Page 444 - Corrosion Engineering Principles and Practice

412 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 413

Bacteria
Bacteria are generally small, with lengths 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.
Bacteria can exist in several different metabolic states. Bacteria that
are actively respiring, consuming nutrients, and proliferating are said
to be in a growth stage while bacteria that are simply existing, but 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 extreme temperatures and long periods without
moisture or nutrients, yet produce actively growing cells quickly when
conditions again become acceptable. 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.

Sulfate Reducing Bacteria SRBs have been implicated in the corrosion
of cast iron and steel, ferritic stainless steels, 300 series stainless steels
and other 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. The key symptom that usually indicates
their involvement in the corrosion process of ferrous alloys is localized
corrosion filled with black sulfide corrosion products.
SRBs 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 an
ideal environment supporting their metabolism and multiplication.
SRBs 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.
SRBs reduce sulfate to sulfide, which usually shows up as hydrogen
sulfide or, if iron is available, as black ferrous sulfide (Fig. 10.10). 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.