Mechanistic Aspect of Non-Steady Electrochemical Characteristic During Stress Corrosion Cracking of an X70 Pipeline Steel in Simulated Underground Water

The stress corrosion cracking (SCC) and the electrochemical behavior of an API X70 pipeline steel in a simulated acidic soil solution were investigated using slow strain rate testing (SSRT), electrochemical polarization, and electrochemical impedance spectroscopy (EIS). The SCC mechanism of the X70 pipeline steel was found to be dependent on the range of applied potential, including anodic dissolution (AD), hydrogen embrittlement (HE), and the combination of AD and HE (AD+HE). The potential range for each mechanism can be determined by comparing the current shift obtained from potentiodynamic curves at different scan rates. When both currents of the fast scan (i_f) and the slow scan rate polarization curve (i_s) are positive, i.e., i_f > 0 and i_s > 0, the SCC mechanism follows AD. When i_f > 0 and i_s < 0, the SCC mechanism is controlled by both AD and HE. SCC obeys the HE mechanism when i_f < 0 and i_s < 0. In this study, we propose a methodology for calculating SCC susceptibility (S_ψ). S_ψ is related to the currents obtained by the fast and slow scan rate polarization curves and by the results of SSRT. This method facilitates the immediate identification of SCC susceptibility.