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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Wren, Jungsook Clara

Abstract

Corrosion is a key long-term degradation process for metallic components in nuclear industry infrastructure. Accurately predicting the corrosion rate of a component in its service environment is considered a grand challenge in the corrosion community. Corrosion is a multi-step electrochemical process, with elementary steps involving mass transfer across more than one stage. Due to the multi-step, multi-phase nature of corrosion systems, predicting corrosion behaviour over time is complex. Existing corrosion models can describe electron transfer and mass transport steps independently, under narrow sets of conditions or over a specified duration but cannot account for the changes that occur over time due to the coupling of elementary steps. Developing a detailed understanding of the interfacial chemical dynamics from the earliest stages of corrosion is critical to developing a rate model that adequately describes the evolution of corrosion systems over a wider range of environmental conditions and durations.

This thesis identifies and describes the initial interfacial chemical dynamics in corrosion systems and presents a corrosion model framework and mechanism that describe the initial corrosion behaviour as a function of solution redox and transport parameters. The rate-controlling elementary steps in the metal oxidation and oxidant reduction half-reactions are identified and decoupled. Electrochemical polarization tests on carbon steel are used to identify the independent effects of the purging gas (e.g., Argon or 21% O2), solution pH (pH 6.0 – 8.0), solution ionic strength, and concentration of radiolysis products (e.g., H2O2) on the rates of each elementary step. Overpotential and pH-dependent rate equations for metal oxidation and oxidant reduction half-reactions are developed. The proposed mechanism for the initial stage corrosion is shown to be well supported by the experimental observations and its extension to other metals (e.g., Co, Ni) is discussed.

This research provides a mechanistic understanding of corrosion at the metal-solution interface in its early stages and offers an approach to describe the evolution of corrosion in various solution conditions. A better understanding of fundamental interfacial processes will enhance the general understanding of corrosion and improve the process of developing predictive models for a wide range of interfacial systems.

Summary for Lay Audience

Transition metals like iron, cobalt, and nickel are found in alloys commonly used in the nuclear industry. Long-term degradation of these alloys by aqueous corrosion could damage these materials and require costly maintenance. Aqueous corrosion, an electrochemical process, involves the transfer of electrons and metal atoms at the interface where the metal and solution are in contact. However, corrosion is a complex process that involves many different steps that can change with time or as corrosion products accumulate at the interface. The rate of each of step depends on solution properties like pH, volume, temperature, and the types of chemical species that are dissolved in the solution. Additionally, the rate of one given step in the corrosion process may also affect the rate of a preceding step, generating feedback behaviours or cyclic coupling. Cyclic coupling is particularly important in the nuclear industry, as solution environments exposed to γ-radiation become more complex corrosion environments, when chemical species produced by the interaction of γ-radiation and water change the solution chemistry.

Corrosion is a slow process that can occur over many years; corrosion experiments, on the other hand, are restricted by laboratory time scales. Furthermore, the description of the overall corrosion process in one solution environment cannot be used to describe the corrosion of the same material in a different environment. Thus, predicting the progression of corrosion in different solution environments to avoid excess testing would be desirable.

Developing a predictive corrosion model that can be applied to different solution environments requires a chemical understanding and a mathematical description of the rate of each step in the overall process. This research presents a framework for modelling the corrosion of carbon steel that can be extended to cobalt and nickel. Rate equations for each step are proposed and coupled to describe the overall corrosion rate. Ultimately, this research contributes to the general understanding of corrosion, and the development of predictive models for many different interfacial systems.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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