W-2&3 Corrosion and Its Types
W-2&3 Corrosion and Its Types
W-2&3 Corrosion and Its Types
Elbow
7. Intergranular Corrosion
•Intergranular corrosion occurs preferentially along
grain boundaries for some alloys and in specific
environments.
•The net result is that a macroscopic specimen
disintegrates along its grain boundaries. This type of
corrosion occurs in some stainless steels.
•When heated to temperatures between 500 and
800C for sufficiently long time periods, these alloys
become sensitized to intergranular attack.
•It is believed that this heat treatment permits the
formation of small precipitate particles of chromium
carbide (Cr23C6)by reaction between the chromium
and carbon in the stainless steel.
Chromium carbide particles that have precipitated
along grain boundaries in stainless steel, and
the attendant zones of chromium depletion.
•Both the chromium and the carbon must diffuse to the
grain boundaries to form the precipitates, which leaves a
chromium-depleted zone adjacent to the grain boundary.
Consequently, this grain boundary region is now highly
susceptible to corrosion.
•Intergranular corrosion is an especially severe problem in
the welding of stainless steels, when it is often termed
weld decay.
•Intergranular corrosion of stainless steels near welded
areas; caused by chromium carbide precipitation along
grain boundaries of alloy subject to prolonged heating in
the temperature range 400-850°C.
Weld decay in a stainless steel.
Preventions
(1) subjecting the sensitized material to a high-
temperature heat treatment in which all the
chromium carbide particles are re dissolved.
(2) lowering the carbon content below 0.03 wt% C so
that carbide formation is minimal.
(3) alloying the stainless steel with another metal
such as titanium, which has a greater tendency to
form carbides than does chromium so that the Cr
remains in solid solution.
8. Selective Leaching
Selective leaching (dealloying, demetalification,
parting and selective corrosion) is found in solid
solution alloys and occurs when one element or
constituent is preferentially removed (leached) as a
consequence of corrosion processes.
The most common example is the dezincification of
brass, in which zinc is selectively leached from a
copper–zinc brass alloy.
Similar process in other alloy system such as loss of
nickel, tin and chromium from copper alloy.
Preventions
Use alloys not susceptible to grain boundary
depletion,
Use a suitable heat treatment,
Altering the environment (e.g. lowering oxygen
content),
Use cathodic protection.
Selective corrosion on Cast Iron
9. Fretting Corrosion
It is refers to corrosion damage at the asperities
(roughness) of the contact surfaces. This damage is
induced under load and in the presence of repeated
relative surface motion, as induced for example by
vibration.
Overall steps for
Corrosion Preventions
1. Material Selection
2. Environmental Alteration
3. Design
4. Coating
5. Cathodic Protection
Cathodic Protection
One of the most effective means of corrosion
prevention is cathodic protection; it can be used for
all eight different forms of corrosion as discussed
above, and may in some situations, completely stop
corrosion.
Oxidation or corrosion of a metal M occurs by the
generalized reaction
M --- Mn+ + ne-
Cathodic protection simply involves supplying, from
an external source, electrons to the metal to be
protected, making it a cathode; the reaction above is
thus forced in the reverse (or reduction) direction.
1. Galvanic Coupling
The metal to be protected is electrically connected to
another metal that is more reactive in the particular
environment. The latter experiences oxidation, and,
upon giving up electrons, protects the first metal from
corrosion. The oxidized metal is often called a
sacrificial anode, and magnesium and zinc are
commonly used as such because they lie at the anodic
end of the galvanic series.