A highly localized form of corrosion that produces small, often deep cavities (pits) on a material surface following local failure of a protective film or coating.

Principle

Initiation occurs when a passive protective film (e.g., oxide) is locally disrupted (mechanically or chemically), exposing a small anodic area where metal dissolution proceeds. Ion migration and hydrolysis within the pit create an autocatalytic acidic, chloride‑rich microenvironment that sustains rapid anodic dissolution while the surrounding surface acts as the cathode.

Demonstration

Stainless steel in chloride‑containing environments (e.g., seawater) frequently exhibits pitting: tiny initiation sites grow into deep pits that penetrate quickly relative to background corrosion. Pipelines and heat‑exchanger tubes can develop through‑wall pits from seemingly minor initial damage.

Misapplication

Confusing general or uniform corrosion with pitting; attributing a large, shallow loss of material to pitting; declaring a pit to be the result of mechanical damage when electrochemical initiation and autocatalysis explain its growth.

Consequence

Even very small pits can lead to rapid perforation, reduce fatigue life, and act as crack initiation sites for stress‑corrosion cracking. Pits are difficult to detect and predict; their presence changes inspection and maintenance priorities and may require localized repair or material change.

Reversal

Reversed behavior would be sustained passivation: intact uniform protective film prevents initiation and pit growth, causing slow uniform corrosion only. Active repassivation halts pit propagation.

Boundary

Occurs primarily in metals capable of forming passive films (stainless steels, titanium, aluminum). It is distinct from crevice corrosion (which is geometry‑driven) and from intergranular attack; it may overlap with microbiologically influenced pitting but differs mechanistically. Environmental aggressors (chlorides, bromides) and local heterogeneities are necessary conditions.

Semantic Tension

Tension exists between pitting and crevice corrosion because both are localized; practitioners must distinguish open‑surface pit initiation from occluded crevice mechanisms. Pitting is also often conflated with erosion or mechanical perforation when inspection lacks electrochemical context.

Synthesis

Pitting corrosion is the localized destruction of a passive surface film that leads to rapidly growing, deep cavities sustained by an autocatalytic, aggressive microenvironment; small initiations can produce disproportionate structural risk.