Intergranular corrosion in aluminum is most likely to occur when the alloy has been subjected to which condition?

• A. Improper heat treatment.
• B. Exposure to ultraviolet light.
• C. Constant friction.
• D. Submersion in seawater.

Answer: (A)

Intergranular corrosion occurs when grain boundaries become electrochemically different from the surrounding grains, creating paths for corrosion along those boundaries. This is most likely when the alloy has been heat-treated improperly, which can cause sensitization—forming precipitates that line up along the grain boundaries and create a continuous network. That boundary-specific chemistry makes the boundaries more susceptible to attack in an electrolyte, so improper heat treatment is the condition that best explains intergranular corrosion in aluminum.

Ultraviolet light does not alter the metal’s grain-boundary chemistry to promote this form of corrosion. Constant friction leads to wear or tribocorrosion rather than a grain-boundary–focused attack. Submersion in seawater can cause general corrosion or pitting, but not the characteristic intergranular path produced by sensitization from improper heat treatment.