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 . 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.