Chloride attack
The specific reason stainless steel does rust. Chloride ions break the passive film locally, and the damage concentrates in a pinpoint.
In detail
Stainless steel resists corrosion because chromium in the alloy forms a very thin oxide film that repairs itself in the presence of oxygen. Chloride ions are the household chemistry that defeats it. They penetrate the film at weak points, and because the attack is local rather than general, the corrosion concentrates into deep pits rather than spreading as a even rust layer — which is why a stainless sink can look perfect apart from a few dark pinpoints. The sources of chloride in a house are ordinary: table salt, food residues, bleach and chloride-containing cleaners, and de-icing salt carried in on shoes and vehicles. The practical rules follow directly. Do not leave salted water standing in a stainless pan, do not add salt to water before it boils, do not leave bleach in contact with a stainless surface, and rinse and dry after cleaning rather than leaving a film to concentrate as it evaporates. The same mechanism explains why winter is hard on vehicle sills, garden tools and outdoor metalwork, and why rinsing them matters more than their appearance suggests.
Why
- Road salt is explained by Chloride attack
- Stainless steel is explained by Chloride attack
- Stainless steel cleaner is explained by Chloride attack
Sources for this page
Sources
Care and Maintenance of Stainless Steel
British Stainless Steel Association, Stainless Steel Advisory Service, 2001 · Trade or professional body
Relied on for: That chromium forms a self-repairing passive layer on stainless steel, that chlorides attack that layer, and that the result is pitting corrosion. (states this directly)
Right to Know Hazardous Substance Fact Sheet: Sodium Hypochlorite
New Jersey Department of Health · Government or regulator
Relied on for: That sodium hypochlorite is a corrosive chloride-bearing oxidising agent. (inferred from the source; not stated directly)