3. • It is also known as uniform attack corrosion.
• It is the most common type of corrosion and is caused by a
chemical or electrochemical reaction that damages the entire
exposed surface of a metal.
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A.GENERAL CORROSION
4. 1. Intergranular corrosion:
It is a corrosion of a chemical or electrochemical attack on the
grain boundaries of metal. It often occurs due to impurities in
the metal which tends to be present in higher content near the
grain boundaries.
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B.LOCALISED CORROSION
5. 2.Pitting corrosion
Pitting corrosion occurs when a small hole or cavity is formed
in the metal, usually as a result of de-passivation of a small
area. This area becomes anodic, while a part of the remaining
metal becomes cathodic, producing a localized galvanic
reaction. This form of corrosion is often difficult to detect
because it is generally relatively small and can be covered and
hidden by compounds produced by corrosion.
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6. 3. Crevice corrosion:
• Crevice corrosion: Similar to pitting, crevice corrosion occurs at a specific
location or crevices. The solution get deposited to crevices and drying of
liquid takes more time. As a result crevice corrosion occurs. This type of
corrosion is often associated with a stagnant micro-environment. Acidic
conditions or a depletion of oxygen in a crevice can cause crevice
corrosion.
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7. 4. Stress corrosion
• When tensile stress is greater than yield stress, then
cracks develop on the surface. For example: cold
formed brass develop cracks in presence of ammonia
environment.
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8. 5. Fretting Corrosion
• Fretting corrosion occurs as a result of repeated wearing or due to sliding or
vibration of metals on an uneven rough surface.
• The corrosion occur due to generation of frictional heat which produce
oxide films which continuously removed and cause corrosion at fresh
surface.
• This can be avoided by minimizing friction or by proper lubrication of
equipment.
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9. 6. Erosion Corrosion
• This occur due to destruction of metal by combination of
abrasion and attrition caused by flow of liquid or gas. The
factors, such as alloy content of steel, pipe system design,
component geometry and water composition, influence
erosion.
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10. 7. Cavitation Corrosion
• Cavitation erosion occurs due to formation of vacuum bubbles
in liquid stream when passed over the surface. The bubbles on
metal surface get collapsed and cause damage and further
corrosion. This type of corrosion generally occur around
propellers. This can be avoided by using more resistant metal
and by using protective coating.
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11. 8. Galvanic Corrosion
• Galvanic corrosion occurs when two different metals are found
together in a corrosive electrolyte. A galvanic cell is formed
between the two metals, where one metal becomes the anode
and the other the cathode. The anode corrodes and deteriorates
faster while the cathode deteciorates more slowly. Protective
oxide film reduce galvanic corrosion.
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12. 9. Oxygen concentration cell
• Oxygen concentration cell: This type of corrosion
occur due to difference in amount of oxygen in
solution at one point when compared with other. The
least oxygen concentration increase rate of corrosion.
Redesign of equipment avoid such type of corrosion.
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13. 10. Corrosion fatigue
• Due to repetition metal surface get corroded by corrosive
media. The protective oxide film that prevent corrosion get
ruptured at higher rate as compared to new protective film.
Therefore ,surface loose its fatigue resistance and finally
equipment get deteriorate.
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14. 11. Impingement attack or corrosion
• It is also called velocity accelerated corrosion. This type of
corrosion is accelerated by removal of oxide films.
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15. 12. Hydrogen Embrittlement
• Penetration of hydrogen into metals cause loss in ductility. At
elevated temperature, cracking of carbon steel occur. As a
result, failure occur by embrittlement, cracking and blistering.
The solution of hydrogen sulphide and hydrocyanic acid is
susceptible to this form of attack.
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16. C. STRUCTURAL CORROSION
• In this type of corrosion, structural strength of metal get
reduced due to corrosion. It is of two type
• a. Graphite corrosion: Graphite corrosion generally involve
Gray Cast iron in which metallic iron get converted into
corrosion product. This produce residue of intact graphite
which is mixed with iron corrosive product and insoluble
components of cast iron.
• b. Dezincification: This is observed in brass which contain
15% zinc. The major constituent of corrosion is metallic
copper. The mechanism behind corrosion is
(1) Redeposition of copper over corrosion products
(2) Formation of zinc corrosion products leaving copper residue.
This type of corrosion can be decreased by adding little
amount of arsenic, antimony, phosphorus to the alloy
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17. D. BIOLOGICAL CORROSION
• Corrosion occur due to metabolic activity of microorganism
which cause deterioration of metal.
• The biological corrosion is due to:
a. Changing resistance to surface film
b. Developing electrolyte-concentration cells on metal surface
c. Developing corrosive environment
d. Altering rate of anodic/cathodic reaction.
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