## 2 The Design of Hydrogen-Charging Experiment  
### Experimental Design  
#### 2.1.1 Specimens Preparation and Pretreatment  
In electrochemical hydrogen-charging experiments, two types of specimens are used: block specimens and tensile specimens. These specimens are typically made of low-alloy steel, and they are specifically chosen for their suitability in hydrogen production reactors. To ensure consistent and comparable hardness measurements across different experiments, the block specimens are primarily used for hardness testing after hydrogen charging. The block specimen has specific dimensions of 20 mm \(\times\) 20 mm \(\times\) 8 mm. The dimensions and configuration of the tensile specimen are determined by the experimental approach standard \"ISO 6892-1:2019, Metallic materials-Tensile testing-Part 1: Method of test at room temperature\" [31], which are shown in Figure 1. Specimens with different surface roughness and surface residual stresses are used in hydrogen-charging experiments. Each part consists of four sets of experiments. In order to minimize the experimental error, three block specimens are used for each set of experiments, while 24 block specimens and eight tensile specimens are used in the hydrogen charging experiment. Before pretreatment, all specimens undergo annealing and are stored in a controlled environment set at room temperature and in dry conditions. The surface of the specimen is rust removed and cleaned with anhydrous ethanol before pretreatment to prevent impurity particles from affecting the experiment results.  
To achieve varying initial surface roughness before hydrogen charging, the hydrogen-charging section of one side of the block specimen and both sides of the tensile specimens undergo a preparatory process consisting of grinding using files, 80\(\mu\)sandpaper, 1200\(\mu\)sandpaper, and polishing. After the grinding process, the surfaces of the specimen are cleaned with anhydrous ethanol to prevent impurity particles from affecting the experiment results. To achieve different surface residual stresses, the block specimens undergo shot peening on one side of the hydrogen-charging section, while the hydrogen-charging sections of tensile specimens undergo shot peening on both sides. The shot peening process parameters include a steel pellet diameter of 0.5 mm, a shot peening time of 120 s, and  
Figure 1: The dimensions and configuration of specimens. (**a**) Block specimen; (**b**) Tensile specimen; (**c**) Dimensions of tensile specimen (Unit: mm).  
the applied shot peening pressures of 0 MPa, 0.2 MPa, 0.3 MPa and 0.4 MPa, respectively. Meanwhile, after shot peening, all specimen surfaces are lightly ground by 80# sandpaper to ensure the uniform surface roughness of all specimens. After the shot peening strengthening and grinding steps, the surface of the specimen is cleaned with anhydrous ethanol to prevent impurity particles from influencing the experiment results. After treatment and surface cleaning, the surface topography, surface roughness, surface residual stress, surface microstructure and section hardness of specimens are measured.  
Meanwhile, to ensure the validity and effectiveness of the experiment, we reduced the period of the hydrogen-charging experiment and controlled the hydrogen-charging sections and current density during the hydrogen-charging process. After pretreatment of the specimens' surface, the untreated sections of specimens are covered with special anti-corrosion tape, which consists of four untreated side faces and the bottom face of block specimens, only leaving the treated square area of 400 mm\({}^{2}\) for hydrogen charging. The two clamping parts and untreated sections of tensile specimens are also similarly covered, leaving only a rectangular area of 1560 mm\({}^{2}\) for hydrogen charging. The wrapped specimens are shown in Figure 1a,b.