PSI - Issue 54

Magdalena Eškinja et al. / Procedia Structural Integrity 54 (2024) 123–134 M. Eškinja et al./ Structural Integrity Procedia 00 (2019) 000 – 000

130

8

3.3. Gaseous Hydrogen Experiments Figure 6 illustrates hydrogen uptake at room temperature in H 2 /H 2 S/CO 2 and H 2 /H 2 S gas mixtures with addition of 3.5% NaCl. These mixtures were chosen to test the specimens under very severe conditions. Hydrogen uptake was shown to be higher in case of HMoS alloy, what is in good agreement with the results of TDS measurements. Moreover, in case of both alloys specimens loaded in plastic region had higher hydrogen content. In H 2 /H 2 S environment no fracture was obtained under any condition, while in more severe conditions of H 2 /H 2 S/CO 2 mixture almost all specimens failed. In this gas mixture CO 2 and H 2 S are both corrosive species which form H + ions and lower the pH, furthermore HS - act as an recombination poison inducing the hydrogen uptake. These results of field conditions simulation highlight the importance of revisiting microstructural properties of studied steels prior to their use for hydrogen transportation and storage.

Fig. 6. Results of gaseous hydrogen experiment in the mixture of H 2 /H 2 S/CO 2 and H 2 /H 2 S with addition of 3.5% NaCl after 720 h. Red cross indicates specimens which resulted in failure under given conditions, while green tick mark indicates that the sample passed the test without failure. 3.4. Diffusion Coefficient Measurements The effective diffusion coefficient was determined from the permeation experiments by using the time lag method and results are illustrated in Figure 7. The first charging cycle for HMoS exhibited effective diffusion coefficient of 4.07× 10 -8 and for LMoS 1.03 × 10 -7 . The estimated values are comparable to data in the literature. The lower effective diffusion coefficient for HMoS is presumably originating from the higher number of deeper hydrogen traps, in this case Mo carbides, which are filled with hydrogen prior to weak traps and subsequently diffusion is slower.