Study On Corrosion Behaviors Of Carbon Steel In The Presence Of Citrobacter Farmeri In Artificial Seawater

With the development of marine economy,the corrosion of metallic materials in marine environment has been paid more attention.Microorganisms in seawater could attach on surfaces of metallic materials are easy to form biofilms,change the physical and chemical properties of the seawater/metal interface,and affect the corrosion process of metals.Microbiological influenced corrosion(MIC)is a significant issue that cannot be ignored in marine corrosion.According to the traits of corrosive microorganisms in ocean environment,species can be divided into sulfate-reducing bacteria(SRB),acid-producing bacteria(APB),iron-oxidizing bacteria(IOB),iron-reducing bacteria(IRB),and slime-producing bacteria(SPB).The typical species can participate in the circulation of iron,sulfur and other elements,and thus changing the cathodic and anodic process in corrosion.Among these corrosive microorganisms,the APB are typical microbes which can adhere to the metal surface and secrete organic acids via their acidic metabolites,and the influence of APB on the corrosion process of metal materials is complex.However,the mechanism of APB affecting metal corrosion in seawater environment is still unclear.The corrosion of carbon steel Q235 by C.farmeri under different acid production capacity was studied,and the corrosion mechanism was discussed in this paper.The acid-producing ability of APB-C.farmeri in different cultures was detected.Surface analysis technology and electrochemical technology were used to study the microbial adhesion and biofilm composition on the surface of carbon steel,the microscopic morphology and chemical composition of corrosion products was detected,the corrosion morphology of carbon steel surface was observed,and the corrosion process of carbon steel in different bacteria containing-medium was analyzed.The effects of adhesion and metabolites of C.farmeri on the corrosion process were analyzed.The results of surface analysis show that C.farmeri could secrete citric acid,and the acid-producing ability of C.farmeri was influenced by the different carbon source.The concentration of citric acid was high under the condition of strong acid-producing ability of bacteria,and the slight bacteria adhesion was observed on the surface of carbon steel.A compact corrosion products layer was formed on the surface of carbon steel,and the higher content of carbon element was detected in corrosion products.The evident local corrosion was not found under bacteria adhesion and compact corrosion products layer.The results of electrochemistry revealed larger values of charge transfer resistance and polarization resistance,and a smaller corrosion current density.These suggest that the corrosion of carbon steel was inhibited by the high concentration of acidic metabolites.The inhibition effect of high concentration of citric acid which produced by C.farmeri was analyzed.C.farmeri exhibited weak acid-producing ability in pure 2216E mudium,and low concentration of citric acid was secreted by C.farmeri.A thick,compact layer formed by sessile bacterial cells was observed on the carbon steel,which indicated that the amount of bacterial adhesion increased significantly in this condition.Severe local corrosion was found beneath the biofilm.The corrosion products were divided into two layers including inside and outside.The outside layer was composed of lepidocrocite,and the inside layer was composed of magnetite.The results of electrochemistry revealed lower values of charge transfer resistance and polarization resistance,and a larger corrosion current density.These suggested that the corrosion behaviors of carbon steel in the presence of C.farmeri were decided by the synergistic effects of biofilm formation and low concentration of acidic metabolites.The transition of the cathodic reaction process and the effect of H~+accumulation beneath the biofilm on the carbon steel were analyzed.