Design of an EIS-based sensor for indirect non-invasive corrosion inspection

Electrochemical Impedance Spectroscopy (EIS) is a powerful quantitative technique to study metal corrosion. However, conventional EIS measurements require immersing the sample in liquid electrolytes and direct electrical connection to the metal substrate, limiting its feasibility for non-destructive testing and field use. This study introduces a novel non-invasive EIS sensor with a compact dual-compartment structure and an agar-based solid electrolyte, enabling a good fit for different surfaces and rapid measurements without sample immersion or wired contact with the substrate. The sensor’s performance has been tested on bare and coated metals under laboratory conditions and compared quantitatively with conventional EIS. The reliability and repeatability of the obtained impedance spectra have been validated, and the effects of key sensor parameters and experimental settings (e.g., measurement distance, reference electrode, working electrode, and AC potential amplitude) on impedance response have been systematically investigated. In addition, appropriate experimental protocol and data processing methodology have been established. The results demonstrate that the proposed method provides high-quality, analyzable impedance spectra comparable to conventional EIS after removing the contribution of the sensor’s self-impedance. Impedance analysis of coated metals indicates that the current path traverses the coating interface and conducts through the metal substrate rather than solely through the coating. Furthermore, the sensor successfully determined the coating thickness consistent with the sample characteristics. This work advances indirect impedance measurements and offers a viable solution for rapid, non-invasive corrosion inspection and coating assessment.