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Copper and copper alloys have found applications in various industries. One of the main reasons Cu and its alloys are utilized widely is that they have sufficient corrosion resistance in key environments, such as seawater and anoxic solutions; however, localized corrosion processes might occur in the presence of aggressive anions, oxygen, or an increase in solution pH. In critical applications of Cu, the susceptibility of Cu to localized corrosion, specifically pitting, must be carefully considered, as it could lead to material failure. In this study, the pitting probability of Cu in unary (sulfate) and binary (sulfate + bicarbonate) solutions was investigated using electrochemical techniques in conjunction with statistical analysis. We determined pitting probabilities based on two different defining criteria for pitting susceptibility, one based on the probability that the corrosion potential, Ecorr, could exceed the passivity breakdown potential, Eb, and the other, a more conservative approach, based on the likelihood that Ecorr would be greater than the repassivation potential, Erp. The pitting probability of Cu did not change significantly with sulfate concentration at pH 8 but was found to increase with increasing [ SO 4 2 − ] up to 0.005 M at pH 9 and then to decrease with a further increase in [ SO 4 2 − ].

One concern regarding the used nuclear fuel containers proposed for use in a Canadian deep geological repository (DGR) is the possibility that a small amount of hydrogen might be absorbed into their copper coating, potentially altering its mechanical properties. Reported herein is a study of hydrogen absorption into 50 nm of copper, coated on 4 nm of Ti using in situ neutron reflectometry (NR) and electrochemical impedance spectroscopy (EIS). NR results show that hydrogen is absorbed when the copper is cathodically polarized below the threshold for the hydrogen evolution reaction (HER), but that the hydrogen concentrates in the underlying titanium layer rather than concentrating in the copper coating. The hydrogen concentration in titanium rapidly rose when the HER was initiated and was observed to reach a steady state at TiH1.5. Over the course of 55h of cathodic polarization, the concentration of hydrogen in the copper remained below the NR detection limit (2 at %). The portion of hydrogen atoms produced that diffused through the copper layer was initially 3.2%, suggesting a possible upper limit for hydrogen uptake by the copper coating of the UFC, although definitive conclusions can only be drawn from studies on 3 mm copper-coated steel samples.

Under some conditions, copper and copper alloys are either immune from corrosion or undergo slow uniform corrosion, generally considered a favourable situation, since predicting the damage incurred by the metal during a period of uniform corrosion is relatively straightforward. However, under conditions leading to surface passivation of Cu, localized corrosion might occur in the presence of aggressive oxidants. As a result, the susceptibility of Cu to localized corrosion must be considered carefully to avoid unpredictable failures in Cu-based structures. Understanding the pitting probability of Cu is important for various applications, including the use of Cu-coated containers for the permanent disposal of used nuclear fuel. In this study, the pitting probability of Cu in chloride-containing solutions crudely representing the groundwater that might be found in a deep geologic repository (DGR) was investigated using electrochemical techniques and statistical analysis. The probabilities of both pitting and repassivation of Cu were found to increase with increasing [Cl-]. The surface morphologies of copper electrodes in the same solution were also evaluated using scanning electron microscopy (SEM). The passive film on the surface of the copper electrode with the highest breakdown potential (Eb) was found to be more protective than that on the electrode with the lowest Eb.

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. As shown by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and Raman spectroscopy, the extent of steel corrosion decreased significantly in the presence of bentonite slurry, favouring mild surface etching over oxide growth. Electrochemical impedance spectroscopy (EIS) 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.

