Long-Term In Situ Corrosion Investigation of Zr Alloys in Simulated PWR Environment by Electrochemical Measurements

Abstract

The corrosion behavior of Zircaloy-type alloys with different tin contents of 1.55, 0.70, and 0.55 wt% was studied at 350°C and 17 MPa in an environment similar to PWR primary water. For this non-interrupted test, a special autoclave system was used that was equipped with electrical feed that allowed followup on the growth of oxide layers by impedance spectroscopy and corrosion potential measurement at high temperature and pressure. As a reference electrode, a platinum wire was used that works as a hydrogen electrode according to the hydrogenated environment established during the start-up procedure. The test ran without interruption for 471 days. Impedance spectra were taken at time intervals and evaluated for thickness and morphology of the oxide layer as well as for its electrical resistance. The tests without any temperature and pressure cycling showed similar oxidation behavior with repeated transitions as in discontinuously performed standard autoclave tests. Early in the pre-transition range, a dense oxide layer is formed, and fast changes of corrosion potential and electrical resistance are observed. The dense layer increases in thickness and homogeneity up to the transition, where a sudden breakdown occurs. Abrupt changes of the corrosion potential and electrical resistance were observed also at those points. After transition, a new dense layer is built up. The corrosion potential changes are caused by a decrease of the electrical corrosion current with increasing oxide layer thickness, by the formation of a potential drop over the high-resistance dense oxide layer, and by structural changes at the transition points. In general, alloys with different tin contents show similar behavior. However, they show differences in the times to transition, the kinetic constants deduced from their impedance spectra, and in the ionic and electronic resistance of the dense inner layer controlling corrosion.