A working hypothesis on oxidation kinetics of Zircaloy

A kinetic hypothesis of zirconium oxidation has been developed in a closed form by assuming a constant strain-energy gradient as a diffusional driving force in addition to an oxygen chemical potential gradient, and verified quantitatively via thermogravimetry in the air atmosphere over a temperature range of 400–800 °C. The hypothesis explains quite precisely the crossover of kinetics from parabolic to cubic before the breakaway of ZrO2-scale, yielding an intrinsic diffusion coefficient of component oxygen as DO/m2 s−1=6.3×10−9exp(−1.59 eV kT−1). The open-circuit potential measurement across the protective scale indicates that the scale is a mixed ionic electronic conductor with an ionic transference number of tion≈0.5. The intrinsic diffusivity is thus interpreted as being a Nernst-type combination of the partial conductivities of oxide ions and electrons. The hypothesis also yields a strain-energy gradient across the protective layer of 1011 to 1010 J m−1(mol-O)−1 as temperature increases from 400 to 800 °C. The strain-energy gradient allows one to evaluate the characteristic thickness where the oxide breaks away, and the strain energy right at the ZrO2/Zr interface to be very reasonably on the order of 1010 J m−3 that is insensitive to temperature. Possible origins of the stress are discussed.