Influence of temperature and composition of SO2/O2 atmosphere on the surface morphology of alumina scales formed on FeCrAl steel

The paper deals with the oxidation of an FeCrAl-type steel in SO2 and in SO2/O-2 gas mixtures in the temperature range 800 - 850degreesC. It has been found that at 850degreesC, independently of the composition of the oxidizing atmosphere, the alpha-Al2O3 scale is strongly deformed and poorly adherent to the metallic substrate. At lower temperatures (800-830degreesC), the scale is adherent and composed of alpha-Al2O3 and theta-Al2O3, with a characteristic plate-like morphology of the external surface. At 820degreesC, in the atmospheres containing less than 60% SO2, the plate-like structure tends to disappear and the rate of scale formation increases with the concentration of O-2 in the SO2/O-2 mixtures. In SO2 and in the SO2/O-2 mixtures containing more than 95% SO2, a well-developed plate-like structure is observed and its mass is comparable to the mass of the inner compact layer of alpha-Al2O3.