What is caused by differences in concentrations of materials?

• A. Uniform corrosion
• B. Erosion
• C. Crevice corrosion
• D. Fretting corrosion

Answer: (C)

Differences in concentrations of dissolved species between a confined crevice and the surrounding solution set up a local electrochemical cell that drives metal loss inside the crevice. In the tight gap, the solution exchanges slowly with the bulk, so oxygen is consumed inside the crevice faster than it can be replenished. This creates a reducing, oxygen-depleted environment with possible accumulation of chlorides and a drop in pH. The metal surface inside the crevice becomes anodic and dissolves, while the external surface remains relatively cathodic. The result is localized corrosion focused within the crevice, even though the rest of the metal may appear relatively uncorroded.