## 3 Results and Discussion
### Hydrogen Permeation into Steel under the Simulated Wet-Dry Cycle Condition  
It is observed from Figure 9 that the hydrogen permeation current does not vary monotonically during both the wetting and drying stages. To further verify the hydrogen permeation behavior of steels in the wet-dry cycle condition and investigate its mechanism, hydrogen permeation test was performed under the simulated wet-dry cycle condition. Figure 15a,b show the variations of the hydrogen permeation current density and open circuit potential of the sensor in laboratory simulated tidal cycling environment, respectively. Figure 15c shows changes in the indicator potential, indicating which state the hydrogen permeation sensor is in (wetting state or drying state). When the indicator potential shows 0 mV, it indicates that the sensor is in the drying state, otherwise the sensor is in the wetting state. It is found that changes in the hydrogen permeation current are nonmonotonic in the drying or wetting stages (Figure 15a). During the drying stage, the hydrogen permeation current firstly decreased and then increased, while during the wetting stage, it showed the  
Figure 13: Relationship between the time-averaged hydrogen permeation current density (\(j_{alt}\)) and the time-averaged corrosion thickness loss rate (\(r\)) of the hydrogen permeation sensors in the entire duration of the outdoor hydrogen permeation test. opposite characterization. In addition, it is obvious that the overall hydrogen permeation current in the wetting stage is higher than that during the drying stage, which is consistent with the outdoor experiment results (Figure 9). Figure 14: XRD spectra of the corrosion products formed on the surface of sensors exposed to different marine corrosion zones. Figure 15: Variations of the hydrogen permeation current density (**a**) and open circuit potential (**b**) of the sensor, and indicator potential (**c**) under the simulated wet–dry cycle condition. Under the wet-dry cycle condition, the anodic dissolution of iron during the electrochemical corrosion process can be balanced by the hydrogen reduction reaction Equation (5), the dissolved oxygen reduction reaction Equation (6) and the reduction reactions of corrosion products represented by Equations (7) and (8) [61; 62; 63]:  
\[\mathrm{H}^{+}+\mathrm{e}^{-}\rightarrow\mathrm{H}_{\mathrm{ad}} \tag{5}\]  
\[\mathrm{O}_{2}+2\mathrm{H}_{2}\mathrm{O}+4\mathrm{e}^{-}\rightarrow 4\mathrm{OH}^{-} \tag{6}\]  
\[\mathrm{\gamma}\text{-FeOOH}+\mathrm{H}^{+}+\mathrm{e}^{-}\rightarrow\mathrm{ \gamma}\text{-FeOOH} \tag{7}\]  
\[\mathrm{3\gamma}\text{-FeOOH}+\mathrm{H}^{+}+\mathrm{e}^{-}\rightarrow\mathrm{ Fe}_{3}\mathrm{O}_{4}+2\mathrm{H}_{2}\mathrm{O} \tag{8}\]  
Under the wet-dry cycle condition, the dominant cathodic reactions of steel corrosion in different states possess some differences. [PERSON] [64] and [PERSON] et al. [65; 66] proposed that the corrosion process of steel in the simulated atmospheric corrosion environment can be divided into three stages: wetting of the dry surface, wet surface and drying-out of the surface. Meanwhile, they found that the cathodic reduction reactions of ferric species within the corrosion products were significantly enhanced when the oxygen reduction reaction was weakened in the wetting stage, whereas the oxygen reduction reaction was dominant in the wet stage. During tidal cycles, the oxygen is prone to diffuse through a thin electrolyte film to steel surface when it is exposed to atmosphere. In this case, the oxygen reduction reaction (Equation (6)) and ferric species reduction reactions (Equations (7) and (8)) are dominant in cathodic reactions. However, this does not mean that hydrogen reduction reaction (Equation (5)) is inactive in this stage, in fact the opposite is true, because the hydrolysis of corrosion products can result in acidification of the electrolyte beneath the rust layer (Equations (9) and (10)).