In the initial pre-transition regime of water-side corrosion at 360◦C the oxidation of zirconium alloy fuel cladding can be described by a power law. The post-transition regime can be divided into several periods of corrosion that cyclically reproduce the pre-transition regime. Careful analysis of the pre-transition oxidation kinetics of zirconium alloys has shown a range of value of n from as low as n∼ 0.2 for ZrCu, to n∼0.3 for Zircaloy-type alloys and as high as n∼0.5 for ZrNb alloys. This range of sub-parabolic kinetics deduced from power law fitting of oxidation kinetics in the pre-transition regime is obtained on alloys that have very small differences in their alloying content. This is rather uncommon in metal oxidation theory and the underlying mechanism is still not well understood. It is important to note that the value of n is related to the alloy composition, such that Zr–Nb alloys generally exhibit close to parabolic oxidation kinetics, whereas Zircaloys exhibit behavior closer to cubic or even sub-cubic kinetics (n ≤ 0.3).
To address this problem, the application of space charge theory to zirconium oxidation by the development of the Coupled Current Charge Compensation (C4) model based on Fromhold’s framework is investigated in this study. A special case of the model is solved in which space charge compensation by embedded Nb aliovalent ions is assumed. This assumption is validated for actual experiments by data from microbeam XANES experiments. A mechanism of zirconium alloy oxidation resulting from the model is also developed.
Related publications
- A. Couet, A. T. Motta, and A. Ambard, “The Coupled Current Charge Compensation Model for Zirconium Alloy Fuel Cladding Oxidation: I.Parabolic Oxidation of Zirconium Alloys,” Corrosion Science, vol. 100, pp. 73-84, 2015.