The sulphidation behavior of MoAl alloys with low aluminum contents

Abstract

The beneficial effect of alloying with aluminum to enhance the sulphidation resistance of molybdenum using sputter-deposited amorphous Mo-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (1.7, 6.2 and 10.7 at% Al) and sulphur vapor pressure (20–4000 Pa) in He-S 2 gas mixtures. It has been shown that the sulphidation process follows parabolic kinetics, being diffusion controlled. The reaction rate of Mo-Al alloys with low aluminum contents is lower than that of pure molybdenum and decreases with increasing aluminum content in the alloy. Because sulphide scales formed on Mo-Al alloys containing small amounts of aluminum are homogeneous and composed only of MoS 2, the lower sulphidation rate of those alloys is explained in terms of the aluminum doping effect in the MoS 2 scale. It is believed that the better protective properties of the sulphide scale on Mo-Al alloys in comparison with those of the MoS 2 scale on pure molybdenum result from a lower defect concentration in aluminum-doped MoS 2 phase. The experimentally obtained dependence of the sulphidation rate of Mo-1.7Al alloy on the sulphur pressure at 1173 K is in accordance with that predicted from theoretical considerations.