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

Abstract

The results of an x-ray diffraction and metallographic study of the kinetics of scale formation during the oxidation of Ni(Ta), Ni₃Ta, and NiTa in air at 600–1000°C are analyzed. The free energies, equilibrium oxygen pressures, and mass balances of the oxidation reactions were calculated, and conditions for the formation of NiO·Ta₂O₅ and NiO on the alloy determined. It is shown that the oxidation process is controlled primarily by the diffusion of oxygen and counter-diffusion of Ni⁺² in the scale, and involves oxidation, reduction, and synthesis reactions. A multilayer scale is formed, consisting of an outer layer containing only oxides (NiO, NiO·Ta₂O₅, Ta₂O₅) and an inner one which additionally contains nickel. The protective ability of the outer scale depends upon the concentrations of NiO and NiO·Ta₂O₅ in it. Preferential oxidation of tantalum is responsible for the appearance 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, particularly in the inner scale on Ni₃Ta. A change in the oxidation mechanism occurs at T ≥ 850°C as a result of the p → n transition in Ta₂O₅, which leads to retarded oxygen diffusion and the appearance of Ta⁺⁵ diffusion in the intermetallic. This, as well as the diffusion 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.