During initiation, pit initiation occurs at which locations?

• A. At smooth, defect-free areas of the metal surface
• B. At defects or imperfections in a protective or passive film
• C. In the bulk electrolyte away from the film
• D. At regions with uniform protective film

Answer: (B)

Pit initiation happens where the protective passive film has defects or imperfections, because those spots break the barrier that normally protects the metal. An intact passive film greatly reduces uniform corrosion, but real films aren’t perfect. Scratches, pores, inclusions, grain boundaries, or mechanical damage create local weaknesses that chloride-rich environments can exploit, causing the film to break down at that spot. Once passivity is locally lost, the underlying metal becomes anodic and begins to dissolve, forming a pit nucleus. The surrounding film and environment then hinder repassivation, allowing the pit to grow. Smooth, defect-free areas or regions where the film is uniform and intact don’t provide a weak point for initiation, which is why they aren’t typical pit initiation sites.