PSI - Issue 13

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 13 (2018) 1215–1220 Hryhoriy Nykyforchyn / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Long-term operation of natural gas transit pipelines implies aging, hydrogen-induced and stress corrosion cracking and it causes hydrogen embrittlement of steels, degradation of mechanical properties associated to a safe serviceability of the pipelines, and failure risk increase. The implementation of effective diagnostic measures of pipelines steels degradation would allow planning actions in order to reduce a risk of fracture. The metal condition is influenced by a number of operational factors; in particular, changes in the metal condition are intensified under long-term mutual effect of mechanical stresses and corrosion and hydrogenating media as it was shown by Nykyforchyn et al. (2010). Corrosion-hydrogen degradation of long-term operated pipeline steels is manifested, first of all, in hydrogen embrittlement and decrease in resistance to brittle fracture and to stress corrosion cracking, as it was was demonstrated in numerous issues by Ch uvil’deev (2006), Gabetta et al. (2008) and Nykyforchyn et al. (2010). It is known that process of in-service degradation of metal occurs non-uniformly on a micro scale. Therefore the fracture propagates along the path with the minimum energy consumption; that is the fracture pass through local sites with the maximum degree of in-service degradation of the metal. Taking into account that characteristics of brittle fracture resistance of pipeline steels decreased significantly due to long-term operation of transit gas pipelines, it should be expected that fracture surface of specimens made of operated pipeline steels and tested for brittle fracture resistance estimation should most of all represent those changes in the metal that led to its degradation. It is known that any changes in a metal – environment system will cause a change in electrochemical behaviour of metal and its electrochemical properties. Taking into account that long-term service of metal structures causes changes in metal state, as indicated by changes in its microstructure and mechanical properties degradation, it can also be expected that its electrochemical characteristics will be changed too. The studies carried out by Nykyforchyn et al. (2011, 2017), Zvirko et al. (2017), Zvirko (2017) showed that electrochemical properties of the metal are sensitive to the change in its state caused by long-term operation. Electrochemical properties of operated metal determined on fracture surface should differ from electrochemical properties obtained on its polished surface taking into account that fracture surface reflects most of all changes in metal associated with its operational degradation. In this paper, a new scientific and methodical approach for evaluation of in-service degradation of long-term operated pipeline steels based on determination of electrochemical properties of fracture surface obtained under testing specimens to estimate resistance to brittle fracture has been developed. This approach has been verified on ferrite-pearlite X52 API 5L pipeline steels. In this work carbon steels with different carbon content and pipeline steels were investigated. Carbon steels with different carbon content C C (metal in the as-received state): steel 20 (0.20% C); steel 35 (0.35% C); steel U8 (0.80% C); steel U12 (1.20% C) were studied. Low-alloyed ferrite-pearlite API 5L X52 pipeline steel was tested. The specimens were cut from real pipes: 1) reserved pipe made of X52 steel (marked as X52) and 2) two pipes made of X52 steel after 30 years operation (natural gas transit pipeline) with wall thickness t = 10 mm (marked as X52-10) and t = 12 mm (marked as X52-12). Impact testing of Charpy V-notch specimens was performed to evaluate impact toughness KCV of investigated steels as characteristic of their brittle fracture resistance. The specimens were machined in the longitudinal direction, in which the axis of the specimen lay in the rolling direction. The electrochemical tests were carried out in 0.3% aqueous NaCl solution. The specimen potential (working electrode) of investigated metal was measured against an Ag/AgCl (saturated KCl) reference electrode by means of potentiostat. Time dependencies of electrode potential (open-circuit potential) were recorded. Electrode potential E pol of polished steel surface and electrode potential Е fr of fracture surface of specimens made of studied steels were determined. The working carbon steel electrodes were in the form of bars 1.0 × 1.0 cm and length of 3 cm, of polished all surfaces. The electrochemical experiments of pipeline steels were performed using Charpy V-notch specimens with polished surfaces. Potential E pol of polished surface of Charpy V-notch specimen was determined and then impact testing of specimen was performed. After impact testing, insulating waterproof coating was subsequently applied on all surfaces of the fractured specimen, except the fracture surface for electrochemical studies and the selected area to be connected to potentiostat, and then potential E fr of the fracture surface of specimen was determined. 2. Objects, materials and methods