Why does dry boiling discolor steel cookware? I have a stainless steel kettle that I let boil dry. That is, I left it on a hot coil burner with nothing in it, and noticed a few minutes later that the bottom was red hot and smoking. To make matters worse, I panicked and quenched it with cool water.

Both the inner and outer surface were left discolored with various shades of yellow and purple. This seems to be a known consequence of overheating, but I can't find an explanation of what actually happened.


I understand why heat (especially combined with quenching) would warp the metal, but why would it cause discoloration (that persists after the metal has cooled)?
After hunting around, I saw several suggestions that boiling white vinegar could remove the discoloration, and to my suprise, it worked pretty well. Why does this work?
Stainless steel have many different compositions. If you heat it up the surface undergoes oxidation. Diffusion at higher temperatures plays also its roll - shifts surface composition of the kettle. The various shades of yellow and purple are metal oxides. The vinegar removes the oxides, thus removes the coloration. Vinegar should remove the oxides even without boiling. It will just take more time.