The failure behavior of epoxy glass flake coating under the synergistic effect of cathodic protection and sulfate-reducing bacteria in seawater

Abstract

Organic coatings are widely used in combination with cathodic protection (CP) to protect marine structures against corrosion. Coating failure is associated with CP and microbial factors in marine environments. However, detailed studies on coating failure due to the co-existence of CP and microorganisms are lacking. In this study, we investigated the failure behavior of epoxy glass flake coatings immersed in seawater containing sulfate-reducing bacteria (SRB) at different CP potentials. The results revealed that the coating remained intact under the effect of CP or SRB acting independently but failed under the combined action of SRB and CP, forming blisters. The presence of CP promotes biofilm formation on the coating surface. As the CP potential decreased from −0.85 VCSE to −1.05 VCSE, the thickness of the biofilm increased. The synergistic effect of CP and SRB enlarges microscopic defects on coating surface and cracks between resin and glass flakes, and promote sulfide generated by SRB penetrating through the coating, which accelerates corrosion of substrate steel leading to coating blister. At CP potentials of −0.85 VCSE, −0.95 VCSE, and −1.05 VCSE, coating blisters were observed after 30 d, 20 d, and 5 d of immersion, respectively, in an SRB medium. The results demonstrate the existence of a synergistic effect between CP and SRB that facilitates coating failure, which increases in speed as the CP potential becomes more negative.