PSI - Issue 22

Olha Zvirko et al. / Procedia Structural Integrity 22 (2019) 299–304

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Olha Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

Keywords: pipeline steel; hydrogen assisted degradation; electrochemical analysis; brittle fracture resistance

1. Introduction Natural gas transmission pipelines are an important component of worldwide energy supply. However, many gas pipelines are near the end of their design life. Long-term operation of pipelines leads not only to an appearance of macro defects but also to in-bulk damaging of pipeline steels at nano- and microscales as it was shown by Gabetta et al. (2008), Nykyforchyn et al. (2017). Hydrogen embrittlement, degradation of mechanical properties associated to a safe serviceability of the pipelines, stress corrosion cracking are the main problems for long-term operated pipeline steels ; they were investigated in numerous issues by Krasowsky et al. (2001), Tsyrul’nyk et al. (2004), Maruschak et al. (2014), Zvirko et al. (2016) and others. A sharp decrease in characteristics of brittle fracture resistance of pipeline steels under long-time operation, as it was demonstrated by Gabetta et al. (2008) and Krasowsky et al. (2001), increases significantly a failure risk. Deterioration of pipelines under operation calls for effective methods for current condition evaluation. Pipelines are subject to periodic in-service inspection by non-destructive testing methods, which only detect and size defects and damages. However, for the correct assessment of the residual lifetime of aging pipelines it is important to take into account the current technical condition of the pipeline steel. The studies carried out by Tsyrul'nyk et al. (2008), Zvirko et al. (2017) and Nykyforchyn et al. (2018) showed that both mechanical and electrochemical properties of pipeline steels due to long-term operation are significantly deteriorated. The influence of internal and external corrosion environment on resistance of pipeline steels to corrosion and stress corrosion cracking was analyzed by Tsyrul'nyk et al. (2008) and Voloshyn et al. (2015). Therefore, a usage of an electrochemical method to evaluate degradation of mechanical properties of long-term operated pipelines steels was proposed. Electrochemical tests can be performed on any structure segments without a need of extracting material specimen for laboratory testing. This paper highlights the electrochemical characteristics useful for characterization of degraded material and also possibility of an evaluation of in-service degradation of impact toughness of pipelines steels under operation by a non-destructive electrochemical method. 2. Objects, materials and methods The research objects were low-carbon low-alloyed ferrite-pearlite pipeline steels with different strength: 17H1S (0.17C-Mn-Si, strength grade API 5L Х 52), API 5L Х 52, API 5L Х 60 and API 5L Х 70 in different states – as received and after long-term operation (25 – 53 years). Sections of pipes being investigated were cut from gas transit pipelines after different time of operation: 17H1S – 28 – 53 years, Х 60 – 25 years and Х 70 – 37 years. The Х52 steel (code X52 for as-received state) was in service for 30 years (code X52-10 and X52-12 for wall thickness 10 and 12 mm respectively). Taking into account aggressive influence of the condensed water, accumulating on the pipe bottom, on the pipeline steel degradation, the top and bottom sections of the pipe were also distinguished in some cases. In order to compare the behaviour and properties of steels, samples were also taken from reserved pipes made of steels of different strength grades (X52, X60 and X70). Impact testing of Charpy V-notch specimens was performed to evaluate impact toughness KCV of the investigated steels as characteristic of their brittle fracture resistance. The Charpy specimens of the cross section of 10x10 mm with sharp V-type notch of radius of about 0.25 mm were used. The specimens were machined in the transversal direction of the pipe section. Notch was cut out from side of the internal pipe surface. To study the electrochemical behaviour of the steels and to determine their tendency to corrosive degradation during long-term operation, a series of electrochemical tests were carried out using a potentiodynamic method. The tests were performed on Bio-Logic SP-300 and IPC-Pro potentiostates, using a standard three-electrode electrochemical cell consisting of working electrode, Ag/AgCl (saturated KCl) reference electrode and auxiliary Pt electrode. The working electrodes were made from the studied steels in the form of bars with polished all surfaces. Insulating waterproof coating was applied on all surfaces of the working electrodes, except the selected area of about 0.5 cm 2 for electrochemical studies and for current supply. Potentiodynamic polarisation curves were performed by sweeping potential from - 1.1 V to corrosion potential (Е corr ) Е corr + 0.6 – 0.7 V vs. Ag/AgCl at a sweep