High temperature corrosion of iron in O 2 + 4% SO 2/SO 3 at 500–800°C

Abstract

The reaction of high purity iron with a gas mixture of O 2 + 4% SO 2/SO 3 has been studied in the temperature range 500–800°C. Reaction kinetics were followed by thermogravimetry and reacted specimens were characterized by X-ray-diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The reaction rate increases with increasing temperature in the range 500–640°C, but at around 640°C the rate constant drops by about an order of magnitude and then continues to increase with increasing temperature. The rapid reaction rate at lower temperatures takes place under conditions such that iron(III) sulphate is formed on the surface. The inner layer of the scale then consists of Fe 2O 3, Fe 3O 4 and Fe 1−yO (depending on the temperature) and Fe 1−xS. It is suggested that the sulphide phase is distributed as a three-dimensional network in the inner layer of the scale and that it serves as a path for rapid outward diffusion of iron, thereby producing the rapid reaction rate. At higher temperatures the scale has the same composition as that of iron oxidized in oxygen, i.e. Fe 2O 3, Fe 3O 4 and Fe 1−yO. In addition, small amounts of Fe 1−xS are detected at the metal-scale interface. This sulphide is probably formed during the initial reaction of the iron with the gas mixture.