Which corrosion occurs as local corrosion in small surface irregularities, often with pitting in stagnant water?

• A. Flow accelerated corrosion
• B. Pitting corrosion
• C. Crevice corrosion
• D. Microbiologic corrosion

Answer: (B)

Pitting corrosion is a highly localized form of attack that starts at tiny surface defects where the protective oxide film on a metal, such as stainless steel, breaks down. In water that is stagnant and contains chlorides, this breakdown at the defect creates a small anodic area while the surrounding metal remains passive and relatively oxygenated, forming a differential aeration cell. Inside the developing pit, oxygen is scarce and chloride ions are concentrated, promoting continued dissolution of metal and pit growth downward. The pit can become deep while the surrounding surface looks intact, making it particularly insidious. This type is common in stainless steels and aluminum in chloride-containing environments when water is stagnant. In contrast, flow accelerated corrosion involves erosion under flow, crevice corrosion occurs at gaps where differential aeration exists within a crevice, and microbiologic corrosion is driven by microbial activity; those scenarios describe different conditions than localized pits in small surface irregularities.