HOW does chromium protect steel from corrosion?

Answer 1

Stainless steel contains chromium that was put there by diffusing it in there with high heat and a concentration difference. Metals corrode because they get converted from neutral, solid form to ions (they are oxidized, that's why oxidation is a synonym for corrosion) that combine with negatively charged ions (usually oxides or chlorides) and are no longer metal, and many times soluble, hence taking a chunk out of your original metal. Chromium is a more active metal than the iron that makes up most of the steel. This means when both are present on the same surface, the chromium is more likely to dissolve into ions and electrons than the iron.

When stainless is exposed to dissolved oxygen in water (for example) the chromium atoms will dissolve their way towards the surface of the steel. Then, they will be oxidized:

Cr(s)-->Cr3+ + 3e-

What little iron that may have been oxidized before the chromium reached the surface can gain these 3 electrons and join the surface as solid iron again. Otherwise, once the Cr reaches the surface, it'll corrode to form chromium oxide, a greenish black powdery ceramic that doesn't stick well to the surface, and can be easily rinsed away or knocked off. And while the Cr will be oxidized, the iron will not because the Cr is more active. The steel itself remains shiny and well, stainless (without rust). However, when the steel runs out of Cr atoms to sacrifice to protect the iron (once they become chromium oxide, they won't go back to being regular Cr metal, and the oxide does NOT stick to the surface to protect it), the iron will rust.

Answer 2

The chromium atoms keep the oxygen atoms from getting close enough to the iron in the steel to make the steel rust.

Answer 3

This has to do with the property of metals, you have to think of them as a sorta sea of electrons. When one electron is ripped off it allows for a electron deficiant region. This happens to steel when oxygen hits it or any radical for that matter. The Chromium acts as a barrier because it has plenty of room to give electrons off due to very small band gaps and lots of exra electrons in the anti bond region, so in turn the oxygen takes the electrons from Chromium rather then the steel. you really need to study MOT and band gaps to understand it.