The role of secondary precipitates and sulfate reducing bacteria in microbially influenced steel corrosion : implications for fuel tank degradation
Maday, Christine Anne
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Microbially influenced corrosion causes wide reaching economic impacts. One example is the United States Navy's recent setbacks after converting to biofuels. The presence sulfate reducing bacteria in fuel tanks has caused contamination of biofuel and accelerated corrosion processes although the exact mechanism is not well understood. Steel corrosion processes include precipitation of secondary minerals in the form of iron sulfides and oxides. Interaction of sulfate reducing bacteria with secondary precipitates has not been previously investigated. The purpose of this study is to document microbe-mineral interaction and characterize products of such interaction. One iron oxide, goethite, and two iron sulfides, pyrrhotite and pyrite, were chosen as analogues to alteration products associated with steel corrosion. These minerals were polished and characterized with an array of analytical techniques prior to and after exposure to sulfate reducing bacteria, Desulfovibrio indonensiensis. X-ray diffraction was performed to confirm mineral phase. Standard scanning electron microscopy and field emission scanning electron microscopy were used to obtain secondary and backscatter electron images to display cell attachment and discriminate phases. Spot analyses using energy dispersive spectroscopy was used to obtain elemental information to help identify phase of mineral substrate, mineral inclusions, and secondary precipitate on the surface. Surface chemistry was further investigated with x-ray photoelectron spectroscopy to identify chemical states of elements present. The x-ray diffraction data confirmed goethite and pyrite to be free of significant contamination. Pyrrhotite, however, had other minerals associated with the sample. Backscatter images of the sample confirmed goethite to be slightly heterogonous, pyrrhotite to have greater heterogeneity and pyrite to be completely homogenous. After exposure to Desulfovibrio indonensiensis for a period of 7, 14, and 30 days, the samples were analyzed again. Goethite was found to be inert with respect to bacteria while pyrrhotite and pyrite had cell attachment and overlying precipitate indicating microbe-mineral interaction. The results of this study indicate the negative feedback mechanism associated with sulfate reducing bacteria and goethite. Sulfide minerals can create a positive feedback mechanism for the corrosion process if localized sites of oxidation occur on the secondary precipitates to allow for sulfate reducing bacteria interaction.