Investigating localized degradation of organic coatings. Comparison of electrochemical impedance spectroscopy with local electrochemical impedance spectroscopy

The degradation of polyester coil-coated galvanized steel was compared using both conventional (macroscopic) and localized electrochemical impedance techniques on the same specimen and within a time interval (hours) much shorter than the total immersion period (days). Specimens containing a central 250 μm laser-ablated defect in the organic coating layer were immersed in a 10 mM NaCl solution for up to 30 days. The local multifrequency impedance was determined by placing a novel impedance probe, either directly above the coating defect or above an area of intact coating. In addition, single frequency impedance mapping of the specimen surface was carried out at 1 kHz and compared with optical microscopy of the surface. The results demonstrate clearly that macroscopic electrochemical impedance provides a surface-averaged measurement of the properties of the coating, plus any defects, and that where several time constants are apparent, they are not uniquely separable into physical processes. Thus, macroscopic impedance spectra convolute the separate responses of the coating and defect together. However, local electrochemical impedance can effectively separate the local properties of the organic coating from the local electrochemical behavior at a coating defect.