Many countries throughout the world have adopted a plan for the permanent disposal of high level nuclear waste that includes sealing it in metallic containers and burying it in a suitable deep geologic repository (DGR). For thick-walled containers, gamma (γ) radiation fields on the outside of the container will have a negligible influence on container corrosion. However, to overcome fabrication issues and to reduce costs, steel containers with a 3 mm outer Cu coating are being designed in Canada making a reassessment of the influence of γ-radiation on container corrosion a potential licensing requirement.
A combination of radiolysis calculations and electrochemical/corrosion experiments are underway in the key environments anticipated in the early stages of disposal when radiation fields are significant and aerated vapour conditions will be evolving to anoxic saturated conditions. Nitric acid (HNO3) is potentially one of the most influential radiolytic oxidants in the aerated vapour phase and could concentrate in condensed water on the container exterior surface potentially resulting in corroded locations [1]. The goal of this project is to investigate the mechanism and determine the kinetics of this corrosion process with the longer term goal of developing a model to assess the total damage expected over the period when the gamma radiation field is significant.
Nitrate (NO3-(aq)) could act as a cathodic reagent on Cu and be reduced in a multi-step reaction which would consume up to 8H+(aq), depending on the final product, for each NO3-(aq) consumed [2][3]. Oxygen (O2)reduction could also act as an oxidant, contributing to the corrosion process and consuming 4H+(aq) per O2 molecule consumed [3]. This enables us to investigate the corrosion process both electrochemically and by monitoring the evolution of pH with time in small volumes of solution.
A combination of these techniques suggests Cu corrosion in solutions containing HNO3 can either proceed under active conditions or be inhibited by corrosion product formation. This is a result of the complex behavior of NO3-(aq) on Cu surfaces. NO3-(aq), when the only anion present, has been shown to chemisorb on the Cu surface and possibly form a thin Cu2O film, which effectively inhibits corrosion [4]. However, it has been proposed that chemisorbed NO3-(aq) can be activated as an oxidant when Cu+(aq) is generated [5], and our recent studies support this view [6].
In this study we investigate the influence of the dissolved O2 concentration and anions such as chloride (Cl-(aq)) and sulfate (SO42-(aq)) on the corrosion process in HNO3 solutions. Since all three anions can chemisorb on Cu, the corrosion behavior is expected to be influenced by their competition for surface adsorption sites. Cl-(aq) is of particular interest since it stabilizes the Cu+(aq) in solution as CuClx(x-1)-(aq) with x being dependent on [Cl-(aq)].
Experiments are being conducted in small and large volumes of solution and the corrosion process followed by measuring the corrosion potential and the linear polarization resistance. The condition of corroded surfaces is being examined using scanning electron and Raman spectroscopies and the possibility of passivation by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Our results show that the corrosion behavior is influenced by the anions, in particular the order in which they are added to the solution. The role of low concentrations of dissolved O2, and their influence on Cu corrosion relative to that of NO3-(aq), remains to be resolved.
References
[1] F. A. King, L; Taxén, C; Vuorinen, U; Werme, L in Copper Corrosion Under Expected Conditions in a Deep Geological Repository, Vol. Svensk Kärnbränslehentering AB, Stockholm, Sweden, 2001.
[2] G. E. Dima, A. C. A. de Vooys and M. T. M. Koper, Journal of Electroanalytical Chemistry 2003, 554–555, 15-23.
[3] H. Ma, S. Chen, B. Yin, S. Zhao and X. Liu, Corrosion Science 2003, 45, 867-882
[4]. S.-E. Bae, K. L. Stewart and A. A. Gewirth, Journal of the American Chemical Society 2007, 129, 10171-10180.
[5] E. V. Filimonov and A. I. Shcherbakov, Protection of Metals 2004, 40, 280-285.
[6] J.Turnbull, R.Szukalo, M.Behazin, D.Hall, D.Zagidulin, S.Ramamurthy, J.C.Wren and D.W.Shoesmith, Corrosion 2018, 74, 326-336.
Copper, Corrosion, Nitric Acid, Radiolysis