Oxidation of plasma surface modified zirconium alloy in pressurized high temperature water

摘要

Oxidation of a zirconium-alloy, zircaloy-2 in the untreated (non-surface treated) condition and after ion implantation with nitrogen, carbon, and oxygen has been examined after exposure for 168 h to an environment of pressurized water at 25 MPa at temperatures of 300 and 500 deg C. At this pressure, water is in subcritical state at 300 deg C and supercritical state at 500 deg C. Cross-sectional scanning electron microscopy showed that oxidation occurred in the form of mainly an oxygen diffusion layer for the 300 deg C exposure, but for the 500 deg C exposure a uniform stoichiometric oxide layer and a sub-surface oxide diffusion layer were observed. For the 300 deg C exposure, carbon and nitrogen ion implantation resulted in a thinner oxygen affected layer compared to the untreated alloy. The oxygen ion implanted samples showed a thicker oxygen affected layer, but this included the thickness contributions from both thermal oxidation and prior oxygen ion implantation. After exposure at 500 deg C, the carbon and nitrogen ion implanted samples exhibited a thicker, and the oxygen ion implanted samples a thinner oxygen affected layer compared to the untreated alloy. A separate experiment involving exposure to supercritical water at 400 deg C for 168 h showed lower levels of oxidation in zircaloy-2 samples that were sputter coated with an yttrium film and subsequently bombarded with xenon ions prior to exposure.