In anoxic groundwaters, the major threat to the durability of copper nuclear waste containers will be corrosion by sulphide, produced dominantly by the action of sulphate reducing bacteria and/or partially by the dissolution of sulphide minerals (e.g., pyrite) in the bentonite. The corrosion of copper in anoxic groundwaters containing sulphide leads to the formation of copper sulphide films (chalcocite, Cu2S) on the Cu surface, whose structure and properties will depend on the groundwater chemistry in the deep geologic repository (DGR) in which the containers will be emplaced.
Electrochemical and long-term corrosion studies have shown that the corrosion deposits varied greatly with the exposure conditions. When the bulk [SH–] was low and the SH– flux and concentration at the copper surface consequently was even lower, the deposit was very porous and non-protective. However, when the bulk [SH–] was high, creating a higher SH– flux at the copper surface, the corrosion product deposit formed was compact and at least partially protective. Scanning electron microscopy examination of focused ion beam (FIB) cut cross sections revealed only general corrosion at lower [SH–] (5 ´ 10–5 mol/L). However, at a sufficiently high [SH–] (≥ 5 ´ 10–4 mol/L), corrosion, interpreted as micro-galvanic corrosion, with a limited penetration depth was observed. No evidence for the formation of a passive barrier layer was obtained and no pitting was observed at [SH–] in the range 5 ´ 10–5 mol/L to 10–3 mol/L.
While inspection of FIB-cut cross sections showed no evidence of pitting, such cross sections show only a limited two-dimensional section of the overall corroded surface. To investigate the wider spread distribution of corrosion damage, corrosion deposits were removed by chemical pickling without causing significant additional corrosion damage to the corroded Cu surfaces. Analyses of these cleaned surfaces confirmed the observations on FIB-cut cross sections with only general corrosion observed for [SH–] ≤ 5 ´ 10–5 mol/L. Corrosion interpreted as micro-galvanic corrosion was only observed for [SH–] ≥ 5 ´ 10–4 mol/L. No pitting was observed. The extent of micro-galvanic corrosion (i.e. the number of the corroded locations at which it occurred, and the depth to which it penetrated the copper surface), decreased as [SH–] was decreased.
Souhaitez-vous présenter une affiche ?Copper; Sulphide; Corrosion; Nuclear waste disposal