## 4 Effect of Surface Roughness on Hydrogen Resistance
### The Mechanical Properties Analysis of Materials after Hydrogen Charging  
Figure 10 compares the specimen's tensile stress-strain curves before and after hydrogen charging for 5 h with different surface roughness values. Figure 11 illustrates the mechanical property parameters obtained through the tensile testing of hydrogen-charged specimens with different surface roughness and the calculated hydrogen embrittlement sensitive index. Based on the analysis of Figure 10, it can be inferred that the tensile mechanical properties of all specimens subjected to hydrogen charging exhibit varying degrees of deterioration. This phenomenon indicates that this paper's hydrogen-charging experimental condition is effective for different surface roughness specimens. It is worth noting that under the test conditions provided in this paper, the mechanical properties of specimens with relatively large or small surface roughness show significant degradation after hydrogen charging. By analyzing Figure 11, it can be found that as the surface roughness of the hydrogen-charged specimens increases, the measured tensile strength and elongation ratio both show the trends of first increasing and then decreasing. Furthermore, the surface roughness Ra 0.17 \(\upmu\)m specimen has the largest tensile strength and elongation after hydrogen charging for 5 h; the tensile stress is 576.35 MPa, which decreases by 5.69% before hydrogen charging, and the elongation is 19.68%, which decreases by 5.22% before hydrogen charging. Figure 11 illustrates that with the increase in the specimen's surface roughness, the hydrogen embrittlement sensitivity of specimens first decreases and then increases. The specimen with a surface roughness of Ra 0.05 \(\upmu\)m has the largest hydrogen embrittlement sensitivity index, which is 40.08%. In contrast, the specimen with a surface roughness of Ra 0.17 \(\upmu\)m has the lowest hydrogen embrittlement sensitivity index, which is 20.96%. Figure 10: Stress-strain curves of samples with different surface roughness before and 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.  
The reasons for the above phenomena are that after 5 h of hydrogen charging, specimens with larger surface roughness are more likely to have stress concentration, resulting in the triaxial tensile stress at the stress concentration point reducing hydrogen's chemical potential energy in steel. The hydrogen atoms would diffuse from high chemical potential energy sections to low chemical potential energy sections due to the effect of the chemical potential difference. Furthermore, the permeated hydrogen atoms accumulate at the stress concentration sections under the stress-inducing effect until reaching a chemical potential equilibrium, which leads to an increase in the hydrogen mass fraction at the stress concentration sections. Moreover, the larger surface roughness specimens have a larger ratio surface and specific surface energy, making it easier to combine with hydrogen ions, which would penetrate more hydrogen ions. Additionally, the specimens with large surface roughness have high hydrogen-induced brittleness, and the fracture morphology is more flush and smooth. In contrast, the specimens with small surface roughness have fewer brittle fracture features on the fracture surfaces. It is attributed to the permeation and fusion of hydrogen atoms within the specimen material, which enhances the material's brittleness, hardness and reduces the material's plasticity, significantly reducing its tensile strength. Although the specimen with a surface roughness of Ra 0.05 \(\upmu\)m has a small surface roughness, its tensile strength decreases significantly after hydrogen charging, exhibiting the largest hydrogen embrittlement sensitivity index. It can be attributed to the surface of the specimen with a surface roughness of Ra 0.05 \(\upmu\)m being relatively smooth, which does not easily generate stress concentrations.