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

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

Answer: (A)

Water in the bearing environment creates an electrolyte that can drive corrosion along the grain boundaries of the metal. When this happens, the corrosion attacks sites where grains meet, so you get staining or etching that follows the network of grain boundaries rather than spreading evenly across the surface. That pattern—staining that traces the grain structure—is a hallmark of intergranular corrosion.

Uniform corrosion would show as a general, even attack over the whole surface; pitting would produce small, localized holes; galvanic corrosion requires two dissimilar metals in electrical contact and an electrolyte, producing corrosion at the more anodic metal and not the characteristic grain-boundary staining seen here.