Tension Tests Behavior of API 5L X60 Pipeline Steel in a Simulated Soil Solution to Evaluate SCC Susceptibility
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Tension Tests Behavior of API 5L X60 Pipeline Steel in a Simulated Soil Solution to Evaluate SCC Susceptibility A. Contreras1*, S. L. Hernández1, R. Galvan-Martinez2, and O. Vega-Becerra3 1
Instituto Mexicano del Petroleó, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, C.P. 07730, México. * Email: [email protected] 2 Unidad Anticorrosión, Instituto de Ingeniería, Universidad Veracruzana, Ave. S.S Juan Pablo II S/N, Ciudad Universitaria, Fracc. Costa Verde, Veracruz, C.P. 94294, México. 3 Centro de Investigación en Materiales Avanzados, S.C. Unidad Monterrey Alianza Norte 202. Parque de Investigación e Innovación Tecnológica. Apodaca, Nuevo León, C.P. 66600, México.
ABSTRACT In this work slow strain rate tests (SSRT) were used for the evaluation of API 5L X60 in contact with a simulated soil solution called NS4 in order to evaluate stress corrosion cracking (SCC) susceptibility. SSRT were carried out in NS4 solution at room temperature to simulate dilute ground water that has been found to be associated with SCC of low carbon steel pipelines. A strain rate of 1x10-6 sec-1 was used. According to the analysis of SSRT results, the X60 pipeline steel is highly resistant to SCC at the conditions studied. A combine fracture type it was observed: ductile and brittle with a transgranular appearance. Some pits close to the fracture zone were observed. The failure process and mechanism of X60 steel in NS4 solution are controlled by anodic dissolution and hydrogen embrittlement which was revealed with the internal cracks observed in the surface fracture. There is a relation between the strength of the steel and the SCC susceptibility, thus, increasing strength in the steel, the SCC susceptibility increases as a function of the pH solution used. Keywords: corrosion, steel, fracture, microstructure, embrittlement. INTRODUCTION It has been only a few years since it was recognized that stress corrosion cracking (SCC) can occur in buried pipelines. SCC on the external surface of pipelines has occurred in several countries throughout the world [1-3]. The failures provoke by SCC generally are catastrophic and almost always cause human injuries and economical losses. SCC has been experienced in line pipes with a wide range of chemical compositions, strengths, grades, sizes and coatings [4]. Additionally, it was seen that SCC has occurred in a wide variety of soils. Pipeline steels are known to be susceptible to two types of stress corrosion cracking: intergranular (high pH-SCC or classical) and transgranular SCC (near neutral pH or nonclassical). The great majority of those failures are associated with intergranular cracking, although transgranular cracking also have been observed. Intergranular cracking is suggested to occur by a localized dissolution process in a carbonate-bicarbonate solution. For high pH SCC it is well accepted that the mechanism involves anodic dissolution for crack initiation and
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propagation. In contrast, it has been suggested that the low pH SCC is associated with the dissolution of t
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