## 4 Corrosion Potential Behavior with Test Parameters  
### OCP Evaluation with the Averaged OCP  
The OCP measurement for two years is shown in Figures 7-9 with increasing chloride concentration. Although data fluctuation occurred due to the non-homogeneity of concrete, the OCP value increased in the negative direction with higher chloride concentration. Some OCP values were recovered due to the changes in temperature and local saturation condition. In particular, when corrosion occurs, the OCP value recovers or the corrosion rate decreases during corrosion progress [32]. This is because the corrosion product (rust) generated locally on the surface hinders the further oxidation of steel reinforcement. When long-term measurement is performed, the OCP decreases again in a direction that propagates corrosion. In addition, the dry condition was maintained before exposing the specimens to saltwater for increasing the inflow of oxygen. As the OCP values measured during this period showed large fluctuations due to a reduction in potential, the average values of three samples were used after data filtering. Figure 5: Accelerated corrosive conditions for the corrosion monitoring test. Figure 6: Open-circuit potential (OCP) monitoring stage and equipment. (**a**) OCP monitoring during accelerating conditions; (**b**) OCP measurement equipment. Figure 8: Open-circuit potential (OCP) evaluation under 3.5% chloride exposure conditions. Cover depths: (**a**) 60 mm, (**b**) 45 mm, and (**c**) 30 mm. Figure 7: Open-circuit potential (OCP) evaluation under 0.0% chloride exposure conditions. Cover depths: (**a**) 60 mm, (**b**) 45 mm, and (**c**) 30 mm. The critical corrosion potential of the used electrode was \(-450\) mV [33], but the OCP that meets the value was measured from a cover depth of 30 mm after one year at a chloride concentration of 7.0% regardless of the \(w/c\) ratio. After one year of corrosion monitoring, some specimens were dismantled, and the steel reinforcement in the corrosion-induced part was collected. The rust on the surface was then removed using citric acid. Afterward, the surface condition of the steel was examined using a microscope (DEM-MUT) with 10 times magnification. For the sample that contained no chloride in the aqueous solution, the degree of corrosion was insignificant. In the two cases with high chloride concentrations (3.5% and 7.0%), however, significant pitting-type corrosion was evaluated. Figure 10 shows the surface corrosion condition after one year. Figure 10: Photos of the corroded area with increasing chloride concentration after 1 year: (**a**) 0.0%, (**b**) 3.0%, and (**c**) 7.0% (cover depth: 45 mm, \(w/c\): 60%). Figure 9: Open-circuit potential (OCP) evaluation under 7.0% chloride exposure conditions. Cover depths: (**a**) 60 mm, (**b**) 45 mm, and (**c**) 30 mm. In this study, the progress of pitting corrosion with increasing chloride concentration was only observed, and the corrosion weight and area were not evaluated. Unlike the corrosion pattern due to carbonation, local corrosion pits are caused by chloride penetration and are accelerated in the cracked area [34]. In the previous studies [35,36], the flexural capacity of an RC beam was evaluated considering the pitting corrosion effect. The residual steel area and distribution function of the corrosion pit were modeled. The probability of failure was evaluated with the pit interference effect, which showed significant increases in corrosion probability at the initial uniform corrosion condition. After the 2-year test, the agar material showed a stable form without changing color and swelling. The agar-melting process for 120 min and distilled water for cement mixing with high alkalinity were effective for bacteria growth control. Careful investigation of preventing bio-deterioration from bacteria is required for the long-term test over 2 years and mass production of the agar-based sensor. The reproducibility test after the corrosion test was performed for 150 s with 1 M KNO\({}_{3}\) and a reference of Ag/AgCl.