Which statement about initiation and propagation is true?

• A. Pitting initiates immediately and propagates slowly
• B. Pitting may not initiate for long periods, but once it does it can propagate rapidly.
• C. Pitting cannot propagate once initiated
• D. Pitting initiates only at high temperatures

Answer: (B)

Pitting corrosion is driven by two stages: initiation and propagation. Initiation often requires a local breakdown of the protective passive film at a flaw, inclusion, or site of stress, which can take a long time depending on the environment and material. Once a pit forms, the local chemistry inside the pit becomes highly aggressive: metal dissolves, the solution inside becomes acidic, and chloride ions keep the pit active. The geometry of a pit also limits oxygen access, which helps sustain the anodic dissolution at the pit mouth and slows overall corrosion elsewhere, allowing the pit to grow rapidly relative to the surrounding surface. This combination means pits can stay dormant for long periods and then advance quickly once they start. Temperature is not the sole driver; pitting can initiate at ambient temperatures if the chemical conditions favor breakdown of the passive film.