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Microorganisms in clay barriers could affect the long-term performance of waste containers in future deep geological repositories (DGR) for used nuclear fuel through production of corrosive metabolites (e.g., sulfide), which is why clay materials are highly compacted: to reduce both physical space and access to water for microorganisms to grow. However, the highly compacted nature of clays and the resulting low activity or dormancy of microorganisms complicate the extraction of biomarkers (i.e., PLFA, DNA etc.) from such barriers for predictive analysis of microbial risks. In order to overcome these challenges, we have combined culture- and 16S rRNA gene amplicon sequencing-based approaches to describe the functional diversity of microorganisms in several commercial clay products, including two different samples of Wyoming type MX-80 bentonite (Batch 1 and Batch 2), the reference clay for a future Canadian DGR, and Avonlea type Canaprill, a clay sample for comparison. Microorganisms from as-received bentonites were enriched in anoxic 10% w/v clay microcosms for three months at ambient temperature with addition of 10% hydrogen along with presumable indigenous organics and sulfate in the clay. High-throughput sequencing of 16S rRNA gene fragments indicated a high abundance of Gram-positive bacteria of the phylum Firmicutes (82%) in MX-80 Batch 1 incubations. Bacterial libraries from microcosms with MX-80 Batch 2 were enriched with Firmicutes (53%) and Chloroflexi (43%). Firmicutes also significantly contributed (70%). Sequence analysis revealed presence of the bacterial families Peptostreptococcaceae, Clostridiaceae, Peptococcaceae, Bacillaceae, Enterobacteriaceae, Veillonellaceae, Tissierellaceae and Planococcaceae in MX-80 Batch 1 incubations; Bacillaceae, along with unidentified bacteria of the phylum Chloroflexi, in MX-80 Batch 2 clay microcosms, and Pseudomonadaceae, Hydrogenophilaceae, Bacillaceae, Desulfobacteraceae, Desulfobulbaceae, Peptococcaceae, Pelobacteraceae, Alcaligenaceae, Rhodospirillaceae in Canaprill microcosms. Exploration of potential metabolic pathways in the bacterial communities from the clay microcosms suggested variable patterns of sulfur cycling in the different clays with the possible prevalence of bacterial sulfate-reduction in MX-80 bentonite, and probably successive sulfate-reduction/sulfur-oxidation reactions in Canaprill microcosms. Furthermore, analysis of potential metabolic pathways in the bentonite enrichments suggested that bacteria with acid-producing capabilities (i.e., fermenters and acetogens) together with sulfide-producing prokaryotes might perhaps contribute to corrosion risks in clay systems. However, the low activity or dormancy of microorganisms in highly compacted bentonites as a result of severe environmental constraints (e.g., low water activity and high swelling pressure in the confined bentonite) in situ would be expected to largely inhibit bacterial activity in highly compacted clay-based barriers in a future DGR., Bentonite microcosms; Bacterial diversity; High-throughput sequencing; QIIME (quantitative insights into microbial ecology); PICRUST (phylogenetic investigation of communities by reconstruction of unobserved states); Potential bacterial activity.

Lifetime predictions of used nuclear fuel containers (UFCs) destined for permanent storage in deep geological repositories are challenged by the uncertainty surrounding the environment and resultan...

RACE has been developing a Blanket Transporter concept for the replacement of tritium breeding blankets within the EUROfusion DEMO tokamak. This operation will be challenging due to the scale of the blankets (˜10 m & 80 tonnes) and the current vessel design, which necessitates complex blanket kinematics within the vessel prior to a high vertical lift through the upper port. The blanket transporter is a key high technical risk system and break-down rescue and recovery has been highlighted as a significant unaddressed risk. This paper outlines the processes used to identify the key hazards resulting from failure scenarios and the improvements made to the concept design to mitigate them. Failure modes were identified via industrial expert input, failure and hazard analyses. Mitigation of the failure scenarios was achieved by providing secondary load paths in the main structure, redundancy in key components and the inclusion of additional features that allowed secondary rescue equipment to engage to assist rescue.

DEMO is a key part of the EU fusion roadmap, and the programme reaches the end of the pre-conceptual phase with a gate review in 2020. As part of the work to complete this phase, eight Key Design Integration Issues (KDII’s) have been identified as critical to the programme. Two of these KDII’s identified a requirement for a more detailed architectural study. Within KDII#3 (advanced divertor configurations) a double null configuration has been developed; in parallel KDII#4 (vertical segment architecture) identified a need to evaluate a split breeding blanket architecture. These two requirements were combined into a single study that assesses the in-vessel architecture for a double null DEMO featuring split breeding blankets. This paper presents the configurations developed and an evaluation of their feasibility with respect to integration of key in-vessel components (breeding blankets, divertors), port hardware (service pipes, shielding, vacuum pumping), and maintenance strategy. Furthermore, wider considerations such as the impact of the port orientation on the building architecture are also included. Finally, this paper will identify the risks and further work required to advance the double null configuration.

