## 2 Hydrogen-Metal Interaction and Fatigue Mechanisms
### Overview of Hydrogen Embrittlement Phenomena
In addition, HIC decreases with increasing pH and decreasing H2S partial pressure. 3. Chemical reactions of hydrogen with metals: Hydrogen forms compounds with metal surfaces that may lead to increased brittleness of the material. These compounds may be brittle and may lead to the formation and extension of surface cracks. The interaction of hydrogen with metals is the cause or basis of many phenomena, ranging from the chemisorption of hydrogen on surfaces, the dissolution of hydrogen in metals, the catalysed reaction of hydrogen as a reactant or stoichiometric constituent, etc., to the formation of metal hydrides. Hydrogen-induced corrosion and hydrogen embrittlement of steel are well known in the chemical processing industry and metallurgy [46; 47]. The researchers [48] describe optical observations and x-ray diffraction measurements of the reaction of iron and hydrogen at high pressure to produce iron hydride. These results greatly extend the pressure range characterised by the technically important Fe-hydrogen phase diagram and have implications for issues ranging from hydrogen degradation and ferrous metal embrittlement to the presence of hydrogen in Earth's metal cores. 4. Influence of hydrogen on dislocation motion: Hydrogen atoms may influence dislocation motion in metals and thus the plastic behaviour of materials. This effect may cause the material to be more susceptible to brittle fracture upon loading. In 310S stainless steel, [PERSON] et al. [49] observed that the presence of hydrogen reduces the elastic interactions between the barrier and both full and partial dislocations, thus improving the mobility of the dislocations. In high-purity aluminium, it was observed that the introduction and removal of hydrogen from the system leads to a reversal of the direction of motion of dislocations stacked on the barrier, consistent with the reduction in elastic interactions by solute hydrogen. These observations provide direct support for a hydrogen shielding mechanism [49]. [PERSON] et al. [50] present a new framework to quantify the effect of hydrogen on dislocations using large-scale three-dimensional (3D) discrete dislocation dynamics (DDD) simulations. In this model, the first-order elastic interaction energy associated with hydrogen-induced volume changes is considered [50; 51; 52]. Furthermore, other factors for material property degradation can also be considered in the existing hydrogen embrittlement mechanism. 1. Hydrogen-induced stress corrosion cracking: This similar failure morphology is shown in Figure 2a. In the presence of stress, hydrogen promotes corrosion of the metal and may lead to crack formation and expansion in areas of stress concentration, which can initiate brittle fracture. This mechanism is particularly significant in specific environments, such as in salt water or in environments containing chloride ions [53; 54]. 2. Hydrogen interacts with the electronic structure of materials: Hydrogen atoms may interact with the electrons in metals, changing the electronic structure and properties of the material, thus affecting its mechanical properties. The molecular structure of this interaction is shown in Figure 2b. This change in electronic structure may lead to an increase in the brittleness of the material [55; 56; 57]. 3. Hydrogen-promoted crystal defect formation: Hydrogen atoms may promote the formation of defects in metal crystals, such as vacancies, interstices, dislocations, etc., which may lead to increased brittleness of the material [14; 15; 16]. Figure 2c shows crystal-plasticity contours for Pd-H alloys: as the hydrogen concentration rises from 0 to 7500 appm, the local fraction of screw dislocations at a logarithmic strain of 0.35 increases markedly, confirming that hydrogen enhances dislocation multiplication and heterogeneity. 4. Diffusion and aggregation of hydrogen within metals: Diffusion and aggregation of hydrogen atoms in metals may lead to localised stress concentrations that increase the brittleness of the material.