Hydrogen Absorption Kinetics During Zircaloy Oxidation in Steam

The oxidation and hydriding kinetics in 400 and 500°C steam have been determined for various grades of Zircaloy. Over a wide range of corrosion rates, the hydriding kinetics essentially depend on the oxidation rate. Two types of parameters can be distinguished: those that modify only the corrosion rate, with a concomitant effect on the hydriding kinetics, and those that also have an intrinsic influence on the latter. Among factors of the first type are the final heat treatment, the “cumulative annealing parameter,” the tin content, and the iron/chromium (Fe/Cr) ratio. The nickel content falls in the second category. When the oxidation rate increases, the hydriding rate also rises, but the fraction of the available hydrogen that is absorbed decreases. These kinetic observations explain the sometimes opposite variations of the integrated parameters that are usually measured, for instance, the change in hydrogen absorption as a function of weight gain. The interpretation of conventional integrated data requires consideration of the successive modifications in the kinetics during corrosion.