Developing an elevated temperature Pourbaix diagram for zirconium and multi-element Pourbaix diagram of Zircaloy-4

Abstract

Zirconium and Zircaloy-4 are used in applications that require corrosion resistance under extreme conditions. For example, the fuel bundles in coolant systems of CANDU™ reactors operate between 533 to 583K (260 to 310 °C). However, thermodynamic and hydrolysis properties of aqueous zirconium species have not been measured above standard conditions. This lack of information is addressed through solubility measurements and the development of elevated temperature Pourbaix (E-pH) diagrams for zirconium and Zircaloy-4. These Pourbaix diagrams of zirconium and multi-element diagram (Sn, Zr, Cr) of Zircaloy-4 were developed at 373.15K (100 °C). Solubility measurements were made using a batch-style pressure vessel and concentration measurements were made using ICP-OES and ICP-MS, for zirconium and Zircaloy-4, respectively. For Zr(OH)6²-, Zr(OH)4 (aq), and Zr(OH)2²+ the hydrolysis constants were found to be -19.39±1.98, 1.77±1.60, and 3.72±1.63, and the Gibbs energy of formation were: -2177.40±8.49 kJ/mol; -1704.86±1.46 kJ/mol; and -1095.05±2.72 kJ/mol, respectively.