Intergranular corrosion is most likely to occur along which part of aluminum alloys?

• A. Along the grain boundaries of aluminum alloys that have been improperly heat-treated.
• B. Along the surface oxide layer on all aluminum alloys.
• C. At locations with dissimilar metal contact.
• D. Uniformly across the entire surface of properly heat-treated alloys.

Answer: (B)

Intergranular corrosion in aluminum alloys happens where the grains meet, along the grain boundaries. This occurs because heat treatment can cause precipitation or segregation of elements at those boundaries, changing their electrochemical properties relative to the grain interiors. The boundaries become anodic compared to the surrounding metal, so the corrosion propagates along them, producing a path that follows the grain structure rather than attacking uniformly through the grains.

The surface oxide layer on aluminum is protective, so corrosion along that layer would not be the characteristic route of intergranular attack. Dissimilar-metal contact creates galvanic corrosion at interfaces, which is a different mechanism and not the preferential path along grain boundaries. Uniform corrosion across the surface describes general, not intergranular, attack.

So, intergranular corrosion is most likely to occur along the grain boundaries in aluminum alloys that have been improperly heat-treated, where boundary precipitates or segregations promote localized attack.