PSI - Issue 79

Giuseppe Macoretta et al. / Procedia Structural Integrity 79 (2026) 508–516

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3.2. Hydrogen diffusivity and trapping characterization

Permeation results are shown in Fig. 4a). Both buildup and decay transients follow a similar pattern: after a short initial plateau, the measured oxidation current significantly increased (reduced) at first before gradually stabilizing at a new steady state value. The shapes of the buildup and decay curves are very similar. The material apparent diffusivity, D, can be calculated using the common D 63 procedure [14–16]: =   6  (1) Where t 63 is the time when 63% of the flux rise is achieved, and L is the sheet thickness. The apparent diffusivity, D , which measures the global diffusion speed for the material in the tested condition, was estimated to be 2.53 10 -11 and 3.11 10 -11 m 2 /s for the rising and descending transient, respectively. Those values are similar to others reported in the literature for QP1180 [17] and for martensitic steels such as 22MnB5 [18]. Representative TPD results are shown in Fig. 4b). A single main peak was invariably measured, with no significant subsequent peaks. For some tests, like the 10 °C/min shown in the picture, a minor peak around 400-600 °C was detected. For each heating rate,  , a peak position was found by Gaussian fitting. Peak temperatures, T p , can be used with Kissinger’s equation to estimate trapping energy, E a, [19]: log       1   = −    (2) A dominant trap binding energy, E a , of 37.5 kJ/mol was identified. It can thus be considered a trap with moderate strength, generally compatible with values reported in the scientific literature for grain or lath boundaries and dislocations, [14]. The absence of significant and repeatable high-temperature peaks further suggests that RA did not trap significant hydrogen in the measured conditions. Indeed, Zhou et. Al. [17] argued that RA can function as a reversible or irreversible hydrogen trap depending on its specific morphology in the considered material.

Fig. 4. H permeation test results: buildup (blue) and decay (orange) permeation transient. (a), TPD results at different heating rates (b).