Corrosion Properties of Zircaloy-4 and M5 Under Simulated PWR Water Conditions

Zircaloy-4 has been widely used as a nuclear fuel cladding material. However, recently, several European countries have gradually replaced Zircaloy-4 cladding material in pressurized water reactor (PWR) nuclear power plants with a Zr-Nb alloy called M5 and other new zirconium alloys with Nb added that are expected to have relatively longer operating lives. Although improved corrosion resistance of the advanced zirconium alloys was demonstrated in various conditions, the origin of this resistance has not yet been elucidated. In this study, corrosion tests were performed on Zircaloy-4 and M5 under simulated PWR water conditions to explore the origin of the better corrosion resistance of the advanced zirconium alloys. Alloy specimens were exposed to simulated PWR conditions, and the increase in oxide film content was analyzed by weight gain and microscopy observations. Electrochemical impedance spectroscopy (EIS) was performed on Zircaloy-4 and M5 in the pretransition period of oxide film to compare their corrosion properties. The EIS results obtained in this study show that the electrochemical behavior of M5 is significantly different from that of Zircaloy-4 in the early period of the initial stage in the pretransition oxidation process. To explain the result, a multilayer circuit model is assumed. The resistance of the diffusion layer comprising multiple layers restricts the rate of oxidation in the M5 response system. The occurrence of this process caused by multilayered oxide film would contribute to improved corrosion resistance of M5 under PWR water conditions.