Which may not be detectable even by careful visual inspection of the surface?

• A. Uniform corrosion
• B. Pitting
• C. Surface rust
• D. Intergranular corrosion

Answer: (D)

Some corrosion forms hide inside the metal rather than showing up as obvious surface damage. Intergranular corrosion runs along the grain boundaries within the metal and can progress beneath the surface, leaving the exterior looking relatively fair even as internal weakening occurs. Because it doesn’t always create obvious surface pits, discoloration, or thinning that you can see with the naked eye, it may not be detectable by careful visual inspection alone. Detecting this type often requires nondestructive testing or disassembly to reveal internal damage.

In contrast, uniform corrosion tends to thin the surface evenly and becomes noticeable as the surface looks worn or dull; pitting forms small visible pits; surface rust is plainly visible as rust on the exterior.