## 4 Effect of Surface Roughness on Hydrogen Resistance
### The Analysis of Tensile Fracture Morphology  
Figure 9 illustrates the tensile fracture morphology of specimens with different surface roughness after hydrogen charging. Figure 9 shows that after 5 h of hydrogen charging, the brittle fracture features, such as cleavage planes and intergranular fractures appear on the different surface roughness specimens to some extent. Figure 9a reveals that the fracture mode of the Ra 3.01 \(\upmu\)m surface roughness specimen is mainly quasi-cleavage fractures, and the fracture surface had plenty of brittle fracture features distributed such as quasi-cleavage planes, tear ridges and intergranular fractures on the fracture surface. As shown in Figure 9b, many river-like textures, plenty of small areas of quasi-cleavage planes and intergranular fracture features are distributed on the fracture surface of the specimen with a surface roughness Ra1.21 \(\upmu\)m. As shown in Figure 9c, the fracture surface of the specimen with a surface roughness of Ra 0.17 \(\upmu\)m has a large number of distributed dimple features and a small number of quasi-cleavage planes. The fracture form of this specimen is a ductile-brittle composite fracture dominated by ductile fracture. As shown in Figure 9d, the fracture surface of the specimen with a surface roughness of Ra 0.05 \(\upmu\)m distributes the dimple features, and it also obviously distributes a great number of brittle  
fractures as large area cleavage planes and intergranular fractures. The fracture form of this specimen is ductile-brittle composite fracture dominated by brittle fracture. Based on the above fracture morphology observation, it can be seen that after 5 h of hydrogen charging, the specimen with different surface roughness appears to have different degrees of embrittlement. As the specimens' surface roughness decreases, the proportion of brittle fracture features in the specimens' fracture morphology tends to decrease and then increase.  
When the specimen has a large surface roughness, the hydrogen permeation resistance of the specimen surface is relatively weak, which increases the permeation of hydrogen atoms into the specimens. However, the permeated hydrogen atoms in the specimen's metal material would accumulate, forming hydrogen molecules and generating high pressure. The acting force generated by hydrogen molecules' aggregation pressure, material internal residual stress and external force may exceed the yield limit of the material. The acting force exceeding the yield limit of the material would lead to hydrogen embrittlement fracture in the material [36]. Therefore, the fracture of specimens with a surface roughness of Ra 3.01 \(\upmu\)m and Ra 1.21 \(\upmu\)m is dominated by brittle fractures. As the surface roughness of the specimen decreases to Ra 0.17 \(\upmu\)m, the adsorption capacity of the specimen surface for hydrogen atoms reduces, leading to a decrease in the content of hydrogen atoms permeating into the interior of the specimen material. Consequently, the brittle fracture features of the specimen gradually reduce with the reduction in surface roughness. In contrast, the surface energy of the specimen with a surface roughness of Ra 0.05 \(\upmu\)m reduces due to the adsorption of hydrogen atoms, reducing the required crack propagation, which makes the specimen more susceptible to fracture. Meanwhile, the crack diffusion fracture point would accumulate hydrogen atoms, which are more likely to produce a hydrogen embrittlement phenomenon, leading to an increase in hydrogen embrittlement fracture features in the fractured specimen.  
Figure 9: Fracture topographies of samples with different surface roughness after hydrogen charging for 5 h. (**a**) Ra 3.01 \(\upmu\)m; (**b**) Ra 1.21 \(\upmu\)m; (**c**) Ra 0.17 \(\upmu\)m; (**d**) Ra 0.05 \(\upmu\)m.