Copper Corrosion in Aqueous Sulfide Solutions under Nuclear Waste Repository Conditions
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Copper Corrosion in Aqueous Sulfide Solutions under Nuclear Waste Repository Conditions J. Chen, Z. Qin and D. W. Shoesmith Department of Chemistry, The University of Western Ontario London, Ontario, N6A 5B7, Canada
ABSTRACT The corrosion behavior of oxygen-free copper in anoxic sulfide solutions under nuclear waste disposal conditions was studied using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) equipped with a focused ion beam (FIB), X-ray photoelectron spectroscopy (XPS) and Micro X-ray diffraction (PXRD). The film growth process and mechanism were elucidated using an Au marker test, and the contribution of solution diffusion to corrosion was demonstrated in magnetically-stirred experiments. The effect of groundwater chemistry, particularly chloride content on copper corrosion and film properties was characterized using long-term corrosion experiments. INTRODUCTION The selection of copper for the fabrication of nuclear waste containers is based on its thermodynamic stability in the anaerobic saline groundwaters anticipated in Swedish/Finnish/ Canadian nuclear waste repositories. The only potential oxidant able to drive the corrosion of copper is expected to be sulfide (SH-) present in groundwater containing chloride due to either mineral dissolution or microbial production from sulfates. It is also known that the concentration of chloride influences the properties of sulfide films formed on copper surfaces when the solution contains both sulfide and chloride [1]. However, the synergistic effects of sulfide and chloride on the mechanism of film growth and film properties remain unresolved. In this paper, we present results on the film growth process and mechanism on copper surfaces using long-term experiments and a marker test. Our primary goal was to determine the nature of the rate controlling step in the overall corrosion reaction and whether or not it changed as sulfide films developed. Of key importance is whether or not the corrosion process will, over extended exposure times, be changed or controlled by the properties of the copper sulfide film when the groundwater chemistry changes. EXPERIMENTAL Phosphorous-doped (40-60 ppm), oxygen-free copper (Cu-OF) provided by the Swedish Nuclear Fuel and Waste Management Co. (SKB), Stockholm, Sweden was used in all experiments. Working electrodes were copper disks, with a diameter of 1 cm, threaded into a
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stainless steel shaft. Non-conductive lacquer was painted on the sample to prevent contact of the Cu/steel junction with the aqueous electrolyte. After painting, electrodes were heated at 60qC for 12h to promote adhesion between the paint and the sample. Cu surfaces were polished successively with 240, 600, 800, 1000, 1200 grade SiC paper and then to a mirror finish using 1 Pm, 3000 Å, and 500 Å Al2O3 suspensions. Prior to experiments, electrodes were washed with deionized Millipore water (18.2 M:cm), ultrasonically cleaned in methanol, washed again with deionized water, and finally dried using argon gas. All experiments
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