Microbiologically
Influenced CorrosionINDOCORNer
Microbiologically
Influenced Corrosion
Classification
of Microorganism in Corrosion
MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)
MIC refers to corrosion that is influenced by the presence and activities of microorganisms and/or their metabolites (the products produced in their metabolism). Bacteria, fungi and other microorganisms can play a major part in soil corrosion. Spectacularly rapid corrosion failures have been observed in soil due to microbial action and it is becoming increasingly apparent that most metallic alloys are susceptible to some form of MIC. The mechanisms potentially involved in MIC are summarized as:
Cathodic depolarization, whereby the cathodic rate limiting step is accelerated by micro-biological action.
Formation of occluded surface cells, whereby microorganisms form "patchy" surface colonies. Sticky polymers attract and aggregate biological and non-biological species to produce crevices and concentration cells, the basis for accelerated attack.
Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of corrosion pits, driven by microbial activity and associated with the location of these colonies.
Underdeposit acid attack, whereby corrosive attack is accelerated by acidic final products of the MIC "community metabolism", principally short-chain fatty acids.
Certain microorganisms thrive under aerobic conditions, whereas others thrive in anaerobic conditions. Anaerobic conditions may be created in the micro-environmental regime, even if the bulk conditions are aerobic. The pH conditions and availability of nutrients also play a role in determining what type of microorganisms can thrive in a soil environment. Microorganisms associated with corrosion damage are classified as:
Anaerobic bacteria that produce highly corrosive species as part of their metabolism.
Aerobic bacteria that produce corrosive mineral acids.
Fungi that may produce corrosive by products in their metabolism, such as organic acids.
Apart from metals and alloys they can degrade organic coatings and wood.
Slime formers, that may produce concentration corrosion cells on surfaces
Refferences: http://www.corrosion-doctors.org
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Microorganisms pervade our environment and readily "invade" industrial systems wherever conditions permit. These agents flourish in a wide range of habitats and show a surprising ability to colonize water rich surfaces wherever nutrients and physical conditions allow. Microbial growth occurs over the whole range of temperatures commonly found in water systems, pressure is rarely a deterrent and limited access to nitrogen and phosphorus is offset by a surprising ability to sequester, concentrate and retain even trace levels of these essential nutrients.
Many engineers continue to be surprised that such small organisms can lead to spectacular failures of large engineering systems. The microorganisms of interest in Microbiologically Influenced Corrosion (MIC) are mostly bacteria, fungi, algae and protozans.
Bacteria are generally small, with lengths of typically under 10 µm. Collectively, they tend to live and grow under wide ranges of temperature, pH and oxygen concentration. Carbon molecules represent an important nutrient source for bacteria.
Fungi can be separated into yeasts and molds. Corrosion damage to aircraft fuel tanks is one of the well-known problems associated with fungi. Fungi tend to produce corrosive products as part of their metabolisms; it is these by-products that are responsible for corrosive attack. Furthermore, fungi can trap other materials leading to fouling and associated corrosion problems.
Protozans are predators of bacteria and algae and therefore potentially mitigate microbial corrosion problems.
MIC is responsible for the degradation of a wide range of materials. Bacteria can exist in several different metabolic states. Those that are actively respiring, consuming nutrients, and proliferating are said to be in a "growth" stage. Those that are simply existing, not growing because of unfavorable conditions, are said to be in a "resting" state.
Some strains, when faced with unacceptable surroundings, form spores that can survive extremes of temperature and long periods without moisture or nutrients, yet produce actively growing cells quickly when conditions again become acceptable.
The latter two states may appear, to the casual observer, to be like death, but the organisms are far from dead. Cells that actually die are usually consumed rapidly by other organisms or enzymes. When looking at an environmental sample under a microscope, therefore, it should be assumed that most or all of the cell forms observed were alive or capable of life at the time the sample was taken.
Refferences: http://www.corrosion-doctors.org
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CLASSIFICATION OF MICROORGANISMS IN CORROSION
Microorganisms can be categorized according to oxygen tolerance:
Strict (or obligate) anaerobes, which will not function in the presence of oxygen (see Sulfate reducing bacteria or SRB).
Aerobes, which require oxygen in their metabolism.
Facultative anaerobes, which can function either in the absence or presence of oxygen.
Microaerophiles, which use oxygen but prefer low levels.
Strictly anaerobic environments are quite rare in nature, while strict anaerobes are commonly found flourishing within anaerobic microenvironments in highly aerated systems.
Another way of classifying organisms is according to their metabolism:
The compounds or nutrients from which they obtain their carbon for growth and reproduction.
The chemistry by which they obtain energy or perform respiration.
The elements they accumulate as a result of these processes.
A third way of classifying bacteria is by shape. These shapes are predictable when organisms are grown under well defined laboratory conditions. In natural environments, however, shape is often determined by growth conditions rather than pedigree.
Examples of shapes are:
"Vibrio," for comma shaped cells.
"Bacillus," for rod shaped cells.
"Coccus," for round cells.
"Myces," for fungi like cells.
Refferences: http://www.corrosion-doctors.org
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SRB are anaerobes that are sustained by organic nutrients. Generally they require a complete absence of oxygen and a highly reduced environment to function efficiently. Nonetheless, they circulate (probably in a resting state) in aerated waters, including those treated with chlorine and other oxidizers, until they find a "ideal" environment supporting their metabolism and multiplication.
SRB are usually lumped into two nutrient categories, those that can use lactate and those that cannot. The latter generally use acetate and are difficult to grow in the laboratory on any medium. Lactate, acetate, and other short chain fatty acids usable by SRB do not occur naturally in the environment. Therefore, these organisms depend on other organisms to produce such compounds.
SRB reduce sulfate to sulfide, which usually shows up as hydrogen sulfide or, if iron is available, as black ferrous sulfide. In the absence of sulfate, some strains can function as fermenters and use organic compounds such as pyruvate to produce acetate, hydrogen, and carbon dioxide. Many SRB strains also contain hydrogenase enzymes, which allow them to consume hydrogen. Most common strains of SRB grow best at temperatures from 25° to 35°C. A few thermophilic strains capable of functioning efficiently at more than 60°C have been reported.
Tests
for the presence of SRB have traditionally involved growing the
organisms on laboratory media, quite unlike the natural
environment in which they were found. These laboratory media
will only grow certain strains of SRB, and even then some
samples require a long lag time before the organisms will adapt
to the new growth conditions. As a result, misleading
information has been obtained regarding the presence or absence
of SRB in field samples. SRB have been implicated in the
corrosion of cast iron and steel, ferritic stainless steels, 300
series stainless steels (also very highly alloyed stainless
steels), copper nickel alloys, and high nickel molybdenum
alloys. They are almost always present at corrosion sites
because they are in soils, surface water streams and waterside
deposits in general. Their mere presence, however, does not mean
they are causing corrosion. The key symptom that usually
indicates their involvement in the corrosion process of ferrous
alloys is localized corrosion filled with black sulfide
corrosion products.
Refferences: http://www.corrosion-doctors.org
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