Effects of sulphate-reducing bacteria on crevice corrosion in X70 pipeline steel under disbonded coatings

• Electrochemical and crevice-corrosion behaviour of X70 steel are studied.
• Samples are exposed to abiotic and biotic media at various polarization potentials.
• Sulphate-reducing bacteria lower the equilibrium potential of the steel.
• Biofilm formation and sulphide deposition increase potential difference in crevice.
• pH value in the bacterial medium is higher than that in the sterilized medium.

The electrochemical and crevice corrosion behaviour of X70 steel were studied in the absence and presence of sulphate-reducing bacteria (SRB) at different polarization potentials. The SRB lowered the equilibrium potential of the steel. The steel was under cathodic protection at an applied potential of −775 mV (vs. saturated calomel electrode) in the sterile solution; however, it was anodically polarized in the SRB-containing solution. Biofilm formation and sulphide deposition caused the potential difference in the crevice to increase. The pH in the bacterial medium was higher than that in the sterilized medium, owing to the hydrogenase secreted by the SRB.