The effect of preoxidation of some Ni, Fe, and Co-Base alloys on subsequent sulfidation at 982�C in sulfur vapor

Abstract

The effect of preoxidation was studied on the subsequent sulfidation in sulfur vapor at a pressure of 0.1 atm at 982°C on numerous iron, nickel, and cobalt-base alloys which were either chromia or alumina formers. In general, alumina films were much more protective than chromia films, but the efficacy of preoxidation in reducing sulfidation rates depended more upon perfection of the films and whether cracking and/or spatting occurred. Increasing oxidefilm thickness had a beneficial effect until either penetration of the films by sulfur or cracking occurred, after which sulfidation rates were sometimes greater than for nonpreoxidized samples. The enhanced sulfidation rates are attributed to sulfidation of a solute-depleted substrate, the solute having been selectively removed by oxide formation. One alloy, MA 956, containing 0.5 Y2O3 as fine dispersions which normally provide spatting resistance, still exhibited extensive cracking and spalling of the oxide and was not much better than alloys without dispersoids or reactive-metal additions. The use of preoxidation to reduce sulfidation rates is not viable under the extreme conditions used. Preoxidation is conceptually a good method for inhibiting sulfidation at lower temperatures and much lower sulfur pressures.