In Canada, the Nuclear Waste Management Organization (NWMO) is responsible for the long-term management of spent nuclear fuel, which involves sealing used fuel bundles in copper-coated carbon steel used fuel containers (UFC) and emplacing them ~500 m underground in a deep geological repository (DGR). In this plan, copper plays a vital role in ensuring the safety of the DGR as it is intended to serve as a corrosion barrier for greater than one million years. Veins and pods of natural copper have long been known to occur in the Lake Superior region of North America where they have been culturally significant to Indigenous peoples for millennia. The natural Lake Superior copper deposits were emplaced close to one billion years ago in lithosphere which has since had a protracted history of glacial overrides and related isostatic adjustment events. In light of this longevity, structural history and exposure to crustal fluids, it has the potential to hold many lessons for DGR specialists and society in general. We present two aspects of our approach. The first is an outline of our efforts to increase cultural competency of non-indigenous scientists in our group while building an understanding of the Indigenous Knowledge system and how it can be respectfully and effectively applied to research. This includes respectful ways in which to collect, and learn from, copper samples. This work directly relates to the important relationships in the context of the NWMO Indigenous Knowledge policy, Reconciliation policy and how both policies apply to the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP). In this context we will also present our early field and laboratory observations of natural copper properties through an integrated analysis of chemical and orientation microstructure in tandem with electrochemical behaviour. Techniques in the laboratory workflow include electron microscopy (EDS, EBSD) and atom probe tomography to map impurities and microstructure relative to manufactured wrought copper (phosphorus-doped oxygen free); Auger electron and X-ray photoelectron spectroscopies to determine surface and near-surface composition and chemical environment; and electrochemical methods such as corrosion potential measurements and potentiodynamic polarization scans to probe the corrosion performance. These measurements may inform container design aspects such as optimal fabrication and the role of impurities in corrosion behaviour of electrodeposited and cold spray-deposited copper in a DGR over geological timescales. Taken together, our aim is to come to an appropriately comprehensive understanding of the cultural, geological and material corrosion aspects of natural copper that has persisted for a time span three orders of magnitude greater than the DGR design requirements. It is our hope that this learning approach to ancient natural copper will play a positive role in seeking social license for DGR planning, while having value for societal education in the global challenge of geostorage.

Cu piping previously used to carry process air was harvested from the now shutdown National Research Universal reactor at the Canadian Nuclear Laboratories site in Chalk River, Ontario. Metallurgical and microscopy examinations found no quantifiable differences between irradiated and non-irradiated sections of the pipe. This provides evidence for the safety case for the use of the Cu-coated used fuel container proposed by the Nuclear Waste Management Organization for the permanent disposal of high-level nuclear waste.

Copper corrosion in aerated water vapour has been studied as a function of temperature, relative humidity in the presence and absence of a low gamma radiation dose rate. On surface areas covered by a thin water layer corrosion ceased after ≤ 30 days by the formation of CuO on the surface of a Cu2O layer. At the locations of condensed water droplets corrosion was more extensive since the corrosion product developed porosity. Radiation accelerated the formation of Cu2O on corroded areas and increased the number and size of the more extensively corroded locations. The distribution of condensed water was the dominant feature controlling corrosion penetration.

The corrosion of a number of coarse- and fine-grained copper materials has been studied in an aerated aqueous nitric acid (100 mM) solution. The materials properties and the consequences of corrosion were characterized by scanning electron microscopy, electron back scatter diffraction and confocal laser scanning microscopy. The corrosion of the coarse-grained materials was shown to be strongly influenced by the crystallographic orientations of grains. For fine-grained materials grain boundary etching at high angle grain boundaries and grain boundary pores (for cold sprayed copper) overrode the influences of texture on the corrosion rate.

The exposure conditions experienced by copper-coated high-level nuclear waste containers in a deep geologic repository will evolve with time. An early exposure period involving the gamma irradiation of aerated humid vapor could lead to the formation of nitric acid condensed in limited volumes of water on the container surface. The evolution of the corrosion processes under these conditions have been studied using pH measurements in limited volumes of water containing various concentrations of nitric acid. The extent and morphology of corrosion was examined using scanning electron microscopy on surfaces and on focused ion beam cut cross sections. The composition of corrosion products was determined by energy dispersive x-ray analyses and Raman spectroscopy. In the absence of dissolved oxygen only minor corrosion was observed with the reduction of nitric acid inhibited by the formation of either chemisorbed nitrate and nitrite species or the formation of a thin cuprite (Cu2O) layer. When the solution was aerated, both oxygen and nitric acid acted as cathodic reagents. After extensive exposure periods corrosion was stifled by the formation of corrosion product deposits of Cu2O, CuO (tenorite), and Cu2NO3(OH)3 (rouaite).

