Fracture Behavior on SCC of API X52 Pipeline Steel Under Cathodic Protection
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Fracture Behavior on SCC of API X52 Pipeline Steel Under Cathodic Protection A. Contreras1* M. A. Espinosa-Medina2 and R. Galvan-Martínez3 1
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Instituto Mexicano del Petróleo, Eje central Lázaro Cárdenas Norte 152, San Bartolo Atepehuacan, C. P. 07730, México. * E-mail: [email protected] 2 Escuela de Ingeniería Mecánica de la Universidad Michoacana, Morelia, Mich. México. Unidad Anticorrosión, Instituto de Ingeniería, Universidad Veracruzana, Av. S.S. Juan Pablo II s/n, Fracc. Costa Verde, Veracruz, México, CP 94294.
ABSTRACT This paper analyzes the fracture behavior and mechanisms of stress corrosion cracking (SCC) of API X52 pipeline steel. Susceptibility and mechanism of SCC was investigated using slow strain rate tests (SSRT) performed at strain rate of 1 x 10-6 in/sec in a glass autoclave containing a soil solution with pH of 8.5 at room temperature. Cathodic polarization potentials of -100, -200 and -400 mV referred to Ecorr was applied in order to establish the effectiveness of cathodic protection in mitigating SCC of X52 pipeline steel. To study the effects of several over potential in SSRT were performed at different cathodic potentials. The results of reduction area ratio (RAR), time to failure ratio (TFR) and plastic elongation ratio (PER) indicate that X52 pipeline steel was susceptible to SCC. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The failure and SCC mechanism of X52 steel in the soil solution was hydrogen based mechanism. This mechanism was confirmed through the internal cracks observed in these specimens. All cracking tests indicated that the SCC rate was enhanced by plastic deformation. Keywords: Fracture, Corrosion, Embrittlement, Stress/Strain relationship.
INTRODUCTION Stress corrosion cracking (SCC) from the external surface of a buried pipeline is a serious problem and it can cause significant economic, environmental and human losses. Despite of many research works to understand the crack initiation and propagation mechanisms, these fracture mechanisms are still being debate. For high pH SCC it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH SCC is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen in the steel [1-3]. The hydrogen concentration plays an important role in this type of cracking. It is believed that applied stresses to the pipe surface are the cyclic nature (fatigue) due to the internal pressure fluctuations and cracking occurs due to synergistic interaction between hydrogen embrittlement corrosion (HIC) and fatigue loading. Cracks propagate as a result of anodic dissolution in front of their tip in SCC process, due to the embrittlement of their tip by hydrogen based mechanism. It was revealed that cracking behavior of pipeline steel in the soil environment depends of the cathodic protection applied. Studies of potential effects on SCC w
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