The effect of preoxidation on the corrosion of some superalloys in coal char

Abstract

Preoxidation of several chromia-forming and of several alumina-forming alloys at temperatures ranging from 982 to 1260°C in either water-saturated argon or in air was performed prior to corrosion in coal char. Chromia films offered no protection to subsequent sulfidation in char. The chromia films were adherent but permitted rapid inward penetration of sulfur and outward penetration of cations to form extensive sulfides. Alumina films offered protection for much longer times than did chromia films, but localized cracking and/or spalling of the alumina permitted rapid attack at the exposed areas. The addition of yttrium as either a metallic element or as an oxide dispersion improved the mechanical stability of the films and prolonged the period of protectiveness. Alumina films would provide an excellent protective barrier, if thick crack-free films could be grown at reasonable temperatures. The low growth rates at very high temperatures and the susceptibility of alumina films to crack and spall during thermal cycling suggest that preoxidation will not offer adequate corrosion resistance for alloys exposed to char in goal gasifiers.