## 3 Experimental Results and Discussions  
### Measurement Results of the Coupled Potential and the OCP  
After the electrodes were transferred to the test solution for 3 h, the OCP of the electrodes became stable. The switches were kept closed and the sensor system began to work. The OCP measurement results of the electrodes at different test periods are shown in Table 2 and the coupled potential measurement result is shown in Figure 3a. It can be seen the coupled potential was nearly \(-\)220 mV at the initial 28 h. A sudden decrease of the coupled potential from \(-\)220 mV to \(-\)650 mV can be seen from the curve. After the inhibitor NaNO\({}_{2}\) was introduced into the solution at 60 h, the coupled potential recovered to \(-\)250 mV and gradually decreased to \(-\)280 mV. It is observed that the OCP of the electrodes were all above \(-\)280 mV at the initiation of the test in Table 2. According to ASTM standard and the previous studies [29; 40], the corrosion probability of carbon steel rebar in a reinforced concrete structure is 90% if the corrosion potential of the rebar is below \(-\)280 mV vs. SCE. Therefore, the OCP of the electrodes at the initiation of the test means a low corrosion probability. After the electrodes were coupled for 24 h, the OCP of the WE 1 and WE 8 both had a little rise to \(-\)173 mV and \(-\)169 mV indicating well protective films form on the BM areas. The OCP of the WE 2 and WE 7 nearly had no changes compared to the initiation period showing a low corrosion risk. However, the OCP of the WE 3 to WE 6 both had different degrees decreases after were coupled together. Especially for WE 3, its OCP had a significant decrease to \(-\)327 mV suggesting a high localized corrosion potential. Detailed information of the coupled potential at the initial 28 h is shown in Figure 3b. It is seen that, although the coupled potential was kept at \(-\)220 mV, frequent negative potential transients can be observed. The fluctuation of the coupled potential was nearly 10 mV in the initial 28 h. This signal may correspond to the metastable corrosion process on the electrodes. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Time** & \multicolumn{6}{c}{**The OCP of the Electrodes (mV)**} \\ \cline{2-9}
**(h)** & **WE 1** & **WE 2** & **WE 3** & **WE 4** & **WE 5** & **WE 6** & **WE 7** & **WE 8** \\ \hline
0 & \(-\)180 & \(-\)210 & \(-\)275 & \(-\)219 & \(-\)247 & \(-\)240 & \(-\)224 & \(-\)185 \\
24 & \(-\)173 & \(-\)207 & \(-\)327 & \(-\)221 & \(-\)263 & \(-\)253 & \(-\)237 & \(-\)169 \\
48 & \(-\)158 & \(-\)237 & \(-\)697 & \(-\)283 & \(-\)310 & \(-\)279 & \(-\)258 & \(-\)166 \\
72 & \(-\)150 & \(-\)200 & \(-\)289 & \(-\)190 & \(-\)224 & \(-\)240 & \(-\)175 & \(-\)148 \\
96 & \(-\)161 & \(-\)235 & \(-\)313 & \(-\)232 & \(-\)247 & \(-\)259 & \(-\)203 & \(-\)157 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The OCP of the electrodes at different times. As shown in Table 2, the OCP of WE 3 decreased to \(-697\) mV after the electrodes were coupled for 48 h suggesting stable localized corrosion occurred on the HAZ 1. The OCP of the WE 1 and WE 8 were still higher than \(-180\) mV. It demonstrates the passive film on the BM zone was intact. The OCP of the other electrodes all had significant decreases during this period. The OCP of the WE 4, WE 5, and WE 6 reached \(-283\) mV, \(-310\) mV, and \(-279\) mV, respectively. It suggests localized corrosion may also happen on the HAZ 1 and WM zone. It can be seen from Figure 3c that the potential transients disappeared except for a sudden positive potential change. After the introduction of the inhibitor, the coupled potential recovered to \(-250\) mV soon. The OCP of the electrodes also had dramatic increases which can be observed from Figure 3a. The OCP of WE 3 recovered to \(-289\) mV indicating the repair of the passive film. The OCP of the other electrodes were all higher than \(-250\) mV at \(72\) h which the corrosion process of the weldment was effectively inhibited by the NO\({}_{2}\)\({}^{-}\).