## 4 Effect of Surface Roughness on Hydrogen Resistance  
### Hydrogen Hardening Effect Analysis  
Figure 8 illustrates the hardness test results along the depth direction and the variation in hardness for specimens with different surface roughness after hydrogen charging. From Figure 8a, it is evident that after the pretreatment by hydrogen, the material hardness of the surface and subsurface exhibit a decreasing trend with an increasing test depth. The hardness of specimens with different surface roughness eventually approaches 210 HV at 125 \(\upmu\)m from the surface. Within the range of 0-100 \(\upmu\)m from the specimen's surface, the hardness of the hydrogen-charged specimens increases with surface roughness. The hardness values of the specimens with a surface roughness of Ra 0.17 \(\upmu\)m and Ra 0.05 \(\upmu\)m are close at a distance of 100 \(\upmu\)m from the surface of the specimen. From a distance of 75-125 \(\upmu\)m from the surface, the decrease in the specimen's hardness with a surface roughness of Ra 0.05 \(\upmu\)m slows down. The hardness at a distance from the surface of 75 \(\upmu\)m is only 1.45% higher than that of 125 \(\upmu\)m; thus, it could be assumed that hydrogen charging produces a considerable hardening effect in the distance of 0-75 \(\upmu\)m from the surface of the Ra 0.05 \(\upmu\)m surface roughness specimen, while the hardening effect in the distance of 75-125 \(\upmu\)m is relatively small. The hardness variation at different depths before and after hydrogen charging is analyzed to avoid the interference of specimen surface pretreatment hardening to the specimen hydrogen-induced hardening analysis. By comparing the measured hardness at different depths of specimens before hydrogen charging in Figure 7a with the corresponding measurement in Figure 8a, the variation in hardness at different depths is shown in Figure 8b. From Figure 8b, it is evident that the change in hardness of specimens with different surface roughness at different depths before and after hydrogen charging shows a decreasing trend as the depth increases. Ultimately, the hardness variation of different surface roughness specimens stabilizes to 3 HV at 125 \(\upmu\)m from the surface. With the increase in specimen surface roughness, the hardness of the specimen surface and subsurface region after hydrogen charging also increases after hydrogen charging. Among these, the surface hardness of the specimen with a surface roughness Ra 3.01 \(\upmu\)m reaches 251.25 HV, which is increased by 13.8% before hydrogen charging. This specimen has the largest hardness after hydrogen charging within the test range of 0-100 \(\upmu\)m, with both the surface hardness and variation in hardness being the largest among the four groups of specimens with different surface roughness. The specimen with a surface roughness of Ra 0.05 \(\upmu\)m achieves a surface hardness of 235.23 HV, which is 11.8% higher before hydrogen charging, and the hardness variation is the lowest. Figure 8: Hardness test results and hardness changing trends. (**a**) Hardness test result after hydrogen charging; (**b**) increase in hardness at different depths. The reason for this phenomenon is that after hydrogen atoms penetrate the metal material, the penetrated hydrogen atoms produce brittle hydrides with impurity elements in the metal. The elasticity and plasticity of the produced brittle hydride differ from the material's properties, resulting in a certain increasing trend in material hardness after hydrogen charging. The specimens with larger surface roughness possess more surface defects, such as peaks, valleys, surface cracks, holes, and scratches, which increase the contact areas and the atomic density between the specimen and the hydrogen-containing medium. Meanwhile, it also has a higher specific surface energy than a smooth surface [34], representing the lack of neighboring atoms around the specimen material surface atoms.