## 4 Results and Discussion
### EIS Analysis  
Various electrical equivalent circuits have been proposed; however, one of the characteristics in these circuits is ambiguous, thus leading to interpretation difficulties. In other words, the measured impedance data can be fitted to not only to one circuit, but to several circuits. A complex circuit, e.g., a transmission line, maybe more accurate in fitting the data. Figure 9: Impedance spectra obtained at different counter electrode locations for active and passive states. Conversely, simplified circuits can easily describe mechanistic behavior, but the accuracy is sometimes low. In this study, one simple electrical circuit proposed by the previous studies was extended instead of introducing new ones. As shown in Figure 10, the electrical circuit was thus used for all impedance spectra, irrespective of the steel condition and counter electrode types. The circuit involved four series which consisted of a parallel R-CPE combination. The physical meanings indicate the properties of bulk concrete representing R\({}_{\text{Conc}}\) and CPE\({}_{\text{Conc}}\), properties of interfacial effects between the concrete and electrode representing R\({}_{\text{IF}}\) and CPE\({}_{\text{IF}}\), properties of corrosion products or a passive film representing R\({}_{\text{CP}}\) and CEP\({}_{\text{CP}}\), and properties of steel representing R\({}_{\text{Steel}}\) and CPE\({}_{\text{Steel}}\), respectively. First, two sets corresponded to an ohmic resistance (high-frequency arc) in the impedance spectra, and the other two sets corresponded to polarization resistance (low-frequency arc). The concrete response (high-frequency arc) was subdivided into the interfacial effect in the frequency \(\sim\)1 kHz to 100 kHz and bulk concrete resistance (\(>\)\(\sim\)100 kHz). The steel responses (low-frequency arc) involved corrosion products or a passive film ranging up to c.a. 1 Hz, and the response regarding the charging transfer ranged from c.a. 1 Hz to a low-frequency limit. Although it was reported that the low-frequency tail attributed to diffusion control was present in chloride-induced corrosion processes [39], the electrical response cannot be extracted directly because 'a straight line' response was only confirmed without a defined semicircle. Note that this electrical circuit cannot be unique, but it is believed that a simplified one can be useful to refine the understanding of the phenomenon because of accessibility. The simulated values for impedance data are tabulated in Table 5 for both passive and active states. To estimate the corrosion rate, the ohmic and polarization resistances are required. For comparison purposes, the simulated values for resistance were multiplied by the exposed area; in the case of the CPE coefficient, they were divided by the exposed area. Figure 11 shows the ohmic resistance variation for different variables in the passive and active states based on considerations of the exposed area. The ohmic resistance of the SS part ranged from 8.7 to 14.5 k\(\Omega\cdot\)cm\({}^{2}\), and was lower than that of the LS part, which ranged from 10.7 to 16.5 k\(\Omega\cdot\)cm\({}^{2}\) except for contact pressure. The high values in the resistance of the LS part may result in the estimation of the polarized area caused by the lengthy working electrode. On the other hand, provided that ohmic resistance measured with internal counter electrodes are reference values (10.7 k\(\Omega\cdot\)cm\({}^{2}\) and 8.7 k\(\Omega\cdot\)cm\({}^{2}\) for active and passive states), the most important consideration is intimate contact between the concrete and electrode electrically. As shown in the results, although the various contact solutions were used for electrical connection between the concrete and the external counter electrode, the ohmic resistance in the cases of external counter electrodes were higher (\(>\)\(\sim\)50%) than that in the cases of internal counter electrodes. It is possible to induce additional errors when contact is not made appropriately with contact pressure.