Effects of microbial biofilms and microbial metabolic activity on the localized corrosion of steel

Microbial biofilms and the metabolic activity of attached bacteria, can cause localized corrosion of metal. Studies on the mechanisms of microbiologically influenced corrosion (MIC) are essential in mitigation of MIC problems. MIC studies have been difficult due in part to the difficulties in analyzing local activities of bacteria. In addition, most electrochemical techniques for analyzing corrosion are destructive and provide only indirect information on localized corrosion. In this research, bacteria were isolated from corrosion tubercles. The isolates were characterized, and reexposed to steel samples. The samples were analyzed using nondestructive electrochemical techniques.; Potential applied during electrochemical impedance spectroscopy (EIS) did not significantly alter the numbers or the activity of sessile bacteria. Results of EIS analyses indicated that the bacterial biofilms accelerated the rates of corrosion with respect to sterile controls. Oxidizing biocides increased the corrosion rates, even when most of the biofilm was removed.; The scanning vibrating electrode technique (SVET) was used to map nonuniform current densities over corroding steel samples. The results of the SVET studies demonstrated that in sterile medium small pits formed on carbon steel samples and subsequently became inactive (repassivated). In the presence of microbial biofilms, pits would initiate and repassivate for a certain amount of time, then would propagate. The time required for pit propagation was dependent on the number and viability of the bacteria.; SVET studies in sterile media demonstrated that phosphate and buffer promoted repassivation of pits. The bacterial biofilms interfered with this repassivation.; Incorporation of {dollar}sp{lcub}14{rcub}{dollar}C-acetate into insoluble cell material, and exposure of the labelled bacteria to X-ray film was used to localize microbial biosynthetic activity. Microautoradiography was used to detect the individually labelled bacteria. Most of the label observed with the X-ray film was associated with the anodic sites of the steel. The {dollar}sp{lcub}14{rcub}{dollar}C-acetate may have become chemically bound to the corrosion products. However, killed controls showed little label on the electrode surface. Alternatively, bacteria either caused initiation of pits by localized colonization, or preferentially attached to corrosion products, enabling pits, initiated by chemical corrosion, to propagate.