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Corrosion Fundamentals

Microbial Corrosion

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.

There are a number of mechanisms associated with this form of corrosion, and detailed explanations are available at the web sites listed at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated.

The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow

Biofilm on a Metallic Condenser Surface
(Courtesy of www.asm.org)

corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive.

Microbial Corrosion of a Fire Hydrant
(Courtesy of www.micscan.com)

MIC can be a serious problem in stagnant water systems such as the fire-protection system that produced the pits shown above. (see Pitting Corrosion). The use of biocides and mechanical cleaning methods can reduce MIC, but anywhere where stagnant water is likely to collect is a location where MIC can occur.

Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible chemicals that allow corrosion to occur. That's how the limited corrosion that was found on the hull of the Titanic occurred. The picture below shows a "rusticle" removed from the hull of Titanic. This combination of rust and organic debris clearly shows the location of rivet holes and where two steel plates overlapped.

Microbial Corrosion of the Titanic Hull
(Couresy of www.dbi.sk.ca)

Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures exposed to air. The pictures below show a spillway

Hydroelectric Dam on the Columbia River

Stress Corrosion Cracks

(Courtesy of www.meic.com)

gate from a hydroelectric dam on the Columbia River. The stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was necessary.

In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MIC.

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