Zirconium alloys are used as fuel cladding materials in nuclear reactors. Their stability in reactor coolant depends on characteristics of oxide, formed on cladding surface under high temperature conditions. Currently, the only method that enables in-situ characterization of corrosion processes at high temperatures and pressures is Electrochemical Impedance Spectroscopy (EIS). As long as it operates in wide enough range of frequencies, it provides information of both dielectric properties of oxide layers growing on the surface during the exposure in reactor coolant as well as the kinetics of the corrosion process and the associated mass transfer processes. In the presented paper, corrosion of different types of zirconium alloys was investigated in-situ under high pressure high temperature conditions using EIS. Chemical conditions were those applied in VVER type of reactor coolant (boric acid, potassium hydroxide, lithium hydroxide). Experimental impedance spectra were approximated by equivalent circuits (EC) based models. Optimal model was selected and parameters characterizing time evolution of oxide parameters were obtained. EC parameters were correlated with those obtained by independent methods (weight gain, electron spectroscopy).
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