Nonlinear Dynamics of Carbon Steel Corrosion under Gamma Radiation
Abstract
Corrosion of materials is still an unresolved problem affecting a multitude of industries. One of the grand challenges facing the corrosion community is the development of high-fidelity models for corrosion in actual service environments. The difficulties arise since corrosion involves transfer of metal atoms between the solid and solution phases thus making the system non-adiabatic. Interfacial transfer of atoms increases the chance of establishing systemic feedback between chemical reactions and transport processes, which results in chemical oscillation and periodic patterns on the corroding surfaces. These oscillating behavior in electrochemical measurements and pattern formation on corroding surfaces have been reported in certain solution environments in corrosion research. However, these corrosion phenomena have been interpreted incorrectly based on linear chemical and transport dynamics.
This work presents metal oxide formation in concentric wave patterns and/or in discrete solid bands during corrosion of carbon steel. It establishes that these oxide formations are a Liesegang phenomenon occurring via strongly coupled reaction-diffusion kinetics, requiring a slow transport medium. Transition metal ions easily form hydroxides which, being hygroscopic, grow in colloidal forms or a hydrogel network. Formation of this slow transport medium induces systemic feedback between FeII/FeIII redox reactions and hydrolysis and diffusion of metal cations, and also between the processes in the gel medium and metal oxidation at the surface. Metal hydrogel formation has never previously been identified as the key condition for feedback processes and oscillation to arise in corrosion. This work is the first to demonstrate unequivocally that non-uniform deposition of metal oxides during corrosion can occur via strongly coupled solution reaction and transport processes, and not simply as a result of metallurgical non-uniformity and/or localized solution environments. This study expands our understanding of corrosion by exploring the individual processes and their non-linear interactions thus providing more insights into the development of a corrosion dynamics model.
In the presence of systemic feedback, the overall corrosion dynamics cannot be expressed by a linear combination of individual elementary processes involved. To develop a reliable corrosion model when strong systemic feedback can exist, it is important to identify the key elementary processes that control the overall corrosion rate and to establish the kinetics of the elementary processes as a function of solution parameters. This study is a step toward developing such a model.