Effect Of Vibrio Natriegens On Metal Corrosion In Sea Water And Research On Anti-pitting Method

Environmental factors play a significant role in influencing the corrosion rate of metal materials in marine environment. The adsorption of microbes always enhances corrosion and is vital for metallic local corrosion. A layer of bacterial film could be formed in several hours on material surfaces after the immersion in sea water, and then the formation of biofilm occurs which favors the subsequent attachment of macroscopical marine organisms. The loss of material even for short-term exposures is important in part because protective measures are not always wholly effective. Consequently, it is very meaningful to research the effect of bacterial film formed on the material surfaces after the early period of immersion, and the identification of the preponderant bacterium, and the influencing behaviours and mechanisms of this bacterium on metal corrosion.In present work, the preponderant bacterium was obtained from the surfaces of mild steel, stainless steel and copper which were staticly hung in natural sea water from Qingdao for different times. The preponderant bacterium was identified as Vibrio natriegens (V. natriegens) through biological method namely PCR. The survival habit and growth curve were studied using nephelometery.Through the comparatively electrochemical experiments of different metals in natural and sterile sea water, the effect of the preponderant bacterial film on the changes of metallic surface properties was studied. The results showed that the influence of this bacterium on mild steel corrosion could be neglectable and that the biologic activites of bacteria greatly influenced the open potential (E_corr) of metals and induced the increasing corrosion rates of two metals.The growth process of V. natriegens biofilm covering on stainless steel surface was observed by atomic force microscopy (AFM) after different immersion in bacteria culture medium and these AFM images confirmed that biofilms were dynamic structural entities in which cell attachment and growth, the formation of micro-colonies, and subsequent detachment took place. Through the E_corr test, it could be found that the attachment of V. natriegens could change the electrochemical properties of stainless steel surface and that the proliferation of the bacteria could induce the reactivation of the passive surface and the occurrence of active corrosion. The results from electrochemical impedance spectra (EIS) showed that the impedance value decreased with immersion time and the anticorrosion abilities of stainless steel were weakened. Utilizing the equivalent circuit models to interpret the EIS data, it could be obtained that the passive film on stainless steel was not intact and pitting corrosion took place. The corrosion behaviours were also inspected using dynamic polarization curve. The passive current density increased and the passive film was thinned, resulting in the anticorrosion ability was weakened. The changes of anodic polarization curve slope were more significant and decreased with immersion time which demonstrated that the resistance of anodic polarization decreased and the anodic dissolution was accelerated. The corrosion product was analyzed by SEM, revealing the porous biofilm and corrosion product layer. The EDS analysis of the corrosion product proved the existence of biologic S,P from bacterial metabolite. The pitting on the stainless steel after the removal of corrosion product validated the conclusions obtained from electrochemical experiments.The growth process of V. natriegens biofilm covering on copper surface was observed by AFM after different immersion in bacteria culture medium. The results from E_corr test showed that the proliferation of these bacteria on copper surface induced the break of the protective oxide film formed previously and the start of active corrosion. The EIS data showed that the impedance value decreased with immersion time and the anticorrosion abilities were weakened. Utilizing the equivalent circuit models to interpret the EIS data, it could be obtained that the oxide film on copper surface was not intact and pitting corrosion took place. The dissolution of copper was accelerated with immersion time which was obtained from dynamic polarization curve test. The SEM images revealed that the corrosion product layer was loose and that the surface consisted of small copper particles and fell off unevenly with the bacterial metabolism. From the EDS analysis, biologic elements S,P were found in the corrosion product.In order to confirm whether the N2-fixation influenced the corrosion of stainless steel and copper, four kinds of media were designed in this work. The corrosion degree of stainless steel and copper coupons immersion in different media for 7 days was tested using electrochemical experiments and the corrosion morphology was characterized by surface analysis. The results showed that the existence of V. natriegens destroyed the protective passive film formed on stainless steel. N₂-fixation had significantly acceleration on corrosion, and the real cause was N₂- fixation not NH3 produced in this process. Element Ni from the matrix was extracted by the bioflim formed on the surface which destroyed the restoration of passive film on stainless steel. These results from copper coupons also demonstrated that N₂-fixation could enhance the corrosion. It was conferred that some aggressive materials were secreted in this process and the final corrosion was more serious than the corrosion induced by NH₃.Pitting corrosion is the most familiar way of local corrosion for mild steel. And 1mol/L HCl is the most commonly used mimic medium for the environment in pit, due to low value of pH and high concentration of Cl^-. In this work, we tried to find the effective additive in coating to inhibit the pitting corrosion. In order to evaluate the feasibility of adding Carboxymethylchitosan (CMCT) into coating as the additive, the pit environment was broadened and 1mol/L HCl was treated as the experiment medium to test the inhibitive effect of CMCT on mild steel. The inhibition of bacterial growth was tested by the epifluorescence microscopy observation. Through weight loss experiment and electrochemical tests, the inhibition of CMCT was determined. The results showed that the inhibiting efficiency increased with the rise of CMCT concentration and reached the maxium at 200mg/L. The adsorption of CMCT on mild steel obeyed Langmuir rule, some thermodynamic parameters were obtained and the adsorption model was established. The correctness of the model was proved by quantum chemical calculation and optimization of molecular structure. The proper addition of Cu²⁺ could improve inhibiting efficiency and reached the maxium at 20mg/L CMCT +10^-4 mol/L Cu²⁺. The mechanism was determined as that Cu²⁺ and CMCT could interact to form a protective layer on mild steel, increasing the charge transfer resistance and decreasing the double layer capacitance, and inhibit the electrochemical corrosion.