Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.