Page 59 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MICROBIOLOGICALLY INFLUENCED CORROSION (MIC) 37

The slime can also act as a diffusion barrier and change the concentrations of different
elements and pH at the electrochemical interface (41).

1.6.2 Environments
A great variety of microscopic organisms are present in all natural aqueous envi-
ronments such as bays, estuaries, harbors, coastal, and open ocean seawaters,
rivers, streams, lakes, ponds, aqueous industrial effluents, and waste waters. Bar-
nacles and mussels are also present in many environments. In natural conditions,
sulfate-reducing bacteria (SRB) grow in association with other microorganisms and
use a range of carboxylic acids and fatty acids, which are common by-products of
other microorganisms. Biological slimes are commonly found in water phases of
industrial process plants. A wide range of common bacteria is thought to secrete
large amounts of organic matter under aerobic and anaerobic conditions (41).

1.6.3 Biological Corrosion in Freshwater Environments
There exist bacteria and algae (yeasts and molds) in fresh water media. The organisms
attach themselves to and grow on the surface of structural materials resulting in the
formation of a biofilm. The film can range from a microbiological slime film on fresh
water, heat-transfer surfaces to heavy encrustation of hard-shelled fouling organisms
on structures in coastal seawater. The presence of a biofilm does not mean that it will
always have a significant effect on corrosion (41).
Microbial films will affect the general corrosion only when the film is continu-
ous. This is not the case in general and microorganisms form in discrete deposits or
colonies and the resulting corrosion is localized.
A uniform slime film formation on the piping of potable water systems and on the
heat-transfer surfaces of low-temperature heat exchangers is inconsequential unless it
obstructs the flow leading to a health hazard by growth of the organisms or localized
corrosion (41).

1.6.4 Biological Corrosion in Marine Environments
A heavy fouling of macroorganisms such as barnacles and mussels decreases the
amount of dissolved oxygen at the interface and acts as a barrier on structural steel in
the splash zone and shields the metal from damage caused by wave action. A continu-
ous film of bacteria, algae, and slime (microorganisms) can have the same beneficial
effect as the macroorganisms. In the majority of cases, the microbial films are not
continuous and an oxygen preferential cell is created. These films are suspected to
be capable of inducing pit initiation on stainless steels and copper alloys in marine
environments. Natural seawater is more corrosive than artificial saline water because
of the living organisms in seawater (41).
Incomplete coverage by barnacles is more likely to initiate pitting and crevice
corrosion. The barnacles are attached to the periphery of a high-strength steel rudder,
which is coated with an antifouling paint. During use, the paint around the edges has