THE EFFECT OF YTTRIUM ON SCALE FORMATION AND BREAKDOWN OF FE-CR-AL IN MIXED GAS

The effects of 1 wt. % yttrium addition to an iron-base model alloy, Fe-25Cr-6Al, on the scale formation and breakdown mechanisms in a mixed gas at temperatures 500-700°C were studied. In this environment, the equilibrium gas contained sulfur and oxygen at high and low vapor pressures, respectively. During the exposure, the formation of an alumina scale was favored kinetically on the surfaces of the alloys. The oxide scale formed on the base (Y-free) alloy had a fine-grained structure which exhibited excellent sulfidation/oxidation resistance. However, because of severe segregation along the grain-boundaries of the Y-containing alloy, which resulted in second-phase precipitation rich in Fe and Y, the morphology of the surface oxide on this alloy exhibited a distinction along the wide substrate grain boundaries. These Fe- and Y-rich regions served as short circuits for Fe diffusion from the underlying substrate to the scale/gas interface and, eventually, allowed iron sulfide to form on the oxide scale. With a pre-oxidation treatment, the transient stage from oxidation to sulfidation was prolonged; however, the sulfidation process was not eliminated. Based on thermodynamic calculations, the probable phases forming on the alloy surface in the oxidizing/sulfidizing environment were predicted, and a qualitative model for the corrosion mechanisms was proposed. © 1990 Plenum Publishing Corporation.