Jungsook Clara Wren

Aggregation of oxide crystals of different compositions in concentric wave patterns on corroding metal is not due to metallurgical non-uniformity, but sustained cyclic feedback between solution reaction and transport processes of metal cations.

This paper presents the “dual electrochemical cell” (DEC) method, a new technique that allows monitoring of corrosion current ( i corr ) in real time with a corrosion potential E corr that may change with time. In this method, the E corr of a metal corroding in a solution containing the oxidant of interest is measured in the 1^st cell. This potential is then continuously applied in real time to a second cell using the same metal electrode in the same solution but free of the oxidant, and the current of the 2^nd cell, which represents the i corr of the 1^st cell, is monitored. This setup allows direct measurement of i corr without having to polarize the corroding electrode away from E corr . The advantages of DEC over conventional methods are that it does not require the corroding system to be at steady state, and avoids any ambiguities associated with the extrapolation of the measured current-potential relationship to extract i corr . The i corr values obtained using the DEC method were compared with those obtained using conventional polarization techniques, using dissolved metal concentrations and surface analysis observations, and the results showed that the DEC method provides the most accurate measurement of i corr .

Amorphous chromium-containing solid nanoparticles and hollow spheres were formed as the intermediate product during the synthesis of Cr₂O₃ by gamma radiolysis of dichromate solution. Scanning transmission X-ray microscopy (STXM) is applied to investigate the electronic structure and chemical composition of this intermediate product (solid nanoparticles and hollow spheres). The STXM result obtained at O K-edge and Cr L_3,2-edge confirms that chromium in the sample exists as Cr³⁺ and that the solid nanoparticles and hollow spheres are both Cr(OH)₃. Combined with other techniques, a mechanism for gamma radiation induced formation of Cr₂O₃ nanoparticles via Cr(OH)₃ intermediates from dissolved dichromate is proposed.

Crevice corrosion of carbon steel was investigated in different exposure environments by performing coupon exposure and electrochemical tests. The extent of corrosion on the bold surface of a carbon steel crevice coupon was more severe at 80°C than at 21°C, in aerated rather than dearated solutions, and with γ-radiation present. In contrast to normal crevice corrosion, we observed ‘inverse crevice corrosion’ behavior, the phenomenon where it is the corrosion on the bold surface that is accelerated when coupled, rather than that on the crevice surface. The coupling current measured between a crevice and a bold electrode in an electrochemical cell was also negative. This inverse crevice corrosion behavior is attributed to a significantly lower metal cation dissolution capacity of the small occluded water volume in the crevice, compared to that of the bulk water volume over the bold surface. The reduction in dissolution capacity results in faster and earlier formation of a protective oxide layer. Corrosion of the bold and crevice surfaces evolves at different rates, leading to galvanically accelerated corrosion of the bold surface. The effect of γ-radiation on corrosion evolution in different solution environments leading to inverse crevice corrosion is discussed.

Copper-coated steel containers are part of the engineered barrier system to permanently store Canadian nuclear fuel waste in a deep geological repository. This work models the dose rates (D_Rs) at the container surfaces as a function of fuel age. It also utilises a humid-air radiolysis model to study the effects of D_R and humidity on radiolytic oxidant production for conditions where unexpected early water intrusion reaches clay seal materials. Radiolysis of humid air produces HNO₃. The HNO₃ production rate in a condensed water droplet formed on a container surface was conservatively estimated by assuming that every •OH produced by primary radiolytic processes was immediately converted to HNO₃ in the gas phase and that all of the HNO₃ was absorbed in the water droplet. Also assuming that all of the nitric acid absorbed in the water droplet is consumed in corroding copper and using a hemispherical water droplet geometry, the corrosion depth of the copper coating induced by humid-air radiolysis is conservatively estimated to be 9.4 μm over the permanent storage time. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Magnetite nanoparticles with a narrow size distribution were produced via gamma-radiolysis of FeSO ₄ solutions. The average particle size increased with [Fe ²⁺ ] ₀ . A multistage mechanism (supported by radiolysis kinetic modelling) is described that can explain the observed dependence of particle size on [Fe ²⁺ ] ₀ .

A mass and charge balance (MCB) model that can simulate oxide growth and dissolution kinetics simultaneously during corrosion of an alloy has been recently developed. In this study, the MCB model was applied to the corrosion of the Co-Cr alloy Stellite-6. The construction of the model and the assignment of values to the rate parameters for the model are presented. The model simulation results are then compared with experimental corrosion data collected as a function of pH, temperature and polarization potential. The data include the current during potentiostatic polarization, the corrosion potential under open-circuit conditions, and post-test analyses of the oxide formed and the amount of metal dissolved. Excellent agreement between the model results and experimental data are found. This is evidence that the MCB model is a useful tool for predicting time-dependent corrosion while an oxide film is evolving.

The kinetics of γ-radiation-induced Co₃O₄ nanoparticle formation occurs in four distinct stages. Scavengers and [Co²⁺]₀ affect the kinetics in different stages, and consequently influence the final particle sizes. Radiolysis model calculations on the effects of radical scavengers provide insights into their role in controlling particle formation.