Effect of intermetallic compound oxidation on corrosion resistance of Zircaloy-4

The role of precipitates in the corrosion of Zircaloy-4(Zr-1.5 mass%Sn-0.22%Fe-0.11%Cr) has been studied by high resolution transmission electron microscopy/energy dispersive X-ray spectroscopy (HRTEM/EDS) with electron microbeam and field emission scanning electron microscopy (FE-SEM/EDS). In Zircaloy-4, the solubility of Fe and Cr is low, and Fe and Cr precipitate as an intermetallic compound Zr(Fe, Cr)(2). The size of the precipitate changed with temperature in the intermediate annealing process. The size of precipitates in the specimen annealed at 780 degrees C are significantly larger than that in the specimen annealed at 630 degrees C. The corrosion rate in pressurized steam at 400 degrees C was higher in Zircaloy-4 with finer Ze(Fe, Cr)(2) particles precipitated by the intermediate annealing at 630 degrees C. The observation of the corroded layers by HRTEM/EDS showed that the surfaces of precipitates oxidized together with the matrix and decomposed to dissolve into ZrO2 during the corrosion. Ils the precipitates oxidize, the atomic ratio of Fe/Cr in the precipitates changes from 2 in the metal to 0.5 in the oxide. This phenomenon suggests that Fe component predominantly diffused into the matrix. The cations of Fe and Cr generated by oxidation and diffused into the matrix ZrO2, were thought to replace Zr4+ sites. The higher corrosion rate in Zircaloy-4 with finer precipitates was attributed to larger amounts of Fe and Cr cations that are made available during the early stage of corrosion.