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Microbiologically-Induced Corrosion (MIC):
What it is and How to Prevent It
MIC is a common problem in industrial processes due to the presence of microbes that form colonies on the surface of metal. This eventually leads to a formation of crevices, with oxygen and ion concentration cells, allowing corrosion to progress.
If left untreated, piping systems will be significantly weakened, often forming holes in its pipe walls resulting leaks and loss of liquid.
Treatment can be done by protecting metal piping with cathodic protection or chemically treating the liquid, which in itself can compound corrosion.
Either method requires additional expenditures that may only delay failure.
Click here to learn more about MIC
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Under the Microscope: Understanding, Detecting, and Preventing Microbiologically-Induced Corrosion
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Richard A. Lane
AMPTIAC
Rome, NY
INTRODUCTION
A renewed interest in corrosion prevention and control has
resulted in a major push within the DOD to help bring down
the Department’s enormous maintenance costs attributed to corrosion.
Much of these rising costs can be directly attributed to
extending the useful life of systems well beyond their original
specifications. However, one type of corrosion that can produce
unexpected problems, premature failures, and costly repairs is
microbiologically influenced corrosion (MIC). Microorganisms
have long been known to influence corrosion, causing throughwall
corrosion of piping and heat exchanger tubes 10-1000 times
faster than normal.[1] Effective prevention and control of MIC
involves an underlying knowledge of the microorganisms
responsible for increased corrosion rates as well as methods that
can be implemented to detect and prevent microbial growth.
MIC is not a form of corrosion, but rather is a process that
can influence and even initiate corrosion. It can accelerate most
forms of corrosion; including uniform corrosion, pitting corrosion,
crevice corrosion, galvanic corrosion, intergranular
corrosion, dealloying, and stress corrosion cracking. In fact, if
unfamiliar with MIC, some corrosion problems may be
misdiagnosed as conventional chloride-induced corrosion. One
prominent indicator of MIC is a higher rate of attack than one
would normally expect. MIC can affect numerous systems, and
can be found virtually anyplace where aqueous environments
exist. It is not exclusive to water-based systems, as it occurs in
fuel and lubrication systems as well. Table 1 lists applications
where MIC has been found to be prominent while Figure 1
shows one such location.
TYPES OF MICROORGANISMS
The types of microorganisms with species attributable to MIC
include algae, fungi, and bacteria.[3] Algae can be found in most
any aquatic environment ranging from freshwater to concentrated
salt water. They produce oxygen in the presence of light (photosynthesis)
and consume oxygen in darkness. The availability of
oxygen has been found to be a major factor in corrosion of metals
in saltwater environments. Algae flourish in temperatures of
32 - 104°F and pH levels of 5.5 - 9.0. Fungi consist of mycelium
structures, which are an outgrowth of a single cell or spore.
Mycelia are immobile, and can grow to reach macroscopic
dimensions. Fungi are most often found in soils, although some
species are capable of living in water environments. They metabolize
organic matter, producing organic acids.
Bacteria are generally classified by
their affinity to oxygen. Aerobic species
require oxygen to carry out their metabolic
functions, while anaerobic species
do not live or metabolize in the presence
of oxygen. Facultative bacteria can grow
in either environment, although they
prefer aerobic conditions. Microaerophilic
bacteria require low concentrations
of oxygen. Oddly enough, aerobic
and anaerobic organisms have often
been found to co-exist in the same location.
This is because aerobic species
deplete the immediate surroundings of
oxygen creating an ideal environment
for anaerobes. Bacteria are further classified by shape into spherical
(bacillus), rod (coccus), comma (vibrio), and filamentous
(myces) species. Figure 2 is an example of rod-shaped bacteria
observed using transmission electron microscopy.
Microorganisms in the planktonic state refer to those organisms
floating freely in the aqueous environment or in air. They
can resist harsh chemicals
including acids, alcohols,
and disinfectants, and can
withstand drying, freezing,
and boiling conditions.[6] Some spores
have the ability to last
hundreds of years and
then germinate once
favorable conditions exist.
