Electrochemical Impedance Spectroscopic Study of Passive Zirconium in High Temperature De-aerated Aqueous Solutions

Abstract

The Point Defect Model (PDM) was further developed to account for the properties of passive films comprising oxide barrier layers and porous oxide outer layers that form on zirconium and Zircaloys under simulated boiling water reactor operating conditions. The model parameter values were extracted from electrochemical impedance spectroscopic data for zirconium in de-aerated, borate buffer solution (pH 6.94) at 250°C by optimization. The results indicate that the corrosion resistance of zirconium in high temperature, de-aerated aqueous solutions is dominated by the outer layer. The impedance model based on the PDM provides a good account of the growth of the bi-layer passive films, and the extracted model parameter values might be used, for example, for predicting the accumulation of general corrosion damage to Zircaloy fuel sheath in BWR operating environments.The zirconium-based alloys Zircaloy-2 and Zircaloy-4 are the most frequently used structural materials for reactor fuel sheaths in Boiling Water Reactors (BWR), because of their low thermal neutron capture cross-sections, excellent mechanical properties, and good corrosion resistance. However, materials degradation under high burn-up condition is inevitable, due to the severe conditions that exist within a BWR core, including the high temperature (288°C), high pressure (6.8 MPa), and high redox potential (> 250 mV SHE ). The principal threat to the integrity of the fuel sheath is oxidation and corrosion, leading to more-or-less uniform thinning and, in some instances, to localized corrosion in the form of nodular attack. Such failure can pose a great threat to normal reactor operation. Therefore, understanding the corrosion mechanism of Zircaloy fuel cladding degradation under BWR coolant conditions and predicting the accumulation of corrosion damage to the cladding over the operating history of the fuel is a matter of considerable importance to the nuclear power industry. Considerable research has been carried out to investigate the kinetics and mechanism of the oxidation and corrosion of zirconium and its alloys (1-7). But little effort has been made to develop deterministic (as opposed to empirical and semi-empirical) models that can be used to predict the accumulation of corrosion damage to the fuel sheath. Those attempts to develop algorithms have employed semi-empirical, parabolic or cubic models to extrapolate oxide thickness data to longer times, but the validity of these models is highly uncertain because there are lacks of a sound electrochemical basis for corrosion process under free corrosion conditions. The objective of this work is to apply the PDM to develop an impedance model for the growth of the bi-layer passive films formed on zirconium and Zircaloys under BWR coolant conditions. In this system, the passive film comprises a nonporous defective oxide barrier layer (ZrO 2-x ) and a precipitated porous oxide outer layer. We have employed a different method of formulating the model, and using an unconstrained optimization procedure, we have e...