Permanent disposal of high level nuclear waste in containers that employ steel as a structural element and copper as a corrosion barrier material is a favoured design concept in several countries. This review considers possible corrosion processes that can affect these packages and demonstrates the methodology used to calculate corrosion depth over package service life in a Canadian deep geological repository environment. Four corrosion sub-allowances are considered: oxic corrosion due to trapped oxygen, radiation-influenced corrosion, anoxic aqueous corrosion, and microbiologically-influenced corrosion. Additional corrosion and container failure mechanisms are discussed, including stress corrosion cracking, creep ductility and fracture, hydrogen embrittlement, coating debonding for copper-coating technologies, and failure at weld regions. Several areas for future study are suggested. Exact corrosion allowances will depend on site-specific geological and groundwater properties, especially the far-field sulphide concentration. For the generic Canadian site conditions described, the expected depth of corrosion is evaluated at 270 μm in 1,000,000 years. Including additional, highly conservative considerations for each possible corrosion reaction, the maximum corrosion damage is 1.2 mm in 1,000,000 years. This is significantly less than the 3 mm coating thickness in the Canadian used fuel container design or the 25–50 mm thickness of copper in dual walled vessels.

The formation of sulfide and oxide films on copper has been studied in sulfide solutions containing chloride and buffered to pH = 9 with borate over the temperature range 20 °C to 80 °C. The primary goal was to investigate the susceptibility of copper to pitting corrosion over a range of temperatures expected in a deep geological repository. Films were formed electrochemically and characterized using scanning electron microscopy (SEM), energy dispersive X-ray and Raman spectroscopy and X-ray diffractometry (XRD). Chalcocite (Cu2S) formation was observed to occur under partially transport-controlled conditions in the potential range −0.9 V to −0.3 V vs SCE. At less negative potentials (≥ −0.3 V vs SCE), a transition from active dissolution to partial passivation by a copper oxide film was observed. Temperature had only a minor effect on the formation of the sulfide film, but the onset of active dissolution and oxide film formation shifted to lower potentials as the temperature increased.

