“The coating metal is anodic to base metal is called anodic coating”.
Ex: - Galvanization.
Galvanization:-
“The process of coating zinc on the base metal [Fe, Steel] surface by hot dipping is known as galvanization”.
The metal surface is washed with organic solvent to remove organic matters.
The metal surface is treated with dil.H2SO4 to remove rust and other deposits.
Then it is well washed with water & it is air dried by passing with hot air.
The metal is treated with a mixture of zinc chloride and ammonium chloride solution which prevents the oxidation
of the coated metal.
Then the metal is dipped in molten Zinc at 4500c
Excess of zinc on surface is removed by passing a pair of hot rollers, which removes excess of zinc and produced
thin coating.
Application:- Galvanized materials are used in fencing wire, buckets, bolts, nuts, nails, screw etc.
Note:- Galvanized sheets cannot be used for preparing (or) storing food because zinc dissolves in acidic medium and
forms toxic compounds.
2. Cathodic coating:-
“The coating metal is cathodic to base metal is called cathodic coating”.
Ex: - Tinning.
Tinning:-
“The process of coating a thin film of molten tin on the base metal surface by hot dipping “is called tinning.
The Iron sheet is washed with organic solvent to remove oil (or) grease.
The Iron sheet is passed through dil.H2SO4 to remove rust (or) scale and other deposits.
Then it is well washed with water.
The Iron sheet is treated with a mixture of ZnCl2 + NH4Cl for dried.
It is then passed through a tank contains molten Tin [2190-3100c].
Then the excess of tin is removed by rolling.
The coated tin is passed through palm oil which prevents the oxidation of the coated tin.
Advantages:-
1. Tin is non-toxic in nature and more nobel than the base metal.
2. Tinning gives a film which completely covers the surface and provides a corrosion resistant coating.
b. Inorganic coating:-
These coatings are produced at the surface of the metal by chemical (or) electrochemical reactions.
Anodizing:-
Anodizing coatings are generally produced on non-ferrous metals like Al, Zn, Mg and their alloys by anodic
oxidation processes in which the base metal is made as anode.
Anodizing of Aluminium:-
When aluminium metal is made anodic in an electrolytic bath with H2SO4 (or) chromic acid as the electrolyte, a
thin layer of Al2O3 is formed on the surface. This process is called Anodizing (or) Anodic oxidation of Aluminium.
The porous coating is obtained by anodic oxidation; it is carried out by making it anode in an electrolytic bath
containing a suitable acid like chromic acid (or) H2SO4 at 350-400c. A plate of lead (or) stainless steel is made the cathode.
When current of moderate density is passed, the O 2 liberated at the anode combines with it to form oxide which takes the
form of thick film Al2O3 deposits on the surface of the object.
The non-porous coatings are produced by using non-corrosive electrolytes like boric acid and borax. These
coatings are applied on electrolytic condensers.
Phosphating:-
Phosphating is the process of depositing a coating of metal phosphate on metal surface by chemical (or)
electrochemical reactions.
The PH of bath is maintained in the range of 1.8-3.2, the temperature is maintained at 350c. The reaction involves
dissolution of metal as ions, resulting in the formation of surface film consisting of crystalline Zn-Fe (or) Mn-Fe
phosphates.
Ex:- coating of mixture of iron & zinc phosphate on steel fabrications such as refrigerators and car bodies.
Cathodic Protection:-
Cathodic protection is the technique of offering protection to a specimen against corrosion by providing electrons
from an external source.
In this method the metallic structure to be protected is connected to a more anodic metal using a metallic wire.
More active metal gets corroded, while the parent metallic structure is protected from corrosion. Since the more active
metal (or) anodic metal are sacrificed and hence it is known as Sacrificial Anode method.
The commonly used sacrificial anodes are Mg, Al, Zn, and their alloys.
Metallic Wire
In this method, the article to be protected is made as cathode by connecting it to a –ve terminal of the DC
supply and an auxiliary e-de made of nobel metal (or) graphite is connected to the +ve terminal of the Dc supply, acts as
anode. The anode is buried in the soil, the anode is usually surrounded by backfill consisting of gypsum (or) coke breeze,
which improves electric contact b/w anode and surrounding soil. Current passes to the article & corrosion is suppressed.
Tank
[Cathode]
Advantages:-
1. Requires low installation cost and minimum maintenance cost after installation.
2. Method protects large area of the base metal.
This notes was prepared by,
M.PRAVEEN, MSc., M.Tech.,(Ph.D)., according to VTU norms new syllabus -2015
For any doubts contact: saintypraveen@gmail.com
Tuesday, May 05, 2015 14
It is used for protecting water storage tanks and oil pipe lines.