Carbon steel (CS) vessels coated with ∼3 mm of Cu have been proposed for the permanent disposal of used nuclear fuel in a deep geological repository (DGR) in Canada. In the event of an undetected defect in the Cu coating that exposes the underlying CS to groundwater, the possibility of galvanically accelerated corrosion of CS arises. In this work, the impact of O 2 availability, NaCl solution concentration, and cathode:anode area ratio on the galvanic corrosion behavior of Cu/CS couples was evaluated by monitoring the galvanic potential of the couple and the galvanic current passing between Cu and CS. The corrosion products and surface damage were analyzed using Raman spectroscopy and SEM/EDX. Varying the Cu:CS area ratio from 1:1 to 2500:1, the [Cl − ] from 0.001 to 3.0 M, the sparging gas from air to Ar, and monitoring the resulting changes in the galvanic current, galvanic potential, corrosion products, and surface damage showed that the galvanic corrosion of CS was most severe when it was exposed to air-sparged solution with a moderate [Cl − ] (0.1 M) as part of the couple with the largest Cu:CS area ratio.

The long-term containment plan for Canada’s used nuclear fuel employs copper-coated carbon steel (CS) containers encased in compacted bentonite clay buffer boxes. In the hypothetical case of a through-coating defect, galvanic interactions between the Cu coating and the CS vessel are dependent on the conditions at the container surface. A zero-resistance ammeter was used to measure the galvanic current between Cu and CS electrodes at various area ratios in the presence of aerated 1 M NaCl and obstructive layers of bentonite slurry. The presence of bentonite resulted in an order of magnitude decrease in the corrosion rate of the steel. Through scanning electron microscopy with energy dispersive X-ray analysis and Raman spectroscopy, we showed that in the presence of bentonite slurry, the extent of steel corrosion decreased significantly, favoring mild surface etching over oxide growth. Electrochemical impedance spectroscopy measurements showed that the total impedance on carbon steel generally increased with bentonite layer thickness, but the total impedance on Cu plateaued when the layer exceeded 6 mm, consistent with trends in galvanic current density on carbon steel. The benign conditions produced by the bentonite slurry are attributed to the slightly alkaline pH and the hindrance of O2 diffusion.

Deep geological containment of used nuclear fuel in Canada will rely on both engineered and natural barriers. The engineered barrier system consists of copper-coated used fuel containers and the compacted bentonite clay buffer boxes in which the containers will rest. This work focuses on a possible degradation mode known as galvanic corrosion, between the Cu coating and the carbon steel (CS) substrate of the container, which may occur at a hypothetical through-coating defect. Corrosion of the Cu/CS couple was studied in the presence of various amounts of chloride, bentonite, and oxygen, using electrochemical tests complemented by surface characterization and 3D X-ray imaging. Cu-coated CS specimens with small holes drilled through the coatings (to simulate defects) were galvanically corroded, and corrosion potential and linear polarization resistance measurements, coupled with in situ X-ray micro-computed tomography and post-mortem Raman spectroscopy and scanning electron microscopy/energy dispersive X-ray analysis, were used to evaluate the nature and extent of CS corrosion.In the absence of bentonite, the CS demonstrated the ability to form a protective oxide barrier. Formation of a protective film was associated with higher polarization resistance values, though the linear increase in corrosion volume over time suggests that the protection was not effective, and asymmetric growth of the corroded volume suggests that the film coverage was not uniform. The oxide layer, when cross-sectioned and viewed by scanning electron microscopy, was observed to be thick but porous, and composed largely of lepidocrocite (γ-FeO(OH)).In the presence of bentonite, the very negative corrosion potential and high polarization resistance values observed suggest that the bentonite was acting as a barrier to O2 diffusion to both the Cu and the CS surfaces. Such benign conditions were also reflected in the X-ray micro-computed tomography analysis, which showed minimal corrosion damage and >80% reduction in corrosion rates over bentonite-free conditions. The bentonite also acted as a barrier to transport of aqueous ferric and ferrous species, which allowed the formation of solid corrosion products on the CS at the base of the hole by preventing their upward transport and deposition atop the Cu coating.The results also provide a comparison of the corrosion susceptibility of the Cu/CS interfaces created by cold spray and electrodeposition techniques. In all cases, the Cu/CS interface was more susceptible to corrosion than the bulk CS, due to residual stress and plastic deformation induced by CS surface modification before or during coating application. The lower adhesion of cold-sprayed coatings made the Cu/CS interface of cold-sprayed specimens slightly more susceptible to corrosion than that of electrodeposited samples. The progression of corrosion at an artificial defect in cold spray Cu-coated CS observed by X-ray micro-computed tomography is shown in Figure 1 as an example. In cold-sprayed specimens, interfacial corrosion occurred ...

Carbon steel vessels coated with ∼3 mm of Cu have been proposed for the permanent disposal of used nuclear fuel in a deep geological repository (DGR) in Canada. In the event that a container is emplaced in the DGR with an undetected defect in the Cu coating that exposes the steel substrate, galvanically accelerated corrosion of steel is, in principle, possible. To investigate this scenario, the progression of steel corrosion at the base of novel simulated through-coating defects was monitored electrochemically and imaged non-destructively using X-ray micro-computed tomography (micro-CT) as a function of time, O2 availability (including anoxic conditions), and coating method (cold spray deposition (with and without heat treatment) and electrodeposition). The corrosion products and surface damage were analyzed using Raman spectroscopy and scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX). These analyses showed how the corrosion damage to steel evolved over time and how it was affected by the method used to coat steel with Cu and the amount of O2 available. The results showed that steel exposed at the base of a through-Cu coating defect corroded and became covered by corrosion products, while there was no visible loss of Cu. The supply of O2 to the sample surface governed the corrosion rate, while the distribution of damage to steel at the base of the defect depended on the Cu coating method and the resulting quality of the Cu/steel interface. Cold spray Cu-coated steel specimens exhibited a radial spread of corrosion along the Cu/steel interface, while electrodeposited Cu/steel specimens experienced preferential interfacial corrosion in the direction in which the steel substrate was machined prior to the electrodeposition of Cu. An example of the progression of corrosion observed by micro-CT is shown in Figure 1. Less extensive corrosion along the Cu/steel interface was observed on samples that were shown by electron backscatter diffraction (EBSD) and adhesion tests to have a less stressed, more uniform, and more well-adhered interface. However, less extensive corrosion was typically at the expense of deeper penetration into the steel. In the absence of O2, the quality of the Cu/steel interface greatly affected both the overall amount of corrosion damage to steel and the distribution of damage. The corrosion rates were significantly lower under anoxic conditions than under oxic conditions, and tended to decrease over time. The influence of a wide range of cathode:anode area ratios and Cl− concentrations, and the availability of O2, was evaluated by monitoring the galvanic current passing between separate Cu and steel electrodes, connected through a zero-resistance ammeter (ZRA), and the galvanic potential of the couple. The galvanic corrosion of steel was most severe when it was exposed to air-sparged solution with a moderate [Cl−] as part of the couple with the largest Cu:steel area ratio. Figure 1