Accelerated degradation of cathodic protected epoxy coating by Pseudomonas aeruginosa in seawater

Abstract

The combination of coatings and cathodic protection (CP) is one of the most commonly used methods for corrosion protection of metal materials. However, the protection life of this method is difficult to match the original design, which mechanism is still not well known. Previous work reports that coatings experience degradation mainly due to cathodic disbondment and microbial factors in marine environments. To further explore the possible reason for this question, we studied the degradation behavior of epoxy coatings in the co-existence of CP and Pseudomonas aeruginosa in seawater using Electrochemical impedance spectroscopy (EIS), Confocal laser scanning microscope (CLSM), Scanning electron microscope (SEM), 3D microscopy, Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analyzer (TGA), and Energy disperse spectrometer (EDS). The EIS results indicate that coating resistance drops bellows 106 Ω cm2 after 35 d of immersion under the effect of P. aeruginosa alone, but the degradation rate of coatings increases in the co-existence of CP and P. aeruginosa. The coating resistance drops bellows 106 Ω cm2 after 7 d of immersion in the P. aeruginosa containing seawater at − 1.05 VCSE. This may be due to P. aeruginosa can cause benzene ring removal from the epoxy coating, and the presence of CP promotes biofilm formation on the coating surface. The thickness of biofilm is about 33.3 μm at − 1.05 VCSE, which is about 1.5 times larger than at OCP, and the diameter of microscopic defects is about 9 times larger than in sterile seawater at the same CP potential. The presence of P. aeruginosa accelerate Cl− penetrating through the coating causing pitting corrosion of the substrate steel. This study revealed that the shorter protection life of organic coating can be attributed to the combined effect of CP and microorganisms in marine environments. The findings of this study can be referenced for better understanding of the coating failure mechanism in marine environments, and provide guidance for selecting appropriate organic coatings with anti-fouling and anti-corrosive properties.