Page 61 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MICROBIOLOGICALLY INFLUENCED CORROSION (MIC) 39

Oxygenated seawater

Aerobic bacteria
consume oxygen:
0% 100%
Oxygen produce carbon sources
tension
Fouling H 2 Anaerobic bacteria S 2–
use carbon sources:
SRBs produce sulfides
S
H 2 2–
Metal

Figure 1.13 Association of anaerobic and aerobic bacteria. (Reproduced with permission of
NACE International from Reference 53.)

and then through their metabolism create a microenvironment favorable for anaerobic
bacteria.
Aerobic organisms near the outer surface of the film consume oxygen and create
a habitat for the SRB at the metal surface (52) (Fig. 1.13). The accompanying flora
delivers the nutrients SRB need such as acetic and butyric acids and consumes the
oxygen that is toxic for SRB. Sulfate-reducing bacterial corrosion is encountered in
the oil and gas industry (51).

1.6.7 Attack by Organisms Other than SRB
Ammonia and amines may be obtained by microbial decomposition of organic mat-
ter under aerobic and anaerobic conditions. These compounds are oxidized to nitrite
by aerobic bacteria such as Nitrobacter. Nitrobacter destroys the corrosion inhibition
properties of nitrite-based corrosion inhibitors by oxidation in the absence of a bioci-
dal agent in the formulation. The release of ammonia at the surfaces of heat exchanger
tubes has a detrimental effect (50).
Thiobacillus oxidizes inorganic sulfur compounds such as sulfides to sulfuric acid.
Some Thiobacilli leach metal sulfide ores as follows:

4FeS + 15O + 2H O → 2Fe (SO ) + 2H SO 4
2
4 3
2
2
2
2
It is useful to note that it is necessary to have oxygen and reduced sulfur
for Thiobacilli to act on and produce sulfuric acid. The growth of anaerobic
sulfate-reducing bacteria in sewage produces H S, which migrates to the air space
2
at the top of the line, where it is oxidized to sulfuric acid in the water droplets at the
crown of the pipe by Thiobacilli. The corrosion problem is because of the dissolution
of alkaline mortar by the sulfuric acid produced by the bacterial action, followed by
the corrosion of the ductile iron in the sewage pipe (50).