PSI - Issue 54

Daria Pałgan et al. / Procedia Structural Integrity 54 (2024) 322 –331

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Daria Pałgan et al./ Structural Integrity Procedia 00 (2023) 000 – 000

while still pressured with gaseous H 2 . Once the autoclave was cooled down the pressure was released, and the samples were removed and stored in liquid nitrogen until hydrogen analysis. Thermal desorption spectroscopy from Bruker (Galileo G8 model) coupled with mass spectrometry from InProcess Instruments (TDMS) and infra-red quartz tube furnace from Brucker (IR700 model) were used to analyse the reversible i.e., the “diffusible” and irreversible i.e., “trapped” hydrogen content in non-charged and charged samples. The instrument was calibrated using pure H 2 gas and three defined volumes. The samples were tested directly after removing them from liquid nitrogen and cleaning them for 3 minutes in ethanol and ultrasonic bath and drying them with cold air. The samples were heated from room temperature (RT) to 800 °C using 1 °C/s heating rate for the A516 samples and 0.25 °C/s and for 304L and 316L 1 °C/s. The hydrogen contents were then quantified by integrating the area below the desorption rate vs. temperature TDMS curves (diffusible hydrogen RT-300 °C, trapped hydrogen above 300 °C to about 800 °C and total hydrogen as sum of diffusible and trapped). 3. Results and Discussion 3.1 Thermal desorption spectroscopy with mass spectrometry Figures 2 and 3 show TDMS curves recorded before (black) and after cathodic and gaseous hydrogen charging (red and blue). The charging conditions are listed above each diagram. Selected TDMS curves are shown with the intention to clarify the differences observed in respect to the shape of the curves as a function of used charging method. In general, all TDMS curves confirm hydrogen uptake due to charging regardless of the charging method used. Moreover, it is also evident that there is a difference in the shape of the TDMS curves when comparing the curves recorded after cathodic and gaseous hydrogen charging. Regardless of steel type the cathodic charging resulted in main hydrogen peak visible in the temperature range from room temperature to about 300 °C, and a more complex hydrogen peak consisting of several subpeaks in the temperature range above 300 °C to about 800 °C. These peaks evolve because of hydrogen pick-up due to charging into various trapping sites present in the studied steels. It is well established that hydrogen traps in metallic materials can be classified as reversible and irreversible based on the hydrogen binding and/or de-trapping activation energy (Lin et al., 2021). For simplicity to the readers, these traps in the present work are denoted as “diffusible” and “trapped” hydrogen traps. Note that at this stage of the work the intention was not to ascribe the hydrogen peaks to specific trapping sites present in the investigated steels, but rather qualitatively to obtain insight which trapping sites are influenced by the used charging method. In that context, looking at the TDMS curves obtained from the gaseous charging it is clearly visible that only for the A516 steel two distinct well separated hydrogen peaks can be seen one up to 300 °C and one above 300 °C for the sample charged for 72 hours. Also, a notable difference was observed between cathodic and gaseous charging of A516 steel in terms of preferred trapping sites with respect to the used charging method. Generally, it seems that gaseous charging promotes preferential uptake of trapped hydrogen rather than diffusible hydrogen as compared to cathodic charging, see hydrogen uptake trends of A516 measured after cathodic charging at elevated electrolyte temperature in Fig. 4c. In contrast, the gaseous charging of 304L and 316L steels resulted in a completely different shape of the TDMS curves as compared to the cathodic charging, compare Figs. 2b-c with Figs. 3b-c. Gaseous charging results in broad TDMS curves with not well separated hydrogen peaks as in the case of cathodic charging. Separation of the peak below 300 °C and above 300 °C evidence that only a small portion of the absorbed hydrogen is diffusible and most of the hydrogen is with trapped origin. In contrast, the TDMS curves recorded after cathodic charging show that most of the hydrogen is diffusible and only a small part is trapped. To have relevant comparison of hydrogen contents and hydrogen trapping type all recorded curves were analyzed in a similar way. The content of diffusible and trapped hydrogen was measured only when the peaks in the curves below 300 °C and above 300 °C could be separated. Thus, for all samples, the total hydrogen content is calculated as sum of the diffusible and trapped hydrogen contents. Since a clear separation of the hydrogen peaks for the TDMS curves after gaseous charging of 304L and 316L steels was not possible, for these steels only the total hydrogen content is evaluated.