PSI - Issue 68

Olha Zvirko et al. / Procedia Structural Integrity 68 (2025) 868–873 Olha Zvirko / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The hydrogen economy is advancing yet confronts significant challenges, particularly in hydrogen transportation via pipelines. Many countries, including the European Union, need a sufficiently developed hydrogen network. As a solution, there is considerable interest in utilizing existing natural gas pipelines to transport hydrogen or hydrogen natural gas mixtures (Haeseldonckx and D’haeseleer (2007), Nykyforchyn et al. (2021), Pluvinage et al. (2021), Poberezhnyi et al. (2024)). According to the European Hydrogen Backbone, 70% could be repurposed by 2030. With its extensive gas transmission network connected to the European Union, Ukraine presents a unique opportunity for hydrogen transportation. However, repurposing these pipelines for hydrogen transport is complex. A significant concern is hydrogen's negative impact on steel's mechanical properties, which raises questions about pipeline integrity when transporting hydrogen. Transportation of hydrogen or its admixture with natural gas via existing natural gas pipelines can increase the risk of failure due to hydrogen embrittlement (HE) and hydrogen induced cracking of steels (Depover et al. (2014), Mohtadi-Bonab and Eskandari (2017), Boukortt et al. (2018), Hoschke et al. (2023)). Moreover, the existing pipeline network often has a long operation time, which leads to a deterioration of the mechanical properties of steels, especially its resistance to brittle fracture (Zhu (2015), Maruschak (2016), Zvirko (2021), Zvirko et al. (2023), Hrabovskyy et al. (2024), Nykyforchyn et al. (2024), Tsyrulnyk et al. (2024)). Analyzing material aspects of hydrogen transport, it is suggested that mixtures with up to 12–17% of hydrogen should not cause issues to the existing distribution network (Haeseldonckx and D’haeseleer (2007), Pluvinage et al. (2021)). HE of as-delivered pipeline steels is extensively investigated, showing a reduction in plasticity, fracture toughness, fatigue resistance, and transition of fracture of the hydrogen-charged specimens from ductile to brittle, as demonstrated by Depover et al. (2014), Alvaro et al. (2019), Dadfarnia et al. (2019), Pluvinage et al. (2021), Nykyforchyn et al. (2022), Kappes and Perez (2023), Cao (2024), Tsyrulnyk et al. (2024), and others. The presence of inclusions in steel plays a critical role in hydrogen-induced cracking, with non-uniform distribution increasing susceptibility to embrittlement. One of the main factors of the negative impact of hydrogen on operated gas pipe integrity is its in-service degradation and damage, as shown by Nykyforchyn et al. (2021, 2022). Evaluating the effect of hydrogen on the mechanical behavior of pipe steels, two mechanisms of steel hydrogen uptake should be considered: hydrogen evolution as a result of electrochemical corrosion on the pipe's internal surface due to the presence of moisture in the transported environment and molecular hydrogen dissociation (Nykyforchyn et al. (2021)). Additionally, long-term operation of steels may increase sensitivity to HE and enhance the anisotropy of mechanical properties in pipeline steels (Nykyforchyn et al (2021, 2023)), which can complicate testing and evaluation due to potential delaminations between non-metallic inclusions and the steel matrix. Several methodologies are used to assess the susceptibility of steels to HE, with tensile testing with ex-situ or in situ hydrogen charging being the most prevalent (Depover et al. (2014), Mohtadi-Bonab et al. (2021), Nykyforchyn et al. (2022)). These tests often reveal a decrease in plasticity due to hydrogen exposure. Evaluation of the degradation degree of steel and determination of the current mechanical properties are essential to predict the serviceability of long-term operated pipes and to investigate maintenance and repairs for the pipeline system. Therefore, the sensitivity of steels to HE should be assessed, taking into account the degree of operational degradation. The research is aimed at assessing the sensitivity of pipeline steels to HE, considering their operational degradation, anisotropy, and different hydrogen charging intensities. 2. Materials and methods The research object is low-carbon steel (Ukrainian code 17H1S, equivalent to API 5L Х52, chemical composition in wt. %: C 0.2; Mn 1.3; Si 0.4; S £ 0.04; P £ 0.01; Al £ 0.04, Fe balance). Steel specimens were machined from two pipes: a reserve pipe (vintage) and after 38 years of operation at the Ukrainian gas transit pipeline (Zvirko et al. (2024)). The manufacturing technology was the same for both steels (pipe outer diameter is 1220 mm and wall thickness is 12 mm). The investigated steels had a microstructure which consisted predominantly of ferrite and pearlite. The properties of the operated pipeline steel were compared to those of the reserved steel, focusing on the