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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corrosion can be absorbed by a metal, therefore most of them in molecular form remains in transported environment. The material ability to absorb hydrogen differs for different zones of weld joint (base metal, heat affected zone and weld), the highest ability is inherent to heat affected zone and the lowest one – to weld, as shown by Capelle et al. (2013). At the presence of hydrogen atoms in a metal some of its mechanical properties degrades, especially its ductility, leading in some cases to embrittlement. Additionally, hydrogen atoms diffuse through metals and coalesce to form hydrogen molecules at certain preferred locations such as inclusions. It is still unclear whether the presence of hydrogen gas in the pipe will affect the degree of its hydrogenation as a result of electrochemical corrosion. Theoretically, this is possible because the equilibrium between hydrogen atom, which are absorbed by the metal and hydrogen evolved into the transported environment (hydrogen or natural gas hydrogen mixture), should be shifted towards intensification of the first factor. In this case, the role of electrochemical corrosion as a source of hydrogenation will increase. Since the condensation of moisture on the inner surface of the pipe is a necessary condition for corrosion, it was suggested by Nykyforchyn et al. (2016, 2017) that this process is more intensive for the aboveground part of the pipeline due to climatic changes in temperature. In that case temperature of the inner surface of the pipe can differ significantly from that of the transported gas. Namely, temperature of the metal on the pipe inner surface will determine the conditions needed for moisture to condensate on it, rather than temperature of the transported gas. Accordingly, a possible intensification of corrosion due to climatic changes in temperature will be a factor in intensifying hydrogenation of a pipe metal. This factor should also be taken into account at transporting hydrogen or hydrogen mixture through a pipeline. Accordingly, reducing their humidity will reduce the likelihood of a metal hydrogenation. Gaseous hydrogen is another source of hydrogenation. Molecular hydrogen should firstly dissociate to atomic state and only then it can be absorbed by a metal. Dissociation energy of molecular hydrogen is lower on a metal without any corrosion product or other films. The inner surface of pipeline is covered with such films, but they can be breached by applied stresses, especially, cyclic loading, exposing the freshly formed metal surface, and thus conditions for the dissociation of molecular hydrogen can be created. Cyclic overloading of pipes is especially dangerous in this case. Atomic hydrogen absorbed by a metal, regardless of a source of hydrogenation, due to its high mobility diffuses into a pipe wall towards an outer surface. This process is facilitated by stress gradient through pipe thickness caused by internal pressure of the transported product. Atomic hydrogen facilitates brittle fracture of steels. Moreover, it can recombine to molecular state in microdefects. As a result, high pressure hydrogen inside traps results in an increase in inner stress generated in a steel in vicinity of defects, which could reach value commensurate with stress induced by gas pressure in pipe. Hence, combination of high stress generated by molecular hydrogen in defects and low resistance to brittle fracture of material hydrogenated by atomic hydrogen lead to development of damages by mechanism of hydrogen induced cracking. Therefore, it is very important to distinguish state of hydrogen in a metal. It should be noted that hydrogen recombination in defects in pipeline steels can result in extensive macrodelamination inside a pipe wall. This phenomenon is typical for crude oil and gas gathering pipelines, transporting sour product. However, in the recent years there are such cases revealed in natural gas transit pipelines, shown by Nykyforchyn et al. (2016, 2017), Kharchenko et al. (2016) which are associated with operational degradation of pipeline steels. 3. Operational damaging as a main factor of the negative impact of hydrogen on pipe integrity The significant decrease in the resistance to brittle fracture of pipeline steels during operation is caused by intensive development of dissipated nano- and microdamages in in-bulk metal. In Fig. 2 fracture surface of specimen made of the 17H1S pipeline steel after 40 years operation fractured during Charpy impact test is demonstrated. As it can be seen in Fig. 2, there are chains of pores in the rolling direction on the fracture surface. Considering roundness of pores, it can be suggested, that these defects were formed as a result of high pressure of molecular hydrogen in them. Thus, the negative impact of such type of defects consists not only in the multiple creation of stress concentrators, but also in the decrease of cross-section.