The Effect of Solution Parameters on the Interfacial Chemical Dynamics of Early-Stage Corrosion

Brianna K. Rector, The University of Western Ontario

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.