Microbiologically Influenced Corrosion Of Steels In Seawater And Seamud Containing Sulfate-reducing Bacteria

Microbiologically influenced corrosion (MIC) has been given severe attention in marine environment concerned of all kinds of structure constructions, such as pipeline, platform for petroleum, etc. In the paper, sulfate-reducing bacteria (SRB) influenced corrosion was studied in seawater and seamud environments, concerned of stainless steel and low alloyed steels, respectively. The influence of SRB on cathodic protection (CP) was also conducted in seamud. Given corrosion factors including SRB, the corrosiveness of regional marine sediment was classified based on the fuzzy clustering analysis technology (FCA).SRB was cultured and enriched originated seamud from Bohai gulf using medicated Postgate's C medium. Biofilm containing SRB was observed on the surface of 316L SS by fluorescence microscopy in medicated Postgate's C medium inoculated SRB. The changes of Electrochemical impedance spectra (EIS) could be ascribed to the SRB biofilm and the metabolism activities. SRB biofilm and proliferation resulted the debasement of passivity of stainless steel. The distinct anodic current peak was observed in the seawater containing SRB. It was presumed that it was the result of electrochemical oxidation of sulfide.In anaerobic environment containing SRB, iron sulfide as depolarization reagent accelerated the depolarization of cathodic reaction, resulting the increase of anodic dissolution. The result of X-ray photoelectron spectra (XPS) indicated, after exposure to SRB, the ratio of Cr/Fe had an increase, Mo and S were detected in the surface layer of passive film. Organic molybdenum sulphide (Mo(V)-Cysteine) was found in passive film. The increase of Cr and the appearance of Mo were thought an auto-repair behaviour.In seamud containing active SRB, the corrosion rate of low-alloyed steels decreased firstly and then increased. According to EIS and surface analysis, the corrosion process of steels in seamud containing active SRB was presented. In the first stage, anaerobic hydrogen depolarization is predominant and produced iron oxide corrosion product layer; in the second stage, due to bio-produced hydrogen sulphide, iron oxide was biomineralized and translated iron sulphide. The iron oxide and initial iron sulphide had a certain extent protect properties, so the corrosion rate decreased and kept stable. Subsequently, the crystal structure and type of iron sulphide changed gradually, there existed crannies in corrosion products, resulting the increase of corrosion rate. According to the characteristics of the EIS change, it is an alternative mechanism that bioorganic sulphide accelerated the corrosion.The optimal CP potential shifted to negative direction in seamud containing active SRB, -1030 mV (vs. saturated Cu/CuSO4 electrode, CSE ) or lower potential was needed.Accordingly, the CP current density was about 11 mA/m2. Given corrosion factors including SRB, the corrosiveness of regional marine sediment was classified based on FCA. The corrosion rate were predicted by FCA. The chart of corrosiviness was plotted. Compared to the factors analysis, FCA can give better results...