# Time-Dependency in the Corrosion Process of Reinforced Concrete Subjected to a Chloride Solution
## 4 Results and Discussion
### Responses for Elements in Equivalent Electrical Circuit  
The electrical impedance of RC is roughly divided into two parts: the steel and the concrete responses. In addition, each part can be separated into subdivisions, such as interface, presence of passive film, sponge effect, and pitting corrosion process; however, response interpretation is not straightforward. The corrosion of steel induced by chloride in concrete is described by a series of processes, including chloride transport, depassivation, and corrosion propagation; thus, different impedance responses for steel and concrete are shown at each stage. Based on these differences, the practicability of electrically equivalent circuits was suggested. Unlike concrete, the steel embedded in concrete cannot be easily controlled under experimental conditions. Therefore, the impedance response of the steel was analysed through monitoring rather than through one-off experiments with controlled variables. To minimise errors caused by unexpected variables [34], such as the conditions of the sponge, location of the counter electrode, and contact pressure, EIS data were analysed for the samples using an internal counter electrode (i.e., steel embedded in the concrete sample). Figure 8 shows examples of the impedance measurements for the active and passive states. Two parts (i.e., the steel and the concrete responses) in both figures were confirmed. For the concrete response, the cusps represent the ohmic resistance. However, a difference in the steel response between the active and passive states was observed. The impedance shapes of the passive and active states differed. For the active state, semi-circular shapes were formed in the steel response (i.e., from a 1 kHz frequency), whereas the steel response for the passive state was almost a straight line. To clarify the steel response, the reference frequency was set to 1 kHz to roughly divide it into two parts (i.e., the concrete and the steel responses) because the impedance at this frequency was closer to the real axis. This frequency was also sugg  
Figure 8: Examples of electrochemical impedance spectrum of steel in (**a**) passive state and (**b**) active state. Figure 7: Time constant of steel with respect to corrosion potential. limit for estimating the concrete resistance [35]. Figure 9 shows the impedance spectra, which were redrawn with the origin of impedance at 1 kHz to describe the impedance response of steel with respect to time, based on the data in Figure 8. The results confirmed that the impedance response was time dependent. Thus, the slopes for the passive state and radii of the semicircles for the active state decreased with exposure time. The reduced slope indicates the depassivation process induced by the chloride at the steel depth, while the straight line appears to be a part of a large semicircle. The expected semicircle was formed after depassivation. Two hypotheses were formulated from the results: (1) the depassivation process is relatively slow compared with the corrosion propagation; thus, the change in slope is marginal; (2) the impedance response in steel contains at least two phases, corresponding to the response of the steel itself and that of the corrosion products. Performing an impedance spectroscopy analysis using an electrical equivalent circuit is crucial. Considering a series of simplified Randle circuits, the former corresponds to one Randle circuit for the steel response in the circuit. For the latter, the electrical equivalent circuit is formed with a minimum of two series of simplified Randle circuits for the steel response. Determining the more accurate circuit based solely on the data is difficult owing to this invisible phenomenon. This study fitted the data with a circuit. The results confirmed that the latter had better fit values. Notably, the fitting accuracy always improved when additional circuits were added. In addition, the value of the element varied with the arrangement of elements in an equivalent electrical circuit.