Microbially influenced corrosion metabolism

The choice of the appropriate material is decisive for resistance against microbially influenced corrosion. This means that before the choice of material can be made, what kind of impacts is has to resist needs to be considered. Microbial influencing factors must also be considered. Accordingly, in the presence of volatile sulfur compounds, e.g., in sewage pipelines, it is recommended not to use materials like unprotected concrete which may be destroyed by the end product of the microbial degradation process (in this case, sulfuric acid formed by Thiobacilli). Another example would be the choice of a stainless steel or of an alloy that cannot be attacked under the conditions of a biofllm and the complex metabolic processes occurring underneath it. If, for instance, a material has to be chosen for static reasons, this material has to be protected by a coating or a liner made of an inert material. All these examples are based on the consideration that all attack factors have been identified by a complete inventory. 

In summary, it becomes obvious that most reactions of materials with the environment are influenced by microorganisms and are often even controlled by them [36]. Thus, in the case of microbially influenced corrosion, the participation of microoiganisms needs to be anticipated [54,95]. Furthermore, cases are known in which excreted metabolic products, e.g., EPS free from microbial cells, cause corrosion. 

Depending on the bacteria and soil conditions which can be described appropriately as an extremely heterogeneous system, these transformations may be assimilatory or dissimilatory metabolic functions. Based on the recent field surveys and laboratory studies, the bacterial-environmental interactions, with reference to the cycles of sulfur and other elements, in corrosion on buried pipes are shown as Figure 1. This Figure demonstrates that microbiologically influenced corrosion (MIC) results from the activities of a microbial community. 

Diagrammatic representation of the formation of microbial consortia and their influence on the corrosion processes (a) When the film becomes sufficiently thick, its inner part will be anaerobic with the possible development of SRB microcolonies (black cells), (b) The SRB attracts secondary colonizers by its metabolic products and forms a consortium with them, (c) The development of local areas with varying physiochemical parameters leads to pitting corrosion. (From Costerton, J.W. and Geesey, G.G., The microbial ecology of surface colonization and of consequent corrosion. Proceedings ofBiologically Induced Corrosion, S. C. Dexter, ed., NACE-8, Houston, TX, pp. 223-232,1986.)... 

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