Abstract
Sulfidation behavior of Fe-Cr alloys containing up to 25 at%Cr was investigated under the sulfur pressures above a dissociation pressure of ferrous sulfide at temperatures of 1023, 1073 and 1123 K using a thrmo-gravimetry and an electron micro-probe analysis.The surface sulfide scale was composed of (Fe, Cr)1−δS and its growth kinetics was parabolic. The parabolic rate constants (kp) increased with increasing Cr content and sulfur partial pressure, while they showed an abnormal temperature dependence at a 10−3 Pa sulfur pressure. These could be due to a competitive effect of a dynamic factor (cation diffusivities) and a driving force factor (sulfur activity difference across the scale).It was found that Cr in the scale enriched near an alloy/scale interface and then decreased gradually toward a scale/gas interface, and these cation profiles became steeper with increasing sulfur pressure. Diffusional analyses of cation distributions showed that Cr is a slower-diffusant in the (Fe, Cr)1−δS sulfide, with the DCr⁄DFe ratios ranging from 0.1 to 0.5. Scale structures and cation distributions could be elucidated by paths of change in composition on the Fe-Cr-S phase diagram.
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