The preferred disposal method for used nuclear fuel is to seal it in containers emplaced in a deep geologic repository (DGR) in a suitable rock formation. In Canada, the container is a robust steel vessel coated with 3 mm of Cu as a corrosion barrier. Eventually, the DGR environment will become anoxic, and in the absence of oxygen copper is thermodynamically stable in water. However, since exposure conditions will initially be oxidizing due to the presence of trapped O2 upon sealing the DGR, there is a possibility of localized corrosion to occur. Mass balance calculations show that uniform corrosion during this period will be within the designed corrosion allowance, however, since the Cu coating is relatively thin and the required lifetime is long, the risk of pitting corrosion during this period must be carefully evaluated. Pitting corrosion susceptibility is determined by the corrosion potential of the material (Ecorr, the potential at which the rates of all anodic and cathodic reactions are equal), and the breakdown potential (Eb, the potential at which the copper oxidation rate on the surface increases abruptly due to the breakdown of the protective film), both of which are distributed parameters. This is due to the stochastic nature of passive film rupture and any uncontrollable variations in the structure and local environment at the metal surface. It is important to note that the analysis is based on the concept that pitting is only possible if the Ecorr is equal to, or more positive than the Eb. Our previous studies showed a passive behavior of copper surface when exposed to high pH solutions. In this work, a multielectrode array was used to obtain a statistically meaningful set of potential measurements through the simultaneous monitoring of 30 electrodes to determine their corrosion and breakdown potentials. From the extensive database that acquired, the Ecorr and Eb distributions were defined for the copper electrodes exposed to solutions containing various chloride concentrations and a more thorough evaluation of the relative values of Ecorr and Eb has undertaken to determine the susceptibility to pitting. Experiments were conducted using a multielectrode array in chloride and sulfate containing solutions at 25 °C and pH 8 and 11. In alkaline solutions, the copper oxide is more stable and can potentially form a passive film, whereas chloride and sulfate promote the breakdown of the passive film. As the concentration of aggressive anions increased, the values of Ecorr and Eb decreased, suggesting that the solubility of copper increased with in the presence of both chloride and sulfate. However, there is a critical sulfate concentration (~5 × 10-3 M) that above which the pitting probability of copper decreased. In general, the possibility of pitting increased with growing overlap between the distribution curves of Ecorr and Eb. To evaluate the possibility of pitting, histograms and distribution curves of Ecorr and Eb in the solutions that contain different chloride and sulfate concentrations were compared. In lower chloride concentrations, there was a very small overlap between distributions of Ecorr and Eb values; therefore, the possibility of pitting under these conditions is very low. On the other hand, a high chloride concentration at high pH indicated a higher probability of pitting, as the overlap between Ecorr and Eb distributions increased. Also, the extent of the passive zone decreased with increasing chloride concentration. In solutions containing sulfate content, the overlap between Ecorr and Eb increased with increasing sulfate concentration up to a certain content (~5 × 10-3 M), after this point the overlap decreased as the sulfate concentration increased. The results from potentiodynamic experiments indicated that increasing the chloride concentration resulted in decreased the extent of the passive zone; however, the sulfate did not seem to play a significant role in changing the extent of the passive zone. Figure 1

The Nuclear Waste Management Organization (NWMO) is preparing to implement the safe long-term disposal of Canada’s used nuclear fuel in robust, corrosion-resistant used fuel containers (UFC). The current UFC design employs a strong carbon steel vessel, coated with copper for corrosion protection. This project explores corrosion processes that could happen on the surface of a container with a through-coating defect that exposes a small area of the underlying carbon steel, in contact with the copper coating, which corrosion scientists would consider a risk for galvanic (dissimilar metal) corrosion. High resolution ex situ and operando observations of Cu-coated carbon steel specimens with an artificial defect penetrating the copper to the steel, exposed to saline solutions with and without dissolved oxygen, have been made using X-ray micro computed tomography (μ-CT) to reveal the three-dimensional form of the corrosion damage and follow its progression with time (as shown for example in Figure 1). These measurements have shown that the form of the corrosion damage is dependent on both the type of additive manufacturing procedure used to create the Cu coating (electrodeposition or cold spray deposition) and the quality of the Cu/steel interface. In the presence of oxygen, the volume of corrosion damage increases linearly with time, independent of the type of coating, but with a rate dependent on the availability of oxygen. In the absence of oxygen, the corrosion rate is low and decreases with time. Post-experiment characterization of corrosion products by Raman spectroscopy indicated the presence of akageneite (β-FeOOH) and lepidocrocite (γ-FeOOH), depending on the solution conditions within the corroding defect. Figure 1

