Microbiologically-influenced corrosion

Influence of Some Microbiological Species on Corrosion.64 Some bacteria are involved directly in the oxidation or reduction of metal ions, particularly iron and manganese. 

Some microbes can produce organic acids, such as formic and succinic, or mineral acids such as sulfuric acid. Some bacteria can oxidize sulfur or sulfide to sulfate or reduce sulfates, very often to hydrogen sulfide as end product. (Dexter)14 

The ferrous sulfide him is not continuous and the base iron can corrode. Hydrogen sulfide can also be produced. The SRB have been identified to contribute to the corrosion of stainless steels, copper and aluminum alloys. (Dexter)14 

Aerobic bacteria consume oxygen produce carbon sources 

Anaerobic bacteria use carbon sources SRBs produce sulfides 

Under certain conditions, bacterial colonies change the chemistry of an alloy s surface and induce rapid corrosion. One common example involves 

Fundamentals of Metallic Corrosion Atmospheric and Media Corrosion of Metals 

MIC is generally associated with stagnant or low-flow aqueous systems over a range of pH values from 1.0 to 10.5 at temperatures of 32°F (0°C) to 212°F (100°C). There are 50-60 bacteria species believed to be associated with MIC, both aerobic and anaerobic species. The corrosive attack can be rapid, often occuring within weeks of introducing the bacteria. One solution that suffered MIC involved dissolved polymeric organic materials and ammonium phosphate compoimds, so the affected solutions can be quite varied. 

There are various treatments used to prevent or alleviate MIC, including the addition of biocides such as ozone or hydrogen peroxide to the water. This, however, will be ineffectual if the bacteria have previously formed protective nodules as their habitat. These nodules must be mechanically removed to kill the bacteria. 

Bacteria must always be assumed to be present in imtreated water, so it is obvious that untreated hydrotest water should be removed from a system or vessel as soon as possible. If this is not possible, the water should be kept flowing at velocities over 5 ft/s (1.5 m/s) because bacteria require low-flow conditions to colonize. 

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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. 

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 ... 

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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