Abstract
Microbially influenced corrosion (MIC) of iron (Fe0) by sulfate-reducing bacteria (SRB) has been studied extensively. Through a mechanism, that is still poorly understood, electrons or hydrogen (H2) molecules are removed from the metal surface and used as electron donor for sulfate reduction. The resulting ferrous ions precipitate in part with the sulfide produced, forming characteristic black iron sulfide. Hydrogenotrophic methanogens can also contribute to MIC. Incubation of pipeline water samples, containing bicarbonate and some sulfate, in serum bottles with steel coupons and a headspace of 10% (vol/vol) CO2 and 90% N2, indicated formation of acetate and methane. Incubation of these samples in serum bottles, containing medium with coupons and bicarbonate but no sulfate, also indicated that formation of acetate preceded the formation of methane. Microbial community analyses of these enrichments indicated the presence of Acetobacterium, as well as of hydrogenotrophic and acetotrophic methanogens. The formation of acetate by homoacetogens, such as Acetobacterium woodii from H2 (or Fe0) and CO2, is potentially important, because acetate is a required carbon source for many SRB growing with H2 and sulfate. A consortium of the SRB Desulfovibrio vulgaris Hildenborough and A. woodii was able to grow in defined medium with H2, CO2, and sulfate, because A. woodii provides the acetate, needed by D. vulgaris under these conditions. Likewise, general corrosion rates of metal coupons incubated with D. vulgaris in the presence of acetate or in the presence of A. woodii were higher than in the absence of acetate or A. woodii, respectively. An extended MIC model capturing these results is presented.
Highlights
Corrosion failures occur in water-transporting pipelines, when the anodic dissolution of iron (Fe0 → Fe2+ + 2e−) is coupled to the cathodic reduction of oxygen (2H+ + 1⁄2O2 + 2e− → H2O)
This generally results in much lower corrosion rates, e.g., with linear polarization resistance (LPR) abiotic carbon steel corrosion rates of 0.26 and 0.11 mm/yr were observed under aerobic and anaerobic conditions, respectively, in a defined medium (Caffrey et al, 2008)
During further studies of microbially influenced corrosion (MIC)-associated methanogenesis at this steam assisted gravity drainage (SAGD) water distribution system we found that concentrations of acetate of up to 1 mM were produced, suggesting involvement of hydrogenotrophic acetogens, catalyzing
Summary
Corrosion failures occur in water-transporting pipelines, when the anodic dissolution of iron (Fe0 → Fe2+ + 2e−) is coupled to the cathodic reduction of oxygen (2H+ + 1⁄2O2 + 2e− → H2O). Water transported through these pipelines is degassed and is kept anaerobic through the addition of chemical oxygen scavengers such as sodium bisulfite (SBS). Protons are reduced at the cathode (2H+ + 2e− → H2), instead of oxygen, under the resulting anaerobic conditions. This generally results in much lower corrosion rates, e.g., with linear polarization resistance (LPR) abiotic carbon steel corrosion rates of 0.26 and 0.11 mm/yr were observed under aerobic and anaerobic conditions, respectively, in a defined medium (Caffrey et al, 2008). Sulfate-reducing bacteria (SRB), using cathodic hydrogen to reduce sulfate to sulfide, are often associated with
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