Evaluation of hydrogen embrittlement in notched STS 310S using electrochemical hydrogen charging

2.1 Specimen Preparation

The geometry of the STS 310S notched tensile specimen used in the slow strain rate tensile test is shown in Figure 1. The chemical composition of the specimens (based on weight percentage) was 0.046% C, 0.53% Si, 1.80% Mn, 0.023% P, 0.001% S, 19.1% Ni, and 24.3% Cr. The specimen was machined from a plate and finalized into a tensile specimen with a notch radius of 0.1 mm, gauge length of 50 mm, and thickness of 3 mm.

Figure 1:

Geometry of the notched tensile specimen

2.2 Electrochemical Hydrogen Charging

Electrochemical hydrogen charging was performed using an aqueous solution containing 3 wt.% NaCl and 0.3 wt.% ammonium thiocyanate in accordance with ISO 16573 [12]. A platinum mesh was used as the anode, and a specimen was used as the cathode.

For ex-situ hydrogen charging, high current densities of 10, 30, and 50 mA/cm² were applied to account for hydrogen desorption. In contrast, under the in-situ hydrogen charging conditions, current densities of 0.5, 1, 5, and 10 mA/cm² were applied.

2.3 Slow Strain Rate Test

To evaluate the hydrogen embrittlement susceptibility of STS 310S, slow strain rate tensile tests were conducted under both ex-situ and in-situ hydrogen charging conditions. The tests were performed at room temperature using a universal testing machine at a constant strain rate of 3 × 10^-5 /s. The specimen displacements were measured using an extensometer with a gauge length of 50 mm.

Before the test, the specimens for ex-situ hydrogen charging were precharged for 72 h in accordance with ISO 16573 [12]. Under in-situ conditions, hydrogen was continuously supplied during the test to prevent desorption.

Each test was repeated thrice under the same conditions to ensure the reliability of the experimental results. A schematic illustration of the hydrogen pre-charging process and slow strain-rate tensile test setup for both the ex-situ and in-situ conditions is shown in Figure 2.

Figure 2:

Schematic diagram: (a) Hydrogen pre-charging; (b)

2.4 Hydrogen Concentration Analysis

The total hydrogen concentration in the specimens was measured using an ONH analyzer (ONH2000). The specimens had dimensions of 5 × 5 × 3 mm. This analyzer can detect hydrogen concentrations in the 0.01–1000 ppm range. The specimens were immediately stored on dry ice following electrochemical hydrogen charging to prevent hydrogen desorption, and the hydrogen concentration was measured within 30 min. During the analysis, each specimen was rapidly melted in a high-purity graphite crucible at temperatures exceeding 2000 ℃ using a pulse furnace. The thermally released hydrogen gas was transported by a stream of high-purity inert gas through a copper catalyst to remove interfering gases such as oxygen and nitrogen. The purified hydrogen was quantified using a thermal conductivity detector.

3.1 Results of Slow Strain Rate Tensile Tests

Figure 3 shows the stress–strain curves obtained from slow strain rate tensile tests conducted on notched STS 310S specimens under both ex-situ and in-situ hydrogen-charging conditions.

Figure 3:

Stress–strain curves at different current densities: (a) Ex-situ and (b) in-situ hydrogen charging

As shown in Figure 3(a), under ex-situ hydrogen charging, the notched tensile strength and elongation were similar to those of the uncharged specimen, even when subjected to different applied current densities (10, 30, and 50 mA/cm²), suggesting that the effect of hydrogen on the mechanical properties was limited under ex-situ conditions owing to hydrogen desorption during testing.

However, as shown in Figure 3(b), under in-situ hydrogen charging conditions, the mechanical properties deteriorated with increasing current density. Both the notched tensile strength and elongation gradually decreased from 0.5 mA/cm² to 5 mA/cm², beyond which the values plateaued, showing minimal variation at 10 mA/cm².

This result suggests that STS 310S reaches a saturation level of hydrogen-induced degradation of its mechanical properties at current densities above 5 mA/cm². This behavior implies that fracture in hydrogen environments occurs when a certain critical hydrogen concentration is reached; beyond that point, additional hydrogen charging has a limited effect on mechanical performance. The test results indicate that hydrogen embrittlement susceptibility varies depending on the hydrogen charging method; thus, the influence of the charging conditions should be carefully considered in hydrogen embrittlement evaluations.

3.2 Evaluation of Hydrogen Embrittlement Susceptibility

Hydrogen embrittlement susceptibility is typically evaluated based on the extent to which the notched tensile strength decreases in a hydrogen-charged environment compared with that in an uncharged (air) condition [13]. In this study, the hydrogen embrittlement index (HE index) was defined as the ratio of the notched tensile strength in a hydrogen environment to that measured in air, referred to as the relative notch tensile strength.

Figure 4 shows the HE index of the STS 310S specimens under various current densities for both the ex-situ and in-situ hydrogen charging conditions.

Figure 4:

Relative notch tensile strength of STS 310S at different current densities: (a) Ex-situ and (b) in-situ hydrogen charging

The notched tensile strength of the uncharged STS 310S specimen was measured to be approximately 652.2 MPa. Even after hydrogen charging at current densities of 10, 30, and 50 mA/cm² under ex-situ conditions, the notched tensile strength remained as high as approximately 647.4 MPa. Despite the increase in current density, the mechanical performance exhibited negligible variation, and as a result, the HE index maintained values exceeding 99% at all tested current densities. These results indicate that, under ex-situ hydrogen charging, the degradation of the mechanical properties due to hydrogen embrittlement was minimal.

