The effects of increased bacterial metabolic diversity on the corrosion of carbon steel

Most studies involving microbial influenced corrosion (MIC) of metals use pure cultures of bacteria though consortia are often associated with corroding metals under in situ conditions. This study investigates the corrosion of carbon steel with increasing bacterial metabolic diversity in order to determine the additive effects of consortial members. Three physiologically distinct groups of bacteria: aerobic, fermentative and sulfate reducing were represented by Bacillus sp., Hafnia alvei and Desulfovibrio gigas, respectively. Combinations of these bacteria were used in an aerobic, freshwater system. The open cell potential (OCP) of carbon steel decreased from -250 to -700 mV/SCE for all bacterial communities while the control decreased to approximatley -350 mV/SCE. The increase in corrosion rate, measured in admittance, was greatest with communities containing D. gigas. The admittance with D. gigas plus Bacillus increased from 2.06 x 10^-5 to 1.35 x 10^-3 mhos cm^-2 while the coculture with D. gigas plus H. alvei increased from 3.86 x 10^-5 to 2.28 x 10^-3 mhos cm^-2 while the admittance with all three bacteria increased to 1.37 x 10^-3 mhos cm^-2. Upon completion of the experiments the carbon steel electrodes were removed for bacterial analysis. The plate counts of Bacillus and H. alvei in monoculture were similar, but when grown in diculture, the total population size increased an order of magnitude. The population size of H. alvei also increased when cocultured with D. gigas. however the Bacillus population size decreased when cocultured with D. gigas. The D. gigas population was similar in both diculture combinations. The population size was greatest for all bacteria when grown in triculture. The bacterial communities were also analyzed based on the differences in the branching, saturation and chain length of the phospholipid fatty acids (PLFA). Cluster analysis of the PLFA confirmed the effects of D. gigas on H. alvei and the Bacillus. The second part of this investigation shows the effects of increased metabolic diversity using enrichments, from a naturally corroding pipe, of these same physiologically distinct groups. The corrosion rate was compared between an aerobic plus fermentative enrichment and an aerobic, fermentative plus sulfate reducing enrichment. The OCP decreased from -250 to -700 mV/SCE for all consortia versus -350 mV/SCE for the control. The admittance was greatest for the aerobic, fermentative plus sulfate reducing enrichment at 3.13 x 10^-3 mhos cm^-2. The total numbers of bacteria from the electrodes were similar for both treatments based on plate counts but MPN revealed the presence of approximately 1 x 10³ cells cm^-2 sulfate reducing bacteria in the aerobic, fermentative plus sulfate reducing treatment. Clustering of the PLFA showed distinct patterns of relatedness based on the enrichments used.