Pitting corrosion is a localized attack forming pits; which environment promotes it?

• A. Localized attack forming pits promoted by chloride exposure and passive film breakdown.
• B. Uniform thinning of the metal
• C. Intergranular attack at high temperatures
• D. Surface whitening due to oxidation

Answer: (A)

Pitting corrosion starts where the protective passive film on a metal surface is locally breached, allowing a tiny area to dissolve aggressively while the surrounding surface stays comparatively passive. Chloride-containing environments are especially dangerous because chloride ions readily disrupt and penetrate the passive oxide layer, initiating pits. Once a pit forms, the confined space becomes acidic, which accelerates deep, localized attack. This explains why chloride exposure promoting passive film breakdown is the environment that promotes pitting. In contrast, uniform thinning describes uniform corrosion, intergranular attack occurs along grain boundaries typically at high temperatures, and surface whitening from oxidation refers to a superficial oxide layer rather than pit formation.