## 4 Corrosion Potential Behavior with Test Parameters
### Relationship between OCP and the Parameters
#### 4.2.2 Effects of the Cover Depth and \(w/c\) Ratio on the OCP with Increasing Period  
Owing to the periodic infiltration of saltwater, the chloride content increases inside the concrete, and it yields a condition vulnerable to corrosion despite the continuous cement hydration. In Figure 12, the OCP values for two years were classified into four stages to analyze the OCP value with the exposure period. When the \(w/c\) ratio was 0.4, a significant drop in the OCP value began to be observed under 3.5% and 7.0% conditions only after one year. When the \(w/c\) ratio was higher (\(w/c\): 0.6), values below \(-100\) mV were observed  
Figure 11: Averaged open-circuit potential (OCP) with increasing chloride concentration: (**a**) 0.0%, (**b**) 3.5%, and (**c**) 7.0%. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Averaged OCP = a \(\times\) Cover Depth + b**} \\ \cline{2-4}  & **a** & **b** & \(\mathbf{R^{2}}\) \\ \hline WC-0.4 (0.0\%) & 0.3297 & \(-61.669\) & 0.5075 \\ \hline WC-0.5 (0.0\%) & 0.1771 & \(-65.936\) & 0.9926 \\ \hline WC-0.6 (0.0\%) & 0.384 & \(-93.324\) & 0.9377 \\ \hline WC-0.4 (3.5\%) & 1.1218 & \(-204.33\) & 0.4354 \\ \hline WC-0.5 (3.5\%) & 1.5005 & \(-218.96\) & 0.8921 \\ \hline WC-0.6 (3.5\%) & 4.0884 & \(-414.6\) & 0.9099 \\ \hline WC-0.4 (7.0\%) & 1.2439 & \(-208.4\) & 0.6597 \\ \hline WC-0.5 (7.0\%) & 2.5515 & \(-314.69\) & 0.8382 \\ \hline WC-0.6 (7.0\%) & 3.9919 & \(-459.86\) & 0.987 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Gradient analysis of the cover depth effect on the open-circuit potential (OCP). regardless of the cover depth after six months. At cover depths of 30 and 45 mm, the OCP linearly and significantly decreased and reached the critical level of \(-450\) mV. The OCP reduction over time was mainly caused by the inflow of moisture and oxygen under dry and wet conditions. A rapid reduction in the OCP was measured in the saltwater environment that exceeded the seawater condition (7.0%). When each exposure environment is fixed, a relatively high coefficient of determination can be derived by obtaining multiple correlations of time, cover depth, and OCP. Figure 13 shows the OCP correlations for the seawater concentration (3.5%) condition and the severe seawater condition (7.0%). A higher correlation was derived as the chloride concentration  
Figure 12: Open-circuit potential (OCP) drop with increasing exposure period. Water-to-cement (\(w/c\)) ratios: (**a**) 0.4, (**b**) 0.5, and (**c**) 0.6.  
increased, and each result was shown as a contour and a regression equation. A weak correlation was observed under the wet condition (0.0% concentration), but the correlation was improved and the corrosion potential decreased with high chloride concentration. From the wet condition, as the chloride concentration increased to 3.5% and 7.0%, the multiple correlation coefficient increased from 0.627 to 0.634 (0.0%), 0.789 to 0.905 (3.5%), and 0.783 to 0.878 (7.0%), respectively. The coefficient of determination also increased from 0.614 to 0.771 under the 7.0% condition. The regression equations under each condition are summarized in Table 6, and the OCP contour for each condition is shown in Figure 14. It is well reported that the exposure condition is very important as the exterior chloride concentration governs the surface chloride content, and it determines the required cover depth and concrete properties for the intended service life [1; 6; 37]. Initial chloride ions from sea sand or chemical admixtures usually remain constant without the dissociation of bound chlorides due to the pH drop [37; 38]. The surface chloride content increases with the exposure period and remains constant after 20-30 years [39; 40]. The results from the test showed a clear relationship with longer exposure period and higher chloride content, and they were directly compared with averaged OCP, which meant actual corrosion behavior. The linear OCP relation with cover depth can be used for a referential guide for durability design.