Pitting occurs mainly in the presence of neutral or acidic solutions containing chlorides or other halides. Chloride ions facilitate a local breakdown of the passive layer, especially if there are imperfections in the metal surface.

Initiation sites may be non-metallic inclusions, e.g. sulphides, microcrevices caused by coarse grinding, or deposits formed by slag, suspended solids, etc.

When the metal corrodes in the pit, dissolved metal ions generate an environment with low pH and chloride ions migrate into the pit to balance the positive charge of the metal ions.

Thus the environment inside a growing pit gradually becomes more aggressive and repassivation becomes less likely. As a result, pitting attacks often penetrate at a high rate, thereby causing corrosion failure in a short time.

The pits often appear to be rather small at the surface, but may have larger cross-section areas deeper inside the metal. Since the attack is small at the surface and may be covered by corrosion products, a pitting attack often remains undiscovered until it causes perforation and leakage.

A higher chromium, molybdenum and nitrogen content in the steel increases the resistance to pitting. The relative resistance to pitting is often measured as a critical pitting corrosion temperature, CPT, in a selected media. The stainless steel grades 254 SMO, SAF 2507, and especially 654 SMO, have very high resistance to pitting.