Which corrosion occurs specifically in crevices or under deposits on metal surfaces?

• A. Pitting corrosion
• B. Flow accelerated corrosion
• C. Crevice corrosion
• D. Microbiologic corrosion

Answer: (C)

Crevice corrosion is a localized form of corrosion that happens specifically in crevices or under deposits on metal surfaces. The key idea is the environment inside a crevice becomes different from the outside: oxygen diffuses slowly into the crevice, so the electrolyte inside becomes relatively depleted in oxygen and can grow acidic due to hydrolysis of metal ions and concentration of aggressive species like chlorides. This sets up a potential difference where the metal surface inside the crevice dissolves (anodic) while the area outside remains comparatively protected (cathodic). The trapped, stagnant electrolyte concentrates corrosive species, accelerating dissolution right at the crevice boundary and under any deposited material, gasket, or rough surface.

This form of corrosion often appears where gaskets, deposits, scale, or overlapped edges create a shielded gap, making it a classic scenario for crevice corrosion. In contrast, pitting corrosion is a localized attack that forms pits but isn’t defined by a crevice or under-deposit environment; flow accelerated corrosion relates to high fluid flow removing protective films rather than confinement; microbiologic corrosion involves microbial activity and biofilms, not the crevice-specific chemistry.