PSI - Issue 33

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 33 (2021) 646–651 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Hydrogen will play a significant role in a decarbonised energy system in the coming decades. Extensive infrastructure exists for distribution of natural gas and it is an obvious step to assess it to be used for supply hydrogen or mixtures with natural gas. At the same time, the realization of the prospects of transportation of hydrogen-natural gas mixtures by existing gas distribution networks exacerbates the problem of violation of steel pipe integrity due to the well-known negative impact of hydrogen on the mechanical properties of steels. One of the most important issue is hydrogen embrittlement, as well as a decrease of resistance to brittle fracture of steels as a result of hydrogen impact, which is often responsible for unpredictable low-energy fracture of metal component. Moreover, long-term operation of gas pipelines causes a reduction of physical and mechanical properties of steels as was shown in numerous issues by Nykyforchyn et al. (2009, 2021), Maruschak et al. (2014), Tsyrulnyk et al. (2018), Zvirko et al. (2019), Okipnyi et al. (2020) and Nykyforchyn (2021). Resistance to brittle fracture is usually the most intensively decreased parameter as it was pointed out by Filippov et al. (2013), Zvirko et al. (2018) and Nykyforchyn et al. (2020, 2021). Analysing effect of operational conditions on in-service degradation of steels, both mechanical loading and hydrogen released as a result of electrochemical interaction of inner surface metal with aqueous environment should be considered as the most important factors. There are two approaches for determining a role of hydrogen in a hydrogen/metal interaction. According to the first approach, hydrogen facilitates metal fracture, especially of brittle material, by certain known mechanisms such as was shown in numerous issues by McMahon Jr. (2001), Kan et al. (2017), Ohaeri et al. (2018), Mohtadi-Bonab and Ghesmati-Kucheki (2019) and others. The second one considers mutual effect of hydrogen and operating stresses on in-bulk metal degradation, which is manifested in development of dissipated damages, revealed by Nykyforchyn et al. (2009, 2020, 2021). Accordingly, a risk of integrity violation of in-service degraded pipes will increase at transport hydrogen or hydrogen-natural gas mixture via existing pipelines in a case they will be hydrogenated from their inner surface. In this paper, based on previous research results on corrosion and hydrogen degradation of steels of gas transit pipelines, some issues related to integrity violation of pipelines due to the possible negative impact of transported hydrogen or hydrogen-natural gas mixture on mechanical behaviour of pipeline steels of gas distribution networks in Ukraine are considered. 2. Hydrogenation sources of a pipe steel from inner surface It is well known that steel pipe of underground gas pipeline can be hydrogenated from its external surface due to corrosive action of soil environment when insulation has breaches. At the same time, inner surface of a pipe can electrochemically interact with condensed moisture with evolution of hydrogen (Fig. 1). Thus, it was demonstrated by Tsyrul’nyk et al. (2008), that such a moisture contains bicarbonate ions, which are cathodic depolarizers in corrosion process of steel and reduced with hydrogen evolution: 2HCO 3 – + 2e – = H 2 + 2CO 3 2– .

Fig. 1. The schematic diagram of hydrogen induced damaging of a pipe steel due to hydrogenation from inner surface.

Therefore, transported natural gas due to a certain degree of its humidity can also be considered as aggressive because of corrosion of inner surface of pipe, on the one hand, and, on the other one – hydrogenation of pipe steel, causing decrease in mechanical strength of pipeline. It should be noted that only part of hydrogen atoms generated by