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

Zircaloy-2 and Zircaloy-4 samples were oxidised in an autoclave in water an d in lithiated water (70 ppm) in order to evaluate the influence of both th e alloy compositions and the effect of lithium on the oxidation kinetics. C ross-sectional TEM and electrochemical impedance spectroscopy were used to analyse the samples. The pre-transition oxidation rate is not affected by t he presence of LiOH, but the transition occurs earlier and the post-transit ion oxidation rate is increased. The oxidation rate correlates with the den sity of cracks in the oxide layer and the morphology of the oxide grains. T he impedance measurements show that the oxides formed have a layered struct ure 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 degr aded the hydrogen pickup ratio increases strongly. The effect of LiOH is su ggested to be due to partial dissolution of the oxide and subsequent incorp oration of lithium ions during a dissolution-precipitation process. Newly f ormed oxide is probably more hydrous and the grain boundaries are particula rly liable to dissolution. The up-concentration of LiOH within cracks and p ores could cause the detrimental levels necessary for dissolution. This is supported by the insensitivity in the pre-transition region, to both the co mpositions of the alloy and to the environment. (C) 2001 Elsevier Science B .V. All rights reserved.