The effect of vacuum annealing on the hydriding kinetics of zirconium

The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 atm hydrogen pressure were studied using combined metallographic and kinetic measurements. Two stages were identified in the reaction kinetics: a fast initial stage in which the apparent front velocity was 70 μm min^-1 and a second stage in which the front velocity gradually decreased until it stabilized around 8 μm min^-1. During the fast stage a special topochemical development of the hydride was observed, with the hydride progressing along grain boundaries which were previously oxidized during the vacuum annealing. This relatively fast process continued until a hydride overlayer was formed on the sample surface. The spatial extent of this stage was thus of the order of the grain size (around 40 μm). During the second stage of the hydriding the rate-controlling step was the diffusion of hydrogen through the product layer leading to a decrease in the apparent reaction rate. Hence, the main effect of the vacuum preannealing on the hydriding rate was introduced through grain boundary oxidation, affecting the initial rapid stage.