## 3 Results and Discussion
### Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments
#### 3.1.2 The Relationship between Hydrogen Permeation and Corrosion Loss of Steel  
The residual thickness of the side wall of the sensor was measured by using an optical microscope so as to obtain its thickness loss data, according to the method introduced in the published study [50]. The cross-sectional morphologies of the sensors endured corrosion exposure are shown in Figure S2 (refer to supplementary file). The widest and narrowest sites of corrosion residual thickness in each field of vision were selected for measurement, and the procedures were repeated for 72 times at different sites on the cross-section, as shown in Figure 12. After multiple measurements, the average corrosion thickness loss of the sensors in the marine atmospheric, splash, tidal and immersion zones are 54 \(\upmu\)m, 245 \(\upmu\)m, 127 \(\upmu\)m and 230 \(\upmu\)m, respectively. This result suggests that sensors in the splash and immersion zones have a higher corrosion rate in comparison with that of sensors in the marine atmospheric and tidal zones.  
Hydrogen permeation behavior into the steel during exposure period was continuously monitored using electrochemical hydrogen permeation technique, as displayed in Figures 7-10. Based on the hydrogen permeation data continuously collected for 3 months,  
Figure 12: Residual thickness measurement on the cross-section of the sensors after being exposed for 3 months in the marine atmospheric zone (**a**), the splash zone (**b**), the tidal zone (**c**), and the immersion zone (**d**), respectively.  
the average hydrogen permeation current density (\(j_{AH}\)) and the sub-surface hydrogen concentration \(C_{0}\) were calculated according to Equations (2)-(4):  
\[Q_{H}=\int\lvert i_{H}(t)\rvert\,\mathrm{d}t \tag{2}\]  
\[j_{AH}=\frac{Q_{H}}{At} \tag{3}\]  
\[C_{0}=\frac{L\cdot i_{AH}}{n\cdot F\cdot D_{eff}} \tag{4}\]  
where \(Q_{H}\) represents the quantity of hydrogen permeation charge, \(A\) is the total working area of the hydrogen permeation sensor, \(t\) is the corresponding test duration, \(L\) is the thickness of the steel specimen, \(F\) represents [PERSON]'s constant with a value of 96,485 C/mol, \(n\) is number of transferred electrons in the hydrogen reduction reaction. Table 2 shows results of the \(j_{AH}\) and \(C_{0}\) of steel in different marine corrosion zones. It can be found that the hydrogen permeation into steel in the splash zone is the most severe, while that of the steel in the atmospheric zone is the least. Meanwhile, the amount of the hydrogen permeated into the steel in the immersion zone is about 1.5 times that the hydrogen permeated into the steel in the tidal zone.  
Figure 13 shows the relationship between the time-averaged hydrogen permeation current density (i.e., \(j_{AH}\)) and the time-averaged corrosion thickness loss rate (\(r\)) of steels exposed to different marine corrosion zones. In Figure 13, it can be found that the average hydrogen permeation current density increases with the increase in corrosion thickness loss of steels. This suggests that hydrogen permeation into steel should be enhanced when the corrosion of steel is accelerated in the marine environment, which applies for all marine corrosion zones. The mechanisms of hydrogen embrittlement were summarized as internal pressure autocatalytic, Vacancy-Agglomeration and Lattice decohesion [51]. The hydrogen source is either internal or external. The low internal hydrogen concentration can be achieved by quality control of production. The hydrogen concentration dependence on corrosion rate indicates that the corrosion protection is an important way for hydrogen entry inhibition into steel. Earlier work showed that petrolatum tape cover was effective in inhibiting hydrogen entry under simulated marine splash conditions [52]. The validity of this technique for the hydrogen entry inhibition in other marine corrosion zones will be confirmed in future works.