| In the natural environment and process of industrial production, biofilms formed on the surfaces of metal due to the adhesion of microorganisms. The diversity of biofilms and their corresponding bacteria constituted a complex biological system which caused the change of the physical and chemical properties at the interface of metal/biofilm, such as pH, concentration gradient change of dissolved oxygen, chloride and sulfide, etc. The corrosion of metal which caused or promoted by microorganism living activity called microbiologically induced corrosion (MIC). Hainan Province belongs to the tropics marine climate. For the higher temperature, microorganisms there grow vigorously all year around and increase the corrosion rate. Wu et al. demonstrated that the average corrosion rate of25steel in natural seawater is2.6times higher than that exposed to sterile seawater in tropical marine environment after365days. The bacterial concentration of aerobic bacteria (pseudomonas, thiobacillus thiooxidans) and facultative anaerobes (vibrio, iron bacteria) in corrosion products which adhered to the sample surfaces had reached high level after immersed in natural seawater for7days. This proves aerobic bacteria and facultative anaerobes played significant roles in the corrosion of carbon steel at the initial stage. Nowadays domestic and foreign research mainly focused on MIC induced by sulfate reducing bacteria (SRB) in anaerobic environment, while microbiologically induced corrosion in marine ecosystem is due to the synergistic effect of multiple bacteria rather than single one.In this thesis, a comparative study of the corrosion behavior and mechanism of carbon steel in sterile seawater, single pseudomona, thiobacillus thiooxidans and mixture inoculated seawater was carried out in order to explore the influence of single pseudomonas, thiobacillus thiooxidans and their synergistic effects on the corrosive behavior and electrochemical behavior of the carbon steel. The cross-synergy effect of the mixed bacteria of vibrio and iron bacteria on the corrosive behavior and electrochemical behavior of carbon steel was also studied by comparing the corrosive behavior and electrochemical behavior of45steel in sterile seawater, in seawater containing vibrio, in seawater containing iron bacteria and in seawater containing the mixed bacteria of vibrio and iron bacteria.Within30days, the synergistic effect of pseudomonas and thiobacillus thiooxidans reduced the average corrosion rate of45steel, while with the extension of immersion time, the average corrosion rate in mixed culture system increased gradually. Electrochemical impedance spectroscopy analysis showed that,1-15d, the impedance values of electrodes in pseudomonas-thiobacillus thiooxidans mixture system are larger than the values in sigle bacteria system after immersed for same period of time. This illustrates the presence of mixed bacteria inhibits the corrosion of45steel. With prolonging the corrosion time, the corrosion product film on sample surface had changed into a multilayer film after30days immersion and the magnitude of total impedance of mixed bacterial system is less than that in single bacteria system. Meanwhile, the polarization curve results showed that after30days’exposure, the corrosion current density of electrode in mixed bacterial system was significantly higher than that in single bacteria and sterile system, which indicated with the prolonging of immersion time corrosion rate of metal in mixed bacteria system increased gradually and larger than that in single bacteria system and sterile seawater system. Electrochemical noise analysis results show that localized corrosion had occurred on samples in pseudomonas-thiobacillus thiooxidans mixture system with a small corrosion rate at early corrosion stage. The SEM result of carbon steel coupons exposed to pseudomonas-thiobacillus thiooxidans mixture system for30days showed that the inner corrosion product film was smooth and dense, but some obvious cracks were existed. And the pitting corrosion was more serious than in single bacteria system.The mixture of vibrio-iron bacteria inhibited the corrosion of45steel at the early stage of corrosion. With the prolonging of corrosion time, the synergy of the two microorganisms accelerated the corrosion rate. In seawater, the corrosion rate of45steel immersed in an environment inoculated with vibrio and iron bacteria for7days and30days were13.6%and133.8%of that in a sterile environment respectively. At initial corrosion stage, metabolism of iron bacteria and vibrio consumed large amounts of oxygen, which controlled the corrosion process of metal. From1to15days the capacitance value of biofilm-corrosion product composite film, Qp, increased with the immersion time indicating the formation of biofilm on the metal surface which acted as a barrier. Then, for the localized defects of the composite film such as peeling off, cracking, etc, the value of Qp decreased gradually. After30days’immersion, the total impedance of electrode in vibrio-iron bacteria mixture system reduced compared with the impedance value of sample exposed to mixture for15days. This proves the presence of mixed bacteria accelerated the corrosion of steel45. SEM results indicated that in vibrio-iron bacteria mixture system, Fe(OH)3which produced for the metabolism of iron bacteria bonded with extracellular polymeric substances(EPS) and formed rust tumors deposited on the metal surface after30days’immersion. Some cracks could be observed on corrosion product film as well. Oxygen concentration cells can be easily created for the formation of rust tumors and cracks, which accelerated the localized corrosion of metal. After removing the corrosion product film, serious pitting corrosion could be observed. |
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