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Item Biocorrosion of 1018 steel in sulfide rich marine environments: a correlation between strain and corrosion using electron backscatter diffraction(Montana State University - Bozeman, College of Engineering, 2014) Martin, Joshua Daniel; Chairperson, Graduate Committee: Paul E. GannonMicrobially induced corrosion (MIC) of steel due to the presence of sulfide is a leading cause of pit formation of carbon steel in fuel-seawater environments. While extensively studied, the exact causes of pitting corrosion in naval fuel tanks when exposed to MIC in the presence of fuel and seawater are not completely understood. This thesis focuses on the role that cold-rolling of carbon steel plays on corrosion while subjected to sulfidogenic, suboxic corrosive environments. Particularly, the effects of microscopic residual strain found within 1018 steel on the anodic dissolution of the metal is studied in different MIC sulfide environments using EBSD, AFM, FE-SEM, EDX, and electrochemistry. It is found that regions of increased plastic deformation of the crystalline lattice as a result of cold rolling correlate to an increase in anodic dissolution rates of 1018 steel coupons cut parallel to rolling direction. Image overlay provides a verification of the location of corrosion of samples to the same locations found in EBSD mapping taken prior to corrosion, ensuring the predictive value of EBSD analysis in establishing locations of accelerated corrosion. The effect of different corrosion environments on the corrosion rate of steel is measured through electrochemistry. Values obtained through these measurements are applied using mechanochemical theories to predict the localized dissolution rate of the steel due to strain using computational methods. Mechanochemical analysis of the strained areas results in predicted corrosion rates within an order of magnitude of the corrosion rates measured using AFM for the exposed time period at the same locations. Areas exhibiting increased corrosion rates occur in areas exhibiting increased strain as measured by EBSD analysis. Further electrochemical results show an increase in corrosion rates for suboxic sulfide rich systems containing low levels of oxygen, when compared to anaerobic sulfide environments. Variations in corrosion current density as a function of oxygen presence, as measured by electrochemistry, accurately predict variations in mechanochemical corrosion rates of strained areas well within an order of magnitude. Results from this study support the use of EBSD as a means to further the understanding of pitting corrosion as a function of material properties.Item The role of secondary precipitates and sulfate reducing bacteria in microbially influenced steel corrosion : implications for fuel tank degradation(Montana State University - Bozeman, College of Letters & Science, 2013) Maday, Christine Anne; Chairperson, Graduate Committee: David W. MogkMicrobially 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.