Abstract
Microorganisms, implicated in microbiologically influenced corrosion, were isolated from the deep subsurface at Yucca Mountain. Corrosion rates of iron-oxidizing, sulfate-reducing, and exopolysaccharide (EPS)-producing bacteria were examined in constructed electrochemical corrosion cells for periods up to 109 days. The test system consisted of a 1020 carbon steel (CS) coupon immersed in soft R2A agar prepared with simulated groundwater. A 1% potassium chloride (KCl) bridge was used to connect the test to a reference calomel electrode and a potential was applied with a platinum counter electrode. The corrosion process was measured by polarization resistance methodology. Average corrosion rates were measured in milli-inches per year (mpy) against time. Purified cultures of EPS-producing bacteria and enrichment cultures of iron-oxidizing and sulfate-reducing bacteria were tested separately and in various combinations. An uninoculated control cell was prepared to assess abiotic corrosion. The corrosion rates peaked at 35 days at 1.2 mpy (control), 2.3 mpy (iron-oxidizing bacteria), 3.30 mpy (sulfate-reducing bacteria), and 2.8 mpy (EPS-producing bacteria) before stabilizing. Various microbial combinations demonstrated higher corrosion rates (3.1 mpy to 4.8 mpy) than single groups and peaked at 30 days. The results indicate that Yucca Mountain microorganisms, alone and in combination, are capable of causing corrosion of 1020 CS. Upon completion of these experiments, phospholipid fatty acid analysis detected all of the bacterial groups inoculated into the individual test systems, suggesting that biofilm development had occurred. The examination of mineralized biofilms on the CS surface with light microscopy and scanning electron microscopy/energy-dispersive x-ray analysis (SEM/EDXA) demonstrated that all of the bacterial groups promoted a generalized corrosion process; however, the corrosion experiments containing SRB were particularly effective in biofilm development and pitting.
