Unit 2- Class 2- CORROSION (Electrochemical theory of corrosion)

Electrochemical theory of corrosion:

Most of the corrosion takes place on the basis of electrochemical reactions on the surface of metal such a type of corrosion is known as wet corrosion.

Electrochemical theory of corrosion can be taking iron as an example.
When a metal like iron is exposed to the environment according to electrochemical theory corrosion of metal takes place due to the formation of anodic and cathodic regions on the same metal surface or when the two metals are in contact with each other in a corrosive medium.

These anodes and cathodes are formed due to the heterogeneities at the interfaces of the metal and environment. The heterogeneities on a metal surface could develop due to several factors like
1. On a metal surface if the concentration of the oxygen is different (if in the metal the area which is exposed to more oxygen acts as cathode, the area which is exposed to less oxygen concentration acts as anode).
2. Due to contact of two different metals (egs: if copper and iron are in contact with each other, then Fe acts as anode and copper acts as cathode due to change in electrode potential).
3. If metal surface subjected to stress (area under stress acts as anode).


Thus anodic and cathodic area are formed, in presence of corrosion medium ( like moisture etc.)
At anode oxidation takes place so that metal is converted into metal ions with the liberation of electrons.
M------>Mn+ + ne-

Egs: Fe------->Fe2+ + 2e-


At the cathodic regions, reduction takes place since the metal at cathodic region cannot be reduced further, so some constituents of the corrosive medium take part in the cathodic reaction. Since in the cathodic reaction as the constituents of the corrosion medium are involved, they are more complicated and dependent on the nature of environment. Most common type of cathodic reaction are 1. Liberation of hydrogen 2. Absorption of oxygen.

Let us end this discussion here and take up types of corrosion in next post