Pit initiation occurs at defects or imperfections in what?

• A. Protective or passive film
• B. Bulk electrolyte
• C. The oxide layer not covered by film
• D. The electrolyte solution

Answer: (A)

Pit initiation starts at defects or imperfections in the protective or passive film that forms on the metal surface. This film acts as a barrier, but when it has flaws—scratches, thinning, inclusions, or local breakdown—the metal beneath becomes exposed to the environment. In chloride-rich or aggressive environments, these flaws allow ions to penetrate and disrupt repassivation locally, creating a tiny anodic spot. That localized anodic region dissolves metal while surrounding areas stay relatively protected, setting up the conditions for a pit to form and grow. The bulk electrolyte is essential for ion transport, but initiation specifically hinges on the film's defects, not the solution itself.