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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specimens will be exposed to hydrogen or hydrogen-natural gas mixture for long-time (from 3 to 24 months) in the field (Fig. 3) with the following in-laboratory testing. For prevention of hydrogen desorption at transporting specimens from field to laboratory holding in liquid nitrogen will be applied. The assembled stand (Fig. 3) with two pipes enables holding witness samples in hydrogen or natural gas-hydrogen mixture at a certain pressure under conditions similar to those inherent to above-ground part of natural gas pipelines, subjecting to climate temperature changes.

Fig. 3. Stand with two pipes for exposing witness samples in hydrogen or natural gas-hydrogen mixture.

For research pipes with an outer diameter of 159 mm and a wall thickness of 4.5 mm in the as-received state (reserve pipes) and after operation were used. Pipes were made of the VSt3ps carbon steel of ordinary quality (0.12% C). It is taken into account that metal of welded joint and pipe elbow, especially tensioned part, can be more sensitive to hydrogen embrittlement than steel of linear part of pipe (base metal), as it was demonstrated in numerous issues by Tsyrul'nyk et al. (2011), Nykyforchyn et al. (2017), and others. Therefore, steel properties not only in different state (as-received and operated), but also from different zones of weld joints and different parts of elbows will be determined and compared to define the most susceptible material to hydrogen embrittlement depending on operational and manufacturing factors. Resistance to brittle fracture of a metal (impact strength, fracture toughness, etc.), and sensitivity to hydrogen induced cracking will be determined in laboratory. Hydrogen concentration in steels before and after exposition to hydrogen or hydrogen-natural gas mixture will be also analysed in order to evaluate possibility of hydrogenation of a material in field conditions. Witness samples in pipes with hydrogen or hydrogen natural gas mixture could be hydrogenated from surface, so samples of reduced thickness should be preferred. Flat tensile specimens cut out of pipes in the transversal direction, gage thickness and width are 1.2 mm and 4.0 mm, respectively, will be pre-loaded and then exposed to hydrogen or natural gas-hydrogen mixture. Detailed microfractographic analysis will be also performed, which enables to clarify behaviour of hydrogen in a metal and to evaluate sensitivity of steels in both as-received and operated states to hydrogen embrittlement. 5. Summary Carrying hydrogen or its mixtures with natural gas via steel pipelines can lead to hydrogenation of steel from internal pipe surface due to both hydrogen dissociation and hydrogen evolution under electrochemical corrosion as a result of interaction with condensed moisture. Absorbed hydrogen can negatively effect on steel by both hydrogen embrittlement and creating hydrogen pressure in defects as a result of recombination causing dissipated damaging.