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.

New power generation concepts may contain higher water vapor in the turbine combustion gas due to the fuel or to steam dilution. To assess the effect of higher water vapor content on thermal barrier coating performance, furnace cycle (1h) testing was conducted in air with 10, 50 and 90 vol.% water vapor and compared to prior results in dry O 2. The first series of experiments examined Pt diffusion (γ+γ') and Pt-modified aluminide (β) bond coatings on second-generation superalloy N5 at 1150°C with commercially vapor-deposited yttria-stabilized zirconia (YSZ) top coats. Compared to dry O 2 , the average coating lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor while the Pt-modified aluminide coating average lifetime was reduced by >50% with 10% water vapor, but less reduction was observed with higher water contents. A similar set of coatings on low Re superalloy N515 showed no debit in lifetime with Pt aluminide bond coatings exposed to 10% water vapor. Characterization of the alumina scale thickness at failure showed a thicker oxide beneath the YSZ coating (compared to the scale without a top coating) for both types of bond coatings, and an increase in the oxide thickness with the addition of 10% water vapor. These observations were further studied using analytical transmission electron microscopy. The second series of experiments examined high velocity oxygen fuel (HVOF) MCrAlY and MCrAlYHfSi bond coatings and air-plasma sprayed YSZ top coatings on X4 superalloy substrates with and without Y and La additions. Compared to a dry O 2 baseline, the addition of 10% water vapor decreased the YSZ coating lifetime for either bond coating by ~30% at 1100°C. Substrates with Y and La additions showed no change in the average lifetime in 10% water vapor compared to standard X4. A further increase to 50% water vapor did not further decrease the average lifetime of one group of coatings. To better simulate base-load power generation, one group of specimens was cycled with 100h cycles, which substantially increased the coating lifetime. In each case, higher average lifetimes were observed with Hf in the bond coating. Initial characterization of the alumina scales formed at failure showed little effect of the water vapor addition, bond coating composition or substrate composition. For both series of coatings, the addition of 10% water vapor to the experiment reduced YSZ coating lifetime. However, increasing to 50% or 90% H 2 O showed no additional decrease in average YSZ lifetime.