Influence of microstructure on stress corrosion cracking of X100 pipeline steel in carbonate/bicarbonate solution

The stress corrosion cracking (SCC) behaviors of X100 pipeline steels with different heat-treated microstructures in carbonate/bicarbonate solution at various potentials were investigated in this study. Several analyses were performed using electrochemical measurements, slow strain rate tensile (SSRT) tests, scanning electron microscopy (SEM), and electron back-scattered diffraction (EBSD). The results showed that the SCC susceptibilities of heat-treated microstructures increase as the cathodic potential shifts from −850 to −1200 mV_vsSCE. At a weak cathodic potential, the different microstructures show clear SCC sensitivity, showing that hydrogen cannot be ignored in this circumstance when the alkaline environment is not considered seriously. At a mild cathodic potential, hydrogen sensitivity controls SCC behavior, but at a larger negative potential, both electrochemical effect and hydrogen sensitivity influence SCC behavior. The SCC cracks propagate along the high angle grain boundary (HAGB) at open circuit potential (OCP) and as the applied potential is adjusted negatively, the cracks would either follow low angle grain boundary (LAGB), HAGB or random orientation propagation paths. The synergistic influence of dislocation, grain boundary, grain size, martensite/austenite (M/A) island, and carbide controls the SCC process of the X100 pipeline. SCC susceptibility was shown to be high in microstructures with coarse grain and high dislocation density, whereas grain boundaries such as the martensite/austenite (M/A) island and carbide offer sites for SCC initiation.