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.

Microstructure and Growth Mechanism of Oxide Layers Formed on Zr Alloys Studied with Micro-Beam Synchrotron Radiation

“…For example, it has been previously proposed that periodicities observed in oxide structures were caused by the opening and closing operations of the autoclave to weigh samples rather than by the corrosion process itself. This is not the case in the current study, because the 360°C pure water samples were tested in the same autoclave with the same sample-weighing schedule and yet show vastly different periodicities, in agreement with prior results [31]. Because we examine bulk samples, with the oxide still attached to the metal, the artefacts related to relaxation of stress and consequent tetragonal destabilization due to dissolution of the underlying metal during sample preparation are minimized [1,2].…”

The influence of alloying elements on the corrosion of Zr alloys

“…The fraction of hydrogen produced during corrosion that is absorbed into the metal, is called the hydrogen pick-up fraction (HPUF) and varies with alloy composition. It has been demonstrated that alloys with a more electrically conductive oxide layer exhibit a lower HPUF [14]. This allows electrons to move more freely though the oxide and thus allows hydrogen ion/electron recombination to occur further from the metal/oxide interface, which consequently reduces the probability of the hydrogen reaching the cladding metal [15].…”

Effect of Alloying Elements on Hydrogen Pickup in Zirconium Alloys

“…Although the mechanism of electron transport is still unclear [24,25,28,29], it seems reasonable to consider that both bulk and localized conduction can contribute to the overall rate [29][30][31][32]. The preceding arguments indicate that oxide electronic conductivity is a key parameter in controlling zirconium alloy oxidation [33,34].…”

“…This can be explained by the doping effect of the donor Nb 5þ at the interface with the aqueous medium, which increases the electron concentration at the oxide-water interface, thereby promoting the hydrogen discharge process [26,44]. Electrochemical measurements have also shown that oxide electronic conductivity is increased by Nb additions, promoting hydrogen evolution at the oxide-water interface and reducing hydrogen uptake [34]. However, these effects cannot readily explain the observed variations of f i H as a function of exposure time [13].…”

“…Metallic precipitates would likely enhance the electronic conductivity of the oxide layer, which would in turn reduce the hydrogen pickup relative to pure Zr. It is believed that a material with a homogeneous distribution of fine precipitates has a higher oxide electronic conductivity than pure Zr [34,37].…”