PSI - Issue 28

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 28 (2020) 896–902 H. Nykyforchyn, O. Tsyrulnyk, O. Zvirko, M. Hredil / Structural Integrity Procedia 00 (2019) 000–000

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hydrogen behaviour have been used for an estimation of possible metal damaging, taking into account that hydrogen in a metal is located mainly in defects, which considered by Lunarska et al. (1997) as hydrogen traps. To determine the amount of hydrogen in metal by vacuum extraction, temperature was increased gradually with a certain delay at 200  C, 400 and 600  C. Thus, the “low-energy” hydrogen, i.e. hydrogen located in low-energy traps (from the point of its interaction with defects) leaves metal under comparatively low temperature. Such traps include, for example, dislocations. At the same time, the “high-energy” hydrogen accumulates in deeper traps (pores, nano- and microcracks), therefore, its desorption is expected only at higher temperatures. Basing on this, the metal defectiveness was analysed, as shown in Table 1. Table 1. Concentration of diffusible hydrogen V H in API 5L X52 pipeline steel measured by extraction at different temperatures Steel state Pipe section Total V H [ppm] Amount of H [% from total V H ] 200  C 400  C 600  C As-received - 1.5 92.7 4.6 2.7 It is derived from the Table 1 that the steel after long term operation, particularly the inner bottom pipe section, contained a higher amount of hydrogen comparing to that for the as-received X52 steel. It could be explained by corrosion processes at the inner pipe wall during operation resulting from electrochemical interaction between metal and moisture which is condensed from transported hydrocarbons as it was shown by Tsyrulnyk et al. (2008), and Hredil and Tsyrulnyk (2010). In the case of steels in the initial state, most of hydrogen was desorbed at a low extraction temperature (200 °C), whereas the operated steel is characterized by higher temperatures of desorption indicating more intensive hydrogen trapping. The higher amount of hydrogen in the more tightly trapped state in the operated material means not only the accumulation of hydrogen during operation, but also the formation of new hydrogen traps and/or the transformation of old ones. The electrochemical permeation method for determining the diffusion coefficients of hydrogen in a metal developed by Devanathan and Stachurski (1964) involves the usage of a metal membrane as a specimen clamped between two electrochemical cells. The nessesary conditions for the method realization can be reached by application of cathodic current to one side of membrane (ingress side) while the oposite (egress) side is polarized anodically. Atomic hydrogen formed as a result of the reduction process at the cathodic side of membrane partially penetrates it reaching its passivated egress side. Hydrogen atoms at the egress surface are almost completely ionized by applied anode potential providing ionization current which is proportional to instant rate of hydrogen desorption. Thus, using this method one can determine intrinsic (lattice) D and apparent D * hydrogen diffusion coefficients, and also their relation as a measure of the trapping efficiency. The electrochemical method described above was used for the estimation of metal susceptibility to hydrogen induced cracking and, in this way, metal defectiveness. In such a circumstance hydrogen accumulates mainly in metal defects, and just these defects facilitate hydrogen assisted cracking. For these purposes, the cathodic current density applied to the ingress surface of the metal membrane was increased step by step. Each time this led to the increment of anodic current density in the egress cell. The regularity is maintained until damaging became to develop under combined action of hydrogen and residual stresses induced by intensive hydrogen charging. Then a sharp decline appeared on the ionization curve of the egress side of the membrane, that indicates the trapping of hydrogen with newly formed defects, that is, the development of hydrogen-induced damaging. Thus, the cathodic current density which is correspondent to defect occurrence is considered as the critical cr c i (Fig. 3 a ), it allows determining the metal susceptibility to hydrogen induced cracking. Operated (30 years) Top, near external surface Bottom, near internal surface 1.7 5.1 17.5 35.1 15.7 47.4 81.4 2.9