Microorganisms in the
sessile state are those that
have attached themselves
to a surface and have developed a protective membrane, collectively
called a biofilm. Microorganisms have the ability to reproduce
quickly; some doubling in as little as 18 minutes. When left
untreated, they can rapidly colonize in stagnant aqueous environments,
potentially introducing a highly active corrosion cell.
MICROORGANISMS THAT ACCELERATE CORROSION
Once a microorganism forms a biofilm on a material’s surface, a
microenvironment is created that is dramatically different from
the bulk surroundings. Changes in pH, dissolved oxygen, and
organic and inorganic compounds in the microenvironment can
lead to electrochemical reactions which increase corrosion rates.
Microorganisms may also produce hydrogen, which can cause
damage in metals. Most microorganisms form an extracellular
membrane which protects the organism from toxic chemicals
and allows nutrients to filter through.[6] Biofilms are resistant to
many chemicals by virtue of their protective membrane and ability
to breakdown numerous compounds. They are significantly
more resistant to biocides (chemicals used to kill microorganisms)
than planktonic organisms. Some bacteria even metabolize
corrosion inhibitors, such as aliphatic amines and nitrites,
decreasing the inhibitor’s ability to control corrosion.
Microorganisms’ metabolic reactions attributable to metallic
corrosion involve sulfide production, acid production, ammonia
production, metal deposition, and metal oxidation and reduction.
Several groups of microorganisms have been attributed to
MIC, and are described briefly below.[7] Following these recognized
forms, Table 2 then lists some specific microorganisms
within these categories, along with their characteristics.
Sulfate Reducing Bacteria
Sulfate reducing bacteria (SRB) are
anaerobic microorganisms that have
been found to be involved with numerous
MIC problems affecting a variety of
systems and alloys. They can survive in
an aerobic environment for a period of
time until finding a compatible environment.
SRB (see Figure 3) chemically
reduce sulfates to sulfides, producing
compounds such as hydrogen sulfide
(H2S), or iron sulfide (Fe2S) in the case
of ferrous metals. The most common
strains exist in the temperature range of
25 - 35°C, although there are some that
can function at temperatures of 60°C.
They can be detected through the presence
of black precipitates in the liquid
media or surface deposits, as well as a
characteristic hydrogen sulfide smell.
Sulfur/Sulfide Oxidizing Bacteria
Sulfide oxidizing bacteria (SOB) are an aerobic species which
oxidize sulfide or elemental sulfur into sulfates. Some species
oxidize sulfur into sulfuric acid (H2SO4) leading to a highly
acidic (pH = 1) microenvironment. The high acidity has been
associated with the degradation of coating materials in a number
of applications. They are primarily found in mineral deposits
and are common in wastewater systems. SRB are often found in
conjunction with SOB.
Iron/Manganese Oxidizing Bacteria
Iron and manganese oxidizing bacteria have been found in
conjunction with MIC, and are typically located in corrosion
pits on steels. Some species are known to accumulate iron or
manganese compounds
resulting from the oxidation
process. High concentrations
of manganese
in biofilms have been
attributed to the corrosion
of ferrous alloys,
including pitting of stainless
steels in treated water
systems. Iron tubercles
have also been observed to
form as a result of the oxidation
process (Figure 4).
Slime Forming Bacteria
Slime forming bacteria are aerobic organisms which develop
polysaccharide “slime” on the exterior of their cells. The slime controls permeation of nutrients to the cells and may breakdown
various substances, including biocides. Slime formers have been
responsible for the decreased performance of heat exchangers as
well as clogging of fuel lines and filters. They can prevent oxygen
from reaching the underlying metal surface, creating an
environment suitable for anaerobic organisms.
Organic Acid Producing Bacteria
Some anaerobic organisms also produce organic acids. These
bacteria are more apt to be found in closed systems including gas
transmission lines and sometimes closed water systems.
Acid Producing Fungi
Some fungi produce organic acids which attack iron and aluminum
alloys. Similar to slime formers, they can create environments
suitable for anaerobic species. The widespread corrosion
problems observed in aluminum fuel tanks in aircraft have been
attributed to these organisms.
