Influence of a passive layer on the kinetics of an electron transfer reaction.
Résumé
In France, the spent nuclear fuel reprocessing involves the use of nitric acid at various concentrations and temperatures up to the boiling point. The corrosiveness of these nitric mixtures leads to the use of corrosion resistant materials such as zirconium or austenitic stainless steels. Indeed, these materials are protected by a passive layer which slows down redox reactions at the metal / nitric acid interface. The objective of this work is to study the role of the passive layer on the kinetics of reduction of nitric acid. As the reduction of nitric acid is very complex in concentrated solution, we firstly focused on a single step in the reduction reaction (Fe (III) / Fe (II) couple in acidic solution) on passivated zirconium with different oxide layer thicknesses. A necessary prerequisite was the establishment of a protocol for the growth of the passive layers with a controlled thickness. This was achieved by anodic polarization with monitoring of the thickness by electrochemical impedance spectroscopy (in situ) and XPS (ex situ). A critical analysis of the results obtained with these two different techniques was done. Two methods of exploitation of the electrochemical impedance spectra were used and discussed the power law and Jonscher's approaches. For XPS analyses, the way to estimate the values of the photoelectrons mean free paths (physical parameters required for estimating the oxide thickness) was shown to be a relevant parameter for both zirconium and zirconium oxide and was thoroughly discussed and scanning electrochemical microscopy . The values of the rate constants k0 of Fe(III) reduction estimated by these three methods were in good agreement and it was shown that the rate constant k0 value decreases when the oxide thickness increases. In addition to the kinetics properties, the electrochemical impedance spectroscopy can lead to estimate the semiconducting properties of the film the space charge capacitance (from the Mott-Schottky equation) allowed to estimate the number of charge carriers in the oxide. These values were shown to follow the same trend than k0 when the oxide thickness increases. In a second step, this study performed on the Fe(III)/Fe(II) couple was then extended to the reduction of nitric acid, for which the preliminary results will also be presented.
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