A Surface Analytical Study of the Attachment of Bacteria to Metal Surfaces

The attachment of microbiologically produced organic acids and exopolymers to stainless steel was investigated. The organic acids investigated were lactic, oxalic and citric acid (a metabolic production of fungi) and an exopolymer extracted from the marine bacterium Delaya marina. Although organic acids tend to be weak acids and have a relatively low ionic conductivity they are environmentally common and the effects on corrosion of such acids are poorly understood. The reactivity and ubiquitous nature of lactic acid provides significant interest to the microbial corrosion field and the food industry. In a bacterial consortium, lactic acid is consumed by some bacteria and generated by others. Lactic acid is also common in the food industry, where its reactivity may cause health hazards by leaching of elements from the protective films of metals used in the food preparation, transport or storage. For this inquiry the surface of 304 stainless steel was probed by performing electrochemical polarization in an aqueous 0.1M HCl solution. A modification of the 304 stainless steel passivation behavior in the O.1M HCl was observed when the steel under went prior exposure to lactic acid prior to the potentiodynamic polarization. The steel surface was analyzed using x-ray photoelectron spectroscopy (XPS) to study the chemical composition of the passive film. It was determined that the mechanism by which the lactic, oxalic and citric adds alters the steel passivity was a combination of iron oxide dissolution and bonding of the organic acids to the surface. The dissolution was confirmed using Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES). Additionally the effect of citric acid on the corrosion of 304 stainless steel in the presence and absence of light was examined. The presence of light was found to accelerate the dissolution of the iron oxides and increase the amount of chromium hydroxides in the passive film.