SULFIDATION PROPERTIES OF LOW ALLOY STEELS IN H2S-H2 ATMOSPHERES

Abstract

Sulfidation behaviors of low alloy steels, STBA24 and STB42, as well as the standard alloys of Fe-2.25Cr, Fe-2.25Cr-1Mo, Fe-1Si and -2Si alloys were investigated in H 2 -H 2 S gas mixtures at a temperature range from 673 to 973 K, as a series of investigations on the H 2 S induced corrosion in the boiler furnaces. These STBA24 and STB42 alloys showed similar sulfidation kinetics and scale structures. Their sulfidation rates were 1/3 of those of Fe and standard Fe-2.25Cr and Fe-2.25Cr-1Mo alloys, and were close to those observed for Fe-1Si and Fe-2Si alloys. Sulfide scales were composed of duplex- or triplex-structures, that is, a duplex of inner and outer layers at high temperatures and a triplex layers of outer, intermediate, and inner layers at low temperatures. The outer and intermediate layers were compact, consisting of (Fe, Mn)S and (Fe, Cr, Mn)S phases, respectively. The inner layer was porous, and at high temperatures it was composed of a (Fe, Cr, Mn)S phase and metallic islands containing Si and Mo, with Si predominantly near the alloy/inner layer interface at low temperatures. The slow sulfidation rates observed for both the STBA24 and STB42 alloys are due to the effect of minor element like Si, but not of major alloying elements like Cr and Mo.