## 3 Results and Discussion
### Hydrogen Permeation into Steel under the Simulated Wet-Dry Cycle Condition
Meanwhile, the acidity of the electrolyte is further enhanced with the evaporation of water during exposure, the pH value of which can be even lower than 3 with the presence of chloride ions [11; 67]. \[\mathrm{Fe}^{3+}+\mathrm{Cl}^{-}+\mathrm{OH}^{-}\rightarrow\mathrm{Fe}\mathrm{ CoCl}+\mathrm{H}^{+} \tag{9}\]  
\[\mathrm{Fe}\mathrm{Cl}^{+}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{FeOH}^{+ }+\mathrm{H}^{+}+\mathrm{Cl}^{-} \tag{10}\]  
Based on Nernst equation, the equilibrium potential of the hydrogen evolution reaction at 25 \({}^{\circ}\)C can be expressed as:  
\[E_{\mathrm{H}_{2}/\mathrm{H}^{+}}=-0.0591 pH \tag{11}\]  
The equilibrium potential of the hydrogen evolution reaction in natural seawater (pH = 7.5) is about \(-0.684\) V\({}_{\mathrm{SCE}}\), whereas it is only \(-0.177\) V\({}_{\mathrm{SCE}}\) when the pH of the electrolyte under the rust layer drops to 3 during the drying stage. It is seen from Figure 15c that the value of the OCP of steel is about \(-0.66\) V\({}_{\mathrm{SCE}}\) during the drying stage. Therefore, a lower OCP and pH during the middle and late drying stage can enhance the activity of the hydrogen evolution reaction, thereby resulting in a higher hydrogen permeation current, as shown in Figure 15a. However, the pH of the electrolyte beneath the rust layer is still high in the initial drying stage, which brings difficulties to the hydrogen evolution owing to a higher overpotential, thus leading to the persistent decrease in the hydrogen permeation current. Accordingly, cathodic reactions (Equations (5)-(8)) can actively proceed in the drying stage during the tidal cycles, although the contribution of the hydrogen reduction reaction to corrosion weight loss of steels may be only about 0.1% [11]. In contrast, the pH of the electrolyte beneath the rust layer is basically the same as that of sea water owing to the good ion transport between sea water and the electrolyte beneath the rust layer. Therefore, hydrogen evolution under immersion condition is more difficult to proceed in the case where the OCP of the steel is almost equal during the drying and wetting stages. This reasonably explains the continuous decrease in the hydrogen permeation current in the middle and the last wetting stages. Meanwhile, the supply of oxygen to the surface of the rusty steel is insufficient in the wetting stage. This can be attributed to two aspects. On one hand, the steel surface left in contact with electrolyte by the rust layer is very small, limiting substances exchange. On the other hand, tortuous pores of nanometric width in the thick rust layer add more difficulty to the diffusion of oxygen. Therefore, the corrosion of the steel is relatively mild in the wetting stage, and the corrosion process is controlled by the diffusion of dissolved oxygen [68]. Therefore, the lower corrosion rate of the steel is also another reason for the lower hydrogen permeation current in the wetting stage. Meanwhile, it is also due to the temporary inhibition of the rust layer on the diffusion of hydrogen ions, which causes the brief rise in the hydrogen permeation current at the initial wetting stage, as shown in Figure 14:0