The corrosion of Fe and some Fe-based alloys in S vapour at 500°C

Abstract

The corrosion behaviour of Fe and of some binary Fe alloys containing approximately 5wt.% of Ni, Co, Cr or Al has been studied in S vapour at 500°C as a function of S pressure within the range 0·1–100 torr. Thermogravimetric, metallographic, X-ray and electron probe microanalysis techniques have been employed. With the exception of the FeAl alloy, parabolic kinetics are exhibited by all of the materials. The parabolic reaction rates for the Fe, the FeNi and the FeCo alloys, increase regularly with increasing S pressure; the rate of corrosion of the FeCr alloy, however, is independent of pressure. Approximately linear kinetics are observed with the FeAl alloy, and the reaction rate is also independent of S pressure. All of the alloys corrode more slowly than the pure Fe, but the FeCr, and particularly the FeAl alloys exhibit a substantially lower rate of reaction relative to Fe. This superior corrosion resistance appears to be associated with the formation of duplex protective FeS scales, the inner layers of which contain coarsely dispersed quantities of Cr 2S 3 or Al 2S 3. The interpretation of the sulphidation of all of the materials is considered in some detail.