Page 443 - 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

Strictly anaerobic environments are quite rare in nature. However,
strict anaerobes are commonly found flourishing within anaerobic
microenvironments (under deposits and fouling) even in highly
aerated systems. The pH conditions and availability of nutrients also
play a role in determining what type of microorganisms can thrive in
a particular soil environment. Another way of classifying organisms
is according to their metabolism:
• The compounds or nutrients from which they obtain their
carbon for growth and reproduction
• The chemistry by which they obtain energy or perform
respiration
• The elements they accumulate as a result of these processes
All microorganisms are not equally aggressive to metals. What is
important, from a corrosion standpoint, is the number of
microorganisms of the specifically corrosive types. Microorganisms
associated with corrosion damage are classified as
• Fungi that may produce corrosive byproducts in their
metabolism, such as organic acids. Apart from metals and
alloys these can also degrade organic coatings and wood.
• Slime formers that may produce concentration corrosion cells
on surfaces.
• Anaerobic bacteria that produce highly corrosive species as
part of their metabolism.
• Aerobic bacteria that produce corrosive mineral acids.

Fungi
Fungi can be separated into yeasts and molds. Certain fungi are
capable of producing organic acids and have been blamed for
corrosion of steel and aluminum, as in the corrosion failures of
aluminum aircraft fuel tanks. In addition, fungi may produce
anaerobic sites for SRB and produce metabolic byproducts that help
the growth of various bacteria leading to fouling and associated
corrosion problems. In general, molds are considered to be of greater
importance in corrosion problems than yeasts [13].
Algae
Algae are photosynthetic organisms with relatively simple nutritional
requirements of light, water, air and certain inorganic compounds.
Algae are found in wide ranges of salinity, from seawater to distilled
water, and survive in different degrees of light intensity. Algae have
been associated with fouling and related corrosion problems through
the photosynthetic production of gaseous oxygen when exposed to
light and the production of corrosive species such as organic acids.