How does chloride concentration influence pitting corrosion in stainless steels?

• A. Higher chloride concentrations promote pit initiation and growth
• B. Chloride concentration has no effect on pitting
• C. Lower chloride concentrations accelerate pitting
• D. Chlorides only affect uniform corrosion, not pitting

Answer: (C)

Chloride ions destabilize the protective passive film that forms on stainless steels. The passive layer is most vulnerable at defects and grain boundaries, where chloride ions can adsorb and attack, making it easier for small pits to nucleate. Once a pit starts, the local chemistry inside it becomes acidic and chloride-rich, which slows repassivation and sustains metal dissolution. As chloride concentration increases, these effects become stronger: pits initiate more readily and grow more quickly, because the film is harder to restore and the pit environment remains favorable for continued attack.

Lower chloride levels reduce the likelihood of pit initiation and slow growth, since there are fewer chloride ions available to disrupt the passive film. Chloride’s impact is specific to localized attack like pitting (and crevice corrosion) rather than uniform corrosion, so higher chloride concentrations are the key driver for pit formation and propagation in stainless steels.