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#055 Differential Aeration Corrosion

Category : Corrosion - Corrosion Protection
September10, 2010

Differential Aeration Corrosion is a type of corrosion phenomenon occurring with oxygen concentration differentials in water where a differential aeration battery environment is formed. As an experiment example shown in [Fig.1], a glass vessel divided with an earthenware barrier wall is filled with 3% salt water, and a pair of polished steel plates with a conductor connected to each are submerged. Using glass tubes, one steel plate is exposed to air and the other steel plate is exposed to nitrogen gas. After some time, the steel plate exposed to the nitrogen gas will corrode. The earthenware barrier wall is to keep the liquid in each compartment separated but to allow for electrical conduction. This corrosion occurs due to a differential aeration battery formation where the air exposed steel plate assumes a reduction reaction of dissolved oxygen as shown below, and the nitrogen exposed steel plate dissolves.

[Fig.1] Differential Aeration Battery

O2 + 2H2O + 4e → 4OH-

As an example, let us examine a case of corrosion of a stake driven into water. With passing time, a local corrosion just below the water line will be evident.

This is a case of Differential Aeration Corrosion as shown in [Fig.2]. The water around the stake at the water line is slightly raised due to water surface tension. The water layer is thin at this immediate area and receives rich oxygen from the air in comparison to the water below the water line. This creates a differential aeration battery condition.

Another example is old water plumbing made of cast iron pipes with rust bumps. When these rust bumps are removed from the surface, advanced corrosion can be seen on the cast iron pipe surface below the bumps.

This is also caused by the differential aeration battery effect. The rust bumps are surrounded by rich dissolved oxygen but the surface below the bumps are relatively isolated from the oxygen, causing the formation of a differential aeration battery, and the corrosion advances under the rust bumps.

[Fig.2] Differential Aeration Battery condition with a Stake