Temperature dependent electrochemical equilibrium diagram of zirconium-water system studied with density functional theory and experimental thermodynamic data

Abstract

Zirconium (Zr) alloys are important cladding materials of nuclear fuel in nuclear power industry, which are commonly immersed in complicated aqueous environments. Their corrosion relates critically to the safe operation of nuclear reactors, and is affected by many factors. As a foundation to study such complicated corrosion behaviors, in this work, the electrochemical stabilities of various species in the Zr-water system are directly demonstrated with the electrochemical equilibrium (Pourbaix) diagrams. The diagrams are calculated by applying methods of combining first-principles calculations on solid state phases with experimental thermodynamic data of aqueous species. The calculated Pourbaix diagrams are generally consistent with the results of previous experiments, while the solid phases in the oxide form (passivation region) of the calculated ones are much richer than the experimental counterparts. The stabilities of the solid phases are significantly affected by temperature and acidity (pH value) of the environment, as well as slightly by ionic concentration. To construct the experimental consistent Pourbaix diagrams for the Zr-water system, it is found necessary to correct the relative chemical potentials between solids and aqueous ions. Such a scheme of correction is demonstrated to be robust for the Pourbaix diagrams obtained with different exchange-correlation functionals. This work may help us to understand the corrosion process more clearly and guide us to enhance the corrosion resistance of the cladding Zr in cooling water circuit environment.