High Temperature Reaction of MCrAlY Coating Compositions with CaO Deposits

The reactivity of beta-NiAl + gamma-Ni-based NiCoCrAlY alloys with and without CaO deposits was studied by means of isothermal exposures in air. Reaction with CaO at 1100 A degrees C produced multi-layer scales of Al2O3 and calcium aluminates, and a mixture of liquid calcium chromate and nickel-cobalt oxide particles. Calcium chromate formation was a rapid, transient process, and the transition to a steady-state of slower Al2O3 growth was favored by increasing the alloy beta fraction. The thermally-growing Al2O3 reacted with the deposit to form calcium aluminates in a solid-state diffusion process, which led to an increased oxidation rate. The analysis of Al2O3 growth kinetics in the production-destruction regime was used to account for the increased flux of aluminum entering the multi-layer scale. The effect of temperature on the ability to kinetically sustain an Al2O3 scale was then evaluated on the basis of Wagner's criterion. Predicted results were consistent with the experimentally observed absence of passivation at 900 A degrees C.