How do chloride ions affect stainless steel passivity?

• A. Chlorides can penetrate the passive film and promote pitting and crevice corrosion.
• B. Chlorides strengthen the passive film and prevent corrosion.
• C. Chlorides convert stainless to iron oxide.
• D. Chlorides have no effect on passivity.

Answer: (A)

Chloride ions disrupt the protective passive film that gives stainless steel its corrosion resistance. The film, a thin chromium-rich oxide, forms naturally but is not perfect—defects, inclusions, or damaged areas let chloride ions attack locally. These ions can penetrate or destabilize the film at those flaws, lowering the breakdown potential and initiating pits. Once a pit forms, the environment inside becomes highly aggressive: high chloride concentration, low pH, and limited oxygen flow, which accelerates metal dissolution and pit growth. That localized attack is what we call pitting corrosion (and crevice corrosion can occur in stagnant gaps).

So chlorides don’t strengthen passivity, and they don’t convert stainless steel to iron oxide; they actively promote breakdown of the passive film under the right conditions, leading to localized, not uniform, corrosion.