The four stages of pit development describe which overall process?

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

Answer: (A)

Pitting corrosion is a highly localized form of attack where small pits start at a tiny flaw in the protective film and then grow in a hostile environment, typically in the presence of chlorides. The four stages describe how that localized damage develops from a tiny initiation site to a deep pit that can penetrate through the wall. First, initiation occurs when the passive film is locally damaged or corrupted, allowing the metal to begin dissolving at that spot. Next, propagation follows as the pit becomes a micro-environment with limited diffusion of oxidants and an increasingly acidic, chloride-rich interior, which drives continued metal dissolution inside the pit. Then the pit grows deeper while the surrounding surface may remain relatively intact, because the corrosion remains confined to the pit mouth and interior. Finally, penetration happens when the pit walls become too thin, potentially leading to through-thickness failure. This localized, pit-by-pit process is what sets pitting corrosion apart from uniform, crevice, or galvanic corrosion.