Structure and Properties of Chromium- and Zinc-Oxide Layers Formed on Zircaloy-2 in High-Temperature Water

The corrosion behavior of Zircaloy-2 in the presence of Zn was investigated. Zinc is a possible technological additive to be injected in the coolant to reduce the ⁶⁰Co buildup. However, its influence on the cladding corrosion, alone or in combination with some typical corrosion impurities, as, for example, Cr, has not been considered so far. Because of this, the surface composition and electrochemical properties of Zircaloy specimens were investigated after their exposure to Zn²⁺, CrO₄^2-, and CrO₄^2- + Zn²⁺ aqueous solutions at 250°C. It was found that zinc-containing phases did not deposit from solutions containing on Zn²⁺ ions. Amorphous Cr³⁺-oxide and ZnCr₂O₄ ferrite phases were found on the surface of the samples after their exposure to CrO₄^2- and CrO₄^2- + Zn²⁺ environments, respectively. The amounts of the deposited Cr and Zn + Cr strongly depended on the times of the preconditioning of the Zircaloy specimens in high-temperature water. The rate of the oxide precipitation declined with increasing exposure time to both the CrO₄^2- and CrO₄^2- + Zn²⁺ solutions. The electrochemical measurement showed that the limiting factor of the Cr and Zn + Cr deposition reaction was the reduction of Cr(VI) to Cr(III). The reduction completely depended on the resistance of ZrO₂, Cr, and Zn + Cr oxides, which increased with the time of preconditioning and exposure. A thermodynamic analysis based on oxide solubilities was applied to explain the different deposition pathways in the CrO₄^2- and CrO₄^2- + Zn²⁺ environments. In view of the decreasing deposition rate of the Zn - Cr-oxide phases, it could be concluded that their limited precipitation and presence do not have a significant adverse effect on the fuel cladding corrosion.