Effects of Nitrogen Ion Implantation on the Corrosion Resistance of Zircaloy-4 in High Temperature Water

The influences of nitrogen implantation on the corrosion resistance of Zircaloy-4 were examined by immersion tests in high temperature water at 350 .deg. C, and the results were discussed with structural and compositional variations of implanted layer that were determined by X-ray diffraction(XRD) and Auger electron spectroscopy(AES). Corrosion resistance of nitrogen implanted Zircaloy-4 was very sensitive to the substrate temperature and the ion dose during the implantation. The alloy implanted at 400 .deg. C has the highest corrosion resistance until 45 days of exposure. On the other hand, the corrosion resistance of the alloy implanted at 200 .deg. C and 500 .deg. C was not improved but degraded. At the substrate temperatures of 400 .deg. C, the corrosion resistance of alloy enhanced with increasing the nitrogen dose from 2 x 1017 ions/cm2 to 5 x 1017 ions/cm2, but did not improve for the ion dose of 1 x 1018 ions/cm2. The improvement of the corrosion resistance of Zircaloy-4 caused by nitrogen ion implantation is not attributed to ZrN but ZrO2. ZrN was not stable in high temperature water of 350 .deg. C and reacted with the environment as confirmed by an extreme decrease in a nitrogen content and an increase in oxygen content in the implanted layer. AES and XRD data indicate that the nitrogen in the ZrN layer is replaced by oxygen and nitrogen is released into water during the corrosion tests. The thickest oxide was formed by the implantation at the substrate temperature of 400 .deg. C and the oxide worked as a barrier of oxygen diffusion to hinder oxidation