Due to chemical or electrochemical attack of
the environment on the surfaces of metal, compounds such as
oxides, sulphides, carbonates etc are formed, which is known
as corrosion. It is a slow process.
Types of Corrosion:
1. Oxidation Corrosion:
The metal reacts with atmospheric oxygen
in the absence of moisture to form oxides.
2M + nO2 → 2Mn+
+ 2No2- → M2
Besides oxygen, Gases like Cl2, CO2,
and F2 also react with metals.
2. Liquid metal Corrosion:
It takes place when a liquid metal flows
over a solid metal at high temperature. Eg. In nuclear
reactor sodium metals corrodes the cadmium rods.
3. Galvanic corrosion: When we joined two different metals directly
or by a conductor and exposed it to a conducting solution.
The more electro-positive metal will act as anode and the
metal which is less electro-positive acts as cathode. Due to
flow of current the metal which is acting as anode liberates
its free electrons and its corrosion begins. The corrosion
is more if the potential difference between the metals is
4. Pitting Corrosion: When a metal is covered by some water or dirt,
the part which is covered has low oxygen concentration acts
as an anode. The part which is exposed to oxygen acts as
cathode. The covered area gets corroded and small pits are
formed on it.
Fe2+ + 2OH- →
This type of corrosion is caused by Corrosion
deposits and dirt deposits. This can be noticed near the
rivets, bolts etc
6. Waterline Corrosion:
When water is stored in a metallic tank,
it is observed that the metal below the waterline gets
corroded. It is because the water below the waterline is
poorly oxygenated and acts as an anode. The metal above the
waterline is highly oxygenated and acts as a cathode. Hence
corrosion of metal below the waterline takes place.
7. Stress Corrosion: When the metal undergoes processes like
bending, rolling etc we apply stress on it. The areas of the
metal under stress acts as an anode and the remaining areas
act as cathode. Due to the potential difference set up
between the two areas the corrosion takes place in the
8. Inter granular Corrosion: This type of corrosion generally occurs
in alloys. This type of corrosion occurs along the grain
boundaries. It is due to the fact that the grain boundaries
shows higher electrode potential than the grains at the
centre. The material along the grain boundaries is more
sensitive to corrosion.
9. Erosion Corrosion: This type of corrosion occurs when the streams
of gases, liquids strikes the surface of metal. The striking
gases and liquids remove the protective films of metal
surface. It occurs when the gases and liquids strike at high
Wet or Immersed Corrosion:
This type of corrosion occurs when a metal is immersed in a
liquid for a long time. In this type the liquid slowly
penetrates into the surface of metal or metal ions slowly
dissolves into the liquid.