Which type of corrosion attacks the grain boundaries of aluminum alloys when the heat-treatment process has been improperly accomplished?

• A. Concentration cell
• B. Intergranular
• C. Fretting
• D. Pitting

Answer: (B)

Intergranular corrosion is corrosion that travels along the grain boundaries of a metal. In aluminum alloys, improper heat treatment can cause second-phase precipitates to form at the grain boundaries during aging, creating a continuous network that becomes anodic to the grain interiors. This sets up micro-galvanic cells along the boundaries, so the dissolved metal preferentially attacks and propagates along those boundaries rather than through the grains themselves. This type of attack is especially likely in alloys that rely on precipitation hardening, where the aging process controls where and how precipitates form; when aging is off, boundaries become sites of weakness.

Concentration-cell corrosion, fretting, and pitting involve different mechanisms. Concentration-cell corrosion depends on a gradient in ion concentration within the electrolyte, fretting arises from mechanical wear that removes protective films, and pitting is localized attack from a breakdown of the protective oxide at surface defects.