Effects of Cathodic Potential, Bicarbonate, and Chloride Ions on SCC of X70 Pipeline Steel

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 -1100 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 -1100 mV versus SCE, all fracture surfaces showed semi-brittle behavior with transgranular cracks.