The temperature-dependent changes of the kinetics and morphology of hydride formation in zirconium

Abstract

The hydriding kinetics and hydride phase development in vacuum-annealed Zr were studied at temperatures ranging from 250 to 800 °C, under H 2 at 1 atm. In the lower temperature range a hydride external layer is formed with a well-defined metal-hydride interface. Between 350 and 550 °C the hydriding reaction consists of two stages: initial fast penetration of hydrogen through grain boundaries, and subsequent development of hydride layer resulting in a decrease in the advancing front velocity. The activation energy for both processes is similar (67 kJ mol −1) although the front velocity of the grain boundaries is somewhat faster. Above 550 °C, where the β phase is formed in front of the δ phase, a significant change is observed in the hydriding kinetics and topochemistry. No external reaction front is formed and the hydriding route of the grain boundaries exclusively prevails with simultaneous decrease in the overall reaction rate. The activation energy for the hydride layer advance at this range, due to the diffusion of hydrogen in the β-zirconium product, or the α→ β phase transformation, is 165 kJ mol −1. The huge discrepancies between the results of high temperature hydrogen diffusion in zirconium hydrides previously published are accounted for in terms of the hydriding topochemical mechanism.