Effects of cathodic protection potential on microbiologically induced corrosion behavior of X70 steel in a near-neutral pH solution

Sulfate reducing bacteria (SRB) are considered as one of the main causes for the failures of buried metal pipes. Although many researchers reported that more negative cathodic protection potential was required in environments containing SRB, SRB would increase the concentration of hydrogen adsorbed on steel surface and thus lead to hydrogen embrittlement. In the study, the optimum cathodic protection (CP) potentials of X70 steel in bacterial and sterile media were evaluated with electrochemical impedance spectroscopy. The morphology and composition of corrosion products were characterized by a scanning electron microscope (SEM), an energy dispersion x-ray spectrometer (EDS), and an x-ray photoelectron spectrometer (XPS). The corrosion morphology of X70 steel in NS4 medium was pits and the corrosion in the bacterial medium was more serious than that in the sterile medium. The corrosion products of X70 steel were FeOOH and Fe2O3 in the sterile medium, whereas its corrosion products in the bacterial medium were FeOOH and FeS. When CP potential was -775 mV, SRB growth was promoted and the optimal protection effect on X70 steel was achieved in the bacterial NS4 medium. Pits were still observed under the biofilm and the corresponding corrosion mechanism was extracellular electron transfer (EET). When CP potential was -875 mV, X70 steel realized the optimal protection in the sterile NS4 solution. However, CO2 hydrolysis and SRB metabolism in the bacterial medium resulted in hydrogen-induced pits. When CP potential was -1025 mV, the growth of SRB was inhibited and severe hydrogen evolution corrosion occurred on X70 steel in bacterial and sterile NS4 media. The optimal CP potential for pipeline steel in the sterile medium may lead to hydrogen corrosion in the bacterial medium when H+ concentration was high.