Corrosion resistance of nitrogen-implanted 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°C, and the results were discussed with structural and compositional variations of implanted layer that were measured by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The nitrogen-implanted layer was composed either of ZrN and ZrO2 layers or of nitrogen-implanted layer with ZrO2 layer whether the implanted nitrogen ion dose is greater than the critical value of about 1018 ions cm−2 necessary for the formation of ZrN layer or not. It was found that most of the nitrogen implanted into Zircaloy-4 was released into water when exposed to the high-temperature water, presumably by a decomposed reaction of ZrN by oxygen diffused from water, thereby producing ZrO2 layer the thickness of which was very sensitive to both the substrate temperature and the nitrogen ion dose. The corrosion resistance of the nitrogen-implanted Zircaloy-4 was improved with increasing the ion dose, provided that the ion dose is less than the critical value for the formation of a continuous ZrN layer. The best corrosion resistance of Zircaloy-4 was obtained when implanted with a nitrogen ion dose of 5×1017 ions cm−2 at a substrate temperature of 400°C due to an increased thickness of ZrO2 layer.