The sulfidation and oxidation behavior of sputter-deposited Cr-refractory metal alloys at high temperatures

Cr-Nb and Cr-Mo alloys have been sputter-deposited on to quartz substrate, and their sulfidation and oxidation behavior has been studied as a function of temperature and alloy composition in He-S2 and Ar-O2 atmospheres. The sulfidation of these alloys follows a parabolic rate law, being diffusion controlled. The sulfidation rates of Cr-Nb alloys decrease with increasing niobium content in the alloy, and the sulfidation resistance of the high niobium alloys is comparable with that of niobium. The sulfidation resistance of Cr-Mo alloys is independent of alloy composition, being comprable with that of molybdenum. The sulfide scales formed on these alloys consist of two layers, comprising an outer chromium sulfide layer and an inner layer composed mainly of refractory metal sulfides. The formation of the refractory metal sulfide scales is responsible for the high sulfidation resistance to sulfide corrosion. Under the oxidation condition the Cr-Mo alloys are rapidly oxidized due to the formation of volatile molybdenum oxide. The oxidation of Cr-Nb alloys proceeds accompanying partial breakdown and the restoration of the scales, but the average oxidation rates are almost the same as their sulfidation rates. Consequently, the Cr-Nb alloys possess high resistance to both sulfidation and oxidation at high temperatures.