MIC IN METALS
Since MIC is a mechanism that accelerates corrosion, it should
be expected to occur more often in metal alloys with susceptibilities
to the various forms of corrosion, and in environments
conducive to biological activity. Metals used in the applications
listed in Table 1 include mild steels, stainless steels, copper
alloys, nickel alloys and titanium alloys. In general, mild steels
can exhibit everything from uniform corrosion to environmentally-
assisted cracking, while the remaining alloys usually only
show localized forms. Mild steels, stainless steels, aluminum,
copper, and nickel alloys have all been shown to be susceptible
to MIC, while titanium alloys have been found to be virtually
resistant to MIC under ambient conditions.
Mild Steels
MIC problems have been widely documented in piping systems,
storage tanks, cooling towers, and aquatic structures. Mild steels
are widely used in these applications due to their low cost, but
are some of the most readily corroded metals. Mild steels are protection
may also be used for select applications. Galvanization (zinc
coating) is commonly used to protect steel in atmospheric environments.
Bituminous coal tar and asphalt dip coatings are often used
on the exterior of buried pipelines and tanks, while polymeric coatings
are used for atmospheric and water environments. However,
biofilms tend to form at flaws in the coating surfaces. Furthermore,
acid producing microorganisms have been found to dissolve zinc
and some polymeric coatings.[11] Numerous cases have also been
documented where microorganisms caused debonding of coatings
from the underlying metal. Delamination of the coating, in turn,
creates an ideal environment for further microbial growth.
Poor quality water systems and components with areas that
accumulate stagnant water and debris are prone to MIC. In
some extreme cases, untreated water left stagnant within mild
steel piping has caused uniform corrosion throughout the low
lying areas. This has been seen to occur in underground pipes
that have been left unused for periods of time.[11] Many power
plant piping failures have been found to be the result of introducing
untreated water into a system. SRB has been the primary
culprit in such cases. A change to a more corrosion resistant
material is not always the most appropriate answer when it
comes to solving MIC problems. For example, an upgrade from
carbon steel to stainless steel in a nuclear power plant caused a
change in MIC problems, that in some instances were even more
severe. SRB has also been found in conjunction with underdeposit
corrosion occurring in cooling towers. Wet soils containing
clay have played a major role in the occurrence of underground
MIC problems. Under such conditions, the exterior of underground
piping and storage tanks have experienced coating
delamination and corrosion as a result of biofilm growth.
Stainless Steels
Stainless steels have suffered MIC problems under the same sets of
conditions as mild steels - primarily in situations where water
accumulates. There are two notable problems that have surfaced
with MIC of stainless steel. First, stainless steels corrode at an
accelerated rate, primarily through pitting or crevice corrosion,
which occurs in low lying areas, joints, and at corner locations.
This has been found to occur in tanks and piping systems that
were hydrotested* using well water, and then put in storage before
service without using biocides or drying the system to prevent
microbial growth.[11] In one particular case, a 304 stainless steel
pipeline for freshwater service, failed 15 months after being
hydrotested.[12] The second MIC problem discovered with stainless
steels is that corrosion occurs adjacent to weldments.
Microorganisms readily attack areas around welds due to the inhomogeneous
nature of the metal. In one case, perforation occurred
adjacent to a welded seam in a 0.2 inch diameter 316L stainless
steel pipe after being in service for four months under intermittent
flow conditions.[13] Stainless steels containing 6% molybdenum
or greater, have been found to be virtually resistant to MIC.[11]
Aluminum Alloys
The major applications where MIC has attacked aluminum
alloys have been in fuel storage tanks and aircraft fuel tanks. [11]
MIC problems typically occur in the low-lying areas of the
tanks and at water-fuel interfaces. Contaminants in fuels, such
as surfactants and water soluble salts, have largely contributed
to the formation of biofilms in these systems. Fungi and bacteria
have been found to be the main culprits. Corrosion of
aircraft fuel tanks has been widely attributed to Cladosporium
resinae, a fungus. Its presence decreases the pH to approximately
3-4, which can harm the protective coatings and
underlying metal. The pseudomonas aeruginosa species is also
known to be connected with MIC of aluminum fuel tanks.
