Effect of environment on the oxidation behavior of commercial and model Ni-based alloys

Ni-base alloys are of increasing technological interest for advanced power generation concepts including ultrasupercritical steam and supercritical carbon dioxide (sCO₂) as well as their traditional role in gas turbines. For applications at 650°-800°C, commercial precipitation strengthened alloys contain 0-4% additions of Al and/or Ti and these elements are observed to internally oxidize. In order to understand the effect of environment and composition on their oxidation behavior, commercial and model Ni-(18-22%)Cr alloys were exposed at 800°C to laboratory air, air with 10%H₂O (to simulate combustion exhaust gas) and steam for up to 10,000 h. Particularly for the commercial alloys, the depth of Al and Ti internal oxidation increased parabolically with exposure time in 17 bar steam and changing the environment to laboratory air or wet air increased the depth and volume of internal oxidation. For both model and commercial alloys, those alloys with the highest Ti content tended to have the deepest internal oxidation. The current 500h exposures at 650°-750°C in sCO₂ show very limited internal oxidation and are not directly comparable to the longer exposures at 800°C.