Electrochemical study of pre- and post-transition corrosion of Zr alloys in PWR coolant

摘要

Corrosion properties of Zr-Sn and Zr-Nb zirconium alloys were studied under simulated PWR conditions (or, more exactly, WER conditions - boric acid, potassium hydroxide, lithium hydroxide) at temperatures up to 340°C and 15MPa using in-situ electrochemical impedance spectroscopy (EIS) and polarization measurements. EIS spectra were obtained in a wide range of frequencies (typically 100kHz - lOOpHz). It enabled to gain information of both dielectric properties of oxide layers developing on the Zr-alloys surface and of the kinetics of the corrosion process and the associated charge and mass transfer phenomena. Experiments were run for more than 380 days; thus, the study of all the corrosion stages (pre-transition, transition, post-transition) was possible.Experimental impedance spectra were approximated by equivalent circuits based models. Jonscher-type analysis was applied to estimate frequency independent oxide capacitance. A model based on the integrated Stern-Geary equation was developed to correlate instantaneous and integral corrosion rates of zirconium alloys.