A localized form of electrochemical corrosion that produces small, often deep cavities (pits) on a metal surface following local breakdown of a protective passive film or differential aeration, leading to concentrated metal loss confined to small surface sites rather than uniform general corrosion.

Principle

Local depassivation or heterogeneity (inclusions, crevices, or aggressive anions) creates an anodic site where metal dissolution is self‑accelerated by local chemistry (acidification, metal ion concentration) and restricted diffusion, causing a pit to grow much faster locally than the surrounding general corrosion rate.

Demonstration

Illustrative scenario → Stainless steel component in stagnant chloride-containing electrolyte: Situation: a weld area contains an inclusion and a small crevice where oxygen access is limited. Recognition: a localized bright spot develops and after exposure a small deep pit is observed. Action: pit propagation continues by local acidification and metal dissolution unless inhibited or mechanically removed; engineering responses include inspection, material replacement, design elimination of crevices, or corrosion inhibitors. Consequence: pits reduce cross-section, serve as fatigue crack initiation sites, and can cause sudden perforation or leak paths despite low measured average corrosion rates.

Misapplication

Assuming that a low overall corrosion rate implies negligible risk of pitting; the error is conflating average corrosion measurements with localized susceptibility—materials with low uniform corrosion can still be highly prone to deep pitting under specific local conditions.

Consequence

Pitting can produce critical reductions in load-bearing section, initiate fatigue cracks, create leak paths in pressure-containing components, and lead to sudden, difficult-to-detect failures because pits occupy a small area yet concentrate damage; detection and sizing are challenging and often require targeted inspection.

Reversal

Some alloys, surface treatments, adequate cathodic protection, or maintaining environments that stabilize the passive film (oxygenated flow, absence of aggressive anions) can suppress pit initiation and growth; in crevice geometries the mechanism shifts to crevice corrosion which is driven more by geometry and restricted diffusion than by uniform surface heterogeneity.

Boundary

Clearly within: small, localized cavities formed by anodic metal dissolution after local passive film breakdown. Boundary case: crevice corrosion, which is localized but strongly geometry-dependent and overlaps mechanistically with pitting. Clearly outside: uniform general corrosion where metal loss is roughly even across the exposed surface.

Semantic Tension

Selecting materials or coatings to resist pitting (often more expensive alloys or treatments) competes with cost, manufacturability and other property demands (mechanical, thermal); inspection resource allocation trades early detection effort against lifecycle risk.

Synthesis

Pitting is a failure mode driven by local electrochemical instability rather than average corrosion rate; engineering control requires attention to micro-environment, geometry and passive-film stability, plus inspection strategies focused on small-area defects rather than bulk corrosion metrics.