2.1.1 Materials and Solution Preparation
2.1.1.1 Wrought Copper
Most experiments were conducted using Cu samples machined from a wrought Cu block provided by the Svensk Kärnbränslehantering AB (SKB), Solna, Sweden. These highly pure Cu samples (> 99.99 %) are oxygen-free (< 5 ppm) and phosphorous-doped (30-100 wt.ppm) [1]. The addition of phosphorous (P) limits creep deformation of Cu which could occur due to the high levels of mechanical stress imposed on the container after emplacement in a DGR.
Cu electrodes were machined as disks with a threaded hole drilled in the back of the disk to allow electrical connection to either a steel or a titanium rod. For corrosion experiments, the steel rods were coated with polytetrafluoroethylene (PTFE) heat shrink tubing and several layers of PTFE tape before they were screwed tightly into the Cu disk, thereby isolating the rods from the solution. The Ti rod was used in a Cu rotating disk electrode (RDE) configuration and the details are discussed below in section 2.2.4. Once the metal rod was attached, Cu disk was sealed with red epoxy resin (Hysol EE 4190) leaving only one flat circular surface area of 0.785 cm2 to be exposed to the electrolyte. Typically, Cu disks were mounted into Teflon (PTFE) holders and were either left inside the holder as an RDE, Figure 2.1, or taken out using a benchtop vice to use as a stationary Cu electrode.
Prior to an experiment, the Cu electrode was ground with a sequence of SiC papers ranging from P400 to P4000, and then fine polished to a mirror finish using aluminum oxide (Al2O3) suspensions with decreasing particle sizes (1 μm, 0.3 μm, and 0.05 μm). Once a mirror finish was achieved, the electrode was rinsed with Type I water (18.2 MΩ·cm) provided by a Thermo Scientific Barnstead Nanopure 7143 ultrapure water system, to remove some polishing residues. Finally, electrodes were sonicated with methanol for 1 min and dried in a stream of ultrapure (99.999%) Ar gas.
Prior to an experiment, the Cu electrode was cathodically cleaned at −1.5 V vs saturated calomel reference electrode (SCE) for 1 min to reduce air-formed oxides, and then polarized at −1.15 V vs SCE for another minute to allow the detachment of any H2 bubble which may have formed due to H2O reduction at the more negative potential.
2.1.1.2 Electrodeposited Copper
Some experiments were conducted using the electrodeposited (ED) Cu samples (ED plates NWMO 44-45) supplied by the Nuclear Waste Management Organization, Toronto, Canada. In this thesis, these electrodeposited Cu samples are referred as C2B Cu
The C2B Cu samples were cut from plates into 2.8 mm × 2.9 mm × 72 mm rectangular prisms with a total average surface area of 8.6 cm2. The configuration of a C2B Cu working electrode (WE) is shown in Figure 2.2 (a), in which the major components of the electrode assembly are indicated. The electrical contact was made by connecting the C2B Cu prism to a Cu wire by soldering a wire onto a flat surface of C2B Cu. The junction between the C2B Cu and the wire was then covered with a glass tube and sealed with clear epoxy resin (Epofix resin). C2B Cu samples were grounded with SiC papers ranging from P120 to P 600 grits. All samples were then sonicated in a beaker with technical grade ethanol for 5 min followed by Type I water for another 5 min to remove any polishing residues. Sonication was repeated if needed. Prior to incorporation into the electrochemical cell, the samples were dried with ultrapure N2 gas and stored in a vacuum desiccator. A total of 131 C2B Cu samples were assembled in an electrochemical cell, but, only one was used as a working electrode (WE). The exposed surface area of a WE was 6 cm2.
