DIFFERENTIAL CORROSION RATES OF CARBON-STEEL BY COMBINATIONS OF BACILLUS SP, HAFNIA-ALVEI AND DESULFOVIBRIO-GIGAS ESTABLISHED BY PHOSPHOLIPID ANALYSIS OF ELECTRODE BIOFILM

The corrosion rates of carbon steel by monocultures and various combinations of aerobic (Bacillus sp.), fermentative (Hafnia alvei) and sulfate-reducing (Desulfovibrio gigas) bacterial biofilms in an aerobic, continuously flowing freshwater reactor containing 0.4 mM sulfate showed marked differences. Biofilm formation and electrode colonization resulted in decreases in the open circuit potential (OCP). The corrosion rate was measured non-destructively as the admittance (1/R(ct)) by electrochemical impedance spectroscopy (EIS) on a four sided working electrode which allowed estimates of the reproducibility. The monocultures each induced greater corrosion initially but with time the rates of corrosion decreased to that of the sterile control. The D. gigas was unable to grow in the bulk phase but formed an apparently non-living biofilm. The admittance (the measure of corrosion) was greatest when the sulfate-reducing bacterium, D. gigas, was present in a consortium. The consortia containing D. gigas + H. alvei showed a significantly higher corrosion rate than the triculture or the other dicultures. The microbial biomass and the bacterial community structure actually on the electrode surfaces were examined by viable counts, most probable number (MPN) estimations and phospholipid fatty acid (PLFA) profiles determined after extraction and gas chromatographic analysis. The rate of corrosion was not directly related to the total microbial biomass or the number of species on the coupon. The rate of corrosion did not depend on the ratio of heterotrophic to sulfate-reducing bacteria (SRB) or absolute number of SRB. The PLFA analysis showed the organisms on the working electrodes were more stressed/starved than when grown in the bulk phase for inocula. This study demonstrates that different combinations of bacteria growing on the same substrata (electrode) in the same bulk phase induce very different corrosion rates. The maximum corrosion rate with the isolate combinations was within 70% of an enrichment with a community composition (determined by PLFA analysis) like that of the native corrosion tubercules.