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

Abstract

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 degrees C, the pitting potential of the nitrogen implanted alloy in deaerated 4 M NaCl at 80 degrees C increased gradually with the ion dose from 350 mV(SCE) for the unimplanted sample to 990 mV(SCE) for the alloy implanted with an ion dose of 6 x 10(17) ions cm(-2), and then the alloy was immune to pitting when implanted with ion doses greater than about 1 x 10(18) ions cm-2. In contrast to this, the alloy implanted at 100 degrees 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 degrees C was attributed to the increase in structural defects produced by ion bombardment, and to low atomic mobility in the implanted layer. (C) 1999 Elsevier Science S.A. All rights reserved.