|Ansari, Talha Qasim
|Phase-field modeling of localized corrosion kinetics in metallic materials
|Shi, S. Q. (ME)
|Hong Kong Polytechnic University -- Dissertations
Corrosion and anti-corrosives
|Department of Mechanical Engineering
|xx, 153 pages : color illustrations
|Corrosion is an electrochemical process that results in the degradation of metallic materials in a corrosive environment. Localized corrosion is a destructive form of corrosion that can lead to catastrophic failure of structures. In developed countries, corrosion costs more than 3% of gross national product every year, much more than the costs arisen from all-natural disasters combined. It is very important to understand this process to prevent sudden accidents and also develop high strength corrosion resistant metallic materials. This work focuses on developing a numerical model using phase-field formulation based on the electro-chemical reactions that govern the localized corrosion kinetics. Firstly, a thermodynamically consistent phase field model for the quantitative prediction of the pitting corrosion kinetics in metallic materials is developed with an assumption that no insoluble corrosion products will form on the corroding surface. The Gibbs free energy of the metal-electrolyte system consists of chemical, gradient and, electromigration free energy. A calibration study is performed to couple the kinetic interface parameter with the corrosion current density to obtain a direct relationship between overpotential and the kinetic interface parameter. The phase field model is validated against the experimental results, and several examples are presented for applications of this phase-field model to understand the corrosion behavior of closely located pits, stressed material, ceramic particles-reinforced steel, and their crystallographic orientation dependence. Secondly, a multi-phase-field model is presented to model the evolution of more than two phases. This model is used to investigate the effect of ICP formation on pitting corrosion kinetics. The Gibbs free energy of the metal-electrolyte-insoluble corrosion product system consists of chemical, gradient and, electromigration free energy. The model is validated with experimental results and several representative cases are presented, including the effect of the porosity of ICP, under-deposit corrosion, corrosion of sensitized alloys and microstructure-dependent pitting corrosion. It is observed that corrosion rate and pit morphology significantly depend on ICP and its porosity. Lastly, the multi-phase-field model is extended to study intergranular corrosion (IGC) kinetics in sensitized metallic materials. The MPF model incorporates the difference in the electrochemical properties of the grains and sensitized grain boundaries. Several simulations are performed and validated with the experimental results. The MPF model results show that IGC process usually becomes transport controlled due to the slow transport of ions in the electrolyte through narrow corroded regions. The model also predicts difference in IGC rate in different plane directions, when heat-treated rolled sheets are exposed to a corrosive electrolyte at a fixed applied potential.
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