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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.

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.

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2–12.6 and the H2O2 concentration range of 10−4 to 10−2 mol dm−3. The Tafel slopes for H2O2 reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec−1, respectively, independent of pH and the concentration of H2O2. Both the reduction and oxidation of H2O2 on γ-FeOOH have a first-order dependence on the concentration of molecular H2O2. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH−. For both cases the electroactive species is the molecular H2O2, not its base form, HO2−. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HO OH− and HO for the reduction, and HO2 H+, HO2 , and O2− for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH− and H+ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H2O2 to the corresponding intermediates, a reaction step which involves the use of FeIII/FeII sites in the γ-FeOOH surface as an electron donor–acceptor relay. The rate constant for the H2O2 decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H2O2 decomposition.

The cathodic reduction of H2O2 has been studied at rotating SIMFUEL disc electrodes in sodium chloride solutions over the pH range 1–9. The surfaces were analyzed by X-ray photoelectron spectroscopy. The mechanism and rate of reduction did not change over the pH range 4–9. Within this pH range, H2O2 chemically oxidizes the UO2 surface to UV which is subsequently electrochemically reduced. The UV state is incorporated into a surface layer of UO2+x. For pH values below 3, reduction is catalyzed by the chemical creation of an adsorbed UV state. For solutions from pH 3 to 9, currents rise to the diffusion-limited value at sufficiently negative potentials since the rate of formation of UV (in UO2+x) is sufficiently rapid, and the UV state formed is sufficiently stable, that reduction is never limited by the availability of this state. However, the UV state formed in acidic solutions (pH 1, 2) is less stable and can be further oxidized to the insulating UVI state prior to its dissolution as UO22+, or cathodically reduced at less negative potentials than required in neutral solutions. This instability limits the availability of the catalytic state and the currents are always well below the diffusion-limited value.

The corrosion of high level nuclear waste containers under permanent disposal conditions can occur via a number of processes which change in importance as the environment within a deep geologic repository evolves from oxic to anoxic. The container design involves a copper coating on a steel vessel. The characteristics of these coatings are being assessed using standard analytical methods, including microscopy and electron backscatter diffraction. The corrosion processes on the surface of the coatings and at through-coating defects are also being investigated electrochemically. Although minor differences exist, the surface corrosion of cold sprayed and electrodeposited coatings generally exhibited similar corrosion behaviour to standard wrought copper. Corrosion at the base of a simulated through-coating defect in a cold-sprayed coating was found to proceed via galvanic coupling to oxygen reduction on the coating surface. Besides accumulating damage at the base of the defect, corrosion propagated along the copper/steel interface, most likely a consequence of the damage inflicted during the deposition process.