Cho, Seongkoo (2020-04). The Study of Electrical and Electrochemical Impedance Behaviors by the Distribution of Physical Properties of Coated Materials. Doctoral Dissertation.
Thesis
Impedance spectroscopy has been widely used to understand electric- and electrochemistry-related research. The use of impedance spectroscopy allows fast and reliable determination of material degradation and corrosion protection performance. Despite such advantages, there have been difficulties in the impedance analysis by various superimposed physical processes. The objective of this research was to specify the physical meaning by interpreting non-ideal and non-conventional impedance spectra. A deterministic-probabilistic 2D impedance model was proposed to represent the physical degradation of a multi-layered coating system. The model allowed to overcome the limitations existed in the 1D model, such as limited parameter domain, by interpreting the effect of pore distribution in the 2D coating space. As a result, the effect of electrolyte penetration and self-healing process on impedance spectra was revealed along with the experimental results. Non-ideal impedance spectra influenced by the distribution of electrical properties, such as electrical conductivity and permittivity, of the coating were studied. The distribution of the inter-correlated parameters on the impedance model described the physical meaning of the non-ideal impedance, represented by the combination of local segments, which resulted in distorted spectra of a bulk system. Since the locally distributed parameters could represent the local electrical or electrochemical behavior, it was possible to evaluate localized impedance spectra. Thus, the modified two-dimensional impedance model based on charge conservation was suggested to understand the localized electrochemical impedance spectra (LEIS) responding the local defects. The modified model revealed the influence of electrical conductivity of electrolyte on the resolution of LEIS for local defects. Lastly, non-conventional impedance responses of a zinc-rich coating system were observed in electrical and electrochemical impedance measurements previously considered to have no effect on the system. Non-electrolytic and electrochemical cells on zinc-rich coatings were analyzed to understand the intrinsic response of the coatings with electrical active and passive analogs under electrical and electrochemical cyclic tests. The aforementioned studies will not only be used as a method to interpret the various physical meanings of systems, such as black boxes, through impedance spectra, but also as an important guideline for optimal design of coating materials.
