## 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
It is seen that the hydrogen permeation current increases significantly when the temperature rises. On one hand, the increase in temperature facilitates the corrosion of the steel, thereby promoting hydrogen permeation into steel [13]; on the other hand, an increased temperature can increase the H diffusion activation energy, leading to more hydrogen permeation into steel, which may also be a reason for the increased hydrogen permeation current [35; 36]. In addition, it is found that the hydrogen permeation current is generally low, when it is rainy. The apparent decrease in hydrogen permeation current during rainfall can be attributed to both the loss of pollutants deposited on the steel surface and the decrease in temperature. It is well known that pollutants such as Cl\({}^{-}\) and SO\({}_{2}\) have an accelerated effect on the corrosion of steel, so that the corrosion rate of steel decreases as pollutants are washed off from steel surface when it rains [37]. Meanwhile, it can be seen in later discussions that the hydrogen permeation current has a positive relationship with the corrosion rate due to the hydrolysis of the corrosion products, hence a lower corrosion rate corresponds to a lower hydrogen permeation current. Accordingly, for steels being exposed to the marine atmospheric zone, pollutions deposition and salts loading which are related to the humidity and weather conditions also play an important role in hydrogen permeation behavior. Figure 8 shows transients of the hydrogen permeation current density of the sensor exposed to the splash zone, and changes in temperature and tide. Taking the position of the pressure sensor in the full immersion zone as the reference zero point, the specific position of the sensor in the splash zone is as shown in Figure 8b. A large hydrogen permeation current density was observed, with the value approximately 10 times that of the current density for the sensor exposed to the marine atmospheric zone (Figures 7a and 8a). It is well known that steel exposed to the splash zone is prone to endure more severe corrosion [38; 39]. Therefore, a large hydrogen permeation current of the steel exposed to the splash zone is reasonable, according to the positive relationship between corrosion of steel and hydrogen permeation. For steel exposed to the splash zone, the wet-dry alternation of the steel surface is mainly controlled by splashing waves. It can be observed that the hydrogen permeation current was relatively high when the surface of the steel was in a wetting stage caused by spindrift, suggesting a corrosion accelerated process in the wetting stage. Furthermore, the promoting effect of a high temperature on hydrogen permeation was more obvious in the splash zone, on the basis of a higher hydrogen permeation current, as shown in Figure 8a. Figure 6: Variations of the background current with environment temperature (**a**), and the relationship between the background current and environment temperature (**b**). the sensor exposed to the tidal zone is shown in Figure 9b. The obvious fluctuations of temperature with tides were observed in Figure 9a, owing to the difference in the specific heat capacity between sea water and air, as well as the evaporation of water. Meanwhile, a current peak was observed in each tidal cycle. It is believed that the appearance of current peak is related to the temperature effect on the one hand and may be related to the difference in the cathodic reactions between drying and wetting process on the other hand (details refer to Section 3.2). Furthermore, it was also found that the hydrogen permeation current of the sensor exposed to the tidal zone was approximately one quarter that of the sensor exposed to the splash zone. This indicates that the corrosion of steel in the tidal zone is relatively light in comparison with that of steel in the splash zone. Figure 10a shows transients of the hydrogen permeation current density of the sensor exposed to the immersion zone and temperature.