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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Shoesmith, David W.

Affiliation

The University of Western Ontario

2nd Supervisor

Noël, James J.

Affiliation

The University of Western Ontario

Joint Supervisor

Abstract

The proposed method for the safe disposal of Swedish, Finnish, and Canadian high-level nuclear waste (HLNW) is to isolate it in iron or steel containers with an outer copper (Cu) shell or coating and bury it in a deep geological repository (DGR). Copper has been selected as the corrosion barrier due to its stability in the aqueous anoxic environments anticipated in DGR conditions. The container will initially be exposed to humid aerated conditions which will evolve to cool and anoxic as radiation fields in the fuel decay and heat production ceases. When the DGR is cool and anoxic, sulphide (SH−)-induced corrosion will become the long-term threat to container durability by inducing Cu corrosion supported by the reduction of H2O or H+ from SH−. The predominant source of SH− in a DGR will be remotely produced SH− from SO42− via the action of sulphate-reducing bacteria (SRB) and, possibly, minor production from mineral dissolution (pyrite (Fe2S)). The slow SH− diffusion from remote locations to the container surface will render Cu susceptible to corrosion, thereby making it essential to determine the corrosion mechanisms, rates and the extent of corrosion damage on Cu, all of which must be known if a reliable Cu corrosion allowance is to be specified.

The film growth mechanisms on Cu in anoxic aqueous Cl− solutions containing SH− have been investigated both electrochemically and under natural corrosion conditions. Specifically, the influence of Cl− and temperature on both oxide and sulphide film growth and the susceptibility of Cu to pitting have been studied. Cyclic voltammetry on rotating disk electrodes showed that anodically-formed Cu2S (chalcocite) films were porous and non passivating, with the rate of film growth determined by a combination of SH− transport through the growing film and the competition between SH− and Cl− for adsorption sites on the reacting Cu surface. Scanning electron microscopy (SEM) on corroded surfaces and cross-sections showed no evidence for pitting when only Cu2Sfilms were present.

The properties of Cu2S films on Cu were also studied under freely-corroding conditions using corrosion potential measurements coupled with cathodic stripping voltammetry. Surface analyses demonstrated that, depending on the [SH−]and [Cl−], the chalcocite (Cu2S) film was composed of either one or two layers; a thin base layer and an outer crystalline deposit. At low [SH−], only the base layer was formed, with a dual layer growth developing as the [SH−] increased. While the base layer may have initially been a barrier layer, it rapidly became porous and stopped growing. The crystalline nature of the outer deposited layer was consistent with previous claims that this layer grew at the outer film/solution interface by the transport of Cu (I) species, as complexes and clusters, from the corroding Cu surface through the porous base layer.

Under anaerobic humid conditions, gaseous sulphide (H2S (g)) may also be produced by the microbial SO42− reduction. Since the full saturation of the bentonite clay may take many years, there could be a period during which H2S (g) could be transported through the unsaturated clay and dissolved into a wetted layer that might form on the container. Hence, the corrosion of Cu when exposed to H2S (g) in a humid environment was also investigated. The redox reaction was followed by measuring the production of H2 (g) by corrosion and correlating this amount to the corrosion rate of the Cu.

Keywords: Copper, Corrosion, Electrochemistry, Pitting, Sulphide, Chloride, Film Growth, Mechanism, Nuclear Waste Disposal

Summary for Lay Audience

The internationally adopted plan for the permanent disposal of high-level nuclear waste is to seal it in metallic containers and bury it in a DGR. Copper is the primary candidate material for the fabrication of used nuclear fuel containers in Sweden, Finland and Canada due to its stability in the anoxic environments anticipated in such repositories. The available oxygen trapped upon sealing the DGR will be rapidly consumed by mineral and microbial reactions, and by minor uniform corrosion of Cu. However, sulphide-induced corrosion, dominantly due to the remote production of sulphide by sulphate reducing bacteria, will be the long-term threat to container durability. Container corrosion will be influenced by other groundwater anions, in particular chloride.

At the low sulphide concentrations anticipated in a DGR, the corrosion process is expected to be controlled by the supply of sulphide to the container surface, resulting in the formation of porous copper sulphide films on the Cu. Since the properties of these films will dictate the distribution of corrosion damage, it is critical to demonstrate that corrosion will not lead to deep localized penetrations. This makes it essential to address the susceptibility of Cu to pitting at the slightly elevated temperatures (up to 90 °C) anticipated in a DGR, and the influence of various groundwater species, such as Cl− and HCO3− on the film formation process in slightly alkaline groundwaters (pH 9). The focus of this project was to investigate the properties of copper sulphide films formed electrochemically and by corrosion in aqueous solutions containing sulphide and additional anions, particularly Cl−. The emphasis was placed on determining whether the films formed met the criteria that would render containers susceptible to pitting.

A variety of electrochemical and corrosion measurements, such as corrosion potential, cyclic voltammetry, potentiostatic polarization and cathodic stripping voltammetry have been conducted on both stationary and rotating disk electrodes. Raman spectroscopy and X-ray photoelectron spectroscopy have been used to determine the chemical composition of the corrosion products formed under various conditions. Additionally, SEM/EDX examinations of surfaces and FIB-cut cross sections were conducted to examine film morphologies and the distribution of corrosion damage on the corroded Cu/film interface.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

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