Understanding the Degradation of Organic Coatings Through Local Electrochemical Impedance Methods : II. Modeling and Experimental Results of Normal Field Variations above Disk Electrodes

Abstract

Local electrochemical impedance mapping (LEIM) represents a potentially important tool in the characterization of discrete electrochemical phenomenon on heterogeneous surfaces, such as the degradation of a coated aluminum alloy. Since LEIM is an emerging tool, it is essential to resolve between real material and interfacial changes and a possible sampling artifact to the measured LEIM response. To this end, both analytical and numerical models were developed to calculate the field above an equipotential disk to examine possible LEIM measurement artifacts. Several features in LEI maps which could be the result of an artifact in the measured field above a corroding region were addressed. These included changes in the field due to the following: (i) edge effects, (ii) two closely spaced corroding sites affecting spatial resolution, (iii) a change in radius of a corroding site, including the ability of LEI to resolve small sites. Numerical analysis was found to more realistically model the finite dimensions and discrete change in potential that were used in actual LEIM experiments. The calculated field above the equipotential disk was then compared to experimental LEI maps of gold disk microelectrodes embedded in Edge effects were found to be an unlikely source for experimental observations of reduced admittance. The spatial resolution of the LEI probe was predicted through theoretical modeling and determined experimentally to be sufficient to resolve two disk separated by 35 μm. The probe was also predicted and shown experimentally to have the capability to measure a disk of radius 17.5 μm. The successful demonstration of this numerical model will allow the exploration of more complicated material interfaces, such as materials coated with a dielectric layer. © 2003 The Electrochemical Society. All rights reserved.