Oxidation of Zircaloy-2 and Zircaloy-4 in water and lithiated water at 360°C

Abstract

Zircaloy-2 and Zircaloy-4 samples were oxidised in an autoclave in water and in lithiated water (70 ppm) in order to evaluate the influence of both the alloy compositions and the effect of lithium on the oxidation kinetics. Cross-sectional TEM and electrochemical impedance spectroscopy were used to analyse the samples. The pre-transition oxidation rate is not affected by the presence of LiOH, but the transition occurs earlier and the post-transition oxidation rate is increased. The oxidation rate correlates with the density of cracks in the oxide layer and the morphology of the oxide grains. The impedance measurements show that the oxides formed have a layered structure and for samples oxidised in LiOH solution the inner protective layer is thin. The hydrogen pickup ratio follows the weight gain, not the oxidation rate, up to the second transition. When the protective oxide layer is degraded the hydrogen pickup ratio increases strongly. The effect of LiOH is suggested to be due to partial dissolution of the oxide and subsequent incorporation of lithium ions during a dissolution–precipitation process. Newly formed oxide is probably more hydrous and the grain boundaries are particularly liable to dissolution. The up-concentration of LiOH within cracks and pores could cause the detrimental levels necessary for dissolution. This is supported by the insensitivity in the pre-transition region, to both the compositions of the alloy and to the environment.