Quote Originally Posted by TimFox View Post
So-called stainless steels contain chromium (and usually nickel) in the alloy. Chromium oxide forms on the surface and forms a hard passivation layer (similar to the aluminum oxide layer that forms naturally on aluminum).
One should never use steel wool (or steel wool scouring pads such as Brillo) on stainless steel. Fine iron particles from the steel wool can get under the abraded-off chromium oxide surface passivation and then rust.
Tim is correct. If you have a stainless vessel with rust stains from this practice, it is not difficult to re-passivate. Commercial passivation uses heated nitric acid solutions to dissolve the free iron and therefore passivate stainless, but at home a person could just use plain vinegar and letting it sit for a while, then rinsing with clean water. Heating it would speed up the process. It might take more than once. If it's a stainless pot, even cooking something acid like tomatoes will dissolve the free iron over time.