## 3 Results and Discussion
### Hydrogen Embrittlement of 316L Stainless Steel Under Different Hydrogen Storage Conditions
#### 3.2.1 Tensile Properties Under Different Hydrogen Storage Conditions
The hydrogen embrittlement index (HEI) was calculated at both room temperature and cryogenic temperature based on the total elongation at fracture, using the following equation [7; 9]:  
\[\text{HEI}=[(\text{El}_{\text{un}}-\text{El}_{\text{H}})/\text{El}_{\text{un}}] \times 100 \tag{4}\]  
where \(\text{El}_{\text{un}}\) represents the elongation at fracture of the uncharged specimens, and \(\text{El}_{\text{H}}\) denotes the elongation at fracture of the hydrogen-charged specimens. Figure 17 presents the calculated HEI values, determined using Equation (4), for the as-rolled, as-built, HTI, and HT2 specimens at both room temperature and cryogenic temperature. The HEI quantifies the loss of ductility caused by the introduction of hydrogen atoms into the microstructure during electrochemical charging. Lower HEI values indicate higher resistance to hydrogen embrittlement, demonstrating that hydrogen has a minimal impact on the mechanical behavior of 316L stainless steel. It can be observed that hydrogen embrittlement resistance improves at cryogenic temperature (testing condition 4) across all material conditions compared to room temperature (testing condition 2). Although cryogenic temperatures enhance the propensity for strain-induced martensite formation and deformation twinning, as described by [PERSON] et al. [20], the dominant effect at low temperatures is the reduction in hydrogen diffusion, which mitigates embrittlement. It is evident that hydrogen had minimal impact on post-processed heat-treated (PPHT) specimens. The PPHT process effectively eliminated hydrogen sensitivity and mitigated hydrogen embrittlement (HE) susceptibility by stabilizing the dislocation morphology and enhancing resistance to martensite transformation. This highlights the high efficiency of additively manufactured (AM) 316L stainless steel in resisting hydrogen embrittlement, aligning with the findings of [PERSON] et al. [23]. The results reveal that additively manufactured (AM) specimens exhibit higher hydrogen embrittlement resistance compared to as-rolled specimens. Moreover, the post-processing heat treatments (PPHTs) further enhance the hydrogen embrittlement resistance of AM materials at both room and cryogenic temperatures. This improvement is attributed to the reduced proposity for strain-induced martensite formation in AM 316L stainless steel, which decreases hydrogen embrittlement susceptibility, as reported by [PERSON] et al. [20]. It is observed that the HT2 condition is the most suitable material condition for compressed hydrogen gas storage, as it exhibits the lowest HEI at room temperature. Conversely, the HT1 condition is the most favorable for liquid and cryo-compressed hydrogen storage, demonstrating the lowest HEI at cryogenic temperature. This behavior can be linked to the microstructural characteristics of each condition. In HT2, the quenched microstructure typically contains martensite, high residual stress, and structural defects resulting from water quenching. These features contribute to superior hydrogen embrittlement resistance at room temperature, as they hinder hydrogen diffusion more effectively compared to the as-built and HT1 conditions. On the other hand, the annealed structure of HT1, which has fewer residual stresses, exhibits the best hydrogen embrittlement resistance at cryogenic temperatures. This is because the high residual stress present in the as-built and HT2 specimens tends to increase further at cryogenic temperatures, thereby heightening hydrogen embrittlement susceptibility. This observation highlights the ability of AM 316L stainless steel to maintain its mechanical integrity and surface properties in hydrogen environments at both room and cryogenic temperatures, demonstrating its suitability for hydrogen storage and related applications.