The Effects Of Microorganisms On The Biodegradation And Corrosion Process Of Polyurethane Varnish Coatings In Marine Environment

With the development and utilization of marine resources,marine erosion and bio-fouling have become two major challenges faced by humans.Natural seawater is a strong corrosive medium containing complex corrosive substances and a wide variety of microorganisms.Vessels and marine facilities coated with paint can greatly insulate the contact of various algae and bacteria microorganisms in the ocean,reduce the corrosion of hulls and equipment due to the attachment of marine microorganisms,slow down the speed,increase fuel consumption,and cause huge economic losses..In many anti-corrosion coatings,polyurethane anti-corrosion coating is the most common type of coating after other anti-corrosion coatings.Its amount is second only to alkyd resin coating.It has high surface energy and can effectively reduce the contact angle between seawater and coating.,play a protective role.Microbial corrosion of metals has been extensively documented and reported by domestic and foreign journal media.However,there are relatively few studies on the decomposition and corrosion destruction processes of organic coatings by related microbes.In this paper,epoxy resin varnish coating as the object of study,using electrochemical impedance spectroscopy(EIS),Scanning electron microscopy(SEM),infrared spectroscopy(FTIR)and contact angle test were used to study the decomposition of the marine microbes and the progress of corrosion on the polyurethane varnish coating.The experimental system was kept in a sterile environment at 26?.The results showed that the degree of influence of the three marine microorganisms on the coating was pseudomonas,bacillus,flavobacterium.The EIS results revealed that the pseudomonas,bacillus and flavobacterium all had a effect on the coating impedance which resulted in the reduction of protective performance and service life.Among them,the effect of pseudomonas was remarkable and bacillus was more gently but flavobacterium had no obvious effect.Pseudomonas had the most significant effect on the coating impedance,the impedance value decreased with the extension of soaking time,and the electrochemical impedance spectroscopy showed two capacitive arcs at the thirty-five day,and the impedance value decreased two order of magnitude,indicating that the coating was heavily permeable for thirty-five day.In the immersion of the seawater inoculated with bacillus,the impedance value decreased by an order of magnitude,and the EIS data showed two capacitive arcs at the forty-three immersion day.However,in the case of sterile and flavobacterium,two capacitive-resistance arcs occurred on the one hundred-twenty days and one hundred-fourteenth days,respectively,regardless of the rate of decline in impedance or the time during which the water-permeation failure occurred.The SEM data showed that,compared with the coating in the sterile seawater immersion system,after the three bacteria seawater immersed,a complete biofilm was formed on the coating surface,and a large number of coating surfaces appeared in the pseudomonas seawater solution.Fine pores and chalking traces showed traces of chalking on the surface of the Bacillus seawater coating.The coating in the aqueous solution of flavobacterium sp.was still very smooth and there was no evidence of decomposition and destruction.pseudomonas,flavobacterium,and sterile seawater systems have similar effects on the contact angle of the coating itself,and all have a role in reducing the contact angle.Infrared spectral data(FTIR)showed that three marine microbes destroyed the molecular structure of the polyurethane coating.After soaking in pseudomonas seawater solution,the C-O and the N-H content in the molecular structure decreased.After soaking in bacillus seawater solution,the contents of N-H,C=0 and C-O in ether groups all increased significantly.After soaking in the aqueous solution of flavobacterium,the content of C-O in the ether group increased significantly.Although the decomposition of flavobacteria could change the molecular structure of polyurethane,it did not change the surface integrity of the coating and had no significant effect on the water permeability and the resistance value of the coating.