Studies of Corrosion of Cladding Materials in Simulated BWR Environment using Impedance Measurements

In the present work, the corrosion of three Zircaloy-2 cladding materials have been studied in-situ in an autoclave using electrochemical impedance spectroscopy. Measurements were conducted in simulated boiling water reactor environment at temperatures up to 288°C, both in the pre-transition and the post-transition stage. In addition, impedance measurements were performed at room temperature on samples previously exposed to irradiation. This work clearly demonstrates that impedance spectroscopy can be used for in-situ studies of the corrosion of Zircaloy cladding materials in the pre-transition region at high temperatures. Oxide thickness, oxide conductivity, and effective donor density were successfully evaluated from the impedance data. Less information has been able to be gained from the impedance measurements in the post-transition region compared to the measurements in the pre-transition region. However, the thickness of the inner oxide layer, oxide conductivity, and oxide porosity were successfully evaluated from the impedance data. The measurements on irradiated samples of the same materials as investigated in autoclaves indicate that irradiation introduces defects in the oxide structure, resulting in a higher number of charge carriers and a more inhomogeneous oxide than produced under autoclave exposure.