High temperature corrosion of beta-NiAl intermetallic compound and pseudobinary NiAl-Cr alloys in sulphur-containing atmospheres

Sulphidation behaviour of beta-NiAl intermetallic compound and NiAl-Cr alloys has been studied as a function of temperature (1073 - 1273 K) and sulphur vapour pressure (10(-5) - 10(4) Pa) using various experimental techniques. It has been found that over the whole temperature and sulphur pressure range studied the sulphidation process follows parabolic kinetics, being thus diffusion controlled. The sulphidation rate of beta-NiAl intermetallic compound decreased with decreasing sulphur pressure, down to about 1 Pa, and at lower pressures (1-10(-5) Pa) the rate of this process became virtually pressure independent. The sulphidation rate of NiAl-Cr alloys was higher than that of beta-NiAl intermetallic compound and increased with increasing chromium content in the alloy. In contrast to beta-NiAl, the sulphidation rate of alloys decreased with decreasing sulphur activity over the whole pressure range studied. The scales formed on beta-NiAl and NiAl-Cr alloys were heterogeneous and mostly multilayer. In addition, an internal sulphidation zone was observed in the underlying metallic phase. Nickel sulphide nodules were always present on the scale surface when the sulphur pressure in the environment exceeded the dissociation pressure of this sulphide. The main part of the scale formed on beta-NiAl consisted of NiAl2S4 sulphospinel. Al2S3 inclusions were present in the internal sulphidation zone. On alloys containing up to 10 at% Cr the main part of the scale was composed of two sulphospinel phases, NiAl2S4 and (Cr,Al)(3)S-4, while on alloys with higher chromium content a stratified scale, consisting of Ni3S2 and (Cr,Al)(3)S-4 was formed. (Cr, Al)(3)S-4 and Al2S3 inclusions were present in the internal sulphidation zone of all chromium-containing alloys. According to marker experiments the scales on the investigated materials grow predominantly by an outward diffusion of cations. At sulphur pressures lower than the dissociation pressure of Ni3S2, beta-NiAl intermetallic shows excellent corrosion resistance in sulphur-containing atmospheres, its sulphidation rates being comparable with the oxidation rates of chromia forming alloys.