Corrosion and hydrogen pickup properties were investigated on sheet materials of pure Zr, Zr-0.2 wt%Fe, Zr-0.2 wt%Cr, and Zr-0.1 wt%Fe-0.1 wt%Cr which were given an intermediate anneal of 853 K or 1003 K, and a final anneal at 853 K. To clarify the substantial effects of iron and chromium, impurities were minimized by using crystal bar zirconium and pure alloy additions. Corrosion results in 633 K water showed that Zr had the lowest corrosion resistance and that Zr-0.1Fe-0.1Cr alloy heat treated at 1003 K had the highest. The alloys heat-treated at 1003 K showed higher corrosion resistance that those treated at 853 K for each composition except the Zr-0.2Fe alloy. Zr-0.2Fe alloys had higher hydrogen pickup ratios (80%) than those of Zr-0.2Cr and Zr-0.1Fe-0.1Cr alloys (10 to 20%). TEM and HR-SEM examination of oxide cross sections was also performed. Zr-0.1Fe-0.1Cr alloy formed columnar oxide grains with few fine cracks, while Zr and Zr-0.2Cr alloys had fine-equiaxed grains. Zr-0.2Fe alloys formed columnar oxide grains and included many lateral cracks. The corrosion mechanism of the alloys was associated with the electrochemical characteristics of the precipitates in this study.
Alloying elements such as Fe and Cr are generally considered to be effective even in small quantities for corrosion resistance of Zircaloy-4. The maximum total solubility of Fe + Cr in a Zr-Sn matrix has been reported to be very low [1]. Therefore, most of these elements are observed in the form of ternary Zr-Fe-Cr-type precipitates. To clarify the effects of precipitates on corrosion property, Zr-1.3 Sn-(Fe,Cr) alloys containing Fe + Cr from 45 up to 180 ppm (the Fe to Cr ratio is about 2) were melted from pure zirconium (X-bar Zr and EB-Zr) and pure alloying elements. They were subjected to corrosion testing in 633 K water and microstructural analysis. It was found that precipitate-free materials showed much larger weight gains than precipitate-containing materials even at the same alloy compositions. Subsequently, a corrosion test on the precipitate-free material galvanically coupled with a noble intermetallic compound of Zr(Fe0.66Cr0.33)2 was performed. It was found that the precipitate-free material, having very poor corrosion resistance in itself, was covered with thin and adherent black film under galvanically coupled conditions. In addition, its oxide grain structure was almost the same as that of the precipitate-containing material Zircaloy. From these results, it was concluded that the good corrosion resistance of Zircaloy-4 is attributed to the anodic protection provided by precipitates in the alloy.
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