## 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 can be seen that the hydrogen permeation current of the sensor exposed to the immersion zone was much larger than that of the sensor exposed to the tidal zone. The relatively large hydrogen permeation current in the immersion zone may be caused by the stronger reproduction and metabolic activity of sulfate-reducing bacteria (SRB) on the surface of the steel in the immersion zone. It can be observed that part of the surface of the hydrogen permeation sensor in the immersion zone was covered by a layer of gray-black sediments, both the sediments and relatively low concentration of dissolved oxygen in the immersion zone providing favorable living conditions for SRB. Raman spectroscopy shown in Figure 11b confirmed that the presence of ferrous sulfide (FeS) in the corrosion products formed in the immersion zone, with the characteristic peak at 284 cm\({}^{-1}\)[40]. The formation of FeS is related to the activities of SRB, and published study showed that SRB were tend to be present in the inner black stratum of the corrosion products layer [41]. Meanwhile, sulfate green rust [GR(SO\({}_{4}\)\({}^{-}\))], akaganite (\(\beta\)-FeOOH) and magnetite (Fe\({}_{3}\)O\({}_{4}\)) were also detected in the corrosion products, with the characteristic peaks at 217 cm\({}^{-1}\), 385 cm\({}^{-1}\) and 638 cm\({}^{-1}\), respectively [42; 43]. However, no  
Figure 9: Time variations of the hydrogen permeation current density of the sensor exposed to the tidal zone and the corresponding temperature (**a**), as well as tidal changes (**b**) from 4 August to 9 August during outdoor hydrogen permeation test. FeS was found in the corrosion products formed in the tidal zone, only \(\beta\)-FeOOH, Fe\({}_{3}\)O\({}_{4}\) and lepidocrocite (\(\gamma\)-FeOOH) were detected, with the characteristic peaks at 312 cm\({}^{-1}\), 540 cm\({}^{-1}\) and 706 cm\({}^{-1}\), respectively (Figure 11a) [43]. It is considered that the sulfide products of SRB metabolic activity (\(\mathrm{S^{2-}}\), \(\mathrm{HS^{-}}\) and \(\mathrm{H_{2}S}\)) can act as a \"catalyst\" in facilitating hydrogen ions reduction and a \"poisoning agent\" in inhibiting hydrogen recombination, respectively, thus promoting hydrogen permeation into steels [44; 45; 46]. Many studies have reported the promoting effect of SRBs on hydrogen  
Figure 11: Raman spectra of the corrosion products formed on the surface of hydrogen permeation sensors exposed to the tidal zone (**a**) and the immersion zone (**b**). Figure 10: Time variations of the hydrogen permeation current density of the sensor exposed to the immersion zone and the corresponding temperature (**a**), as well as tidal changes (**b**), from 4 August to 9 August during outdoor hydrogen permeation test. permeation [47; 48]. [PERSON] et al. [49] found that the hydrogen permeation current of steel in sea mud containing SRB was about three times to that in sterilized sea mud in the hydrogen permeation investigation of API X56 steel in sea mud. Furthermore, due to a large specific heat capacity of sea water, it was observed that changes in temperature of sea water in the immersion zone were very small and the maximum fluctuation range was about 2 \({}^{\circ}\)C. However, similarly, based on the background of a large hydrogen permeation current, the fluctuations of hydrogen permeation current caused by small changes in temperature can be obviously observed from Figure 10a.