Intergranular corrosion in aluminum alloy parts can be difficult to detect by surface indications. Which statement best describes this?

• A. It can be detected easily by surface inspection
• B. It cannot occur in aluminum alloys
• C. It cannot always be detected by surface indications
• D. It can be readily detected in all cases

Answer: (C)

Intergranular corrosion can travel along grain boundaries inside the metal, not just at the surface. In aluminum alloys, this attack often occurs beneath the surface or along boundaries where precipitates create tiny galvanic cells, so the surface may look relatively normal even as significant damage accumulates inside. That’s why surface indications aren’t reliable for detecting this type of corrosion—the internal destruction can be well advanced before any surface signs appear. In practice, you’d need cross‑sectioning, etching, or other non-destructive tests that reveal subsurface damage to confirm it.