Which statement about pit formation is correct?

• A. Pits form at defects or imperfections in a protective or passive film.
• B. Pits form uniformly across a perfectly smooth metal surface.
• C. Pits never occur in the presence of a passive film.
• D. Pits only form in the bulk solution.

Answer: (A)

Pit formation is driven by a breakdown of the protective film at a localized spot. Defects such as scratches, inclusions, grain boundaries, or contaminants create sites where the passive oxide is thinner or broken, turning that small area into an anodic region. In chloride-containing environments, this breakdown can be hard to repassivate, so metal dissolution continues at that spot while the surrounding surface remains protected by the film. Because corrosion starts at these discrete defects, a pit grows downward from the surface rather than forming uniformly across a smooth area. The passive film does not guarantee complete immunity; pits can still form where the film is imperfect. So the statement that pits form at defects or imperfections in a protective or passive film is correct.