Abstract
External transgranular cracking was observed on underground pipelines. The phenomenon took place under disbonded coating zones and was induced by the presence of diluted bicarbonate aqueous solution containing dissolved carbon dioxide (CO2). The observed environmental-assisted cracking is very often interpreted as a result of the inefficiency of the cathodic protection. However, some recent local electrochemical potential measurements on buried pipeline surfaces displayed important fluctuations with occasional overprotection conditions. The main objective of this work was to investigate the steel surface behavior under open-circuit potential conditions and over a wide range of cathodic potentials to identify the optimum electrochemical conditions to initiate cracking. Low-frequency cyclic loading tests were performed under various electrochemical conditions: underprotection, overprotection, and alternating stages of under- and overprotection. In low-pH simulated groundwater, the X-52 pipeline steel did not display active-passive electrochemical behavior. Significant general dissolution of the ferrite occurred at open-circuit potential whereas an adherent calcium carbonate (CaCO3) deposit appeared at low cathodic overpotentials. Damage caused by hydrogen was observed for elevated overpotentials. Alternating polarization stages, enhancing either material dissolution, deposit formation, or hydrogen entry, seemed to generate the more favorable conditions for crack initiation. Local hydrogen ingress appeared to be the main mechanism for microcrack propagation.
