The interior of the pit contains which of the following that are more corrosive than the bulk solution?

• A. Acidic, hydrolyzed salts
• B. Basic, alkaline salts
• C. Neutral, inert salts
• D. Oxidizing agents only

Answer: (A)

Localized corrosion in a pit creates a microenvironment that is more aggressive than the bulk solution. As the metal dissolves, ions enter a small, confined space with limited exchange with the outside. Those metal ions hydrolyze with water, producing hydrogen ions and lowering the pH. The result is an acidic interior containing hydrolyzed metal salts and concentrated chloride ions, which destabilize the protective surface film and accelerate dissolution. Because diffusion into the pit is restricted, the acid and aggressive species accumulate, making the interior more corrosive than the surrounding solution.