The preferred disposal method for used nuclear fuel is to seal it in containers emplaced in a deep geologic repository (DGR) in a suitable rock formation. In Canada, the container is a robust steel vessel coated with 3 mm of Cu as a corrosion barrier. Eventually, the DGR environment will become anoxic, and in the absence of oxygen copper is thermodynamically stable in water. However, since exposure conditions will initially be oxidizing due to the presence of trapped O2 upon sealing the DGR, there is a possibility of localized corrosion to occur. Mass balance calculations show that uniform corrosion during this period will be within the designed corrosion allowance, however, since the Cu coating is relatively thin and the required lifetime is long, the risk of pitting corrosion during this period must be carefully evaluated. Pitting corrosion susceptibility is determined by the corrosion potential of the material (Ecorr, the potential at which the rates of all anodic and cathodic reactions are equal), and the breakdown potential (Eb, the potential at which the copper oxidation rate on the surface increases abruptly due to the breakdown of the protective film), both of which are distributed parameters. This is due to the stochastic nature of passive film rupture and any uncontrollable variations in the structure and local environment at the metal surface. It is important to note that the analysis is based on the concept that pitting is only possible if the Ecorr is equal to, or more positive than the Eb. Our previous studies showed a passive behavior of copper surface when exposed to high pH solutions. In this work, a multielectrode array was used to obtain a statistically meaningful set of potential measurements through the simultaneous monitoring of 30 electrodes to determine their corrosion and breakdown potentials. From the extensive database that acquired, the Ecorr and Eb distributions were defined for the copper electrodes exposed to solutions containing various chloride concentrations and a more thorough evaluation of the relative values of Ecorr and Eb has undertaken to determine the susceptibility to pitting. Experiments were conducted using a multielectrode array in chloride and sulfate containing solutions at 25 °C and pH 8 and 11. In alkaline solutions, the copper oxide is more stable and can potentially form a passive film, whereas chloride and sulfate promote the breakdown of the passive film. As the concentration of aggressive anions increased, the values of Ecorr and Eb decreased, suggesting that the solubility of copper increases with in the presence of both chloride and sulfate. However, there is a critical sulfate concentration (~5 × 10-3 M) that above which the pitting probability of copper decreases. In general, the possibility of pitting increases with growing overlap between the distribution curves of Ecorr and Eb. To evaluate the possibility of pitting, histograms and distribution curves of Ecorr and Eb in the solutions that contain different chloride and sulfate concentrations were compared. In lower chloride concentrations, there was a very small overlap between distributions of Ecorr and Eb values; therefore, the possibility of pitting under these conditions is very low. On the other hand, a high chloride concentration at high pH indicated a higher probability of pitting, as the overlap between Ecorr and Eb distributions increased. Also, the extent of the passive zone decreased with increasing chloride concentration. In solutions containing sulfate content, the overlap between Ecorr and Eb increased with increasing sulfate concentration up to a certain content (~5 × 10-3 M), after this point the overlap decreased as the sulfate concentration increased. The results from potentiodynamic experiments indicated that increasing the chloride concentration resulted in decreased the extent of the passive zone; however, the sulfate did not seem to play a significant role in changing the extent of the passive zone.

The Nuclear Waste Management Organization is evaluating the safety and feasibility of the permanent disposal of used nuclear fuel in a deep geological repository. Their current design concept utilises copper-coated steel used fuel containers to isolate the waste from the environment. Immediately following repository closure, a finite quantity of O2 will be trapped inside the repository and could cause some amount of oxic corrosion to the outer copper layer of the containers. On a per container basis, 13 mol of O2 will be trapped in the repository rooms at the time of closure, based on reference design dimensions. This corresponds to a maximum depth of copper corrosion of 81 μm, assuming a uniform distribution. This work also considers the sensitivity of this oxic corrosion allowance to various hypothetical design changes to the repository that may occur before or during construction.

Oxide growth kinetics on the Ni-Cr-Fe alloy Inconel 600 and the Co-Cr alloy Stellite 6 under potentiostatic polarization have been investigated by current measurements augmented by ex-situ surface analyses. The results reveal a mechanism for metal oxidation and oxide formation that is common to both alloys. The reaction thermodynamics for the oxidation of a metal determine whether a certain metal oxidation can or cannot occur. However, the metal oxidation proceeds via two competing pathways, oxide formation and metal ion dissolution. At pH 10.6 where the solubilities of FeII, NiII or CoII species are near their minima, oxide formation is favoured over metal ion dissolution. As the oxide grows, the rate of metal oxidation decreases with time due to an increase in the electrochemical potential barrier. The oxide formation occurs sequentially; the conversion of the preformed Cr2O3 film to chromite (FeCr2O4 or CoCr2O4) proceeds before the next layers of Fe3O4/NiFe2O4 and NiO/Ni(OH)2 grow on Inconel 600, or CoO/Co(OH)2 grows on Stellite 6. The effect of a different EAPP is to limit the oxidation sequence. The pH does not directly affect the driving force for metal oxidation but it strongly influences the relative rates of oxide formation and metal dissolution, thereby affecting metal oxidation kinetics.

