Comment by convolvatron

stainless passivation usually involves an acid to form an oxide layer. wiki says nitric and citric, but we used to use an HF gel which was pretty damn nasty

edit: I rabbit holed a little bit. apparently its not that straightforward. the acid encourages the iron to leave the surface layer (probably through oxidization and dissolution) with just the chromium and the nickel. this then oxides in the presence of air, leaving a protective layer without the surface iron to start to rust