1995 Volume 44 Issue 3 Pages 174-182
A review is given of our work on the development of new, novel alloys with high resistance to high temperature corrosion in sulfidizing and oxydizing environments. The corrosion behavior of sputter-deposited amorphous Al-(34-46)Mo and Al-(31-33)Mo-(6-16)Si alloys has been studied as a function of temperature of 973-1273K in sulfur vapor of 103Pa as well as in oxygen of 105Pa and in air. The sulfidation process has been found to follow parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step, determining the overall reaction rate, is the inward diffusion of sulfur through the inner barrier layer of the scale. Over the whole temperature range studied these binary and ternary alloys have shown excellent resistance to sulfide corrosion, their sulfidation rates being comparable to or even lower than the oxidation rates of chromia-forming materials. No influence of silicon on the sulfidation rate of Al-Mo alloys has been observed. The oxidation resistance of binary Al-Mo alloys has been found to be satisfactory, but only at temperatures below 1073K. Above the melting point of MoO3 (1069K), the scale, consisting mainly of Al2O3 with about 5% of MoO3, becomes unprotective because of evaporation of molybdenum oxide. On the other hand, ternary Al-Mo-Si alloys show excellent resistance to oxidizing environments, up to about 1200K, their oxidation rates being comparable to those of alumina formers. No traces of molybdenum have been found in alumina scales on these alloys.