Water stains on a wheel bearing indicate which type of corrosion?

• A. Intergranular
• B. Pitting
• C. Uniform
• D. Galvanic

Answer: (A)

Moisture acting as an electrolyte promotes corrosion along the metal’s microstructure, especially at grain boundaries. When steel in wheel bearing components has been exposed to heat or aging (a process that can cause sensitization and chromium carbide precipitation at the boundaries), the boundaries become depleted of protective alloy elements and more anodic than the grain interiors. Water then preferentially attacks these boundaries, so the corrosion appears as a network or staining along the grain boundaries rather than as pits or uniform thinning.

Pitting shows isolated small holes; uniform corrosion thins the surface evenly; galvanic corrosion requires two different metals in electrical contact and creates localized attack at the junction. The observed water stains along lines where grains meet point to intergranular corrosion.