The difference in the availability of air or oxygen over a metallic surface leads to formation of oxygen concentration cells.
The less aerated or less oxygenated part behaves anodic while the more oxygenated part cathodic. Since cathodic reactions involve consumption of oxygen, the more oxygenated part behaves cathodic and less oxygenated pan behaves anodic.
The corrosions which are caused due to formation of such oxygen concentration cells are known as differential aeration corrosions.
It is believed that aeration causes the formation of an oxide film on the metal surface resulting in the development of a metal metal-oxide cell or even oxygen electrode-metal electrode system.
Most of the common types of corrosion are due to differential aeration. The following is the examples:
(a) Corrosion of metals partly immersed in solutions :
When a metal piece is partly immersed in a solution or water, the part of the metal piece above the surface of the solution is more aerated and behaves cathodic while the pans immersed in interior has less accessibility to oxygen and behaves anodic. The difference of potential created causes the flow of current between the two regions which results in corrosion.