The effects of nitrogen ion implantation on corrosion behavior of zirconium alloy was studied. The substrate used was Zircaloy-4. Nitrogen ion implantation was performed with varying the ion doses of nitrogen from $3×10^{17}$ to $1.2×10^{18}$ N ions/㎠ at 120 keV. The substrate temperatures during implantation were kept at 100, 200 and $300^oC$ by controlling the current density of ion beam. The influences of nitrogen dose and substrate temperature on localized and general corrosion resistance were examined by measuring potentiodynamic anodic polarization curves in chloride and in $H_2SO_4$ solution. Electrochemical properties obtained from polarization curves were correlated to the change of surface composition and structure analyzed by AES and XRD. Also, compound layer formed by nitrogen implantation was characterized by measuring galvanostatic anodization curves.
The localized corrosion resistance of Zircaloy-4 was improved significantly with nitrogen implantation in chloride solution, which is confirmed by the increase in the pitting potential and the decrease in the passive current density. This improvement of corrosion resistance of Zircaloy-4 is the result of the formation of ZrN and $ZrO_2$ by implantation. Higher ion dose increased the pitting potential due to the increase in ZrN and $ZrO_2$. Galvanostatic anodization tests showed the increase in protective layer thickness with the nitrogen dose.
With the substrate temperature, different localized corrosion behavior was observed. The improvement of localized corrosion resistance of Zircaloy was observed for specimens implanted at $200 - 300^oC$. This is attributed to the increase in the formation of ZrN and $ZrO_2$ and decrease in defects at higher temperature. Galvanostatic anodization tests showed the increase in protective layer thickness with the substrate temperature.
In order to observe the effect of oxide layer formed by implantation on the corrosion behavior two experiments were carried out. Firstly, the effect of implantation of nitrogen and argon ion on localized corrosion resistance was examined. The pitting potential of nitrogen implanted Zircaloy-4 considerably higher than that of argon implanted Zircaloy-4. This may be considered as the result that nitride formed by nitrogen implantation acted as protective layer in cooperation with oxide. Secondly, nitrogen was implanted under oxygen atmosphere and then the localized corrosion resistance was measured. With the increase in the partial pressure of oxygen in chamber, pitting potential increased. This may be due to the increase in uniform oxide layer from the XRD patterns showed that the $ZrO_2$ peak of nitrogen implanted Zircaloy-4 under oxygen atmosphere became stronger.
The general corrosion resistance of Zircaloy-4 was improved by nitrogen implantation, which is confirmed by the passive current density. The passive current density decreased with increasing the nitrogen dose and substrate temperature. This effect may be explained as follows; the ZrN and $ZrO_2$ layer caused by nitrogen implantation operates as a barrier and reduces the passive current density.
