Page 446 - Corrosion Engineering Principles and Practice

414 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 415

They are also found on stone buildings and statues and probably
account for much of the accelerated damage commonly attributed to
acid rain. Where Thiobacillus bacteria are associated with corrosion,
they are almost always accompanied by SRB. Thus, both types of
organisms are able to draw energy from a synergistic sulfur cycle.
Iron/Manganese Oxidizing Bacteria Bacteria that derive energy from
2+
3+
the oxidation of Fe to Fe are commonly reported in deposits
associated with MIC. They are almost always observed in tubercles
(discrete hemispherical mounds) over pits on steel surfaces. The most
common iron oxidizers are found in the environment in long protein
sheaths or filaments. While the cells themselves are rather indistinctive
in appearance, these long filaments are readily seen under the
microscope and are not likely to be confused with other life forms.
The observation that filamentous iron bacteria are omnipresent in
tubercles might be, therefore, more a matter of their easy detection
than of their relative abundance. An intriguing type of iron oxidizers
is the Gallionella bacterium, which has been blamed for numerous
cases of corrosion of stainless steels.
Besides the iron/manganese oxidizers, there are organisms that
simply accumulate iron or manganese. Such organisms are believed
to be responsible for the manganese nodules found on the ocean
floor. The accumulation of manganese in biofilms is blamed for
several cases of corrosion of stainless steels and other ferrous alloys
in water systems treated with chlorine or chlorine/bromine
compounds.
Methane Producers Methane producing bacteria (methanogens) have
only been added in recent years to the list of organisms believed
responsible for corrosion. Like many SRB, methanogens consume
hydrogen and thus are capable of performing cathodic depolarization.
While methane producers normally consume hydrogen and carbon
dioxide to produce methane, in low nutrient situations these strict
anaerobes will become fermenters and consume acetate.
Organic Acid Producing Bacteria Various anaerobic bacteria such as
Clostridium are capable of producing organic acids. Unlike SRBs,
these bacteria are not usually found in aerated macroenvironments
such as open, recirculating water systems. However, they are
a problem in gas transmission lines and could be a problem in closed
water systems that become anaerobic.

Aerobic Slime Formers Aerobic slime formers are a diverse group of
aerobic bacteria. They are important in managing corrosion mainly
because they produce extracellular polymers that make up what is
commonly referred to as slime, a natural polymer that is actually a
sophisticated network of sticky strands that bind cells to the surface
and control what permeates through the deposit.