Oxidation processes for alloys in the Ni - Zr system. III. Oxidation of NiZr

We used the continuous weighing method to study the oxidation kinetics in air for the alloy NiZr at 500–1000°C. We used x-ray diffraction and metallography for layer-by-layer phase analysis of the scale. We have established that the oxidation kinetics is described by a parabolic equation q² = K_pτ (where q is the mass gain per unit area of the sample, K_p is the rate constant, τ is the time). The value of K_p periodically decreased on the kinetic isotherms. In the scale, the phase components are distributed over the layers as follows: top layer, cubic and monoclinic ZrO₂, NiO; inner layer, monoclinic ZrO₂, Ni, and (or) Ni₅Zr. At the boundary with the scale, the alloy layer (underscale) is depleted in zirconium. We have established that oxidation of NiZr is accomplished by predominant diffusion of oxygen through the oxygen vacancies in the lattice of monoclinic ZrO₂. The decrease in q and K_p as the temperature rises from 600°C to 850°C is explained by a reduced concentration of these vacancies and (or) slowdown of their mobility. For T ≥ 850°C, the oxidation mechanism changes: counterdiffusion of Zr⁴⁺ also occurs through interstices in the lattice of monoclinic ZrO₂. The outer layer (NiO), saturated by zirconium dioxide, loses any protective properties and diffusion of oxygen is facilitated. For this reason, both q and K_p increase as the temperature rises to 1000°C.