These are particularly insidious types of corrosion: the attack is limited to a very small area of the metal, the remainder being free from attack. The depth of the attack can increase very rapidly.

Pitting phenomenons can be separated into two stages:
- the initiation: occurs at a point where the protective film is accidentally damaged or weakened (by impurities, scratches, notches, rough surfaces)
- the progression: will increase rapidly if the protective film has difficulty in re-establishing itself.

This can occur when deposits become established on the material Crevice corrosion is a particular from a pitting corrosion and occurs when the surrounding environment remains stagnant on the material, small defects gradually becoming depleted in oxygen. The corrosion products arising from this concentration pile are acid and, themselves, very corrosive.
The following are the essential ingredients likely to lead to pitting and crevice corrosion:
- chlorides, particularly when associated with oxidising conditions (aeration, ferric iron etc.)
- deposits (rust etc.)
- structure anomalies (joints,threads, etc.).

The presence of molybdenum in the stainless steel can diminish or even prevent from crevice and pitting corrosion.