How does a relatively small amount of chromium prevent steel from rusting?
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As several have mentioned, the chromium oxidizes forming a protective layer. The key here is that unlike iron oxide, chromium oxide has a similar molar volume to chromium metal. That is, unlike rust that is more expansive than iron, chromium oxide takes up the same space as chromium metal. Same Delia with aluminum. All those aluminum thingies out there have a nice coating of aluminum oxide protecting the metal underneath
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