## 2 Hydrogen-Metal Interaction and Fatigue Mechanisms  
### Overview of Hydrogen Embrittlement Phenomena  
Hydrogen embrittlement is a phenomenon in which metallic materials undergo the degradation of material properties in a hydrogen-containing environment. Although hydrogen embrittlement has been extensively studied, its exact mechanism is still a complex issue, and there is no unified theory that can explain all cases. A number of possible hydrogen embrittlement mechanisms have been proposed, but these may vary depending on the material, environmental conditions, and stress state. The existing hydrogen embrittlement mechanism is mainly modelled in eight aspects: diffusion of hydrogen atoms in the crystal lattice, hydrogen-induced crack growth, chemical reaction between hydrogen and metal, effect of hydrogen on dislocations motion, hydrogen-induced stress corrosion cracking, hydrogen interaction with the electronic structure of the material, hydrogen-promoted formation of crystalline defects, and diffusion and aggregation of hydrogen within the metal. Four factors of material property degradation can be considered in the existing hydrogen embrittlement mechanism. 1. Diffusion of hydrogen atoms in the lattice: Diffusion of hydrogen atoms in the lattice, which can penetrate the metal lattice and lead to lattice deformation, grain boundary strengthening and interactions between hydrogen atoms and other atoms in the metal crystal. This may lead to a concentration of stresses within the crystal, which increases the brittleness of the material. The mobility of hydrogen in metal lattices is a large topic. This discussion is limited to the kinetics of hydrogen diffusion, relative to temperature, hydrogen concentration, isotopic mass, and concentration of a third element at and above room temperature [42]. Hydrogen-induced intergranular cracking is the most common type of hydrogen embrittlement. Hydrogen atoms will preferentially diffuse into the grain boundaries and then accumulate in the grain boundaries. Different grain boundaries can significantly affect hydrogen migration. As shown in Figure 1a, the ratio of H diffusion barriers in various grain boundaries to the energy barrier in bulk Fe differs markedly. For the \(\Sigma 3\) and \(\Sigma 5\) grain boundaries, the logarithmic form of this ratio is greater than zero, indicating that these boundaries hinder hydrogen migration. In contrast, for the \(\Sigma 9\) grain boundary, the logarithmic ratio in the y and z directions is less than zero, suggesting that this boundary facilitates hydrogen diffusion. Therefore, it is necessary to reveal the diffusion mechanism of hydrogen atoms at grain boundaries, which contributes to a deeper understanding of hydrogen embrittlement. Notably, [PERSON] et al. [43] found that the difficulty of hydrogen diffusion along grain boundaries depends mainly on the connectivity of the low electron density regions along the grain boundaries. 2. Hydrogen-induced crack propagation: The accumulation of hydrogen atoms within the metal may lead to the formation and extension of cracks. This crack growth can occur in a number of ways, such as the combination of hydrogen atoms with metal atoms to form brittle compounds or the release of hydrogen gas. From scanning electron microscopy observations, the researchers found that in these low-Mn + Si high-purity steels [44], the mode of hydrogen-induced cracking was some type of segregation of interfaces, possibly between martensitic laths, rather than rupture occurring in air. Similar behaviour was found in other high-purity steels. [PERSON] et al. [45] presented a comprehensive finite element model for numerical simulation of hydrogen-induced cracking (HIC) in steel pipes exposed to sulphur-containing compounds such as hydrogen sulphide (H2S), as illustrated in Figure 1b. The computational results show that in the extension stage, the crack growth behaves as a trap that attracts more hydrogen, and the hydrostatic stress field at the crack tip accelerates the emergence and extension of HIC-related cracks.