In contrast, under in-situ hydrogen charging, the notched tensile strength begins decreasing at a current density of 0.5 mA/cm², reaching approximately 645 MPa. It further decreased to approximately 621 MPa at 5 mA/cm² and remained at a similar level (620 MPa) at 10 mA/cm². However, even at the highest current density of 10 mA/cm², the HE index was maintained at approximately 95%.

This behavior is consistent with the properties of austenitic stainless steels, which exhibit low hydrogen diffusivity and high hydrogen solubility. These characteristics help confine hydrogen near the surface while limiting its penetration into the bulk material.

In particular, the high chromium (24.3 wt.%) and nickel (19.1 wt.%) content of STS 310S plays a critical role in enhancing hydrogen embrittlement resistance by improving the stability of the austenitic phase. These alloying elements stabilize the face-centered cubic (FCC) structure, thereby suppressing deformation-induced phase transformations, such as martensitic transformation, which is associated with increased resistance to hydrogen embrittlement [14].

Moreover, chromium and nickel contribute to the reduction of hydrogen trapping at microstructural defects, such as dislocations and grain boundaries, which are potential nucleation sites for crack initiation. By minimizing these hydrogen-trapping effects, the alloy maintained its ductility and fracture resistance, even under hydrogen-rich conditions. Previous studies have shown that high Cr-Ni austenitic steels suppress phase transformations and hydrogen-assisted cracking more effectively than low-alloy steels [15].

Accordingly, STS 310S demonstrated superior mechanical performance and structural stability in both ex-situ and in-situ hydrogen environments.

3.3 Hydrogen Concentration Analysis

The amounts of hydrogen charged at each current density via electrochemical hydrogen charging are summarized in Table 1. As the current density increased, the hydrogen concentration also increased. However, above 10 mA/cm², the rate of increase was more gradual. This behavior was presumed to be due to the saturation of the electrochemical reaction at higher current densities, which led to a decrease in the hydrogen charging efficiency.

Table 1:

Measured hydrogen concentration at different current densities

Hydrogen is generally known to initiate at the metal surface and diffuse inwards. Accordingly, the hydrogen concentration distribution was calculated based on Fick’s second law of diffusion, as defined by Equation (1) [16].

where C(x) is the hydrogen concentration at depth x (wppm); C(s) is the surface hydrogen concentration (wppm); D is the hydrogen diffusion coefficient (m²/s) defined in Equation (2) [17]; t is the hydrogen charging time (s); erf is the error function.

where E_B is the activation energy for hydrogen diffusion (53.9 kJ/mol), R is the universal gas constant for hydrogen (8.314 J/molžK), and T is the absolute temperature.

In this study, calculations were performed at room temperature (298 K). Assuming a surface hydrogen concentration of 1 wppm, the theoretically calculated hydrogen concentration distribution is presented in Figure 5.

Figure 5:

Hydrogen concentration profile assuming a surface hydrogen concentration of 1 wppm

The estimated hydrogen diffusion depth was approximately 35 μm. These results indicate that hydrogen tends to accumulate locally near the surface, with diffusion concentrated toward the surface region.

The hydrogen concentration measured in this study was determined based on the total volume of the analyzed specimen (75 mm³). Hydrogen was assumed to diffuse uniformly across all surfaces of the specimen. Under this assumption, the volume into which the hydrogen diffused, excluding the central region (which remained uncharged), was calculated to be approximately 3.79 mm³. Based on the principle of mass conservation, the average hydrogen concentration in the hydrogen-diffused region was calculated as shown in Equation (3).

where $$ {\stackrel{-}{C}}_H$$ is the average hydrogen concentration (wppm), $$ {\stackrel{-}{C}}_{total}$$is the average hydrogen concentration of the entire specimen (wppm), V_total is the total volume of the specimen (mm³), and V_H is the volume of diffused hydrogen (mm³).

Based on Equation (3), the average hydrogen concentration in the hydrogen-diffused region was calculated as shown in Table 2. Using these results, the surface hydrogen concentration was back-calculated according to the formula given in Equation (1), as shown in Equation (4).

Table 2:

Average and surface hydrogen concentration in the diffused region at various current densities

where d represents the depth of the hydrogen diffusion. Accordingly, the hydrogen concentration distribution curves for each current density were calculated using Equation (1), as shown in Figure 6.

Figure 6:

Hydrogen concentration profiles as a function of depth—calculated for various charging current densities

The results indicate that the surface hydrogen concentration increased as the current density increased, suggesting that a greater amount of hydrogen was concentrated near the metal surface, which may have enhanced the influence of hydrogen on embrittlement. In particular, a steeper concentration gradient near the surface implies that hydrogen diffusion is more localized, leading to a higher probability of hydrogen accumulation at microstructural defects such as grain boundaries, dislocations, or inclusions. This localized hydrogen enrichment can serve as an initiation site for microcracks under mechanical stress, thereby increasing susceptibility to hydrogen-assisted cracking. In addition, the limited diffusion depth observed across all current densities confirmed that hydrogen remained confined primarily to the near-surface region. This result indicates the importance of the surface geometry and regions with stress concentration, including notches, when evaluating hydrogen embrittlement behavior.

Author Contributions

Conceptualization, B. K. Hwang and S. J. Cha; Methodology, B. K. Hwang and H. W. Choi; Software, S. J. Cha; Formal Analysis, B. K. Hwang; Investigation, B. K. Hwang and J. H. Kim; Resources, B. K. Hwang; Data Curation B. K. Hwang and S. J. Cha; Writing-Original Draft Preparation, B. K. Hwang; Writing-Review & Editing, J. H. Kim; Visualization, H. W. Choi; Supervision, J. M. Lee; Project Administration, J. M. Lee; Funding Acquisition, J. H. Kim and J. M. Lee.