Understanding of oxide formation and growth on an alloy is very important in predicting its corrosion behaviour. Several models for oxide growth as well as metal dissolution kinetics in the presence of an oxide layer have been developed. These models focus on describing the transport of charged species through the oxide film and the electrochemical reactions at the metal/oxide and oxide/solution interfaces, although the detailed description and formulation of these processes vary. However, these models do not consider the type of oxide that can form. Consequently, the models have limited capability of predicting changes in the corrosion rate with time as corrosion progresses, or the dependence of corrosion kinetics on the solution redox conditions. Recently, we have developed a corrosion model that can predict the rates of metal oxidation, oxide growth and dissolution simultaneously as a function of time. The model imposes reaction thermodynamics constraints, and mass and charge balance (MCB) requirements on corrosion reaction rates and hence is labelled the MCB model [1]. The mass and charge balance requirements dictate that the flux of metal cations created by oxidation of metal atoms at the m|ox interface (the oxidation flux) must be equal to the sum of the fluxes of the metal cations forming an oxide at the oxide/solution interface (the oxide formation flux) and the flux of metal cations dissolving into solution (the dissolution flux). The oxidation flux is calculated by using a modified Butler-Volmer equation with an effective overpotential that is defined as a function of the equilibrium potential of the metal oxidation and the potential drop across the oxide layer that is growing. Both the oxide formation dissolution fluxes have a first-order dependence on the oxidation flux. The first-order rate constant for the oxide formation follows an Arrhenius dependence with an activation energy that increases linearly with oxide thickness. The dissolution rate constant depends on surface hydration and the solution environment (pH and temperature), but is independent of the oxide thickness. Consequently, under constant solution conditions the rate constant for the oxide formation, kMO(t), changes with time as the oxide grows but the rate constant of dissolution, kdiss, is constant with time. Due to a mass balance constraint and competition between the two processes for the metal ions, the oxide formation and dissolution fluxes cannot vary independently. The fraction of the oxidation flux that leads to oxide formation or dissolution depends on their rate constants; fk-MO(t) = kMO(t)/(kMO(t) + kdiss) and fk-diss(t) = 1 - fk-MO(t), respectively. In this paper, we present MCB model simulation results of potentiostatic polarization experiments performed on Co-Cr alloy Stellite-6 [2]. The simulations results are compared with the experimental measurements of the corrosion current as a function of time and the final composition and structure of the oxide(s) that formed. In these simulations, the parameters such as rate constants, exchange current density and field strength (or specific potential drop) across an oxide layer were kept constant for a specific pH and temperature. The main rate parameter that varies with pH and temperature was . This rate constant ratio is higher under conditions which promote oxide formation over dissolution, such as high pHs where the solubility of metal cations is low. The MCB model with the same model parameters was then applied to different sets of experimental data which include measurements of corrosion potential as a function of time and determination of the amounts of dissolved metals in coupon corrosion tests conducted in sealed quartz vials. The excellent agreement between the model results and experimental data over a range of polarization potential, pH and temperature indicates that the MCB model is a valuable method for simulating time-dependent corrosion behaviour while an oxide film is changing. References [1] M. Momeni, J.C. Wren, Faraday Discussions (2015) DOI 10.1039/C4FD00244J. [2] M. Behazin et al. Electrochimica Acta 134 (2014) 399–410.