Effect of environment on the high temperature oxidation behavior of 718 and 718Plus

Ni-base alloys 718 and 718Plus are widely used for high temperature components in aircraft and power generation turbines under various environment conditions. Laboratory experimental rigs were used to simulate turbine exhaust (air with 10%H₂O), steam and laboratory air at 550°-800°C for up to 10, 000 h and compared to oxidation in laboratory air. Because component lifetimes can be much longer than 10, 000 h, the experiments at 800°C were performed in an attempt to simulate longer exposures at lower temperatures but there are concerns about 718 microstructural stability at this temperature. Oxidation in wet air resulted in net mass losses due to the formation of volatile CrO₂(OH)₂ but Cr depletion in the substrate was minimal, even at 800°C. The rate constants for 718Plus in air tended to be slightly lower than 718 but otherwise few differences in oxidation behavior were observed. The higher Al content in 718Plus or the finer grain size in these specimens may help to reduce the reaction rate.