## 3 Results and Discussion  
### Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments  
#### 3.1.1 Hydrogen Permeation Behavior of Steels in Different Marine Corrosion Zones  
Figure 5 shows the macro-morphologies of sensors after being exposed for 3 months in outdoor marine environments. It can be seen that the sensors exposed to different corrosion zones endured varying degrees of corrosion. The sensor exposed to the marine atmospheric zone was less corroded in comparison with the sensors exposed to other corrosion zones. Meanwhile, the sensor exposed to the tidal zone was obviously affected by biofouling, and the senor surface was covered with a large number of barnacles (Figure 5c). To date, the effect of macro biofouling on steel corrosion remains controversial. On the one hand, macro-fouling organisms such as oyster, barnacle and filamentous macroalgae may induce crevice corrosion, or accelerate the corrosion process by facilitating the growth of microbial communities, as elaborated in literatures [30; 31; 32]; on the other hand, based on outdoor measurement data, it was found that there was a negative relationship between biomass data and corrosion mass loss, suggesting that macro-fouling played a positive role in reducing mass loss [33; 34]. In addition, due to the scouring action of sea water and prolonged immersion, it was observed that the rust layer formed on the surface of the sensor exposed to the immersion zone was relatively loose, and the surface was covered with silt-like sediments. Given the positive relationship between hydrogen permeation and corrosion of steel, varying degrees of corrosion for steel may imply differences in the amount of hydrogen permeated into the steel. Figure 4: Photos of the hydrogen permeation test set-up under simulated tidal cycling condition in laboratory (**a**), enlarged view of electrolytic cell (**b**). Figure 6a shows the polarization current of the completely sealed sensor exposed to the atmospheric zone, and environment temperature from 4 August to 9 August. It is observed that there is a positive relationship between the background current and environment temperature. Since there is no electrochemical reaction (including corrosion reaction) occurring on the surface of the completely sealed sensor, it is considered that the variation of the polarization current density in Figure 6a is caused by temperature change rather than hydrogen permeation. Accordingly, for a sensor that is not completely sealed, the polarization current contributed by hydrogen permeation (i.e., hydrogen permeation current) can be obtained by subtracting the polarization current totally caused by temperature (i.e., background current). In other words, the hydrogen permeation current can be expressed as Equation (1). \[i_{H}=i_{a}-i_{b} \tag{1}\]  
where \(i_{H}\) is the hydrogen permeation current, \(i_{a}\) and \(i_{b}\) are the total anodic polarization current and background current, respectively. Based on a large amount of polarization current data of the completely sealed sensor (i.e., background current data) and temperature data, the quantitative relationship between the background current and temperature was obtained by fitting, as shown in Figure 6b. Accordingly, changes in the background current can be determined via the variation of environment temperature during tests. Figure 5: Macro-morphologies 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**). Figure 7 shows the hydrogen permeation current of the sensor exposed to the marine atmospheric zone, temperature and relative humidity, as well as weather records, from 4 August to 9 August. The data show that the relative humidity and temperature of the marine atmosphere were relatively high during this period (Figure 7a), which provided favorable conditions for the corrosion of steel in the marine atmospheric zone.