A study into the viability of a double null concept for DEMO has been undertaken as part of the work to address Key Design Integration Issues within the DEMO powerplant. The objective of this study was to evaluate the feasibility of a double null architecture featuring split blanket segments to enable a wider range of maintenance options. One of the main challenges found was the integration of the service pipes to provide connections to the cooling and tritium breeding systems for the divertor and breeding blanket. By understanding how these pipes might be integrated, the constraints on key details such as the length of the lower port could be understood. This allowed for progression of other areas of the design and investigations into the remote handling of these pipes during maintenance periods. The key requirements identified for the service pipe routing included determining the removal sequence for the blanket and divertor segments during maintenance; access for the cutting and welding tool at the pipe/blanket interface; pipes grouped as modules per component; and optimisation of port space for the vacuum pumps. Two proposals were considered to investigate their impact on the insertion location for the cutting and welding tool: pipes with large radii, and pipes with smaller radii. The former was found to be preferable due to the remote maintenance advantages, versus the limited space gains offered by the latter. This paper will describe the development of these requirements, both proposals, and discuss the challenges uncovered during the study in more detail.

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.

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 oxide chemical composition on metallic Ni and NiCr alloy electrodes has been studied for changes in simulated reactor coolant solution chemistry, through a range of oxidizing potentials and pH settings at 25 °C and 150 °C. Even under strongly reducing conditions, the Ni surface has a 2-nm layer of oxide and hydroxide from which further oxides form, depending on the subsequent oxidative step. For basic (normal operating) pH conditions and under oxidation potentials near 0 volts (vs. SCE), a predominantly Ni(OH)2 layer is formed that appears to remain relatively stable up to at least 48 hours of oxidation at 150 °C. For the NiCr alloy, similar stability is imparted by a thin film of Cr(OH)3/ Cr2O3 and Ni(OH)2./NiO. Under milder oxidizing (but still basic) conditions, the surface is stabilized by a thin film that is mostly Ni(OH)2/NiO. Under neutral solution conditions, the same oxide/ hydroxide films do not seem to be as effective in stabilizing the surface. Copyright © 2017 John Wiley & Sons, Ltd.

The Nuclear Waste Management Organization (NWMO) has developed a comprehensive Proof Test Plan to evaluate the feasibility and safety of its deep geological repository solution for nuclear waste. This work plan includes many active research programs to build confidence in the Canadian used nuclear fuel container design, a copper-coated steel vessel that is optimised for the CANDU waste form. Ongoing research within the Proof Test Plan includes several programs to evaluate the possible extent of damage that may be caused by various corrosion mechanisms: the anoxic corrosion of copper in pure water, concentrated chloride solutions, and in the presence of sulphide; a comparison of the corrosion behaviour of copper coatings on steel with wrought copper; the localised corrosion and surface roughening of copper; the γ-radiolysis-induced corrosion of copper; the galvanic corrosion of through-coating defects; and the localised corrosion of the internal steel weld region.This paper is part of a supplement ...

The corrosion behaviour of nuclear fuel waste containers depends on the near-field environmental conditions, which may differ significantly from those in the host rock, especially during the early ...

Canada is currently considering Cu-coated carbon steel containers for the long-term storage of used nuclear fuel in a deep geological repository. The Cu coating provides a corrosion-resistant barrier, protecting the underlying steel from coming into contact with groundwater. However, galvanically accelerated corrosion of steel is possible if there is a defect through the Cu coating. To investigate this scenario, the progression of steel corrosion at the base of a simulated though-coating defect was imaged using synchrotron X-ray micro-computed tomography. Results show that coatings produced using different methods (cold spray, annealed cold spray, electrodeposition) lead to different corrosion propagation geometries. These findings can be used for modelling steel corrosion at a though-coating defect under deep geological repository conditions.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

The focus of this research is to determine the conditions under which passivity of Cu is (with the possibility of pitting) or is not possible, in anticipated bentonite pore water compositions expec...

