With general corrosion, the anodes and cathodes are caused by...

• A. Uniform external currents.
• B. Minor differences in composition or orientation between small areas on the metal surface.
• C. Differences in ambient temperature only.
• D. Impurities introduced from the environment.

Answer: (B)

General corrosion happens when many tiny galvanic cells form on the metal surface due to microstructural differences. Small areas have slight variations in composition (like impurities or second-phase particles) or crystallographic orientation, making some spots more anodic and others more cathodic in the same electrolyte. The electrochemical reactions cause metal to dissolve at the anodic sites while the cathodic sites facilitate reduction, and because these microcells are spread across the surface, the loss of metal tends to be uniform. External currents, pure temperature differences, or environmental impurities alone don’t explain the distributed anodic/cathodic behavior as clearly as surface micro- variations do.