PSI - Issue 81

Olha Zvirko et al. / Procedia Structural Integrity 81 (2026) 41–46

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Studies of the condition of long-operated gas pipelines indicate a significant deterioration in the initial mechanical characteristics of steels (Meshkov et al. (2015), Nykyforchyn et al. (2016), Zvirko (2017, 2021)). It is manifested by embrittlement of steels, the development of damage in them and a decrease in their resistance to brittle fracture, as demonstrated by Nykyforchyn et al. (2018), Tsyrul’nyk et al. (2018), Zvirko et al. (2021), and others. At the same time, the susceptibility of operated steels to hydrogen embrittlement may increase (Nykyforchyn et al. (2025)). A particularly dangerous result of damage is the initiation and propagation of cracks. In general, the probability of the presence of undetected cracks in pipeline steels is considered already at the design stage of such structures (API 579-1/ASME FFS-1 (2016)), which cannot be avoided even when using modern non-destructive techniques. Operational degradation of steels accelerates the growth of existing cracks and the initiation of new cracks at different scales. Therefore, crack growth resistance is an important characteristic of pipe steels determining the serviceability of pipelines. Under the conditions of hydrogen transportation, it will interact with pipe steels, resulting in hydrogen degradation of steels, primarily due to their hydrogen embrittlement (Campari et al. (2023), Zvirko et al. (2025)). This causes deterioration in the mechanical characteristics of steels, which negatively affects their performance by increasing the risk of integrity loss, particularly in steels pre-embrittled by long-term service. Currently, there are no established criteria for the limit state of metal in reprofiled pipelines, particularly under conditions of hydrogen transportation via existing gas pipelines. This study addresses the development of a serviceability criterion for the transmission of hydrogen through existing natural gas pipeline infrastructure. The research focuses on evaluating material compatibility, structural integrity, and safety requirements under operational pressures and conditions specific to hydrogen transport. 2. Materials and methods 2.1. Material The research object was low-carbon pipeline steel (Ukrainian code 17H1S, equivalent to API 5L Х52 strength grade) after 38 years of operation at the Ukrainian natural gas transit pipeline. The steel had a microstructure which consisted predominantly of ferrite and pearlite. To substantiate the criterion for achieving the critical state in pipe steels, results on their crack resistance as a function of operating conditions, hydrogenation, and deformation rate were used. The crack growth resistance of the metal was determined using the J -integral method, per ASTM E1820, on single-edge notched bending specimens. Pipe steel 17H1S, operated for 38 years on a main gas pipeline (Nykyforchyn et al. (2025)), was studied. Its sensitivity to hydrogen embrittlement was evaluated during tests conducted both before and after preliminary electrolytic hydrogen charging of specimens with cracks, which was implemented in the electrolyte H 2 SO 4 (pH1) with an addition of 10 g/l thiourea at current densities of 0.05 mA/cm 2 and 1mA/cm 2 for 120 hours. The specimens were tested at loading rates v = 0.5, 0.05 and 0.005 mm/min. Specimens were extracted from the pipe sections oriented longitudinally to the pipe axis. 3. Test Results and Discussion The test results of the influence of absorbed hydrogen on crack growth parameter J 0Н for the pipeline steel, which has been in service for 38 years, are shown in Fig. 1 (the J 0H index is the value of the J -integral determined for crack initiation). Considering a correlation established between the hydrogen fugacity during electrochemical charging and the pressure of gaseous hydrogen (Liu et al. (2018), and others), which ensure the same concentration of absorbed hydrogen in a metal, as well as the established relationship between fugacity and current density under charging, equivalent hydrogen pressure values Р Н eq 2 = 0.6 and 9.71 MPa were determined for the applied regimes of electrolytic hydrogen charging of specimens (Fig. 1). The crack growth parameter J 0Н for specimens of the post-operated steel depended on both equivalent hydrogen pressure Р Н eq 2 and loading rate v (Fig. 1). Based on experimental data on the influence of hydrogen on the fracture toughness of pipeline steels, a criterion for assessing the serviceability of steels during hydrogen transportation has been proposed. It is based on determining the fracture toughness of hydrogen pre-charged pipe steel specimens at a low loading rate and comparing the obtained values with the limit one, taking into account the presence of possible crack-like defects in an operated pipeline. As a criterion for the limit state of the steel of the operated pipeline, it is proposed to use the fracture toughness value K H , which is converted from J 0H , determined experimentally in accordance with the approaches of nonlinear fracture mechanics, according to the formula: Н =√ 0Н 1− 2 . (1) 2.2. Experimental Procedures