The influence of molybdenum on the degradation resistance of aluminized iron-nickel-base alloys in sulphidizing environments

Abstract

Since Al2O3−forming nickel-and iron-nickel-base alloys are known to be more effective than Cr2O3−forming alloys in sulphur-containing gases, several commercial Cr2O3−forming alloys have been pack aluminized to establish aluminide coatings for use in mixed-gas environments of high sulphur and low oxygen activity. Results are given for long-term exposures (up to 2500 h) to H2/1.6%H2O/1.1%H2S at 1000°C under thermal-cycling conditions. Uncoated specimens of such alloys are sulphidized completely in very short periods in this environment. Early work indicated that an aluminized molybdenum-containing nickel-base alloy is the most resistant system, so a series of alloys based on nickel and nickel-iron and incorporating various amounts of molybdenum was included in the tests. Most of the aluminized systems showed reasonable resistance for periods of 500 h or more. Subsequently, the protective Al2O3 scale broke down following back-diffusion of aluminium into the alloy substrate and depletion of the surface in thi...