Occurrence of Near-Neutral pH Stress Corrosion Cracking in X80 and X100 Pipe Steels Under Various Cathodic Protection Conditions

In this work, proof ring tests with a duration of 220 d were performed to investigate the effect of cathodic protection (CP) on the occurrence of stress corrosion cracking (SCC) of X80 and X100 pipe steels in near-neutral pH environment. Various CP conditions were used in the study to mimic the possible CP circumstances occurring in the field, i.e., under-protection, standard-protection, and over-protection. Also, the delayed CP conditions, i.e., 110-d without CP followed by 110-d with CP, were applied to simulate the interruptions of CP in the real world. The results showed that a sufficient continuous CP could prevent the occurrence of SCC cracks. Under a given condition X100 steel tended to form deeper cracks than X80 steel. The cracks formed in both X80 and X100 steels exhibited normal/quasi-normal depth distributions. Only a small fraction of cracks would propagate deep into the steel. For X80 steel, with CP no higher than −776 mV, CP exhibited an inhibiting effect on both SCC crack initiation and propagation, and the inhibiting effect increased with CP level. For X100 steel, it was indicated that hydrogen effect played a more significant role in crack initiation compared to that in X80 steel. The application of CP would retard the crack growth in X100 steel, but promote more cracks to form. Under the delayed CP conditions, the growth of cracks developed in X80 steel could be effectively controlled by the delayed CP of −776 mV; for the deeper cracks formed in X100 steel, a CP level higher than −776 mV was needed to achieve an effective control on the growth. For both materials, the delayed CP of −1,126 mV would enhance crack initiation.