What is electrochemical corrosion ?

Electrochemical corrosion involves:

1. The formation of anodic and cathodic areas or parts in contact with each other.
2. Presence of a conducting medium.
3. Corrosion of anodic areas only.
4. Formation of corrosion product somewhere between anodic and cathodic areas. This involves flow of electron-current between the anodic and cathodic areas.

At anodic area oxidation reaction takes place (liberation of free electron), so anodic metal is destroyed by either dissolving or assuming combined state (such as oxide, etc.). Hence corrosion always occurs at anodic areas.

M (metal) → M n+ + n e-

Mn+ (metal ion) → Dissolves in solution

→ forms compounds such as oxide

At cathodic area, reduction reaction takes place (gain of electrons), usually cathode reactions do not affect the cathode, since most metals cannot be further reduced. So at cathodic part, dissolved constituents in the conducting medium accepts the electrons to form some ions like OH and O^2-.

Cathodic reaction consumes electrons with either by

(a) evolution of hydrogen or

(b) absorption of oxygen, depending on the nature of the corrosive environment

The electrochemical corrosion is classified into the following two types:

(i) Galvanic (or Bimetallic) Corrosion

(ii) Differential aeration or concentration cell corrosion.