Iron combines with oxygen to form rust because of how chemistry works. The way electrons are arranged in iron atoms makes it “happy” to combine with oxygen and form “iron oxide”, the fancy name for rust.
Stainless steel is an alloy that adds some Chromium to the iron. Chromium is “even more happy” to bond with oxygen, but when it does that it creates a completely different substance that’s really just cosmetically not nice and not as physically different as rust is. The substance it makes takes the form of a thin film, and that film makes it harder for more oxygen to get at the metal. So once the film forms it’s less likely any more gets made. In higher-quality stainless steel this film is so thin we can’t really see it without powerful instruments. But cheaper stainless steel can usually appear to rust (because it has less of the more expensive metals that make it truly stainless.)
So put more simply: it DOES rust a little. But it makes a different kind of rust, and that rust forms a kind of protective coating that stops more from forming.
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