Influenced Corrosion

Microorganisms may either be freely suspended within the bulk solution (planktonic existence) or attached to a surface (sessile existence). When a material is first immersed in an aqueous solution, a thin layer of organic matter (referred to as the conditioning film) is adsorbed onto 

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Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

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. 

MicrobiologicaHy influenced corrosion, which results from the interaction of microorganisms and a metal, is receiving increased emphasis (1,3,9). The action of microorganisms is at least one of the reasons why natural seawater is more corrosive than either artificial seawater or sodium chloride solutions. Microorganisms attach to the surfaces of metals and can, for example, act as diffusion barriers produce metaboHtes that enhance or initiate... 

See Chap. 5, Oxygen Corrosion Chap. 6, Biologically Influenced Corrosion and Chap. 17, Graphitic Corrosion. ... 

Sulfate reducers. The best-known form of microbiologically influenced corrosion involves sulfate-reducing bacteria.- Without question, sulfate reducers cause most localized industrial cooling water corrosion associated with bacteria. Desulfovibrio, Desulfomonas, and Desulfotomacu-lum are three genera of sulfate-reducing bacteria. 

Thiobacillus thiooxidans is an aerobic organism that oxidizes various sulfur-containing compounds to form sulfuric acid. These bacteria are sometimes found near the tops of tubercles (see Chap. 3, Tubercu-lation ). There is a symbiotic relationship between Thiobacillus and sulfate reducers Thiobacillus oxidizes sulfide to sulfate, whereas the sulfate reducers convert sulfide to sulfate. It is unclear to what extent Thiobacillus directly influences corrosion processes inside tubercles. It is more likely that they indirectly increase corrosion by accelerating sulfate-reducer activity deep in the tubercles. 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Metal-reducing bacteria, such as those that convert ferric to ferrous ion, have been suggested as an accelerant for steel corrosion in oxygenated waters, lb date, evidence of these bacteria influencing corrosion in industrial systems is scarce. 

Active attack is commonly caused by microorganisms. Four factors must be present for a diagnosis of microbiologically influenced corrosion ... 

TABLE 6.2 Typical Microbiological Analysis In a Service Water System Pipe Experiencing Microbiologically Influenced Corrosion ... 

The importance of biologically influenced corrosion (particularly micro-biologically influenced corrosion) has been underestimated for many years. Recently, more attention has been paid to biological forms of corrosion— yet more attention does not always mean an improved situation. 

Many forms of corrosion resemble each other. However, it is just as true that each form of corrosion produces a unique fingerprint by which it can be differentiated from all other forms of attack. Biologically influenced corrosion is no exception. 

The presence of organisms (large or small) in proximity to corrosion by itself is not proof of biologically influenced corrosion, any more than a correlation of lunar phases with stock market fluctuations establishes a lunar-financial connection. It should be stressed vigorously that all evidence must be consistent with any single corrosion mode before a definitive diagnosis can be made (see Critical Factors above). Further, all alternative explanations must be carefully examined. 

A perforation in similar piping was caused by wastage beneath a tubercle. Although potentially aggressive bacteria were present in this system, no evidence of microbiologically influenced corrosion was found on this section. 

This case history illustrates the paradox so often encountered in microbiologically influenced corrosion. Clearly, two corrosion mechanisms were operating in the system, namely, acid attack and microbiologically influenced corrosion. To what degree each mechanism contributed to wastage is difficult to quantify after the fact. This was especially the case here, since other areas of the rolling oil system were attacked by a predominantly acidic form of corrosion. 

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