Rapid DEIS measurements also allowed to directly monitor the effectiveness of the permanganate inhibitor depending on its concentration in the electrolyte. To evaluate the protective ability depending on its concentration, a peristaltic micropump purchased from Lead Fluid was used to gradually increase the concentration of the inhibitor in the electrolyte. In the course of the experiment, for the first 10 min DEIS spectra were recorded in the reference solutions without the inhibitor (pre-conditioning). Subsequently, permanganate inhibitor in the form of concentrated solution (600 мМ KMnO4 in 0.05 М NaCl) was added to the system according to the concentration profile shown in Fig. in the Supplementary information. The experiment was designed in a way that the final concentration of the inhibitor was the same as in the ease of classical EIS measurements (150 mM). The initial and the final volumes of the electrolyte are 45 and 60 mL, respectively. At the end of the experiment, DEIS spectra were recorded for 10 min in the solution containing 150 mM of the inhibitor. The total duration of the experiment was 70 min. The surface area in DEIS experiments was 0.6 cm 2 and the data was recalculated to 1.0 cm 2 during the spectra analysis. All spectra were analyzed using the Nelder-Mead algorithm . More details on the experimental setup and the measurement conditions are reported elsewhere .