In-situ studies of the oxide film properties on BWR fuel cladding materials

The pre-transition oxides formed on five different types of Zircaloy-2 alloys have been characterized in-situ using electrochemical impedance spectroscopy (EIS) in high-temperature water simulating BWR conditions at 300°C. The contribution of the oxide film properties to the impedance spectra has been distinguished from the contribution of the corrosion reaction by performing additional EIS measurements in a gas atmosphere. To obtain a correlation between the oxide film thickness, the oxide growth rate, and the impedance parameters, the EIS data have been fitted to the transfer functions derived from the Mixed Conduction Model for oxide films. As a result, the oxide growth rate of each alloy has been calculated assuming that the growth rate is proportional to the flux of oxygen vacancies through the film. The values of the oxide film thickness based on the model calculations have proved to be in good agreement with the values measured from the cross-section micrographs of the specimens using scanning electron microscopy (SEM). The measured corrosion properties have been qualitatively correlated to the secondary phase particle distributions of the five fuel cladding materials. However, to establish a quantitative correlation between the corrosion rates and the second phase particle distributions of these alloys, more data on the microstructure of the metal-oxide interface are needed.