Correlation between electrochemical properties and corrosion resistance of zirconium alloys

The electrochemical behavior of some zirconium alloys including Zry-2 with various ΣAi from 2.5 x 10-20 to 1.2 x 10-17 (h), modified Zry-2 with iron contents of 0.15, 0.25, and 0.5%, and standard Zry-4 was studied by measuring anodic polarization curves in sulfuric acid solution. The results of these electrochemical tests were compared with those of steam autoclave tests. In Zry-2, the current peak was observed at 1250 mV (versus SCE) on the anodic polarization curve, and this peak area increased with ΣAi and with the size of secondary precipitates. Also, this peak was closely correlated with nodular corrosion resistance as expected from the above results. As iron contents in modified Zry-2 increased, the current peak at 1,250 mV decreased and a new peak at 1,900 mV appeared. The former peak disappeared and the latter peak increased further at 0.5% iron. In Zry-4, the current peak was observed at 1,900 mV, but not at 1,250 mV, and this behavior was the same as that of modified Zry-2 of 0.5% iron. AEM observation of secondary precipitates in these zirconium alloys indicated that the peak at 1,250 mV appeared when the Fe/Cr ratio of Zr(Fe,Cr)2 was below 1.0 and the peak at 1,900 mV appeared when the ratio was above 1.5. In order to clarify the mechanism of these peak behaviors, the anodic polarization properties of intermetallic compounds that precipitated in zirconium alloys were studied. The results suggest that the peak at 1,250 mV is related to the reaction Cr3+ → Cr6+ (soluble in solution) and the peak at 1,900 mV is related to the reaction Fe3+ → Fe6+ (soluble in solution) and oxygen generation on iron-rich precipitates