# Time-Dependency in the Corrosion Process of Reinforced Concrete Subjected to a Chloride Solution
## 4 Results and Discussion
### Electrical Response for Time-Dependency  
The analysis of the GP and EIS techniques is based on an electrical circuit, and these results are comparable. Recently, values represented in the time domain using the GP technique have been transformed into values in the frequency domain [17]. The required parameters for calculating the corrosion rate of steel in concrete are the ohmic and polarisation resistances. The corrosion rate is generally underestimated when the uncompensated polarisation resistance is used to calculate the corrosion rate. Accurate estimation of the ohmic and polarisation resistances is beneficial to engineers. Extracting the values corresponding to the parameters from the EIS measurements is challenging because the electrical equivalent circuit in EIS is more complicated than that in the GP technique. In this study, a simple circuit arrangement (four parallel R-CPE combinations) was employed, roughly comprising the steel response and ohmic drop, to extract the parameters for comparison. In the circuit, the first two R-CPE combinations were considered as ohmic drops, and the other RC combinations were considered as steel responses. The values of the circuit elements corresponding to each part were added and compared with the values obtained from fitting in the GP method. In addition, the corrosion rate and time to the onset of corrosion were determined. In this study, the ohmic drop accounted for the interfacial effect and bulk concrete resistance. The interfacial effect, which indicated that a layer was formed between the  
Figure 10: (**a**) Proposed electrical equivalent circuit and (**b**) fitting examples from passive state to active state. concrete and electrode, was unavoidable owing to the electrode configuration in the polarisation technique. Confirming whether the ohmic resistance was pure concrete resistance was necessary; however, the capacitance was excluded in the comparison because the ohmic resistance in the GP technique was considered only with a resistor. Figure 11 shows the ohmic resistances of the samples. The exposure area (75.40 cm\({}^{2}\)) was considered for calculating the corrosion rate for the EIS measurement and for data for the GP technique. The ohmic resistances of both samples were similar, irrespective of the electrochemical measurements. This discrepancy was approximately 10%. These results confirmed that the ohmic resistances could differ depending on the electrical measurements, leading to errors in estimating the corrosion rate. In addition to the ohmic resistance, the resistance of steel based on EIS, that is, the polarisation resistance, is compared with that in the GP technique in Figure 12. The comparison was performed only for the 0.60 PC concrete because the resistances corresponding to the charge transfer resistance in the circuit were not measurable by fitting in the passive state. The polarisation resistance initially decreased with time. In the passive state or at the early stage of corrosion, the EIS measurements were more than 10 times higher than those from the GP technique, and the gap between both values decreased significantly as the corrosion progressed. Figure 11: Comparison of ohmic resistances depending on electrical measurements. Figure 12: Comparison of polarisation resistances depending on electrical measurements. Considering that the diffusion process in the GP technique was avoided with a short application time, and unmeasurable resistance in the passive state for EIS was observed, the impedance by the diffusion process appeared to be restricted within the low-frequency range. However, a low-frequency tail representing the diffusion process was not confirmed or may have overlapped. This impedance was not always observed in previous studies [39; 40]. Therefore, employing a Warburg impedance in the electrical equivalent circuit is difficult because an additional fitting error is involved when an uncertain value is employed.