Microbiological corrosion (MIC) refers to corrosion and ensuing loss of metal caused by biological organisms. MIC can occur in any aqueous environments, and because of the omni present nature of microbes in fluid systems, MIC is a commonly occurring phenomenon. MIC is a common problem in industrial processes due to the presence of microbes, adequate nutrients and corrosive byproducts.

A number of metals, such as structural steels, copper alloys etc., tend to corrode generally over the entire surface in the absence of crevices or galvanic effects. In such cases, corrosion is determinated by the rate at which dissolved oxygen can be delivered to the metal surface. Biological organisms present in the aqueous medium often have the potential to increase or decrease oxygen transport to the surface; consequently, these organisms have a role in increasing or decreasing general corrosion. Most MIC, however, manifests as localized corrosion because most organisms do not form in a continuous film on the metal surface. Microscopic organisms also tend to settle on metal surfaces in the form of discrete colonies or at least spotty, rather than continuous films. Biological organisms fall under two groups based on the type of corrosion they engender: (a) Anaerobic corrosion (b) aerobic corrosion. Sulfate reducing bacteria (SRB) from the genera desulfovibrio are a typical example of anaerobic MIC. Aerobic sulfur oxidizing bacteria of the type thiobacillus can create an environment of up to 10 percent sulfuric acid, thereby encouraging rapid corrosion.

Cathodic depolarization

• The classic mechanism for MIC of steel and iron proposed by von Wolzgen Kuhr in 1934
• This mechanism is based on the idea that the rate-limiting step in corrosion is the dissociation of hydrogen from the cathodic site.
• It is thought that sulfate-reducing bacteria (SRB) consume hydrogen through the action of their hydrogenase enzymes, and thus "depolarize" the cathode, accelerating corrosion.
• Some investigators still believe that this mechanism is the important one for MIC of iron and steels, despite the fact that numerous experiments using SRB in pure culture gave corrosion rates far less than those seen at field sites and less than those measured in experiments using MIC communities.

Formation of occluded area on metal surface

• This mechanism is based on the observation that when microorganisms form colonies on the surface of a metal, they do not form uniform layers, but rather, local "community centers."
• The sites chosen for initial colonization may be related to such metallurgical features as roughness, preexisting corrosion sites, inclusions, or surface charge.
• Once the colony has formed, it produces sticky polymers which tend to attract and aggregate other biological and nonbiological (metals and chloride, for example) species to the colonization sites.
• This, in addition to the metabolism of available oxygen, iron, manganese, etc., results in conditions within and under the colonies very different from those on the surrounding metal.
• This leads to the formation of crevices and oxygen and ion concentration cells, allowing corrosion to proceed.

Fixing the anodic sites

• This parallels the development of the occluded cell. The presence and activities of the microbes creates a condition under t he colony in which incipient pitting leads to pitting driven principally by microbiological activities.
• This is made possible by the fact that most of the microbiological community usually remains fixed to the colonization site (although progeny may find other colonization sites).
• This causes the anodic site to become "fixed." This is a principal reason for the fact that more than 90% of MIC is seen as pitting-type corrosion

Underdeposit acid attack

• Most of the final products of MIC community metabolism are short-chain fatty acids (acetic acid is the most common).
• Acetic acid is very aggressive to carbon steel when concentrated under a colony or other deposit.
• This is the case both at field sites and in the laboratory.

Microbiologically Induced Corrosion in a Sour Gas Pipeline -

The highly localized corrosion shown in the figure is typical of that resulting from microbial action. One of the features of this type of attack are the elongated pits which tunnel into the specimen often in an irregular manner. The pit was one of several located near the gas/water interface. The pipeline was left for a prolonged period in a shut-in (static) condition which promoted the growth of bacteria and highly localized corrosive attack. Sulfate reducing bacteria were suspected due to the combination of sulfate species in the water and anaerobic conditions. The corrosion was mitigated by a closer control of operating conditions and chemical treatment.