Additionally, heavy fungal growth on interior surfaces of helicopters
has occurred after undergoing depot maintenance.[14]
Fungal growth had been reported in passenger areas of the H-53
helicopter before being returned to field use and as a result it was
slated for cleaning. Fungi could be found on virtually all interior
surfaces of the helicopter. The surfaces were cleaned with
100% isopropanol, treated with a biocide, and followed by
application of a corrosion preventive compound. The procedure
removed most of the microorganisms present and was effective
at killing spores. However, some biofilms remained, which rapidly
reproduced before the aircraft was even returned to service.
Copper Alloys
Copper alloys find use in seawater piping systems and heat
exchangers, which are susceptible to MIC. Microbial products
that can be harmful to copper alloys include carbon dioxide
(CO2), hydrogen sulfide (H2S), ammonia (NH3), organic and
inorganic acids, and other sulfides.[11] MIC observed in copper
alloys includes pitting corrosion, dealloying and stress corrosion
cracking. Higher alloying content in copper usually
results in a lower corrosion resistance. Although MIC has been
found in both, more problems have been documented with
70/30 than with 90/10 Cu/Ni alloys. MIC has also been documented
in Admiralty brass (Cu-30Zn-1Sn), aluminum brass
(Cu-20Zn-2Al), and aluminum bronze (Cu-7Al-2.5Fe).
Ammonia and sulfides have gained considerable attention as
compounds that are corrosive to copper alloys. Admiralty brass
tubes have been found to suffer stress corrosion cracking in the
presence of ammonia. Seawater that is high in sulfide content,
has caused pitting and stress corrosion cracking in copper
alloys. SRB has also been known to attack copper alloys causing
dealloying of nickel or zinc in some cases.
Nickel Alloys
Nickel alloys are often used for applications subject to high
velocity water environments, including evaporators, heat
exchangers, pumps, valves, and turbine blades, as they generally
have a higher resistance to erosive wear than copper
alloys.[11] However, some nickel alloys are susceptible to pitting
and crevice corrosion under stagnant water conditions, so
that downtime and static periods can lead to potential MIC
problems. Monel 400 (66.5Ni-31.5Cu-1.25Fe) has been found
to be susceptible to underdeposit MIC. Pitting corrosion, intergranular
corrosion, and dealloying of nickel have all been
observed with this alloy in the presence of SRB. Ni-Cr alloys
have been found to be generally resistant to MIC.
MONITORING/DETECTION METHODS
Early detection of potential MIC is crucial to the prevention
of equipment failure and extensive maintenance. The most common detection methods involve sampling bulk liquids from
within the system and monitoring physical, chemical, and biological
characteristics. The goal is to identify favorable conditions
for biofilm formation and growth, so that the internal
environment may be adjusted appropriately. Visual inspections
of accessible areas should also be performed on a routine basis.
Additional methods that may be utilized include coupon monitoring,
electrochemical sensor and biosensor techniques.
Monitoring equipment
is available for
measuring a number
of properties of the
bulk system. A common
practice has been
to directly monitor
temperature, pH, conductivity,
and total
dissolved solids, while
taking samples to
evaluate (by portable
or laboratory testing
methods) dissolved
gases and bacteria counts, and to identify bacteria.[2] Bacteria
counting, via cultured growth, may be helpful, but strict conditions
must be set to produce meaningful results. The most
important factor in bacterial counts is observing changes in
trends rather than in actual numbers. Consistency is crucial
where deviations in sample location, temperature, growing
media, growth time, and even changes in technicians can affect
the results. A strict schedule must also be maintained. Changes
in bacteria counts are used to adjust biocide usage, and may
also be indicative of biofilm growth in the case of differences
in counts across a system. Bacteria cultures can also be used to
identify specific species present (Figure 5). Direct bacteria
counts can be performed using a microscope to inspect bacteria
which have been placed onto a slide and may also be stained
for viewing, as shown in Figure 6. Visual inspections should be
performed on exposed surfaces where algae and fungal growth
can occur and on surfaces exposed during maintenance procedures.
