Characterisation by neutron diffraction in high temperature pressurised water of the surface corrosion and hydrogen embrittlement of zircaloy-4

摘要

Zircaloy-4 foils were analysed by in situ neutron diffraction experiments when submitted to a water corrosion process simulating PWR conditions (high temperature and high steam pressure). The oxidation process takes place through two steps and layers. First, a layer of dense tetragonal beta-zirconia is formed when the metal external surface is corroded, then it transforms progressively to an alpha-zirconia external layer. Advancement of the oxidation layer is accompanied by the penetration of some hydrogen in the first metal layers. High temperature formation of a ternary intermetallic hydride involves the fine Zr(Cr,Fe)_2 particles of the C14 Laves phase type which are randomly distributed as precipitates in the zirconium matrix. The hydrides become unstable when the sample is cooled to room temperature. Precipitation of binary implied by-ZrH_2 grains occurs only when the sample is cooled down to 200 deg C. A posteriori experiments reveal some possible local correlation between these two transformations, so an impact of the formation of hydrides on the corrosion process is suggested from the present experiment. The zircaloy-4 was cold-rolled first, and found to be highly textured even after annealing. However, the two types of corroded zirconia formed at the surface were found to have a texture correlated with the metal substrate, and texture effect was observed also for the precipitates.