A stainless steel comprising at least 20 weight % of chromium and at least 1.0 weight % of manganese is adapted to support an overcoating having a thickness from 1 to 10 microns of a spinel of the formula Mn.sub.xCr.sub.3-xO.sub.4 wherein x is from 0.5 to 2. Preferably the overcoating is on chromia and has stability against chemical reaction at temperatures at least 25.degree. C. higher than the uncoated chromia.