The Influences of Traces of Oxygen and Sulfide on the Corrosion of Copper Under Canadian Nuclear Waste Disposal Conditions

The Canadian repository concept for the permanent disposal of used nuclear fuel is to seal it in Cu-coated steel containers and dispose of it in a deep geological repository (DGR) surrounded by compacted bentonite clay. The Cu coating will be directly bonded on the steel vessel and lid using electro-deposition (ED-Cu), with the final container closure weld sealed with a cold-sprayed Cu deposit (CS-Cu). To assess the durability of such a container, long-term corrosion experiments have been performed on CS-Cu and ED-Cu specimens, and compared with those performed on wrought Cu specimens provided by the Swedish Nuclear Fuel and Waste Management Company (SKB-Cu). Experiments were conducted in 3 M NaCl solutions containing traces of sulfide (< 10- 6 M) for exposure periods of ≥150 days. These conditions reflect a generic groundwater that may influence container corrosion in a Canadian DGR Since conditions in a DGR will evolve from initially oxic to eventually anoxic, experiments were conducted under three sets of redox conditions: (i) aerated (oxic) conditions; (ii) Ar-purged (oxic to anoxic transition conditions (with a dissolved [O2] ~ 10- 6 M)); and (iii) anaerobic chamber (anoxic) conditions ([O2] < 1.3 × 10- 9 M). The exposure process was followed using open circuit (i.e., corrosion) potential and polarization resistance (RP) measurements. Specimens were analyzed before and after corrosion using optical microscopy, scanning electron microscopy on surfaces and focused ion beam cut cross sections, Raman spectroscopy, and X-ray photoelectron/Auger spectroscopy. In all environments, all three types of specimens (ED-Cu, CS-Cu, SKB-Cu) showed similar behaviour. Under anoxic conditions, minor corrosion, indicated by large RP values, leading to the deposition of traces of chalcocite (Cu2S) (identified by Raman spectroscopy) and observed by SEM, indicated that corrosion could be attributed to reaction with the trace amounts of sulfide present in NaCl. In Ar-purged solutions, the RP values were approximately one order of magnitude lower (i.e., the corrosion rate was one order of magnitude higher) than those measured under anoxic conditions, and significant corrosion damage was observed in SEM micrographs. Both Cu2O and Cu2S were detected by Raman spectroscopy. The detection of oxidized sulfur species (SO3 2 - and SO4 2 -) on the corroded surface by XPS/Auger confirmed that even trace amounts of dissolved O2 led to sulfide oxidation and the possible formation of potentially more corrosive species such as thiosulfate (S2O3 2 -). Under aerated conditions, extensive corrosion occurred leading to the formation of green CuII products (atacamite (Cu2(OH)3Cl)) with no detectable Cu2S formation. Within the DGR, corrosion of Cu under anoxic conditions will be limited by the available supply of sulfide, which may be produced remotely at the host rock/clay interface and/or in the porewater within the rock fractures. In the presence of even trace of dissolved O2, the corrosion process is significantly accelerated with the formation of potentially aggressive oxidized sulfur species. Under aerated conditions, extensive corrosion due to reaction with O2 leads to the deposition of CuII deposits. On the time scale of these experiments with only trace of sulfide present, no detectable conversion of copper oxide to Cu2S was observed. Keywords: Copper; Sulfide; Corrosion; Nuclear waste disposal

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