Pitting corrosion is caused by the localised breakdown of the passive film that protects a corrosion resistant alloy. It starts at discrete points, wherever there is a weakness in that film or the environment is aggressive enough to attack it.
Carbon and low alloy steels tend to corrode across most of their exposed surface. Stainless steels, nickel alloys and copper alloys resist this because they form a passive film as soon as they meet oxygen. That protection is why the prevalent form of attack on a corrosion resistant alloy (CRA) is pitting rather than gross surface loss.
Where pits start
Several factors decide where a pit begins:
- Inclusions. An inclusion is a compound formed during steelmaking. Because it contains no chromium or molybdenum, it cannot form a protective passive film, so a pit can initiate at that point.
- Mechanical damage without oxygen. Damage alone should not cause corrosion, as the passive film regenerates almost immediately. But if the working environment lacks oxygen, the surface may not re-passivate, or not quickly enough.
Aggressive environments
Pitting also occurs where the conditions themselves are aggressive enough to attack the passive film, such as acidic solutions or the presence of chlorides, for example seawater. Elevated temperatures increase the speed of the reaction.