The Low Temperature Oxidation of Iron

Abstract

An electron diffraction study of the oxide films formed on abraded iron specimens by heating in air at temperatures ranging from 80° to 260°C has failed to detect structural effects due to a ``critical temperature'' in the neighborhood of 200°C. α-Fe2O3 was found on specimens which were oxidized at any temperature above about 130°C provided the period of oxidation exceeded a minimum value which depends upon the temperature. Thus 1 hour at 180°, 4 hr. at 165°, 13 hr. at 150° or 1000 hr. at 130° are just sufficient to produce an α-Fe2O3 film whose diffraction pattern is well defined. Lower oxidation temperatures or shorter times tend to strengthen the diffraction pattern due to the primary oxide film which is assumed to be γ-Fe2O3 (or Fe3O4) and is detectible immediately after the initial abrasive treatment. The strengthening of the primary oxide pattern is not accompanied by appreciable grain growth, however, and traces of iron are present in the pattern after oxidation periods as long as 1700 hr. at 120°C or below. Based on considerations of the presence of these iron rings it is estimated that the primary oxide film cannot be thicker than about 200A over some portions of the abraded surface. At 193°C and above the development of the first order blue interference color is characterized by the appearance of a new α-Fe2O3 film which is more coarsely crystalline and has a different type of preferred orientation than the first-formed α-Fe2O3 film. It is believed that this secondary film is formed by the outward diffusion of iron through the oxide layer. The axis of preferred orientation of the α-Fe2O3 is always symmetrical about the normal to the surface. This orientation cannot be attributed to pseudomorphism of the oxide crystals on the underlying iron as is the case with the primary oxide.