THE SULFIDATION BEHAVIOR OF MO-AL ALLOYS WITH LOW ALUMINUM CONTENTS

The beneficial effect of alloying with aluminum to enhance the sulphid ation resistance of molybdenum using sputter-deposited amorphous Mo-Al alloys has been studied as a function of temperature (1073-1273 K), a luminum content (1.7, 6.2 and 10.7 at% Al) and sulphur vapor pressure (20-4000 Pa) in He-S2 gas mixtures. It has been shown that the sulphid ation 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 co ntent in the alloy. Because sulphide scales formed on Mo-Al alloys con taining small amounts of aluminum are homogeneous and composed only of MoS2, the lower sulphidation rate of those alloys is explained in ter ms of the aluminum doping effect in the MoS2 scale. It is believed tha t the better protective properties of the sulphide scale on Mo-Al allo ys in comparison with those of the MoS2 scale on pure molybdenum resul t from a lower defect concentration in aluminum-doped MoS2 phase. The experimentally obtained dependence of the sulphidation rate of Mo-1.7 Al alloy on the sulphur pressure at 1173 K is in accordance with that predicted from theoretical considerations.