On the sulphidation mechanism of niobium and some of Nb-alloys at high temperatures

Abstract The kinetics and mechanism of niobium sulphidation have been studied as a function of temperature (700–1000 °C) and sulphur pressure (10−4–104 Pa) in pure sulphur vapour and H2–H2S gas mixtures, using microthermogravimetric technique. It has been found that in both different sulphidizing atmospheres the sulphidation process follows parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step of the overall reaction rate is the outward diffusion of cations. No influence of small amounts of impurities on the sulphidation rate has been observed in this study. Excellent agreement between calculated and experimentally determined parabolic rate constants has been obtained under the assumption, that the correct formula of the sulphide scale on niobium is Nb2±yS3 and not Nb1+xS2, as suggested by Gesmundo. It has been found that the rate of niobium sulphidation in H2–H2S gas mixtures is much higher than in pure sulphur vapour, strongly suggesting that the dissolution of hydrogen in the growing scale influences the defect structure in this sulphide.