## 3 Results and Discussion
### Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments
#### 3.1.3 Corrosion Products Analysis and Its Effect on Hydrogen Permeation  
Different corrosive environments between marine corrosion zones may not only lead to differences in the corrosion rates of steel, but also differences in the composition of corrosion products. Figure 14 shows the XRD spectra of the corrosion products formed on the surface of sensors exposed to different marine corrosion zones. It can be seen that the corrosion products were mainly iron oxides, consisting of goethite (\(\alpha\)-FeOOH), lepidocrocite (\(\gamma\)-FeOOH), akaganeite (\(\beta\)-FeOOH) and magnetite (Fe\({}_{3}\)O\({}_{4}\)). No sign of FeS was found in the XRD pattern of the corrosion products formed in the immersion zone, owing to the overlap of some peaks and less content. The XRD pattern shows a high  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Exposure Site** & \(j_{AH}\)/nA\(\cdot\)cm\({}^{-2}\) & \(C_{0}\)/mol\(\cdot\)cm\({}^{-3}\) & \(H_{C}\)/wt ppm \\ \hline Atmospheric zone & 19.6 & 2.46 \(\times\) 10\({}^{-8}\) & 3.15 \(\times\) 10\({}^{-3}\) \\ Splash zone & 435.3 & 5.46 \(\times\) 10\({}^{-7}\) & 7.00 \(\times\) 10\({}^{-2}\) \\ Tidal zone & 128.1 & 1.61 \(\times\) 10\({}^{-7}\) & 2.06 \(\times\) 10\({}^{-2}\) \\ Immersion zone & 206.9 & 2.60 \(\times\) 10\({}^{-7}\) & 3.33 \(\times\) 10\({}^{-2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: The parameters for hydrogen permeation of AISI 4135 steel in different marine corrosion zones, including the average hydrogen permeation current density (\(j_{AH}\)), sub-surface hydrogen concentration (\(C_{0}\)), and diffusible hydrogen content (\(H_{C}\)).  
proportion of \(\alpha\)-FeOOH in the corrosion products formed in the marine atmospheric zone. Previous studies have shown that a corrosion environment with a low content of chloride ions can facilitate the formation of \(\alpha\)-FeOOH in the corrosion products, thus leading to a denser corrosion products layer. This dense corrosion products layer can largely hinder the penetration of O\({}_{2}\) and Cl\({}^{-}\) to steel surface, thereby suppressing electrochemical corrosion and decreasing hydrogen permeation ultimately (Figure 7a) [53; 54]. On the contrary, an environment with a high Cl\({}^{-}\) content favors the formation of \(\beta\)-FeOOH [55; 56]\(\beta\)-FeOOH can alter the structure of corrosion products layer such as facilitating the formation of cracks and pores, thereby promoting corrosion and hydrogen permeation into steel via providing extra channels for the penetration of corrosive substances [57]. Meanwhile, published studies have shown that the corrosion products layer formed on steel surface can suppress the anodic reaction of iron dissolution and simultaneously promote the hydrogen evolution reaction at the cathode, thus enhancing hydrogen permeation [58; 59; 60]. Furthermore, as mentioned above, corrosion product-FeS (as shown in Figure 11b) can simultaneously act as both \"catalyst\" and \"poisoning agent\" during hydrogen permeation process. Accordingly, the category of corrosion products can, in turn, affect the corrosion and hydrogen permeation behaviors of steel in a direct or indirect way.