Models for corrosion influenced by iron-oxidizing bacteria (IOB) in fresh water are specific for material/environment combinations, i.e., 300 series stainless steel exposed to oxygenated chloride-containing potable water and carbon steel exposed in oxygenated fresh water ([Cl-] = 20 ppb) containing dissolved copper. Reports of IOB influenced corrosion in marine environments have been limited to rusticle formation on shipwrecks. IOB involved in corrosion in fresh water include Gallionella, Leptothrix, and Siderocapsa. Historically these organisms have also been thought to be active in marine environments. New isolation and molecular identification techniques are demonstrating the presence of novel IOB in both freshwater and marine environments, and expanding our understanding of their potential role in microbiologically influenced corrosion.
Iron-oxidizing bacteria (IOB) have been implicated in microbiologically influenced corrosion (MIC) since the 1960's.1 IOB derive energy from the oxidation of ferrous (Fe2+) to ferric (Fe3+) at/near neutral pH and in some cases the result is the formation of dense deposits of Fe oxides. Most IOB are microaerophilic, requiring low concentrations of oxygen (O2) for growth. For example, Druschel et al.2 determined that the maximum O2 levels associated with growth of the IOB Sideroxydans lithotrophicus were 15-50 µM. Because of the requirement for low concentrations of O2, IOB are often found in association with other microorganisms or in areas where reduced iron is exposed to an aerobic environment. However, IOB contribute substantially to Fe2+ oxidation rates in low O2 environments with a sustained concentration of Fe2+.3,4 It is difficult to isolate microaerophilic IOB, due to their relatively fastidious requirements for growth. The liquid medium gradient method of Kucera and Wolfe5 uses opposing gradients of Fe2+ and O2 for culturing IOB. This gradient method allows the microorganisms to grow under their preferred oxygen concentration (diffusing from the top of the test tube), with a continuous source of Fe2+ diffusing up from a plug of reduced iron present at the bottom of the tube. A recent modification of Kucera and Wolfe's method5 by Emerson and Moyer4 uses agarose to provide a more solid matrix for establishing the O2 and Fe2+ gradients. The IOB form discrete layers of cells in the agarose at their preferred O2/Fe2+ concentrations. As a result of the development of this agarose gradient tube technique, several new isolates of obligatory lithotrophic IOB have been identified, e.g., Sideroxydans and Mariprofundus.6
The IOB that have received the most attention in MIC are Gallionella, Leptothrix, and Siderocapsa. Most of the documented case histories MIC associated with IOB have involved exposure of a 304 or 316 stainless steel to well water or chlorinated drinking water. The corrosion mechanism is under-deposit corrosion or formation of a differential aeration cell.7-15 Under stagnant conditions, IOB form dense deposits within months, excluding oxygen from the area immediately under the deposit and initiating a series of events that are individually or collectively very corrosive (Figure 1).