Abstract
An electrochemical study of stress corrosion cracking (SCC) of API X70 steel in a simulated soil solution at different pH values (3, 8 and 10) was carried out. The stress conditions were implemented by slow strain rate stress test (SSRT) and the SCC process was simultaneously monitored by electrochemical impedance spectroscopy (EIS). Fracture surface analysis and corrosion product analysis were performed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that the susceptibility to SCC was higher as the pH decreases. In the acid solution, hydrogen evolution can occur by H+ and H2CO3 reduction, and more atomic hydrogen can diffuse into the steel, producing embrittlement. EIS results indicated that the anodic dissolution contributed to SCC process by reducing the charge transfer resistances during the SSRT test. While SEM micrographs shown a general corrosion morphology on the longitudinal surface of samples. At higher pH (pH 8 and pH 10), the SCC susceptibility was lower, which it is attributed to the presence of corrosion products film, which could have limited the process. Using the angle phase (φ) value it was determined that the cracking process started at a point close to the yield strength (YS).
Highlights
It has been known that buried pipelines had experience failures caused by a form of localized corrosion known as stress corrosion cracking (SCC)
At pH 3, the X70 steel was moderately susceptible to SCC, whereas at pH 8 and pH 10, the X70 steel presented low susceptibility to SCC
Fractographic analysis by scanning electron microscopy (SEM) revealed that at pH 3, the fracture surface exhibited a brittle appearance with presence of internal cracks, indicating that hydrogen was involved in the SCC process
Summary
Buried pipelines have been used as one of the most economical and safe ways to transport oil and natural gas to meet energy demand around the world [1]. It has been known that buried pipelines had experience failures caused by a form of localized corrosion known as stress corrosion cracking (SCC). This is a highly important issue, because always the leakage or rupture and failure of the pipelines can pose a potential threat to humans and environment [2,3,4]. SCC is characterized by initiation and propagation of cracks due to the simultaneous effect of tensile stress and a specific corrosive environment [5]
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