This contains eight forms of corrosion which are very often discussed.Chemical Engineers must have proper knowledge about corrosion.

1. Corrosion is the disintegration of an engineered material into its constituent atom due to
chemical reactions with its surroundings. In the most common use of the word , this means
electrochemical oxidation of metals in reaction with an oxidant such as O2.
Corrosion
Eight forms of corrosion
1. Uniform Corrosion
2. Galvanic Corrosion
3. Crevice Corrosion
4. Pitting Corrosion
5. Stress Corrosion
6. Erosion Corrosion
7. Intergranular Corrosion
8. Selective Leaching Corrosion

2. This type of corrosion is first seen as a general dulling of the
surface and, if allowed to continue; the surface becomes rough.
This is common form of corrosion.
Effect produced by most direct chemical attacks.
This corrosion is also called General Corrosion.
1.Uniform Corrosion:

4. 2.Galvanic Corrosion:
Corrosion damage induced when two dissimilar
materials are coupled in a corrosive electrolyte.
Takes place when two metals are in physical
contact with each other and are immersed in a
conducting fluid.
Galvanic Corrosion also called as “dissimilar metal
corrosion”

5.  Plate and screw of different electrical potentials due to differences in processing.
 Multiple component implant using different metals for each component.
 Copper and steel tubing are Joined in a domestic water heater, the steel will corrode
in the vicinity of the junction
Examples:

6. 3.Crevice Corrosion:
Crevice corrosion is a localized form of corrosion usually
associated with a stagnant solution on the micro-environmental
level .
Such stagnant microenvironments tend to occur in crevices
(shielded areas) such as those formed under gaskets, washers,
insulation material , fastener heads, surface deposits, dis-bonded
coatings, threads, lap joints and clamps.
Occurs under gaskets, rivets and bolts , between valve disk and seats.

7. Well-known examples of such geometries including flanges, gaskets, dis-bonded
linings/coatings, fasteners, lap joints and surface deposits.

8. 4.Pitting Corrosion:
Pitting corrosion is a localized form of corrosive attack that produces holes or
small pits in a metal.
The bulk of the surface remains un-attacked.
Pitting is often found in situations where resistance against general corrosion is
conferred by passive surface films.
Localized pitting attack is found where these passive films have broken down .
Pitting attack induced by microbial activity, such as sulfate reducing bacteria
(SRB) also deserves special mention.

10. 5.Stress Corrosion:
This type of corrosion is observed in fabricated articles which are subjected
to various mechanical operations.
Here mechanical operations refers Bending , Hammering and annealing.
This corrosion is usually unpredictable is nature.

11. 6.Erosion Corrosion:
It is the result of relative movement between the corrosive fluid and metal surface .
All types of equipment exposed to moving fluids are subjected to erosion corrosion.
Surface chemistry can play a role in erosion corrosion due to Mechanochemical
effects.
Examples:
Ship propellers
Hydraulic turbines
Pump impellers
Diesel engine cylinder

13. 7.Inter-granuIar Corrosion:
Inter-granular corrosion refers to preferential (localized)corrosion along grain
boundaries or immediately adjacent to grain boundaries, while the bulk of the
grains remain largely unaffected .
This form of corrosion is usually associated with chemical segregation effects
(impurities have a tendency to be enriched at grain boundaries) or specific phases
precipitated on the grain boundaries.
This selective dissolution may lead to the dislodgement of grains .
Inter-granular corrosion in sensitized stainless steels and exfoliation in aluminum
alloys represent industrially significant examples of this form of damage.

15. 8.Seleactive Leaching:
Selective leaching, also called de-alloying , de-metallification, parting and
selective corrosion.
Selective leaching is a corrosion process in which one constituent of an alloy is
preferentially dissolved by the environment ,leaving the de-alloyed metal weak
and often porous.