Sulphide corrosion of pure and chromium-modified β-NiAl intermetallic compound at high temperatures

Abstract

The kinetics and mechanism of high-temperature sulphidation of the intermetallic compound β-NiAl and of pseudobinary NiAlCr alloys have been studied as a function of temperature (1073–1273 K) and sulphur pressure (1.6−2 × 10 3 Pa) in flowing HeS 2 gas mixtures. The sulphidation process for all the materials studied follows a parabolic rate law after a certain initial period. The rate-determining step of the overall scale growth is the outward diffusion of cations. The scale on pure β-NiAl was essentially homogeneous and composed of NiAl 2S 4 sulphospinel only, while that on the NiAlCr alloys also contained another spinel phase, (Cr,Al) 3S 4, the amount of which increased with the chromium content of the alloy. The sulphidation rate of the pseudobinary alloys was higher than that of pure β-NiAl and increased with chromium content because of the less protective properties of (Cr,Al) 3S 4 compared with NiAl 2S 4. The rate of sulphidation for all the materials studied has been found to exceed the oxidation rate of β-NiAl adn FeCrAl or CoCrAl alumina-forming alloys by at least six orders of magnitude.