Electrochemical corrosion measurements in low-conductivity bio-oils: decoupling interfacial film memory from bulk electrolyte aging

Electrochemical corrosion measurements in viscous, low-conductivity organic media are limited by high cell impedance and parasitic artefacts. This work showcases a robust electrochemical protocol validated in a model bio-oil (MBO, 135 cP) and a more viscous pristine fast pyrolysis bio-oil (P-FPO, 7780 cP) to obtain reliable corrosion kinetic parameters. Established best practices, including open circuit potential (OCP) stabilization, low-amplitude perturbations, and electrochemical impedance spectroscopy (EIS) conductedwithin a charge-transfer-focused frequency window were implemented using a four-electrode cell. Experiments were performed in MBO under three aging systems: Same Solution–Same Electrode (SSSE), Same Solution–
Different Electrode (SSDE), and Different Solution–Different Electrode (DSDE). EIS analysis with a physically motivated equivalent circuit resolved the solution resistance (R_s), charge transfer resistance (R_ct), and EIS-derived polarization resistance (R_p(EIS)), thereby decoupling the contributions of interfacial film growth and bulk-solution aging to the measured resistances. Uncompensated linear polarization resistance (LPR) slopes initially overestimated R_p(LPR) due to the contribution of R_s; after ohmic drop (iR) correction using EIS-derived R_s, R_p(LPR) converged with R_p(EIS), enabling rapid yet quantitative corrosion screening. Cross-comparison between MBO and P-FPO systems confirms that a properly wired four-electrode configuration is essential for quantitative corrosion measurements in highly resistive, highly viscous, polymer-rich organic media, particularly when solution resistance is uncertain.