Oxide Films on Alloy Steels

Abstract

IN a recent communication¹, T. Tokumitu concludes that the naturally occurring oxide film on stainless steels is ±-(Fe,Cr)₂O₃, and he bases this conclusion upon the fact that heating the natural oxide layer to 600° C. for one hour suffices to change the electron diffraction pattern from one of blurred rings which cannot be analysed, to a well-defined pattern of ±-(Fe,Cr)₂O₃. The crystalline structure adopted by the film on heating, however, may very well be different from that of the natural oxide, and even the chemical composition may be changed, since, unless the heating is carried out in an exceedingly good vacuum or its equivalent, the film may gain oxygen, and also, as Pfeil² has shown in the case of thick scales, diffusion of metal atoms towards the oxide surface occurs at high temperatures.