Two-electrode electrochemical impedance sensor: Part 1-response to coating degradation on conductive substrates

Electrochemical impedance spectroscopy (EIS) is widely used for understanding and quantifying the corrosion protective quality of coatings. However, the traditional single-cell EIS (SCEIS) configuration has largely been limited to laboratory testing on small panels partly due to the requirement of an electrical connection to the substrate, which implies damage to the protective coating. We have studied a two-cell EIS (TCEIS) configuration that increases the flexibility of EIS, enabling its use in the field testing of coatings without the need to damage the coating to achieve an electrical connection to the substrate. Comparison measurements between the SCEIS and the TCEIS were carried out with coated panels under three different accelerated weathering protocols (ultraviolet IUV], salt spray, and alternating UV/salt spray). Measurements from the TCEIS and the SCEIS showed similar impedance spectral changes with exposure time under the first two exposure protocols but had significant measurement deviations under the alternating UV/salt spray protocol. Further study showed that the alternating UV/salt spray protocol caused more localized coating damage, which increased the chance of severe nonuniform defect distribution over the coating area. The resultant material inhomogeneity, in turn, caused difficulties in the interpretation of a TCEIS sensor measurement due to three possible situations: the local severe defects could be under two, one, or neither of the electrodes. Although the problem caused by the localized defects was identified with the TCEIS, it would also cause the SCEIS spectrum variations depending on whether the sample area contained, or failed to contain, the severe local dejects. Many test procedures based on the SCEIS call for only a single site to be tested on a panel with the SCEIS, which makes the coating quality ranking determined partly by the probability of whether the defects were covered by the monitored area or not.