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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Fig. 3. Scheme of a build-up permeation transient registered under stepwise application of cathodic current to ingress electrochemical cell ( a ) and critical cathodic current density for the API 5L X52 steel in the different states ( b ).

The critical cathodic current density is the highest for the steel in the as-received state (Fig. 3 b ). The operated steel becomes irreversibly damaged under much lower current. For steel specimens cut from the bottom pipe section, especially near the inner surface subjected to corrosion during operation, the value of cr c i is less than that for specimens from the top part of the same pipe. A higher susceptibility to formation of defects due to hydrogenation (decrease in cr c i ) is concerned with a higher trapping efficiency of metal and hence its intensive in-service degradation. The highest degradation degree of the steel of the inner bottom pipe section is confirmed by both hydrogen extraction and hydrogen permeation methods. 4. Argumentation of hydrogen influence on operational degradation of steels A clear evidence of a destructive role of hydrogen in degradation of long-term operated objects derives from a comparison of the properties of different metal parts of the same installation that have the same stress state but different hydrogenation conditions during operation. As an example, Slobodyan et al. (2002) considered the top and bottom sections of the pipe operated on the oil pipeline for 30 years. Dissolving aggressive species from the transported hydrocarbons, a residual water was collected at the bottom of the pipe and formed favourable conditions for corrosion and subsequent hydrogenation of the lower section of the pipe. As a result, steel properties of the pipe bottom were essentially worse comparing to its top section. A significant part of long-term operated structures is made of rolled steels, which degradation are characterized by specific peculiarities illustrated by Student 1 et al. (2018), Nemchuk et al. (2019) and others. Considering the pipeline steels, Zvirko et al. (2018) showed that hydrogen induced defects in a form of microdelamination occurred between structure fibers can lead to a macrodelamination. Such cases happen mainly in aboveground pipeline sections, where temperature changes are especially noticeable and result in moisture condensation at the inner pipe wall, as it was demonstrated by Nykyforchyn et al. (2017). Similar to the previous example, moisture inside the pipe became a reason for corrosion and hydrogenation facilitating steel degradation. As a result, in-service damaging becomes oriented in the direction of rolling. Then the direction of specimen cutting becomes especially important for a correct evaluation of steel properties. Scheme in Fig. 1 has been developed taking into account longitudinal specimens (cut in the rolling direction), as commonly. Using this specimen type in Charpy testing, the fracture plane intersects texture/rolling direction and hence preferable direction of delamination (Fig. 4). This requires additional energy that could result even in an increase in impact toughness, as illustrated as stage IIB (Fig. 1). However, in this case increasing in impact toughness of operated pipeline steel determined using longitudinal specimens does not mean a decrease in degree of metal degradation, rather the opposite. This can be seen by testing short transverse specimens, enabling fracture propagation in the rolling direction, as shown by Nykyforchyn et al. (2017).