Investigating Localized Degradation of Organic Coatings

Abstract

The degradation of polyester coil-coated galvanized steel was compared using both conventional ͑macroscopic͒ and localized electrochemical impedance techniques on the same specimen and within a time interval ͑hours͒ much shorter than the total immersion period ͑days͒. Specimens containing a central 250 m 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.

Development of quantitative Local Electrochemical Impedance Mapping: an efficient tool for the evaluation of delamination kinetics

“…1). The border of LEIM response, where z −1 (r) approaches zero, is at about 5 mm that is 2-3 times larger than the actual radius of the WE (2 mm), which is in a good agreement with the literature [7,[15][16][17][18].…”

“…Maps of the local impedance, z(r), or the local admittance, z −1 (r), at fixed frequency are the typical output used so far in the literature to report on LEIM experiments [7,[15][16][17][18], but in the present work only the local admittance has been used.…”

“…[12] these parameters were D = 10 µm, h = 25 µm and d = 80 µmṄumerical simulations of the primary potential distribution for a disk electrode embedded in an isolator were in a good agreement with experimental results. In the more recent work [13], a spatial resolution of 35 Even though the spatial resolution in the micrometer range can be reached, the precise size of electrochemically active zones did not match their actual size on the admittance maps in LEIM [7,[15][16][17][18]. Moreover, this size was shown to depend on the applied frequency of the AC perturbation [15].…”

“…the position of delamination front) [7,[16][17][18]. Larger apparent size of the delaminated area from the admittance maps in comparison with the optical observations was always reported in the literature: delamination on the steel/epoxy-vinyl interface scanned at 5 kHz [7], vinyl coating on steel and Al alloy 5182 scanned between 100 and 1 kHz [16], industrial coil coated galvanized steel scanned at 1 kHz [17], epoxy based polymers on Al alloy AA2024-T3 scanned at 700 Hz [18]. In several works, LEIM revealed the formation of blisters in polymer even before they were visible by an optical technique [16,18].…”

Local electrochemical impedance spectroscopy: A review and some recent developments

“…This point is important because it emphasizes that, depending on the probe size, information can be obtained at different scales. Using a similar technique, Philippe et al [37] investigated polymer-coated galvanized steel. They showed that the global electrochemical impedance measurements provided surface-averaged responses corresponding to both the polymer properties and its defects, whereas LEIS allowed coating defects to be isolated.…”