Study On Corrosion Of Steel Induced By Iron Bacteria And SRB In Certain Aquatic Environment

Iron oxidizing bacteria （IOB） and sulfate reducing bacteria （SRB） are well known fortheir attaching, growing and metabolizing on the metal surface and introducingcorrosion to the metal thereby by changing the surface status of metal, altering themetal/medium interface and chemical composition of the medium nearby. On theother hand, iron oxidizing bacteria in water environment will ennoble the corrosionpotential of passivated metal, which could be utilized in bio cathode of microbial fuelcell （MFC）.In this study, Q235carbon steel samples have been exposed in Zhoushan corrosiontesting base for3years for continuous dynamic electrochemical tests. In the meantime, the bacteria amount within the rust layer of Q235samples were measured atdifferent time point during the exposion. The influence of bacteria amount oncorrosion speed was studied. Result shows that the initial corrosion speed of Q235steel was relatively high, however the corrosion speed decreased with exposing timebecause of the out rust layer coming into being on the metal surface which protectedinner metal from corroding. After exposing for2months, the corrosion speed of thesample was mutually controlled by temperature and rust layer and temperature wasthe crucial factor. XRD shows the inner rust layer is mainly composed of black Fe₃O₄,and outer layer of red Fe₂O₃due to SRB and iron oxidizing bacteria controlling thecorrosion process in inner and outer rust layer respectively.In this dissertation, the corrosion electrochemical behavior of304stainless steel wasstudied in sterilized tap water with and without inoculation of iron oxidizing bacteriaand SRB. Results showed that304stainless steel was passivated in dechlorinatedsterilized tap water. The microbial film developed on the substrate after iron bacteriawere inoculated in the tap water and protects substrate from corroding in some extent.In tap water system with SRB inoculation, SRB attached on the surface of304stainless steel and reduced SO₄^2-to S^2-after inoculation, which led to corrosion. Ascorrosion proceeded, the microbial films formed by SRB and the corrosion productsacted as protective layer for the inner substrate, therefore the corrosion slowed down.Later when corrosive substance spread inside the layer, corrosion was sped up again. Electrochemical noise （EN） analysis could be used to effectively detect corrosionunderneath outside layer.In ATCC MSVP1917medium, the factors influenced the ennoblement of glassycarbon and304stainless steel by L. discophora SP6were analyzed. Results showthat factors such as substrate material, initial roughness of substrate, oxygenconcentration and so on place different influence on substrate potential ennoblementwhen L. discophora SP6was inoculated. Varied external polarization to the substrateand prepainting certain biological activated matter like PLL on the substrate will alsochange the ennoblement range, which is instructive for utilizing L. discophora SP6orother bacteria biocathode in microbial fuel cell.