A physical model describing oxidation and oxygen diffusion under non-isothermal conditions is presented. It assumes (a) equilibrium at phase interfaces, as is usual in the treatment of diffusion, and (b) oxygen supersaturation of the β-zirconium to precipitate α-zirconium on cooling. An important consideration, not required in isothermal diffusion, is that the equilibrium oxygen concentrations in the α- and β-phases decrease with temperature.

During isothermal oxidation at temperatures above 850°C, the phase distribution is zirconium dioxide (ZrO₂/α/β. This model predicts that on slow cooling, oxygen in the β-region diffuses to the α/β interface leading to a depletion of oxygen in this region with a corresponding increase in the thickness of the α-layer. With more rapid cooling, α-zirconium is precipitated in the β-region near the α/β interface. The oxygen concentration of the β-region is determined not by the maximum temperature in the transient but by the lowest temperature at which appreciable diffusion is still possible.