This paper presents a state-of-the-art analysis of the expected degradation processes of a variety of candidate container materials for the disposal of high-level waste and/or spent nuclear fuel. The work, focusing on the most recent developments, has been performed under the auspices of the Implementing Geological Disposal Technology Platform in the context of an international conference hosted by the Nuclear Waste Management Organisation of Canada (NWMO). The scope of the analysis includes the expected corrosion and environmentally assisted cracking behaviour of copper, carbon steel and titanium in contact with relevant buffer materials (e.g. bentonite, cement) and in conditions expected in an underground disposal facility (long-term anoxic conditions). Considerations relative to the expected evolution of the environmental conditions (especially in the period following backfilling) are also presented. Beyond summarising the current state of knowledge, areas in which opportunities for international collaboration may be present are also highlighted. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

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.

This paper presents the development work undertaken to tailor Cu powder size specifications to produce dense cold spray Cu coatings featuring strong adherence on steel substrates. Through review of historical data generated in the course of the development of Cu coatings for corrosion protection of Used (Nuclear) Fuel Containers, it was found that particle size distribution was particularly critical to meet application adhesion requirements of 60 MPa and prevent nozzle clogging during application; D01 and D90 were fixed at 5 and 60 µm, respectively. The effect of powder size on coating microstructure and adhesion was further investigated with four Cu powder lots presenting different granulometries. The four lots were analyzed for composition, hardness, microstructure and morphology. Their in-flight particle velocities were measured using a coldspraymeter and the impact temperatures and velocities were simulated. Using fixed spraying conditions consisting of a He bond coat and a nitrogen buildup step, 3-4-mm-thick coatings were produced and tested for bond strength. Coating cross sections and fracture surfaces of selected coatings were examined. These trials confirmed that although the various powder sizes tested produced dense and sound coatings, tight control of particle size distributions was required to achieve optimal impact velocities that ensure deformation of the steel and good coating adhesion.

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 Canadian Nuclear Waste Management Organization (NWMO) has chosen a used fuel container (UFC) design that consists of an inner steel core and outer copper coating that resists corrosion in the a...

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 safe long-term disposal of radioactive waste arising primarily from power generation is a complex proposition. Safety assessment modelling typically incorporates up to one million years, which is an engineering requirement without precedent. In order to perform this modelling, it is imperative that the behaviour of the materials comprising the repository are well understood over the timescales in question. This article describes a method developed for studying the behaviour of metals under anoxic conditions via the corrosion reaction end product, hydrogen. At its simplest, test cells are left for periods of time, up to several months, to accumulate hydrogen. This gas is then expelled from the cell and analysed using a hydrogen sensor. As such, the method is limited to characterising gas generation and consequently only uniform corrosion, but does permit direct monitoring of corrosion rates as low as 0.01 nm/year, which is necessary in order to accurately monitor the very low long-term corrosion rates expected in a deep geological repository.

International audience; Carbon steel (C-steel; E24 and S235 grades) and stainless steel (316L) electrodes were corroded in situ in Opalinus clay under anoxic conditions in a vertical descending borehole. The electrodes were exposed at ambient temperature for two years, and then at 85°C for five years. In situ electrochemical impedance spectroscopy showed that the instantaneous corrosion rate of C-steel decreased over time down to 1 µm year−1, followed by steady state. Microbial and chemical investigations showed that sulphate and thiosulphate reducing prokaryotes were present in the porewater and at the metal surface. Post mortem characterisation revealed contrasting corrosion aspects. The E24 corrosion interface was thick and contained magnetite, mackinawite, hydroxychloride and siderite, together with more oxidised species (goethite, greigite, elemental sulphur). The S235 corrosion interface was thinner and contained siderite and mackinawite. Corrosion damage of the 316L electrode was negligible, and the surface was covered by a thin fringe of pyrite.

The EU fusion roadmap defines as a goal the development of a DEMO, which achieves a long plasma operation time and demonstrates tritium self-sufficiency and net electricity output. Eight design issues have been identified as critical: (i) feasibility of wall protection limiters during plasma transients, (ii) integrated design of breeding blanket and ancillary systems, (iii) power exhaust taking advantage of advanced divertor configurations, (iv) tokamak architecture based on vertical blanket segments, (v) direct or indirect power conversion concept, (vi) configuration of plant systems in the tokamak building, (vii) feasibility of hydrogen separation in the torus vacuum pump and direct recirculation, and (viii) plasma scenario. For each of these issues potential solutions have been identified and activity plans have been defined for the associated developments and assessments. Two of these affect in particular the integrated design of the DEMO machine, namely (i) and (iv). These are introduced and discussed in this article providing a summary of identified risks, requirements, and solution concepts. For the remaining six design issues references are provided only due to space limitations in this article.

