PITTING CORROSION

Pitting is a localized corrosion by which cavities or holes or pits that extend deep into the metal develop. This is considered to be more dangerous than the slower general type because the pits may decrease the material strength and also be the nuclei for fatigue failure. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. With some materials pitting rate may increase with time. Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Apart from the localized loss of thickness, corrosion pits can also be harmful by acting as stress risers. Fatigue and Stress Corrosion Cracking (SCC) may initiate at the base of corrosion pits. One pit in a large system can be enough to produce the catastrophic failure of that system.

Steel which normally rusts uniformly upon exposure to atmosphere may, with sufficient time, develop pits.

Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment, which often includes ensuring that the material is exposed to oxygen uniformly (i.e., eliminating crevices)