Thermogravimetric Method to Evaluate the Average Compressive Stress Evolution during Zy-4 Oxidation

The development of an oxide on zirconium alloys is governed by the diffusion of the oxygen ions through the oxide scale and the appearance and growth of compressive stress at the oxide - metal interface. It is admitted that the diffusional driving force is the compressive strain gradient in addition to a chemical potential gradient across the oxide scale. Each of these two contributions to the kinetic curve can be treated separately, allowing the evaluation of their dependency on the oxide scale thickness and the evolution in time of both flows given by the two potential gradients. In the first part of our work we present the theoretical treatment of the kinetical curves in both pre- and post- transition regions. The equations obtained where used in the treatment of kinetic data obtained experimentally on isothermal oxidation of Zy-4 cladding in 873 - 1073 K temperature range. The contribution of chemical potential gradient to the kinetic curve is linear for whole range with different kinetic coefficients for the pre-transition and post-transition regions. The average compressive stress exponentially increases in time, in the pre-transition region which means a linear increase with the oxide scale thickness. For the post-transition region the shape of stress evolution curves became more complicated due to the cracks and pores formation. The periodical stress relaxation can be related to the kinetical behaviour over the transition and explains the multilayered structure of the oxide scale. (author).