## 3 Results and Discussion
### Hydrogen Embrittlement of 316L Stainless Steel Under Different Hydrogen Storage Conditions
#### 3.2.2 Impact Toughness and Fractography at Different Hydrogen Storage Conditions  
Hydrogen Embrittlement Evaluation Using Impact Toughness  
Charpy impact toughness was measured for additively manufactured and post-processed 316L stainless steel specimens, as shown in Figure 18. Impact testing at room temperature (testing condition 1) revealed no significant differences in impact toughness, with a slight improvement observed in post-processed conditions. Figure 17: Hydrogen embrittlement index (HEI) based on the loss of total elongation of 316L stainless steel under different processing conditions. In testing condition 2, where specimens were hydrogen-charged prior to impact testing, a reduction in impact toughness was observed across all material processing conditions. A similar trend was noted in Testing Condition 4 compared to Testing Condition 3, where specimens tested at cryogenic temperature exhibited lower impact values after hydrogen charging. The hydrogen embrittlement factor (HEF) was calculated at both room and cryogenic temperatures to quantify the reduction in Charpy impact toughness after electrochemical hydrogen charging, using the following equation [13]:  
\[\text{HEF}=[(\text{U}_{\text{un}}-\text{UH})/\text{U}_{\text{un}}]\times 100 \tag{5}\]  
where U\({}_{\text{un}}\) represents the Charpy impact toughness of the uncharged specimens (in Joules), and U\({}_{\text{H}}\) denotes the Charpy impact toughness of the hydrogen-charged specimens (in Joules). The hydrogen embrittlement factors, based on the reduction in impact toughness after hydrogen charging at both room and cryogenic temperatures, are presented in Figure 19. The results indicate that post-processing heat treatments positively influence hydrogen embrittlement resistance across both temperature conditions. Notably, the HT2 condition exhibits the highest resistance to hydrogen embrittlement at room temperature, making it the most suitable choice for processing AM 316L stainless steel tanks intended for compressed hydrogen gas storage at ambient conditions. Conversely, the HT1 condition demonstrates superior resistance to hydrogen embrittlement at cryogenic temperatures, making it the optimal material for AM 316L stainless steel tanks used in liquid and cryocompressed hydrogen storage applications. Figure 18: Impact toughness of additively manufactured (AM) 316L stainless steel under different processing conditions. The hydrogen embrittlement behavior of different AM 316L stainless steel alloys can be attributed to the influence of the additive manufacturing process on the hydrogen solubility and diffusivity. As reported by [PERSON] et al. [9], AM increases hydrogen solubility while reducing its diffusivity due to a higher number of trapping sites. This structural characteristic allows the AM material to dissolve introduced hydrogen atoms more effectively, preventing their segregation and thereby mitigating their detrimental effects. The HEF results further validate the trends observed in the HEI measurements. Fractography at Different Hydrogen Storage Conditions  
The environmental embrittlement occurs due to the adsorption of hydrogen molecules on the surface, which subsequently dissociate into atomic hydrogen. This atomic hydrogen can migrate across the surface and diffuse into the material during mechanical deformation. The failure of the metal is influenced by the transport and accumulation of hydrogen within the triaxial tensile stress field ahead of a crack tip. Hydrogen embrittlement (HE) increases the likelihood of sudden brittle failure, compromising the material's mechanical integrity. This section describes the fractography analysis of as-built and HT2 specimens under different testing conditions to provide insights into the fracture mechanisms at various hydrogen storage conditions.