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 substr ate, and their sulfidation and oxidation behavior has been studied as a function of temperature and alloy composition in He-S-2 and Ar-O-2 a tmospheres. The sulfidation of these alloys follows a parabolic rate l aw, being diffusion controlled. The sulfidation rates of Cr-Nb alloys decrease with increasing niobium content in the alloy, and the sulfida tion 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 sulfid e scales formed on these alloys consist of two layers, comprising an o uter chromium sulfide layer and an inner layer composed mainly of refr actory metal sulfides. The formation of the refractory metal sulfide s cales is responsible for the high sulfidation resistance to sulfide co rrosion. Under the oxidation condition the Cr-Mo alloys are rapidly ox idized due to the formation of volatile molybdenum oxide. The oxidatio n of Cr-Nb alloys proceeds accompanying partial breakdown and the rest oration of the scales, but the average oxidation rates are almost the same as their sulfidation rates. Consequently, the Cr-Nb alloys posses s high resistance to both sulfidation and oxidation at high temperatur es. (C) 1997 Elsevier Science S.A.