Biofilms on carbon steel due to pure and mixed bacterial cultures and the resulting corrosion characteristics

Microbiologically-influenced corrosion (MIC) is a costly problem for many industries. Large portions of MIC studies have investigated MIC system by only considering the effect of the sulfate reducing bacteria (SRB) in pure culture. However, many species of bacteria co-exist in natural environments. The role of certain bacterial species such as the iron reducing bacteria (IRB), which are often isolated alongside the SRB in industrial waters, is still unknown. Biofilm formation introduces structural and chemical heterogeneities and alters the metal/solution interface. This objective of this thesis was to investigate biofilms formed by a representative SRB (Desulfovibrio desulfuricans) and IRB (Shewanella putrefaciens) in mixed and pure culture environments and study their resulting effect on corrosion processes of carbon steel. Microscopic, electrochemical and surface characterisation techniques were used in this laboratory study to investigate the effects of pure and mixed culture biofilms for relatively short period of time (72 h and 168h) under batch conditions. The organic nutrient availability for the bacteria was varied in order to investigate biofilms formed under nutritionally rich as well as nutritionally insufficient conditions. Focused ion beam-scanning electron microscopy was used for the first time in a MIC study to investigate the effect of subsurface biofilm structures on the resulting corrosion characteristics of carbon steel. Hydrated biofilms with minimal surface preparation were observed under the environmental scanning electron microscope. The elemental composition of the biofilms was investigated using energy dispersive spectroscopy. ATR-FTIR was used to provide information on the organic and inorganic molecules in the biofilms. Electrochemical techniques used included open circuit potential, linear polarisation and potentiodynamic polarisation. In SRB solutions, H₂S generation and the concurrent precipitation of iron sulfide film was observed to indirectly increase localised corrosion processes. Pitting corrosion occurred due to either breaks in the iron oxyhydroxide film which exposed the carbon steel to iron sulfide species or possible dissolution of iron oxyhydroxide films by the sulfide species. Decrease in organic nutrient availability in the media led to enhanced cell attachment. Following 168 h exposure, extensive pitting could be observed on the carbon steel when grown in lactate containing media. However when pure culture SRB biofilm formed under starvation conditions, deep pitting could not be observed. The pure IRB biofilm contained the highest content of organic molecules due to the extracellular polymeric substances (EPS) and biofilm formation ability of Shewanella putrefaciens. The biofilm was composed of carbohydrates, lipids and phosphate groups. Reductive iron dissolution and biomineralisation could be observed. The deepest pits occurred when the IRB biofilm formed under starvation conditions. The contribution of both the SRB and IRB could be observed in the mixed culture biofilm. The H₂S formation by the SRB occurred in a shorter time due to the IRB removing residual O₂ from solution and thereby setting up favourable conditions for the anaerobic SRB. The adhesive EPS components of the biofilm, due to the activity of the IRB, facilitated the attachment of SRB cells as well as concentration of their metabolic products such as sulfides close to the carbon steel surface. The mixed culture environment led to the highest occurrence of deep open pits, even when the bacteria were cultured under starvation conditions.