Crevice corrosion is caused by which mechanism?

• A. Potential difference caused by ion concentration differences
• B. Differences in electrode material within the same metal
• C. Temperature gradient across a crevice
• D. Ion concentration differences in electrolyte

Answer: (D)

Crevice corrosion happens when a tight, stagnant crevice creates a separate, altered electrolyte environment. Inside the crevice, diffusion is limited, so ion concentrations build up and oxygen is depleted. This sets up a local concentration cell: the metal surface within the crevice becomes anodic and dissolves, while the exterior surface acts as the cathode and reduces species like oxygen. The result is localized dissolution driven by differences in ion concentrations in the electrolyte between the crevice and the surrounding fluid. Other factors like differences in electrode materials or a temperature gradient can influence corrosion in other scenarios, but the distinct rise in ion concentration differences inside the crevice is the primary driver here.