Due to the limited irradiation lifetime of the structural material used for in-vessel components in DEMO, it will be necessary to replace all breeding blankets within the given planned maintenance window in order to meet DEMO availability targets (Romanelli, 2012; Ellis, 2017) [ 1 , 2 ]. It is assumed that failure of in-vessel components cannot be excluded, whilst in-situ repair is unrealistic. Hence the replacement of individual breeder blankets must be technically feasible. As such, remote maintenance replacement of the breeding blankets is a mission critical operation. The baseline concept utilises vertical segment architecture to aid in the removal of the blankets (Iglesias, 2013) [ 3 ]. This choice impacts on the tokamak and plant architecture and also affects operational maintenance strategy. Within the EUROfusion PPPT program efforts have been made to perform cross work package investigations on eight Key Integration Issues needed to show the feasibility of the DEMO pre-concept design (Bachmann, 2017) [ 4 ]. Key Design Issue 4 is an investigation into the feasibility of the Vertical Segment Architecture blanket feasibility The present work documents the approach, current progress and developments within this investigation. This includes the strategy, identified risks and proposed solutions.

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.

Within ongoing work, Canada's Nuclear Waste Management Organization (NWMO) is demonstrating that copper coatings can be developed as viable corrosion barriers for used fuel nuclear containers, emplaced in a deep geological repository. As a means of assessing container design and feasibility, coated samples, manufactured via cold spray and electro-deposition have been produced and characterised. Corrosion testing for coated samples and wrought samples reveals that cold spray samples behave similar to wrought samples in 3 mol L21NaCl solution at room temperature. When oxygen is absent, corrosion has not been observed.

Carbon steel is a candidate container material for the disposal of used fuel in a deep geological repository in sedimentary host rock in Canada. Prediction of the long term anaerobic corrosion behaviour of the container is important, not only because it partly determines the container lifetime but also because the products of the corrosion reaction, Fe(II) and H2, can impact the properties of the clay based sealing materials and host rock. A mechanistically based numerical model is described that predicts both the long term corrosion rate and the effects on the repository environment, including the periodic build-up and release of H2, the precipitation of Fe3O4 within the bentonite buffer, and the alteration of montmorillonite to a non-swelling clay. The results of a reference simulation for a repository in shale host rock are described.

A model has been developed to predict whether the development of acidity is feasible within actively corroding sites on spent nuclear fuel (UO2) surfaces inside a failed nuclear waste container. The model simulations demonstrate that the build-up of acidity is possible within flaws and pores in a corroded UO2 surface, providing the separation of anodes and cathodes occurs. The extent to which the pH can be depressed is determined by the dissolution rate of the fuel, the dimensions of the defect, the local redox conditions which determine the corrosion potential, and the fraction of the fuel surface that is reactive. Based on the anticipated redox conditions established radiolytically in a failed container it is shown that a suppression of the pH sufficient to accelerate dissolution (pH≤5) is very unlikely.

Placement of encapsulated used nuclear fuel in a deep geological repository is recognised internationally as the leading option to achieve containment and isolation from the environment and the public. In Canada, containment and isolation are proposed through an engineered barrier system, which comprises a copper-coated, welded carbon-steel container to encapsulate used fuel, highly compacted bentonite clay buffer, and the geosphere. Critical to regulatory approval and public confidence is the robustness of the engineered barrier system to perturbations capable of affecting the survival of used fuel containers over time scales of interest. However, uncertainty in the evolving conditions of a repository adds to the challenge of used fuel container life predictions. A new probabilistic container survival model is presented, featuring a simple, multi-level Monte Carlo technique to account in part for the unique constraints posed by a deep geological repository layout and an engineered barrier system on lifetime predictions.

The aerobic corrosion of pipeline steel was investigated in an aqueous sulfide solution by monitoring the corrosion potential and periodically measuring the polarization resistance. The properties and composition of the corrosion product deposits formed were determined using scanning electron microscopy, energy dispersive X-ray analysis, and Raman spectroscopy. The establishment of aerobic conditions leads to corrosion and (oxyhydr)oxide deposition beneath the anaerobically-formed mackinawite film originally present on the steel surface. This leads to blistering and spalling of the sulfide film. Chemical conversion of the mackinawite to Fe(III) (oxyhydr)oxides also occurs but is a relatively slow reaction.

Severe corrosion damage may occur when gas transmission pipelines are exposed, at disbonded coating locations, to trapped waters containing sulfide. This scenario has been investigated in long-term...

