A primary cause of intergranular corrosion is

• A. Impurities in the alloy
• B. Excessive mechanical wear
• C. Improper heat treatment
• D. Poor atmospheric exposure

Answer: (C)

Intergranular corrosion occurs when the boundaries between grains in a metal become the most susceptible paths for attack, so the corrosion travels along the grain boundaries rather than through the grain interiors. This is most closely tied to the material’s microstructure being altered by heat treatment. In stainless steels (and some other alloys), exposure to certain temperatures during heat treatment can cause chromium carbides to precipitate at grain boundaries. This “sensitizes” the material by depleting chromium near those boundaries, lowering corrosion resistance specifically along the grain boundaries. When the metal later encounters a corrosive environment, especially chlorides, the grain boundaries corrode preferentially, leading to intergranular corrosion. Proper solution heat treatment and controlled aging dissolve and re-precipitate phases uniformly, avoiding this sensitization.

Impurities can affect overall corrosion resistance but don’t inherently create the distinct grain-boundary attack pattern. Excessive mechanical wear is a physical degradation process, not corrosion. Poor atmospheric exposure changes corrosion rates due to environment but does not by itself induce the grain-boundary vulnerability that causes intergranular corrosion.