The sulphidation and oxidation behaviour of sputter-deposited amorphous AlMo alloys at high temperatures

Abstract

The sulphidation behaviour of sputter-deposited amorphous AlMo alloys has been studied as a function of temperature (973–1273 K) and alloy composition (34–46 at%Mo) in pure sulphur vapour at 10 3 Pa pressure. It has been shown that under these conditions the sulphidation process follows parabolic kinetics, being diffusion controlled. No influence of the alloy composition on the reaction rate has been observed. The scales were heterogeneous and composed of two layers. The outer scale layer was built of aluminium sulphide, while the inner layer was heterogeneous and composed of a mixture of Al 0.5Mo 2S 4, Al 2S 3 and MoS 2 phases, the latter being the major component. It is believed that the slowest step, determining the overall reaction rate, is the inward diffusion of sulphur through the inner barrier layer of the scale. Over the whole temperature range studied the alloys showed excellent resistance to sulphide corrosion, their sulphidation rates being comparable with, or even lower than the oxidation rates of chromia forming materials. The better protective properties of the sulphide scale on AlMo alloys in comparison with those of the MoS 2 scale on pure molybdenum result probably from lower defect concentration in aluminium-doped MoS 2 phase, constituting the major part of the inner barrier layer. Preliminary results indicate that the alloys under investigation show satisfactory oxidation resistance in the temperature range not exceeding about 1123 K. Under these conditions a protective oxide scale is formed composed virtually only of Al 2O 3 and the oxidation rates are lower than those of chromia formers. At higher temperatures aluminium activity in the alloy is too low to suppress the formation and evaporation of molybdenum oxides.