Microbial corrosion microbes

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Sanders PF. Monitoring and control of sessile microbes Cost effective ways to reduce microbial corrosion. In Sequeira CAC, Tiller AK, eds. Microbial Corrosion-1. New York, N.Y. Elsevier Applied Science, 1988 191-223. 

Sanders, P. F, Monitoring and Control of Sessile Microbes Cost Effective Ways to Reduce Microbial Corrosion, in Sequeira, C. A. C., and Tiller, A. K. (eds.), Microbial Corrosion—1, New York, Elsevier Applied Science, 1988, pp. 191-223. 

Microbial corrosion (also called microbiologically influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods. 

Microbiologically influenced corrosion is defined by the National Association of Corrosion Engineers as any form of corrosion that is influenced by the presence and/or activities of microorganisms. Although MIC appears to many humans to be a new phenomenon, it is not new to the microbes themselves. Microbial transformation of metals in their elemental and various mineral forms has been an essential part of material cycling on earth for billions of years. Some forms of metals such as reduced iron and manganese serve as energy sources for microbes, while oxidized forms of some metals can substitute for... 

Once microbes have multiplied and established themselves as a colony, they can adhere to metal parts forming microbial plaquesUnderneath these plaques, severe corrosion of metal is often found. 

Microbial activity is a major concern in systems in which water-based fluids are used. Particularly, glycol fluids provide a good source of nutrition to some types of biological species. In salt-based brines, however, microbes do not survive because of high osmotic pressure. When microbes start to grow inside a system, they create a layer known as biofilm on the walls of the pipes and heat exchangers. This reduces the heat transfer rate. Some microbes are capable of creating acids and hence cause a substantial amount of corrosion in the system. 

Hamilton, W. A. (2003). Microbially influenced corrosion as a model system for the study of metal microbe interactions a unifying electron transfer hypothesis. Biofouling 19, 65-76. 

Corrosion in ships can also be caused by MIC. In this type of corrosion, microbial organisms present in the environment can accelerate corrosion. For example, SRB, which are present in stagnant water of many harbors, can build up on the hulls of ships. Other corrosion-causing bacteria, such as acid-producing and anaerobic bacteria, are also present in ballast tanks as well as in the liquid products that some tankers carry. The microbes cause a localized change in the environment, which can promote aggressive pitting and other types of corrosion. 

When coupled with observation of the surface by various techniques, the chemical data from microelectrodes can be correlated with the positions of microbial colonies and corrosion sites on the metal surface. Franklin et al. [97] have used confocal laser scanning microscopy (CLSM) to image microbes preferentially attached to corroding particles of zinc and iron. Xu [47] has used both CLSM and epifluorescence microscopy to image colonies of microorganisms associated with various combinations of oxygen, peroxide, and manganese revealed by the Au-Hg microelectrodes on passive metal surfaces. 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

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