Galvanic Coupling of Copper and Carbon Steel in the Presence of Bentonite Clay and Chloride

Abstract

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.

Monitoring the galvanic corrosion of copper–steel coupling in bentonite slurry during the early oxic phase using coupled multielectrode arrays

“…Furthermore, a larger current is generated when the Cu:CS area ratio is as large as 1000. [12,13] Our study using Cu electrical resistance sensors corrosion under oxic conditions in bentonite with Aspö groundwater resulted in a corrosion rate of around 1 μm year -1 monitored through 3.5 years of exposure. [14,15] There, the reduction of thickness and impedance measurements were used to evaluate the corrosion rate on copper electrical resistance sensors.…”

“…In the event of an undetected defect in the Cu coating, the underlying CS could be exposed to the groundwater, potentially accelerating the galvanical corrosion of CS. [10][11][12][13] Recent studies have simulated a defect in a coldsprayed coating and reported that corrosion proceeds via the galvanic coupling and oxygen reduction at the surface. [10,11] Besides the accumulation of damage at the base of the defect, corrosion propagated along the copper/steel interface, most likely resulting from damage inflicted during the deposition process.…”

“…In the event of an undetected defect in the Cu coating, the underlying CS could be exposed to the groundwater, potentially accelerating the galvanical corrosion of CS. [ 10–13 ]…”

Corrosion behaviour of heterogeneous antimony-copper layers in chloride media

“…Therefore, a common place for this element in equivalent circuits is a series connection with R ct although the cases of its placement in other positions are not rare. Positioning the Warburg impedance away from the R ct has been proposed by other authors in various equivalent circuits describing some specific processes at Li-ion batteries [44,45] and electroactive species diffusion through modified with polymers electrode surfaces for biosensing [46] or diffusion of O 2 through above 2 mm bentonite clay layer on carbon steel [47]. To achieve better fits to our experimental results, the Warburg impedance is added in series of R Ox , which indicates that diffusion limitations are localised at the electrolyte-oxide film interface.…”