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Title: Electrochemical studies of mobile charged species during zirconium alloy oxidation
Author: Smith, James Stephen
ISNI:       0000 0004 2740 7433
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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This research has used a suite of electrochemical techniques, both in-situ and ex-situ to investigate the mobile charged species in the oxides of zirconium alloys. Limits on the corrosion resistance of existing zirconium alloys used for fuel cladding are a major restriction on the burn-up that can be achieved within a pressurised water reactor (PWR). Developing a full mechanistic understanding of the corrosion process of zirconium alloys in the primary water environment is necessary for prolonging the life-time of fuel cladding. A key component in the corrosion of zirconium alloy is the transport properties of various charged species within the oxide layer, which determine the rate of corrosion. Three distinct electrochemical methods have been employed in this work. The first technique, Electrochemical Impedance Spectroscopy (EIS), has been used to determine the physical properties of the oxide. The second technique, Mott-Schottky analysis, studied the oxide acceptor/donor densities; hence the nature, concentration and activation energy of the major electronic charge carrier could be determined. The final method, a DC method known as Wagner-asymmetric polarization (WAP), separated the ionic and electronic currents and can thus allow the calculation of transport numbers, partial conductivities and activation energies of the ionic species. All the techniques investigated parameters as a function of oxidation time, lithium concentration and alloying elements.Work was carried out on oxides grown under PWR conditions (2 ppm Li and 1000 ppm B) at temperatures of 240-360 °C on specimens of zircaloy-4, ZIRLO, Zr-0.1%Nb and Zr-1%Nb. The EIS was undertaken in-situ to determine the oxide thickness and showed that the electrochemically active oxide grows quicker in a higher lithium concentration. Variances in oxide growth rates are also seen between specimens of differing niobium and tin content.In-situ Mott-Schottky analysis of the electronic charge carriers within the thin oxide showed the oxide to be a n-type semiconductor. Hence the dominant charge carriers, for early stage oxidation of Zircaloy-4 under simulated PWR conditions, were electrons with a donor density of approximately 2 x 1017 cm-3. The addition of lithium to the oxidation environment increases the donor density, which indicates lithium plays a previously unknown active role in the corrosion process.Despite the oxide being highly resistive, this research has also shown that the ionic charge carriers within the oxide can be directly measured using asymmetric-polarisation. However, further work needs to be undertaken to perfect this technique under inert atmospheres.
Supervisor: Lyon, Stuart Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available