Pitting corrosion (Topic 11001)

Pitting corrosion is the most common corrosion form seen on aluminium. It starts from a weak point in the oxide, and after initiation, the reactions within the pit will reduce the pH and increase the chloride concentration and in this way sustain the reaction. Pitting can penetrate several mm during a short period if the conditions are extremely unfavourable. Normally the development of pit depths follows a time function of the form:

D = K · tc

Where D is the depth of the deepest pit, K is an alloy-, surface area- and environment-dependent constant, t is time and c is an environment-dependent parameter which often is close to 1/3. For good alloys immersed in sea water, with t given in years and D in mm, K can typically be close to 0,75. The development of the maximum pit depth will typically follow the form shown below.

Typical development of pitting corrosion rate, the figures on the axes are only examples. [34].

with a rapid increase after initiation followed by a slower growth. The pits can be of different shapes, wide or narrow. The narrow pits can often be dangerous because the attack can be deep and difficult to detect.

The susceptibility to pitting corrosion depends on several parameters.

How to avoid pitting corrosion.