“Stainless” is better understood as corrosion-resistant, not incapable of oxidation. Rust on ordinary steel is porous, allowing water and oxygen to keep reaching fresh metal. Stainless steel contains enough chromium for a chromium-rich oxide or hydroxide film, only a few nanometres thick, to form when the surface meets oxygen and water. The film is thin and transparent, yet it greatly slows the movement of ions and oxygen through the surface. This condition is called passivation.
After a minor scratch, the passive film can usually form again where oxygen is available, so everyday marks on a kitchen sink do not necessarily become rust. The protection is not unlimited. Chloride salts can locally break down the film and initiate pitting. Oxygen-poor spaces beneath washers or inside tight joints can hinder repair. Particles from ordinary steel can also contaminate the surface and produce brown marks that seem to come from the stainless steel itself.
Different stainless-steel grades therefore use differing amounts of chromium, nickel or molybdenum for particular environments, and they do not offer equal corrosion resistance. “Stainless” describes strong protection supplied by a passive film under suitable conditions, not an absolute fact. When a rust spot appears, the useful question is where the film failed and which local condition defeated it.
https://www.nature.com/articles/s41529-023-00392-z
https://pmc.ncbi.nlm.nih.gov/articles/PMC9920300/
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