THE SULFIDATION AND OXIDATION BEHAVIOR OF SPUTTER-DEPOSITED AMORPHOUSAL-NB ALLOYS AT HIGH-TEMPERATURES

The sulfidation and oxidation behavior of sputter-deposited Al-(27-54 at. %)Nb alloys has been investigated as a function of temperature (10 73-1473 K) and alloy composition in He-S-2 atmospheres with partial su lfur pressures (pS(2)) ranging from 5 x 10(1) to 1 x 10(4) Pa and in a n Ar-O-2 atmosphere with partial oxygen pressure (pO(2)) of 2 x 10(4) Pa at a total pressure of 10(5) Pa. Sulfidation of these alloys follow s parabolic kinetics over the whole temperature and pressure ranges st udied. The sulfidation rate increases with sulfur pressure and aluminu m content in the alloy, the apparent activation energy for the reactio n being virtually independent of alloy composition. The scale consists of two layers. The outer layer is composed mainly of aluminum sulfide , while the inner layer is built up of niobium sulfide. It is presumed that the rate-determining step for the overall reaction is the outwar d diffusion of both aluminum and niobium ions through the inner niobiu m sulfide layer. Therefore a higher aluminum content in the alloy is h armful for the sulfidation resistance of Al-Nb alloys. The oxidation r esistance of these alloys increases with aluminum content in the alloy , although even high aluminum alloys such as Al-30Nb show lower oxidat ion resistance than typical alumina-forming alloys. Oxidation of Al-30 Nb does not follow parabolic kinetics, but follows linear kinetics, be cause of the formation of a nonprotective scale composed of Al2O3 and NbAlO4. At lower temperatures these alloys break up during oxidation o r during the cooling period to room temperature. This seems to be due to the ''pest phenomenon''. Copyright (C) 1996 Elsevier Science Ltd