Page 65 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MICROBIOLOGICALLY INFLUENCED CORROSION (MIC) 43

by SRB in estuarine environments has been observed. Copper-nickel tubes from fan
coolers in a nuclear power plant showed pitting corrosion under bacterial deposits.
The slime-forming bacteria acting in concert with iron- and manganese-oxidizing
bacteria were responsible for the deposits. Monel 400 tubing containing Ni, Cu
Fe was pitted severely after exposure to marine and estuarine waters containing
SRB (41).
It has been shown that welds provide unique environments for the colonization of
SRB with subsequent production of sulfides that affect the weld seam surface of the
HAZ. Exposure of sulfide-derived surfaces to fresh, aerated seawater resulted in rapid
spalling on the downstream side of weld seams. The bared surfaces became anodic
to the sulfide-coated weld root, initiating and accelerating localized corrosion (41).

1.6.16 Microbiological Impacts and Testing
Evidence of harmful impact: Microorganisms, including the corrosion-inducing
microorganisms, are present in soils, freshwater, seawater, and air. In a majority
of cases, these organisms influence corrosion. In a few cases, the organisms can
reproduce the attack on introduction into a sterile system (57). Waters that may be
untreated, fresh well waters, have been used in the hydrotesting of fabricated stainless
steel structures. These waters may contain microorganisms such as Gallionella,
which can cause corrosion.

1.6.17 Recognition of Microbiological Corrosion
This is done based on four types of evidence: (i) metallurgical; (ii) microbiological,
(iii) chemical; and (iv) electrochemical evidence (41).
Metallography of the samples shows the types of MIC by the patterns of the cor-
rosion products on the surface.
Microbiological evidence consists of gathering data on wet samples. It is also nec-
essary to obtain photographs of the sample while the organism is live. This is followed
by analysis of biological materials and corrosion products.
Chemical evidence consists of detailed chemical analysis of the corrosion prod-
ucts and any biological mounds present at or near the corrosion site. Evaluation of the
chemistry of the liquid phase and its variability, both spatially and with time in rela-
tion to corrosive attack, is necessary. The important factors are color, texture, odor,
and distribution of materials and organic and inorganic chemistries.
The color change of the corrosion product from black to brown indicates a sulfide
corrosion product. Tests for sulfate, total organic carbon, pH, sulfide, and oxygen
concentrations are good indicators of the potential for SRB growth (4, 41).
Electrochemical techniques such as variation of corrosion potential and corrosion
rates as a function of time and EN may be useful in monitoring biological corrosion
evolution (57).
An effective monitoring scheme for controlling both biofouling and biocorrosion
should involve acquiring data on (i) sessile bacterial counts of the organism in the