The present study aims to investigate the electrochemical reduction of a range of H2O2 concentrations on a 1.5 at.% SIMFUEL rotating disk electrode over the pH range 1–4. The peroxide reduction mechanism is determined to occur either on a UV-containing surface layer of composition UIV1−2xUV2xO2+x or on an adsorbed UV-containing surface intermediate depending on the surface composition which is determined by solution pH and H2O2 concentration. The UIV1−2xUV2xO2+x catalytic surface lattice layer, if formed, is stable and rotation disk studies have demonstrated that H2O2 reduction on this surface achieves the diffusion-controlled limit at sufficiently negative overpotentials. However, the adsorbed UV-containing surface intermediate is unstable and can be destroyed by electrochemical reduction to its original state, i.e. UO2, or by chemical oxidation to UVI prior to dissolution as UO22+. The instability of this surface intermediate limits its availability which prevents significant H2O2 reduction and yields currents below the diffusion-controlled limit. The occurrence of both reduction mechanisms demonstrates the influence of locally established surface compositions and the switch from one to the other appears to be controlled by surface diffusion conditions and the bulk pH and H2O2 concentrations.

Copper surfaces have been investigated by X-ray absorption spectroscopy after electrochemical oxidation and subsequent exposure to sulfide solution. Oxide surface layers on bulk copper surfaces wer ...

A corrosion mechanism has been developed to describe tubercle formation along pipeline steels during successive anaerobic–aerobic cycles. Small concentrations of O2 under nominally anaerobic conditions can lead to the separation of anodes and cathodes. Under subsequent aerobic conditions localized corrosion is then promoted by O2 reduction on the general magnetite-covered surface. Subsequently, the conversion of magnetite to maghemite passivates the general surface, and focuses corrosion within one major tubercle-covered pit. On switching from aerobic to anaerobic conditions, corrosion is temporarily supported by the galvanic coupling of lepidocrocite (γ-FeOOH) reduction (to γ-Fe-OH·OH) to steel dissolution primarily within the tubercle-covered pit.

The influence of anaerobic–aerobic cycling on pipeline steel corrosion was investigated in near-neutral carbonate/sulphate/chloride solution (pH 9) over 238 days. The corrosion rate increased and decreased as exposure conditions were switched between redox conditions. Two distinct corrosion morphologies were observed. The majority of the surface corroded uniformly to produce a black magnetite/maghemite layer approximately 4.5 μm thick. The remaining surface was covered with an orange tubercle, approximately 3–4 mm in cross section. Analysis of the tubercle cross section revealed a single large pit approximately 275 μm deep. Repeated anaerobic–aerobic cycling localized the corrosion process within this tubercle-covered pit.

The corrosion of nuclear fuel under waste disposal conditions is likely to be influenced by the bicarbonate/carbonate content of the groundwater since it increases the solubility of the UVI corrosion product, [UO2]2+. As one of the half reactions involved in the corrosion process, the anodic dissolution of SIMFUEL (UO2) has been studied in bicarbonate/carbonate solutions (pH 9.8) using voltammetric and potentiostatic techniques and electrochemical impedance spectroscopy. The reaction proceeds by two consecutive one electron transfer reactions (UIV → UV → UVI). At low potentials (≤250 mV (vs. SCE) the rate of the first electron transfer reaction is rate determining irrespective of the total carbonate concentration. At potentials >250 mV (vs. SCE) the formation of a UVIO2CO3 surface layer begins to inhibit the dissolution rate and the current becomes independent of potential indicating rate control by the chemical dissolution of this layer.

The reaction kinetics of AgI formation by reaction of Ag 2 O films with aqueous iodide were followed by monitoring the open-circuit potential ( E OC ). Previous studies have shown that complete conversion of Ag 2 O to AgI is indicated by an abrupt transition in the E OC from a value close to the equilibrium potential for the Ag|Ag 2 O|H 2 O system to one close to that for Ag|AgI|I − , allowing an easy determination of the total reaction time, τ f . The solid–solution interfacial kinetic parameters can then be determined by measuring τ f as a function of the initial amount of Ag 2 O and the I − concentration. In this study, the application of the technique was extended to examine the effect of I − (aq) transport to an Ag 2 O surface by measuring τ f as a function of electrode rotation rate. By studying the chemical conversion as a function of electrode rotation rate we are able to separate mass transport effects from the reaction kinetics. The interfacial reaction follows kinetics typical of a sequential binary-reaction system consisting of mass transport to, followed by reaction at, the surface. The mass transport rate was proportional to the square root of the electrode rotation rate, consistent with the well-established diffusion-boundary layer model for a rotating disc. Varying the rotation rate allows a quantitative extrapolation of the data to the infinite rotation rate to determine the chemical reaction rate at the surface.