THE INFLUENCE OF MOLYBDENUM ON THE DEGRADATION RESISTANCE OF ALUMINIZED IRON-NICKEL-BASE ALLOYS IN SULFIDIZING ENVIRONMENTS

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 H-2/1.6%H2O/1.1%H2S at 1000-degrees-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 this element, thereby enabling base metal sulphides to develop rather than the protective oxide to be maintained. The aluminized Inconel 625 system was the most effective, with failure not occurring until after 2000 h, while the corresponding Incoloy 825, Alloy 904L and PK33 systems showed good resistance for 1500 h. These alloys contain 3 to 9 wt% molybdenum and were more resistant than similar alloys containing none of this element. However, aluminized alloys incorporating higher molybdenum concentrations (such as Hastelloy S, Hastelloy B2 and Hastelloy C4) were not as resistant and failed in less than 1000 h. The structures of the various aluminized systems are presented and their performances in the mixed-gas environment are considered in relation to the structures and compositions of the base alloys.