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So does the chromium form an oxide on the surface, or does it merely sacrificially protect the iron that contains it? I note from the reactivity series (https://en.wikipedia.org/wiki/Reactivity_series) that chromium is a bit more reactive than iron and therefore could act as a sacrificial anode, could it not?
does the chromium form an oxide on the surface?
mercury will react with aluminum to form an alloy that will oxidize before your very eyes!
My guess is iron will oxidize to form iron +2 and iron +3. The oxygen will gain two or three iron electrons to form rust. The most common oxidation states of chromium are +2, +3, +6. This means the oxygen can get more electrons from chromium since it will release up to six electrons to oxygen. The chromium acts like a firewall, does for a computer, taking the bulk of the oxidation hit to protect the iron.
The most common oxidation states of chromium are +2, +3, +6.
The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen.
No, it's not just an issue of the chromium being more easily oxidized. If it were that simple, magnesium, aluminum or zinc could be used for the same purpose. Sometimes we do use one of those more easily oxidized metals as a sacrificial metal to protect iron, but stainless steel is a different animal.