Which factor can contribute to reinitiation of pits after rewetting (differential aeration context)?

• A. Differential aeration between the solution in the main pit cavity and solutions in cracks emanating from deeper in the metal
• B. Uniform oxygen concentration
• C. Lower humidity levels
• D. Temperature alone

Answer: (A)

Differential aeration between sections of the metal surface is what drives pit reinitiation after rewetting. When the metal is wetted again, the solution in the main pit cavity is typically well-oxygenated, while cracks or crevices deeper in the metal restrict oxygen diffusion and become relatively oxygen-poor. This creates a local galvanic cell: the oxygen-poor area acts as the anode and dissolves, while the oxygen-rich area is the cathode. The renewed electrolyte connection during rewetting allows aggressive ions to concentrate at the crack tips, promoting metal dissolution and reestablishment of pit nuclei. Thus this differential aeration between the main pit solution and the solution in cracks is the factor that promotes reinitiation. The other options don’t create the necessary electrochemical gradient: uniform oxygen concentration lacks the driving force for localized attack, and lower humidity levels or temperature alone don’t establish the localized anodic regions that lead to pit reinitiation in this context.