## 3 Experimental Results and Discussions
### Measurement Results of the Coupled Potential and the OCP
It can be also found that once stable pitting corrosion is formed on a local area, the current transients will also disappear, suggesting the cathodic current will provide a protective effect for other electrodes and inhibit the potential metastable corrosion process [42]. The fluctuation of the current significantly increased to 10\({}^{-5}\) A during this period. After the introduction of the inhibitor, the maximum anodic current on WE 3 soon decreased to 9 \(\times\) 10\({}^{-7}\) A suggesting the highest localized corrosion rate was 0.01 mm/a. It demonstrates that NO\({}_{2}\)\({}^{-}\) has a remarkable inhibition effect on the localized corrosion of the Figure 3 weldment. The passive layer recovered effectively on WE 3. With the dramatic decrease of the maximum anodic current, WE 5 and WE 6 transferred to anodic areas again. The current transients and current mutations also reappeared which can be seen from Figure 3d, indicating that the breakdown and recovery of the passive film occurred again. Repeat experiments were conducted to verify the experimental conclusion, and the results of the repeat experiments are shown in Figures S1 and S2 in the Supplementary Materials. The anodic and cathodic electric charges of the eight electrodes are shown in Figure 5. It can be seen that WE 3 was the pure anode and the WE 8 is the pure cathode through the whole test. The anodic electric charge of WE 1, WE 2, and WE 7 were quite low which can be neglected. In addition to the WE 3, WE 5 and WE 6 performed as the second major anodic electrodes. The corrosion depth \(d\) of each electrode can be expressed as:  
\[d=Q_{A}\mathrm{M}/A\rho\mathrm{r}\mathrm{F} \tag{2}\]  
where \(Q_{A}\) is the anodic charges. The largest corrosion depth was 2.32 \(\upmu\)m on the WE 3. The corrosion depths of the other electrodes were relatively small. Through the potential and galvanic current measurement results, it can be found that the passive films on the WM zone and HAZ 1 were preferred to be damaged by the aggressive ions in carbonate pore solution. It suggests a high corrosion risk of weldment in marine structures. The OCP of the HAZ 1 and WM zone was nearly 80 mV lower than the BM area in the initial period. It means the HAZ 1 and WM zone will act as an anode in the complete welded structure. The fragile passive film and the weak anodic current will firstly lead to the dissolution of the passive film in these areas. When a stable pit forms on these areas, the aggressive ions will soon come into the occluded pit sites leading to the continued breakdown of the passive film [34]. Consequently, a large potential difference will be generated between the BM area and the WM area or HAZ 1. The large galvanic current will further cause the propagation of the pitting corrosion and lead to a high localized penetration rate. In actual marine structures, the WM zone and HAZ were much smaller than the BM area. The preferential corrosion behavior of these areas will bring high failure risks to the ocean structures. Therefore, the weldment should be intensively monitored to avoid potential failures. The common inhibitor NaNO\({}_{2}\) had an effective inhibition effect for corrosion of weldments in the carbonated pore solution. The localized penetration rate decreased nearly 80 times with the introduction of the inhibitor. However, the corrosion rate of the HAZ 1 remained high. In consideration of the more serious galvanic corrosion risk and high chloride ion concentration in the actual marine environment, the localized corrosion rate may be higher than 0.01 mm/a with NO\({}_{2}\)\({}^{-}\) injected into concrete. Thus, more effective inhibitors should be developed for the inhibition of localized corrosion that occurred in weldments in concrete structures.