## 4 Results and Discussion
### Influences on Electrode Positions  
It is well known that in active states, the self-confinement of the electrical field makes the determination of the true corrosion rate difficult [34]. Figure 8 also shows the electrical impedance spectrum for LS at different sizes of the counter electrode stainless plates (35 \(\times\) 100 and 35 \(\times\) 200 mm). Although different sizes of counter electrodes were used, the impedance responses were almost similar, especially in the low-frequency arc, thus accounting for the steel response. For comparison purposes, when using counter electrode sizes to calculate polarization resistance, the corrosion rate associated with the use of a small counter electrode was twice as high as that used for the large counter electrode. The current applied to the working electrode was concentrated on the corrosion spot which had the lowest polarization resistance on the steel [35]. This would lead to a considerable error when the corrosion rate was calculated based on the electrical measurements, especially in pitting corrosion. To overcome this limitation, the empirical values suggested for the estimation of the corrosion rate in pitting corrosion ranged from four to eight [36]. As shown in Figure 8, the small counter electrode (35 \(\times\) 100 mm) was only positioned on bare steel, but the large counter electrode (35 \(\times\) 200 mm) covered the exposed steel and the part areas masked by heat-shrink insulation. Differences in the shapes of the high-frequency arc were observed, but the cusp points were similar for the small electrode (resistance = 203.85 \(\Omega\)) and large electrode (resistance = 199.46 \(\Omega\)) at 1 kHz. Considering the exposed area of steel, the resistance of the large electrode was 50% higher than that of the small electrode. It seems that the estimation of the polarized area is limited in its capacity to determine the true value. Many studies have used different types of counter electrodes for corrosion measurements, as shown in Table 1. Among them, there is a study on the differences between the external and the internal counter electrodes. Provided that the concrete sample is immersed or placed in saturated environments, any types can be used owing to the high electrical connection. However, concrete structures are mostly unsaturated and exposed to the atmosphere from a practical viewpoint. In the field, a small external counter electrode is also preferred owing to the considerations of its portability and practical use; however, an error can be involved attributed to various factors (contact media, electrical connection between the concrete and electrode, and the estimation of a polarized area), as described above. Meanwhile, noble materials are used for the counter electrodes in the lab; this is beneficial as they prevent corrosion in the cases of the premixed chloride or applied current methods, and because they are directly embedded into concrete to secure intimate electrical contact. This method seems to be impractical in the field because electrodes are costly and it is difficult to install embedded electrodes directly. Figure 8: Impedance spectra obtained when counter electrodes are used at different sizes. Figure 9 indicates the impedance spectra at different counter electrode locations. Firstly, in a passive state, the resistance of the high-frequency arc which used the internal counter electrode (resistance = 173.03 \(\Omega\) at 1 kHz) was considerably lower than that which used the external counter electrode (resistance = 258.98 \(\Omega\) at 1 kHz); in an active state, the resistances were also 142.54 \(\Omega\) and 224.21 \(\Omega\) for the internal and external counter electrodes at 1 kHz, respectively. It was shown that the differences of resistances were made depending on the counter electrode locations in both a passive and active state.