Abstract
Short-term sulfidation-oxidation exposures were conducted under high pS2 and low pO2 conditions for TiCrAl and FeCrAl alloys at 600 and 800°C. Low mass gains and submicron Al-rich oxide scales were formed on the TiCrAl, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of the TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation-oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidation-oxidation environments were the key factors in determining whether or not a protective alumina scale was established. For FeCrAl, no internal oxidation of Al was observed, which suggested that effects related to secondary gettering were not sufficient to yield protective Al2O3 formation in these environments. Rather, it was proposed that additional third-element effect benefits of Cr based on the existence of solid solutions among Al, Cr, and Fe oxides to mitigate rapid transient Fe-oxide formation were rendered ineffective because FeS formation was thermodynamically favored over Fe-oxide in the environment. In the case of TiCrAl, the stability of Ti-oxide in the sulfidation-oxidation environment and the non-classical third-element effects of Cr on promoting protective Al2O3 scale led to good corrosion resistance.
