Effect of pH and Temperature on Stress Corrosion Cracking of API X60 Pipeline Steel
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Effect of pH and Temperature on Stress Corrosion Cracking of API X60 Pipeline Steel A. Contreras1*, S. L. Hernández1 and R. Galván-Martínez2 1
2
Instituto Mexicano del Petróleo, Eje central Lázaro Cárdenas Norte 152, San Bartolo Atepehuacan, C. P. 07730, México. * E-mail: [email protected]
Unidad Anticorrosión, Instituto de Ingeniería, Universidad Veracruzana, Av. S.S. Juan Pablo II S/N, Z. Universitaria, Fracc. Costa Verde, Veracruz, México, CP 94294.
ABSTRACT Stress corrosion cracking (SCC) is the result of the combined influence of tensile stress and a corrosive environment on a susceptible material. This paper analyzes the understanding mechanisms of stress corrosion cracking (SCC) of API X60 pipeline steel. The susceptibility to SCC and mechanism was investigated using slow strain rate tests (SSRT). The SSRT were performed at strain rate of 25.4 x 10-6 mm/sec in a glass autoclave containing a soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Cathodic polarization potentials of -200 mV referred to Ecorr was applied. SCC properties were evaluated by SSRT in simulated soil solution (NS4 solution) that is typical environment for SCC under cathodic charging. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was more susceptible to SCC at pH 3 independently of the temperature. At high pH the SCC susceptibility was higher at 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The failure and SCC mechanism of X60 steel into NS4 solution was hydrogen based mechanism. Hydrogen absorption by X60 pipe steel may result in local corrosion (primarily, SCC) of underground pipeline steel. SEM observations revealed the presence of internal cracks in some of these specimens. Keywords: Steel, Corrosion, Fracture, Scanning Electron Microscopy (SEM), Stress/Strain relationship.
INTRODUCTION Stress corrosion cracking is a type of environmentally assisted cracking that occurs when tensile stresses (mechanical factor) are applied to a susceptible material (metallurgical factor) which is exposed to corrosive environment (electrochemical factor). Environmentally assisted cracking (EAC) is referred to any cracking process which can be enhanced due to environmental effects. The most common types of environmentally assisted cracking observed in steels are called as: Stress corrosion cracking (SCC). Hydrogen embrittlement (HE). Stress oriented hydrogen induced cracking (SOHIC).
Mater. Res. Soc. Symp. Proc. Vol. 1275 © Materials Research Society
In this type of EAC, cracking propagate due to anodic dissolution and due to hydrogen embrittlement mechanism. SCC is a process that consists of three stages: crack initiation, steady crack propagation and fracture failure. SCC can be either intergranular (IGSCC) or transgranular (TGSCC). Transgranular cracks propagate through the grains usually in specific crystal planes and intergranular cracks gr
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