Development and feasibility evaluation of a miniature surface-mounted EIS sensor for chloride-exposed reinforced concrete

A miniature surface-mounted electrochemical impedance spectroscopy (EIS) sensor integrating a platinum wire pseudo-reference electrode and a Type 316 stainless-steel counter electrode was developed for localized monitoring of chloride-related electrochemical changes in reinforced concrete. A cell-resistance-based framework was used to evaluate the effects of concrete moisture, electrode–concrete coupling, and counter-electrode area, followed by benchmarking against an Ag/AgCl-based configuration. The selected sensor exhibited a mean peak working-electrode-to-counter-electrode (WE–CE) voltage demand of 406 ± 34 mV, less than 5% of the ±10 V compliance limit. Following short-term conditioning, the Pt-based OCP correlated strongly with the Ag/AgCl-based values ( R² = 0.944), with a condition-specific offset of approximately 65 mV. Within the usable frequency window of 0.0031–754 Hz, the median absolute relative impedance-magnitude difference and absolute phase-angle difference were 5.90% and 0.55°, respectively, while the fitted solution-resistance and interfacial CPE parameters differed by no more than 3.4%. The interfacial CPE coefficient increased with surface NaCl concentration, supporting qualitative monitoring of chloride-related changes at the steel–concrete interface. These findings establish laboratory-scale feasibility; however, long-term calibration, sensor-component and coupling-interface durability, and validation under variable field environments and active corrosion conditions remain necessary.