Biological Corrosion and Biofilm Effects on Metal Biodeterioration

Abstract

It is widely acknowledged in the literature that metal surfaces immersed in natural or industrial aqueous media undergo a sequence of biological and inorganic changes that alter the metal/solution interface structure. The first stage of the biological changes is the formation of a thin film (approximately 200 to 800 A thick) on the metal surface due to the deposition of inorganic ions and high molecular weight organic compounds. This conditioning film alters mainly the electrostatic charge and wettability of the metal surface (Dexter, 1976) facilitating bacterial settlement. The second stage is caused by microorganisms colonization that results in the development of a biofilm formed by bacterial cells and their extracellular polymeric substances (EPS). The biofilm structure consists to a large extent of water and creates a diffusional barrier to the exchange of elements between the metal and the solution (Characklis, 1984). The inorganic changes correspond to another sequence of events occurring simultaneously with biofilm formation. This sequence is related to the process of metal dissolution (corrosion) and corrosion products formation on the metal surface (passivation). Corrosion and biofilm formation on metals in aqueous media occur within the same time scale, beginning immediately after the immersion of the specimen in the aqueous environment.