## 1 Introduction  
With the development of the iron and steel manufacturing industry, high-strength steel has been gradually used in various industries such as civil buildings, energy transportation and marine engineering due to its superior mechanical properties. However, high-strength steel endures severe corrosion and other potential threats during its service in offshore engineering facilities, owing to the harsh marine environments. Among these threats, the mechanical degradation of steel caused by hydrogen permeation during the corrosion process has received widespread attention in the last few decades [1; 2; 3; 4]. [PERSON] et al. [4] found that diffusible hydrogen led to a decrease in the notch tensile strength for the AISI 4135 steels at 1320 MPa in a power law manner. [PERSON] et al. [5] found that E690 steel exhibited high susceptibility to stress corrosion cracking (SCC) under simulated marine thin electrolyte layer and indicated that the SCC process was jointly determined by local anodic dissolution (AD) and hydrogen embrittlement (HE). [PERSON] et al. [6] found that the change in fracture stress of steels showed apparent correspondence with the change in hydrogen content during the evaluation on delayed fracture property of outdoor-exposed high-strength AISI 4135 steels. To date, it has been recognized that hydrogen plays a critical role in the fracture failure of steels, while the sensitivity of steels to hydrogen increases sharply with strength [7; 8]. Generally, there are two main sources of hydrogen permeated into steels: a part of hydrogen comes from the steel production and subsequent processing process, such as pickling and plating; while most of the hydrogen absorbed into steel comes from the corrosion processes, especially the hydrolysis of corrosion products [9; 10]. It has been found that hydrogen permeation behavior into steel is closely related to the corrosion behavior of steel [11; 12; 13; 14; 15; 16]. Our previous studies [11; 12] found that the amount of hydrogen permeated into steel is positively correlated with the corrosion weight loss of steel under the simulated wet-dry cycle condition. Meanwhile, the positive relationship between the hydrogen permeation and steel corrosion in the automobile moving environment and the atmospheric corrosion environment has also been reported in previous studies [13; 14]. Furthermore, it is well known that the amount of hydrogen permeated into steel increases significantly in the case of salts being loaded on the surface of steels [9; 15; 16]. Therefore, hydrogen permeation behavior into steel should be more serious and complex in the marine corrosive environments according to above analysis. Traditionally, the marine corrosive environments are divided into five zones including the marine atmospheric zone, the splash zone, the tidal zone, the full immersion zone and the seabed sediment zone [17]. For offshore facilities, the steel structures run through different corrosion zones, and the hydrogen permeation behavior caused by varying degrees of corrosion may also be diverse at different locations. Since the amount of hydrogen permeated into steel plays an essential role in the hydrogen embrittlement or delayed fracture of steel, it is significant to clarify how much hydrogen can be permeated into steel during service in a certain environment [17; 18]. Over the past few decades, important progress has been made on understanding the hydrogen permeation behavior of steel in atmospheric environments [14; 19; 20; 21], but relevant information on such behavior of steel in more complex and harsh marine environments remains limited. Therefore, systematic and scientific research should be carried out on the hydrogen permeation behavior of steels in marine environments. Among the methods for the determination of hydrogen content in steel, the electrochemical hydrogen permeation technique is widely used in various scenarios due to its excellent reliability and adaptability [22; 23; 24].