Influence of O2-content (Topic 11036)

Oxygen plays a significant role in the corrosion reactions of aluminium. The most important cathodic reaction is the oxygen reduction, see below;

O2 + 2 H2O +4 e- = 4 OH-

This means that if the access of O2 is limited, the cathodic reaction and hence the corrosion reaction will be confined.

Secondly, the protective aluminium oxide layer needs oxygen in order to be sustained and rebuilt, but the oxide layer can be formed by a direct reaction between water and aluminium, and the oxide layer do probably not require dissolved oxygen in the water. Thus, normally an unrestricted access to oxygen will enhance corrosion on aluminium and in neutral water with limited amounts of oxygen, the corrosion rate will be lower. In acidic or alkaline water other cathodic reactions are more important and will be additional to the oxygen reduction.

An example of an area with high corrosion rates because of unrestricted access to oxygen is the so called splash zone in sea water.

When the access of O2 is uneven, so called oxygen-cells may be formed. Where the cathodic reaction takes place in an area with good access to O2 , while the dissolving of the metal (the anodic reaction) proceeds in an area with little or no O2. An example of this is crevice corrosion.