Defects that cause pit initiation may be which combination?

• A. Randomly distributed, mechanical damage to the films
• B. Uniformly distributed, chemical attack on intact films
• C. Clustered, chemical erosion of the substrate
• D. Randomly distributed, microbial attack

Answer: (A)

Pit initiation happens at flaws in a protective film where the barrier is broken and the underlying metal becomes exposed to the environment. When those flaws are randomly distributed and arise from mechanical damage to the film, each defect creates a discrete local site where corrosion can start. The exposed spot acts as a local anode, concentrating current and dissolving metal to form a pit. The randomness explains why pits appear scattered rather than in a regular pattern. If the film were attacked uniformly chemically on intact areas, you’d see broad, even thinning instead of isolated pits. If defects were clustered, pits would tend to form in groups at those clusters rather than as isolated events. Microbial attack can cause localized corrosion too, but the straightforward mechanism here is random, mechanically induced defects.