Which statement about intergranular corrosion in aluminum alloy parts is correct?

• A. Cannot always be detected by surface indications
• B. Always visible on the surface
• C. Does not affect structural integrity
• D. Is unaffected by heat treatment

Answer: (A)

Intergranular corrosion travels along the boundaries between grains, which are the microscopic interfaces in the metal. This attack can occur beneath the surface oxide layer, so a part can look fine on the outside while the grain boundaries are already compromised underneath. Because of that, relying on surface appearance alone is not enough to detect it.

To actually reveal this type of corrosion, you may need non-destructive tests or metallographic methods that expose or reveal subsurface damage, since surface indications can be absent or misleading.

Heat treatment influences the metal’s microstructure and the precipitation at grain boundaries, which in turn affects susceptibility to intergranular corrosion. So saying it’s unaffected by heat treatment isn’t accurate. And even though a surface may not show obvious signs, intergranular corrosion can still reduce cross-sectional strength and compromise structural integrity, so it’s not something to ignore.

Therefore, the statement that you cannot always detect intergranular corrosion by surface indications is the best choice.