What type of corrosion may attack the grain boundaries of aluminum alloys when the heat treatment process has been improperly accomplished?

• A. Intergranular
• B. Uniform
• C. Pitting
• D. Galvanic

Answer: (A)

Grain-boundary attack from improper heat treatment leads to intergranular corrosion. When heat treatment isn’t done correctly in some aluminum alloys, alloying elements tend to precipitate along the grain boundaries rather than uniformly throughout the grains. Those boundary precipitates create a path where the grain boundaries become anodic relative to the grain interiors, so in the presence of an electrolyte the corrosion progresses along the boundaries instead of across the whole surface. This is why the corrosion is described as intergranular—it's concentrated at the interfaces between grains rather than being uniform across the material.

Other forms of corrosion don’t specifically involve grain boundaries caused by heat-treatment issues. Uniform corrosion would attack the surface more or less evenly; pitting is localized and forms circular pits where the protective film breaks down; galvanic corrosion requires two different metals or phases in electrical contact to drive a current, not a cohesive boundary-based attack from improper aging.