2.1.1.3 Copper Wire
A few electrochemical experiments were conducted using Cu wires (annealed, ≥ 99.9%, 0.25 mm in diameter) purchased from Goodfellow. These wire samples are referred to as C1C Cu. As shown in Figure 2.2 (b), the WE configuration of C1C Cu is similar to that of C2B Cu, except that no soldering was performed. It should be noted that a 250 cm long wire was coiled at the bottom with a surface area of 19.7 cm2 to the electrolyte, Figure 2.2 (b). Each 5 cm long segment wire was cut from the spool of wires and a total of 2600
Figure 2.2: WE configurations: (a) C2B Cu; and (b) C1C Cu. The major components of each WE are labelled from (1) to (9): (1) Swagelok SS 3-way valve; (2) Swagelok SS tube fitting, union tee; (3) junction sealed with resin; (4) Swagelok SS tube fitting, port connector; (5) ground glass inner male joint; (6) Cu wire covered by heat-shrink tubing; (7) glass tube filled with resin; (8) junction soldered with Tin; (9) C2B Cu or coiled C1C Cu wires
C1C Cu segments were made and assembled in an electrochemical cell, corresponding to an exposed surface area of 1024 cm2.
Since it is effectively impossible to grind these thin C1C Cu segments using a mechanical polisher, wires were cleaned immediately prior to testing. The C1C Cu was first degreased by sonication for 5 min in a vial containing technical grade ethanol. The vial containing C1C Cu was then filled with Type I water and swilled to remove any residual ethanol. Wires were then immersed in a second vial containing ~ 10 mM nitric acid for another 5 min of sonication. The procedure described above was repeated three times until the wires appeared to be free of tarnish by the visual inspection with an optical microscope. Eventually, the C1C Cu samples were dried with ultrapure N2 gas and stored in a vacuum desiccator prior to use.
2.1.1.4 Solution Preparation
All solutions were prepared using Type I water (18.2 MΩ·cm), obtained from a Thermo scientific Barnstead Nanopure water purification system, and reagent grade chemicals. Aqueous sulphide solutions were prepared with a regent grade sodium sulphide nonahydrate (Na2S·9H2O, 98.0%, supplied by Sigma-Aldrich). Since Na2S is stored in the hydrated form, crystals were thoroughly dried on a filter paper prior to weighing. For benchtop experiments, solutions were purged with ultrapure (99.999%) Ar for 30 minutes prior to each experiment to minimize SH− oxidation and then continuously sparged throughout an experiment. For experiments performed inside an Ar-purged anaerobic chamber (Canadian Vacuum Systems Ltd.) using an MBraun glove box control system, Type I water was sparged with ultrapure Ar gas for 60 minutes on the benchtop before being transferred into the glovebox. The minimum measurable [O2] indicated by the detection limit of the O2 sensor in the chamber was 1 ppm. Once water was transferred, sulphide solutions were then prepared inside the glovebox to ensure the anoxic environment. Solution pH was measured using either an Orion™ Star pH meter (Thermo Scientific) or Hydrion® Brilliant pH papers (Sigma-Aldrich) in the pH range of 6.5 to 13.0.
2.1.2 Electrochemical Cell
Experiments were performed in either a one-compartment or a three-compartment electrochemical cell. A dense glass frit was used in the three-compartment arrangement to
separate the solution in the working electrode (WE) compartment from that in the reference electrode (RE) and counter electrode (CE) compartments. Commonly, a Luggin probe was used to minimize the ohmic potential drop between the RE and WE. RDE experiments were performed in a one-compartment cell with an exposed wrought Cu surface area of 0.785 cm2 and a solution volume of 450 mL, Figure 2.3 (a). A three-compartment cell was used for experiments conducted using a stationary wrought Cu electrode with a surface area of 0.785 cm2 and a solution volume of 800 mL, Figure 2.3 (b). All experimental measurements were obtained via a conventional three-electrode system, with a Pt plate (99.95%) connected to external circuitry by a Pt wire (0.5 mm diameter) as the counter electrode, a commercial saturated calomel reference electrode (SCE, +0.242 V vs. SHE), and Cu as the working electrode. All cells were housed inside a Faraday cage to reduce electrical noise from external sources.
Electrochemical data was collected using one of the following: a Solartron 1287 potentiostat, a Solartron Analytical Modulab equipped with CorrWare (version 3.4d, Scribner Associates) and a three-terminal XM-Studio-ECS software (AMETEK Scientific Instruments), and a Solartron 1480 MultiStat in conjunction with CorrWare software.
It should be noted that some experiments were conducted in a novel electrochemical cell configuration associated with C2B and C1C Cu samples. This cell setup will be discussed in detail in Chapter 6.
Figure 2.3: Schematics of the electrochemical cells. (a) a three-compartment cell and (b) a one-compartment cell.