Understanding the Degradation of Organic Coatings Using Local Electrochemical Impedance Methods : I. Commonly Observed Features

Abstract

Local electrochemical impedance mapping (LEIM) was used to investigate local underfilm corrosion occurring on organic-coated AA2024-T3 exposed to chloride solution. Films (5-20 μm thick) of neat epoxy, polyurethane, and vinyl resins were studied. Frequently observed features in the LEI maps included: an unexpected decrease in admittance in early stages of development, trenching at the periphery of admittance peaks, both increases and decreases in peak height over time, changes in peak width over time, different amplitudes in admittance over different regions of multilobed blisters, and different admittance magnitudes for different types of defect. These LEIM features could provide significant insight into and documentation of the local breakdown processes of coated metals, if they truly represent electrochemical phenomena associated with changes at the interface. These features were observed on each of the coating chemistries studied. Proposed origins and supportive evidence of these LEIM features is presented. © 2003 The Electrochemical Society. All rights reserved.