A primary cause of intergranular corrosion is

• A. Improper heat treatment
• B. Impurities in alloy
• C. Excessive cold work
• D. Contamination from environment

Answer: (A)

Intergranular corrosion happens when grain boundaries become preferentially attacked because the metal has been sensitized by heat. In stainless steels, exposing the material to certain temperatures causes chromium carbides to precipitate right at the grain boundaries. This uses up chromium in the adjacent grains, creating chromium-depleted zones along the boundaries. Those boundary regions can no longer form the protective chromium oxide film as effectively, so the metal corrodes more readily along the grain boundaries when exposed to a corrosive environment.

Other factors don’t specifically produce this boundary-focused attack: impurities in the alloy can influence overall corrosion resistance but don’t inherently create the boundary vulnerabilities; excessive cold work strengthens and hardens the material without creating Cr-depleted boundaries; and environmental contamination tends to cause surface or uniform attack rather than preferentially along grain boundaries.