MICROBIAL INDUCED CORROSION

Microbial Induced Corrosion in Carbon Steel and Water Quality Effect on MIC

Microbial Induced Corrosion in Carbon Steel and Water Quality Effect on MIC

Although the electrochemical nature of corrosion remains valid for microbiologically influenced corrosion (MIC), the participation of micro organisms in the process nonetheless induces several unique features, the most significant being the modification of the metal-solution interface by bio film formation. Bio films affect interactions between metal surfaces and the environment, not only in bio deterioration processes such as MIC, but also in several biotechnological processes applied to materials recovery and handling. Thus, the key to the alteration of conditions at a metal surface, and hence the enhancement or inhibition of corrosion is the formation of a bio film (Videla, 1996). This can be considered as a gel containing 95% or even more water and a matrix of exopolysaccharidic substances (EPS), in which microbial cells and inorganic detritus are suspended (Geesey, 1982).

Bio films formation on metals is the result of an accumulation process-not necessarily uniform in time or space that starts immediately after metal immersion in the aqueous environment. A thin film (approximately 20-80 nm thick), due to the deposition of inorganic ions and organic compounds of high relative molecular mass, is formed in a first stage. This initial film can alter the electrostatic charges and wettability of the metal surface, facilitating its further colonization by bacteria. In a short time (minutes or hours depending on the aqueous environment in which the metal is immersed), microbial growth and EPS production result in the development of a bio film. This bio film is a dynamic system, and the different transport processes and chemical reactions occurring at the bio fouled interface will thus take place through the bio film thickness (Characklis, 1981).

Microbial colonization of metal surfaces drastically changes the classical concept of the electrical interface commonly used in inorganic corrosion: important changes in the type and concentration of ions, pH values, and oxidation-reduction potential are induced by the bio film, altering the passive or active behaviour of the metallic substratum and its corrosion products, as well as the electrochemical variables used for assessing corrosion rates. Micro organisms influence corrosion by changing the electrochemical conditions at the metal-solution interface. These changes may have different effects, ranging from the induction of localized corrosion, through a change in the rate of general corrosion, to corrosion inhibition. Any biological effect that either facilitates or impedes one of the anodic or cathodic reactions of the corrosion process, or that permanently separates anodic and cathodic sites, will increase corrosion. For instance, stimulation of the anodic reaction by acidic metabolites or the cathodic reaction by microbial production of a cathodic reactant such as hydrogen sulphide, the breakdown of protective films, or the increase in conductivity of the liquid environment enhances corrosion.

A New Biologically Conditioned Interface

Simultaneous with the biological changes that lead to bio film accumulation, a sequence of inorganic changes takes place at the metal surface immediately after its immersion in an aggressive aqueous ...