Relationship between high pH stress corrosion cracking mechanisms and applied potentials of X80 pipeline steel

Most of the current researches are only limited to the evaluation of high pH stress corrosion cracking (SCC) susceptibilities of pipeline steels at some certain potentials, but the division of the sensitive potential ranges controlled by different SCC mechanisms is rarely reported. When the results of SCC susceptibilities adopted by using the ultimate fracture strength, the reduction-in-area and the elongation rate separately are not consistent, how to evaluate the SCC susceptibilities taking into account both the loss of fracture strength and toughness also needs solving. In this work, the slow strain rate tensile tests were conducted to evaluate the SCC susceptibilities of X80 pipeline steel in concentrated carbonate/bicarbonate solution at different applied potentials. The immersion tests at several constant potentials were used to analyze the potential sensitivity of electrochemical corrosion behavior and its influence on the SCC controlling mechanisms. With the aid of potentiodynamic polarization tests and the thermodynamical calculation, the typical potential ranges of different SCC controlling mechanisms were divided. Considering both of the ultimate fracture strength and reduction-in-area loss, a comprehensive index I_∑ for estimating SCC susceptibility was established and verified. The main results showed that at -580 mV, the ferritic intergranular was corroded preferentially, and X80 steel exhibited remarkable intergranular SCC (IGSCC) with anodic dissolution (AD) as the controlling mechanism. At -750 mV, the ferritic grain was prone to be dissolved causing the occurrence of pitting, but the steel showed very low SCC susceptibility, which indicated that high transgranular SCC (TGSCC) susceptibility could not be triggered only by AD. The open circuit potential E_ocp could be seen as a transition potential between pitting and TGSCC zone, at which the SCC susceptibility was slightly enhanced. At -880 mV, the steel showed tremendous TGSCC susceptibility due to the synergistic effects of AD and hydrogen embrittlement (HE), although it was protected by the applied cathodic potential. And the SCC behavior mainly controlled by HE at -1200 mV, showing quasi-cleavage fracture mode. Therefore, the typical potential ranges could be divided into four categories: -600 - -569 mV for IGSCC, E_ocp - -600 mV for pitting, hydrogen evolution potential -1046 mV - E_ocp for TGSCC, and below -1046 mV for HE, i.e., quasi-cleavage cracking. By applying the criteria proposed in acidic soil simulated solution to illustrate the SCC susceptibility in this work, the comprehensive index I∑ was proved to be reasonable, which provided a supplementary method for evaluating the SCC susceptibility.