## 3 Experimental Results and Discussions
### The Morphologies of the Working Electrodes and the Corrosion Coupon  
After removing the loose rust layers on the electrodes, the surface morphologies of the working electrodes were observed and shown from Figure 6a-h. It can be seen from the surfaces of WE 1, WE 2, WE 7, and WE 8 which had minor anodic electric charges were smooth and the scratches can be seen. However, a few tiny pits can still be seen on the electrode surfaces, indicating slight localized corrosion happened on these electrodes. Serious localized corrosion can be found on WE 3 to WE 6. The WE 3 suffered the most severe corrosion with a large localized corrosion area on its surface. Obvious pitting corrosion can be found on the surfaces of WE 5 and WE 6. As the anodic electric charge of WE 4 was relatively small, the localized corrosion level on WE 4 was lower than those on WE 5 and WE 6. The surface morphologies of the electrodes clearly show that the welding metal and heat-affected areas will suffer more serious localized corrosion in a welded structure. The observed results are in accordance with the measurement results obtained by the coupled multi-electrode array sensor.  
Figure 5: Anodic and cathodic electric charges of the electrodes.  
Once the passive film on the steel surface is broken, a stable pit will soon form and a large quantity of Fe\({}^{2+}\) will enrich the pitting corrosion area. Meanwhile, with the enrichment of the Fe\({}^{2+}\), a mass of Cl\({}^{-}\) will also migrate into the pit. It can be seen from Figure 7a that a green rust layer appeared on the steel surface, suggesting the formation of FeCl\({}_{2}\) 4H\({}_{2}\)O [43] according to Equation (1):  
\[\text{Fe}^{2+}+2\text{Cl}^{-}+4\text{H}_{2}\text{O}\rightarrow\text{FeCl}_{2} \text{.4H}_{2}\text{O} \tag{3}\]  
Because \(\beta\)-FeOOH always appears in the corrosion product, the rust layer showed a dark green color. As shown in Figure 7a, an obvious pit can be seen on the corrosion coupon after a longer immersion test. The whole surface morphology as well as the edge and inner morphologies of the pit can be seen from Figure 7b-d. It can be seen that the pitting corrosion area was located in the WM zone and the HAZ bordering the  
Figure 6: Surface morphologies of the electrodes taken 50 h after the inhibitor was added, (**a**) the surface of WE 1 (**b**) WE 2 (**c**) WE 3 (**d**) WE 4 (**e**) WE 5 (**f**) WE 6 (**g**) WE 7 and (**h**) WE 8.  
Figure 7: The photo of the corrosion coupon after immersed in the test solution for 144 h with and without the rust layer (**a**), the whole surface morphology of the pitting corrosion area (**b**), the edge of the pitting corrosion area (**c**), and the inner surface of the pitting corrosion area (**d**).  
WM area. The pitting corrosion area was more serious and concentrated compared with the multi-electrode sensor system. The immersion test result was in accordance with the measurement results obtained by the sensor system that the breakdown of the passive film firstly occurs at the WM zone and HAZ 1. Except for the pitting corrosion area, nearly no obvious corrosion happened on the other areas of the weldment. It further confirms that once a stable pit is forming on the weldment, the galvanic current can provide protection to the other areas and accelerate the localized corrosion at the pitting area.