## 2 Hydrogen-Metal Interaction and Fatigue Mechanisms
### Overview of Hydrogen Embrittlement Phenomena
This process of diffusion and aggregation may be influenced by factors such as material structure and temperature [58; 59]. As shown in Figure 2d, the mobile hydrogen may be located at different interstitial positions. Depending on the potential energy, the atoms can migrate to various positions. Figure 1: Experimental database and numerical model. (**a**) The ratio of the H diffusion barrier at different grain boundaries to the energy barrier in bulk Fe [43]. (**b**) Hydrogen pipeline modelling [45]; (**i**–iii) Schematic diagram of the model geometry. Figure 2: Multiscale schematic of hydrogen–metal interactions. (**a**) Stress corrosion cracking morphology of API-X52 steel in low-pH NS4 solution [60]. (**b**) Full-potential energy calculation cell of the NH–LiH system [57]. (**c**) 3D simulation of Pd–H alloy showing the distribution of screw dislocations at different hydrogen concentration [15]. (**d**) Schematic of typical hydrogen traps such as intragranular vacancies, dislocations, and interfaces [54]. Research on hydrogen embrittlement has gradually evolved from macroscopic tensile testing to advanced in situ techniques such as transmission electron microscopy and atomic force microscopy, as illustrated in Figure 3. Currently, many theoretical models have been proposed to describe hydrogen embrittlement phenomena and mechanisms [61; 62; 63; 64; 65; 66], such as Hydrogen Enhanced Dislocation Emission (HEDE), Hydrogen Enhanced Local Plasticity (HELP), and Adsorption-Induced Dislocation Emission (AIDE) and so on. Several reviews have provided detailed introductions to these topics [67; 68; 69; 70; 71; 72; 73]. A clear summary of these mechanisms is shown in Figure 3. The HEDE model focuses on the interaction of hydrogen atoms with dislocations. In this model, hydrogen atoms adsorb around the dislocations and change the structure and energy of the dislocations, resulting in an increase in the energy required to move the dislocations, which results in a loss of plasticity in the metal. This model is based on the interaction of hydrogen atoms with dislocations and assumes that the hydrogen atoms increase the energy barrier for the movement of the dislocations, thus making the metal more susceptible to fracture. The HELP model focuses on the description of plastic deformations induced by hydrogen atoms in a localised region. In this model, hydrogen atoms adsorb near-local dislocations or grain boundaries and reduce the local yield strength, making the material more susceptible to localised plastic deformation. The HELP model suggests that hydrogen atoms induce plastic deformation in a localised region, which reduces the toughness of the material. The AIDE model focuses on describing the process of dislocation emission caused by the adsorption of hydrogen atoms on the metal surface. In this model, hydrogen atoms adsorbed near the source of dislocation emission weaken the energy barrier to dislocation emission and promote dislocation emission and motion. This model suggests that the adsorption of hydrogen atoms reduces the energy barrier to dislocation emission, which increases the probability of dislocation emission and leads to brittle behaviour of the material. Each of these models has its own focus, but all of them attempt to explain the microscopic changes induced by hydrogen atoms in metals that lead to the loss of toughness and plasticity of the material. Under low to moderate stress, HELP dominates; under high stress, HEDE becomes the primary mechanism. In the transitional region, a mixed-mode behaviour is typically observed. Their study contributes to a deeper understanding of the hydrogen embrittlement phenomenon and provides a theoretical basis for the prevention and control of hydrogen embrittlement. Figure 3: The development of hydrogen embrittlement research methods [67]. One of the common problems in the current explanation of the hydrogen embrittlement phenomenon is the lack of a comprehensive theoretical framework and a consistent mechanism model.