Zircaloy-2 cladding materials: Effect of microstructure on corrosion properties

The earlier results obtained in the controlled distance electrode (CDE) arrangement showed that the thin-layer electro-chemical impedance and the contact electric impedance measurements provided similar information of the corrosion reactions of the oxide films of different types of Zircaloy-2 specimens. To gain information on the properties of the zirconia corrosion films only, the corrosion reactions taking place in the water was eliminated. To improve the statistical reproducibility of the impedance measurements, a new disc type specimen was introduced. The specimens made of Zr and of Zircaloy-2 tube materials were exposed to simulated BWR water at 300°C for four days, after which their thin oxides were measured by performing the contact impedance measurements in the water. Therefore, the autoclave was evacuated at 300ºC and filled with nitrogen. The additional contact impedance measurements in N2 were performed at temperatures ranging from 300°C down to 100°C. As a result of the measurements in N2, the contribution of the conductivity of the oxide film could be distinguished from the impedance spectrum measured in the water. The thickness of the oxide films was determined later from metallographic samples using scanning electron microscope (SEM). However, the observed difference between the conductivity of the oxide films of the two Zr and Zircaloy-2 specimens were not larger than the accuracy of the impedance measurements.