Hello Ashish, Thank you for your question.

Pitting corrosion is a localized type of corrosion. It occurs in the following scenarios – 1. Alloys with oxide films (Stainless steel, aluminium, nickel alloys, etc.) 2. Presence of rust layers (carbon steels, steels in marine environment with calcareous deposits).

In case of oxide films, the chloride ions penetrate the oxide films and come in contact with the metal underneath. They then react to form the respective metal chlorides. This leads to a localized material loss, forming a tiny pit. The area where the pit is formed is the anode, and the surrounding area is the cathode supporting the hydroxide formation and hydrogen evolution reactions.

The pit continues to grow due to an autocatalytic process, which means it will continue to grow irrespective of the presence of more electrolyte.

Stainless steel will undergo pitting corrosion most of the times.

Carbon steel will undergo pitting corrosion only at a high pH (alkaline pH) because only at that pH, it will have a passive film. In chloride environments, carbon steel will undergo pitting only if it has thick rust layers on top. Otherwise, it will undergo uniform corrosion.

Please let me know if you have anymore questions.