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

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 degrees C. At this pressure, water is in subcritical state at 300 degrees C and supercritical state at 500 degrees C. Cross-sectional scanning electron microscopy showed that oxidation occurred in the form of mainly an oxygen diffusion layer for the 300 degrees C exposure, but for the 500 degrees C exposure a uniform stoichiometric oxide layer and a sub-surface oxide diffusion layer were observed. For the 300 degrees 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 degrees 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 degrees 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. (c) 2006 Elsevier Ltd. All rights reserved.