Question

Discuss differential aeration corrosion with the help of a suitable example

Answer 1

Differential aeration: It is the most important type of concentration cell corrosion which

occurs when one part of a metal is exposed to a different air concentration from the other part

of the metal.

As a result, difference in the potential takes place between differently aerated areas. It has

been found experimentally that poor oxygenated (poorly aerated) parts of metals are

anodic and highly oxygenated (highly aerated) parts of metal are cathodic.

Example:

If a piece of metal say Zinc is partially immersed in dilute solution of a salt NaCl & the

solution is not mixed properly, the parts above & closely adjacent to the water line are most

strongly aerated because of the easy access of oxygen to this area & hence become cathodic.

On the other hand, parts immersed to greater depth have less access of oxygen & so they are

poorly aerated i.e. they show a smaller oxygen concentration & thus become anodic.

As a result difference of potential is created which is responsible for the flow of current

between two differently aerated areas of same metal Zinc.

Zinc will dissolve at the anodic area & oxygen will take up electrons at the cathodic area to

form hydroxyl ions.

The reactions involved are as follows,

Zn → Zn⁺⁺ + 2e^- (Oxidation)

½ O2 + H2O + 2e- → 2OH- (reduction)

The circuit is completed by migration of ions, through the electrolyte & flow of electrons

through the metal from anode to cathode.