Oxidation of Ni-Ta alloys. III. Mechanism of oxidation

The results of an x-ray diffraction. and metallographic study of the kineti cs of scale formation during the oxidation of Ni(Ta), Ni3Ta, and NiTa in ai r at 600-1000 degreesC are analyzed The free energies, equilibrium oxygen p ressures, and mass balances of the oxidation reactions were calculated, and conditions for the formation of NiO . Ta2O5 and NiO on the alloy determine d It is shown that the oxidation process is controlled primarily by the dif fusion of oxygen and counter-diffusion of Ni+2 in the scale, and involves o xidation, reduction, and synthesis reactions. A multilayer scale is formed, consisting of an outer layer containing only oxides (NiO, NiO . Ta2O5, Ta2 O5) and an inner one which additionally contains nickel. The protective abi lity of the outer scale depends upon the concentrations of NiO and NiO . Ta 2O5 in it. Preferential oxidation of tantalum is responsible for the appear ance of a subscale consisting of Ni(Ta) + Ta2O5 on the intermetallic Ni3Ta, and Ni3Ta + Ni(Ta) + Ta2O5 on NiTa. Differences in molar volumes of phases result in the formation of pores and cracks at interphase boundaries, part icularly in the inner scale on Ni3Ta. A change in the oxidation mechanism o ccurs at T greater than or equal to 850 degreesC as a result of the p --> n transition in Ta2O5, which leads to retarded oxygen diffusion and the appe arance of Ta+5 diffusion in the intermetallic. This, as well as the diffusi on of Ni+2, promotes the healing of macrodefects in the scale. However, it also results in enrichment of the outer scale in pentoxide, which decreases its protective ability.