Galvanic Corrosion of Copper/Carbon Steel Couples Under Simulated Disposal Conditions for Used Nuclear Fuel

Abstract

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 symmetrically around the drilled hole and corrosion propagated preferentially around Cu splats at the Cu/CS interface. The machining grooves at the interface of electrodeposited specimens caused some preference for interfacial corrosion parallel to those lines, due to introduction of residual stress from plastic deformation. When interfacial corrosion was less extensive, as in the presence of bentonite, corrosion occurred symmetrically around the hole in both electrodeposited and cold-sprayed specimens.These experiments also demonstrated the effectiveness of bentonite slurry in suppressing galvanic corrosion in the Cu/CS couple by creating benign corrosion conditions in the through-coating defect. Figure 2 shows the suppression of corrosion rates by the presence of bentonite.This work provides insight into the conditions which may exacerbate or mitigate galvanic corrosion in the deep geological repository, contributing to the body of research which will support prediction of the long-term performance of the engineered barrier system. Figure 1