Effects of Cathodic Potential, Bicarbonate, and Chloride Ions on SCC of X70 Pipeline Steel
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JMEPEG (2013) 22:1421–1429 DOI: 10.1007/s11665-012-0416-8
Effects of Cathodic Potential, Bicarbonate, and Chloride Ions on SCC of X70 Pipeline Steel A. Shahriari, T. Shahrabi, and A.A. Oskuie (Submitted February 2, 2012; in revised form September 26, 2012; published online October 23, 2012) This is an investigation on the stress corrosion cracking (SCC) behavior of X70 pipeline steel in high pH carbonate-bicarbonate solutions with different concentrations of bicarbonate and chloride ions and at cathodic potential of 21100 mV versus saturated calomel electrode (SCE) using slow strain rate testing. Electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy were used to investigate the electrochemical behavior of X70 pipeline steel in solutions with different concentrations. X70 pipeline steel fracture surface morphology in these different solutions was also studied by scanning electron microscopy (SEM). The results suggested that the susceptibility to SCC for X70 pipeline steel decreased in the most dilute carbonate-bicarbonate solution in the absence of the chloride ion. Also, at potential of 21100 mV versus SCE, all fracture surfaces showed semi-brittle behavior with transgranular cracks.
Keywords
carbon/alloy steel, corrosion testing, mechanical testing
1. Introduction Stress corrosion cracking (SCC) is one form of environmentally assisted cracking (EAC). EAC is a generic term that describes all types of cracking in materials in which the environment and stress act together to reduce the strength or load-carrying capacity of the material (Ref 1). Therefore, the environment is an important factor in formation of the SCC. Investigations showed that SCC susceptibility changes in accordance with the concentrations of ions in the solution trapped between the disbonded coating and the surface of the pipeline steel. SCC has continued toward making a significant contribution to the number of leaks and ruptures experienced by the massive infrastructure of buried pipelines (Ref 2). Two forms of SCC, namely, high pH SCC or classic SCC and near-neutral pH SCC, can happen on the exterior of the buried mild steel pipelines (Ref 2). The characteristics of these two types of SCC are found in many publications (Ref 3). Field and laboratory studies demonstrated that in addition to a susceptible material and tensile stress, the type of SCC occurring is highly dependent on the local environment developed under the disbonded coating. For instance, high pH SCC occurs normally in a concentrated carbonate-bicarbonate solution, with pH varying from 9 to 11 under a permeable coating, while the near-neutral pH SCC is
A. Shahriari, T. Shahrabi and A.A. Oskuie, Department of Material Science and Engineering, Faculty of Engineering, Tarbiat Modares University, Jalal Ave., P.O. Box 14115-143, Tehran, Iran. Contact e-mail: [email protected].
Journal of Materials Engineering and Performance
usually associated with an anaerobic, dilute bicarbonate solution with pH of 6-7, under an imperme
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