Effects of compositional and structural change on the corrosion behaviour of nitrogen implanted Zircaloy-4

The influence of nitrogen implantation on the corrosion behaviour of Zircaloy-4 was examined by potentiodynamic polarization tests in a chloride and an acid solution, and the results were discussed with structural and compositional variations of implanted layer that was determined by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The resistance to localized corrosion of Zircaloy-4 was very sensitive to the ion dose and the substrate temperature during the implantation. At substrate temperatures above 200°C, the pitting potential of the nitrogen implanted alloy in deaerated 4 M NaCl at 80°C increased gradually with the ion dose from 350 mVSCE for the unimplanted sample to 990 mVSCE for the alloy implanted with an ion dose of 6×1017 ions cm−2, and then the alloy was immune to pitting when implanted with ion doses greater than about 1×1018 ions cm−2. In contrast to this, the alloy implanted at 100°C exhibited an inferior corrosion resistance to the unimplanted sample irrespective of the ion dose. A drastic increase in the resistance to pitting corrosion in chlorides as well as a significant reduction in the passive current density in acid solution of the implanted alloy, were found to be associated with the formation of ZrN layer with a stoichiometric ratio of N to Zr. The low resistance to localized and general corrosion of the alloy implanted at 100°C was attributed to the increase in structural defects produced by ion bombardment, and to low atomic mobility in the implanted layer.