The Canadian used fuel container (UFC) for the long-term containment of used nuclear fuel in a deep geological repository (DGR) comprises a 3 mm copper corrosion barrier applied directly to a strong carbon steel container. While a final site for the Canadian DGR has not yet been chosen, the site selection process has identified five candidate locations each with unique groundwater chemistry, particularly with respect to salinity. Therefore, in order to ensure the long-term integrity of the UFC, the effect of the various groundwater chemistries on copper corrosion must be understood. The corrosion of copper in both pure water and in brine remains a contentious issue, from both a thermodynamics viewpoint [1–5], but also in terms of the source of hydrogen from which a uniform corrosion rate is inferred [6,7]
In this paper, we present new data that demonstrates limited oxidation of copper at 75 °C, as a function of pH, salinity and in the presence of small quantities of hydrogen sulfide. Cells were analysed using the purge and collect method with analysis via solid state hydrogen sensor.
In deionised water, six nominally identical cells were found to generate miniscule quantities of hydrogen corresponding to corrosion rates of less than 0.25 nm/year. Within 70 days, the corrosion rates had all declined below the limit of detection for this technique (0.01 nm/year). Changes in electrolyte to 5 molal sodium chloride at pH 7 (un-buffered) had little influence on the corrosion rate. However, similar cells, either buffered with citric acid at pH 5 and with a low chloride concentration (0.1 molal), or at a reduced pH (2.7 to 3.8), un-buffered and with 5 molal sodium chloride, generated significant hydrogen at rates that declined as a function of time as the electrolyte altered. The highest corrosion rate recorded corresponded to 20 nm/year at pH 2.7, declining to less than 0.1 nm/year at pH 5.
There are two potential sources of the detected hydrogen; the egress of hydrogen absorbed during manufacturing and annealing of the copper, and the hydrogen generated through anoxic corrosion. A limited number of cells, containing copper wire in 5 molal sodium chloride buffered with 5 mM citric acid at 75 °C, have been studied over several years. The corrosion rates in these cells were relatively high, of the order nm/year, cumulatively yielding a quantity in excess of what could be absorbed by the copper during annealing under a reducing atmosphere [8]. However, the corrosion rate was found to be highly sensitive to small temperature gradients as placement within the water bath was found to influence behaviour and the actual corrosion rates at 75 °C were found to be closer to 0.5 nm/year. Interestingly, a number of the older cells were found to have developed localised regions of copper nodules, perhaps driven by the temperature differential across the cell.
A number of test cells, containing deionised water or 5 molal sodium chloride were dosed with hydrogen sulfide, equivalent to approximately 12% of a monolayer. A hydrogen response was recorded over the course of several months that was significantly greater than the intended hydrogen sulfide dose, although the corrosion rate never exceeded 0.5 nm/year and there was no visual evidence of localised corrosion.
In summary, the corrosion behaviour of copper wire has been studied under a wide range of electrolyte chemistries and at 75 °C. Corrosion rates under conditions approximating an anticipated Canadian deep geological repository were significantly less than 0.5 nm/year and would invariably decline over the course of several years.
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copper, anoxic, chloride, sulfide, corrosion, hydrogen