Electrochemical and XPS studies and the potential scan rate dependent pitting corrosion behavior of Zircaloy-2 in 5% NaCl solution

Electrochemical studies of Zircaloy-2 in 5% NaCl solution were carried out using polarization and electrochemical impedance spectroscopy (EIS). Scan rate dependent pitting behavior of Zircaloy-2 was observed when polarization experiments were carried out at different scan rates (5 mV/min, 50 mV/min and 500 mV/min). Polarization resistance, double layer capacitance, and the thickness of passive film were calculated from the EIS study. The slope of the Bode plot in the capacitive region showed non-ideal behaviour, suggesting formation of pits. A distribution of chloride concentration was seen at the surface of the electrode when the test solution was sampled from different parts of the sample surface and analyzed for chloride. Inclusion of chloride ions in the passive film was observed from the XPS study and the chloride concentration increases with the higher anodic polarization. The outermost layer of the passive film was rich in hydroxide and oxy-hydroxide. A bi-layer passive film structure on the alloy surface is proposed.