The present work deals with corrosion problems related to conditions which prevail in a VVER primary circuit. We had a two-fold aim: (A) electrochemical methods were applied to characterise the hydrothermally produced oxides of the cladding material (Zr-1%Nb) of nuclear fuel elements used in Russian made power reactors type VVER, and (B) a number of possible reference electrodes were investigated with a view to high temperature applications. (A) Test specimens made of the cladding material, Zr-1%Nb, were immersed into an autoclave, filled with an aqueous solution typical to a VVER primary circuit, and were treated for different periods of time up to 28 weeks . The electrode potentials were measured and electrochemical impedance spectra (EIS) were taken regularly both as a function of oxidation time and temperature. This rendered information on the overall kinetics of oxide growth. By combining in situ and ex situ impedance measurements, with a particular view of the temperature dependence of EIS, we concluded that the high frequency region of impedance spectra is relevant to the presence of oxide layer on the alloy. This part of the spectra was treated in terms of a parallel CPE||Rox equivalent circuit (CPE denoting constant phase element, R ox ohmic resistor). The CPE element was understood as a dispersive resistance in terms of the continuous time random walk theory by Scher and Lax. This enabled us to tell apart electrical conductance and oxide growth with a model of charge transfer and recombination within the oxide layer as rate determining steps. (B) Three types of reference electrodes were tested within the framework of the LIRES EU5 project: (i) external Ag/AgCl, (ii) Pt/Ir alloy and (iii) Pd(Pt) double polarised active electrode. The most stable of the electrodes was found to be the Pt/Ir one. The Ag/AgCl electrode showed good stability after an initial period of some days, while substantial drifts were found for the Pd(Pt) electrode. EIS spectra of the Ag/AgCl reference electrode were found to be capacitive at high frequencies and resistive at low ones, as one would expect. EIS spectra for Pt, Pt-Ir and Pd(Pt) quasi reference electrodes show opposite tendency. The spectra, which can usually be described with a rather simple -R s-R p||CPE- combination in a wide frequency region, are resistive at high frequencies and become capacitive as the frequency decreases. The temperature dependence of the CPE coefficient, however, is opposite for the Pt and Pt-Ir, compared to the Pd electrode.
|Publication status||Published - dec. 1 2004|
|Event||European Corrosion Conference: Long Term Prediction and Modelling of Corrosion, EUROCORR 2004 - Nice, France|
Duration: szept. 12 2004 → szept. 16 2004
|Other||European Corrosion Conference: Long Term Prediction and Modelling of Corrosion, EUROCORR 2004|
|Period||9/12/04 → 9/16/04|
ASJC Scopus subject areas
- Surfaces, Coatings and Films