Pitting Corrosion

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Corrosion & Protection

Definition

A localized form of corrosion that produces small cavities or pits on the metal surface while most of the surrounding area remains unattacked. It is particularly common in passive metals exposed to chloride ions.

Detailed Explanation

Pitting initiates when the passive oxide film breaks down at local defects, inclusions, or heterogeneities, creating a small active anode surrounded by a large passive cathode. The pit interior becomes acidified and oxygen-depleted, creating an autocatalytic environment that accelerates pit growth. Molybdenum additions to stainless steel significantly improve pitting resistance by stabilizing the passive film; this is why 316 stainless steel (with 2–3% Mo) outperforms 304 in chloride-containing environments.