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.
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-
The circuit is completed by migration of ions, through the electrolyte & flow of electrons
through the metal from anode to cathode.