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
(Courtesy of www.asm.org)
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.
(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.
(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
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.