The presence of SRB can be detected by observing black
particles in the liquid media and/or deposited on surfaces (a
result of iron sulfide and/or copper sulfide formation), or by a
distinct hydrogen sulfide odor.[17] Fluorescent dyes can be
used to enhance visual detection, as biofilms absorb some of
the dye, whereby an ultraviolet light is then used to expose the
microorganisms.
Coupons have been found to be quite useful in detecting
MIC, especially when used in conjunction with additional monitoring
techniques. Coupons are small metal samples placed
within the system and periodically extracted to measure corrosion
rates by a weight loss method and possibly to collect
microorganisms from biofilms present on the coupon for identification.
Proper placement of the coupons within the system
plays a key role in MIC monitoring and detection. Coupons
should be placed in locations where MIC is likely to occur.
Electrochemical sensing techniques, such as electrical impedance
spectroscopy and electrochemical noise, are other means of
detecting MIC. Electrochemical sensors detect characteristics of
corrosion reactions, such as changes in electrical conductivity. As
with coupons, strategic placement of the sensors in the systems
is crucial to detecting MIC.
One type of sensor designed specifically for biofilm detection
uses a probe that attracts microbial growth.[1] Utilizing knowledge
of the electrochemical conditions under which biofilms
occur, probes have been developed that replicate these preferred
conditions. The sensor then alerts operators when biofilm activity
is present. Sensors should ideally be placed in areas where
biofilm growth is more likely. Another method that may be used
specifically to detect microorganisms in water systems is the use
of fluorogenic bioreporters.[18] These are compounds (dyes)
that experience a change in their fluorescence upon interaction
with microorganisms. Activity is determined by the ratio of
fluorescence of the reacted dye, extracted from the system or
measured in-service, to the unreacted dye. The ratio increases
with biological activity and can be used to effectively regulate the
use of biocides. This method however, does not distinguish
between planktonic and sessile organisms. Thus, problems could
be growing in the system without being detected.
MITIGATION METHODS
Clearly, the best way to prevent MIC is to prevent the growth
of biofilms altogether. Once a biofilm has formed, it is more
resistant to biocides, and can rapidly grow if not completely
removed. The emphasis is placed on cleanliness and incorporating
established corrosion prevention and control techniques for
the various metals and forms of corrosion. Monitoring and
detection of microorganisms will effectively guide preventive
maintenance procedures.
Maintaining the cleanliness of systems involves monitoring
the quality of water, fuel, or lubricants present in the system.
This includes water content in fuel and lubrication systems.
Water content should be monitored and removed when it
becomes too high. All fluids should be monitored for solid
particles and filtered to prevent particle contamination. Contaminants
increase the likelihood of biofilms, as they can sometimes
be used as nutrients. Bacterial counts and biosensing provide
information that can help adjust the level of biocides
introduced to the system to an optimal concentration. Biocides
are widely used and are effective at killing planktonic microorganisms.
The cost of biocides is significant however, and they
are also quite toxic. Effectively managing their use can reduce
costs and minimize the damaging effects on the environment. Preventive maintenance also includes scheduled cleaning of
exterior components where any debris accumulation has
occurred. Non-abrasive cleaning methods are preferred so as to
not damage coatings. Inspection/cleaning should also be performed
on normally inaccessible components that are exposed
during maintenance and repair activities. Designing systems
that minimize MIC prone areas and providing accessibility for
maintenance helps to promote system cleanliness. This involves
eliminating stagnant and low-flow areas, minimizing crevices
and welds, incorporating filtration, drains, and access ports for
treatments, monitoring/sampling, and cleaning.
Established corrosion prevention and control methods that
are employed to protect metals from the various forms of corrosion
will also help mitigate MIC. This includes designing systems
to minimize stagnant water conditions, proper base material
and coating selection, cathodic protection, sealing crevices
and around fasteners, using gaskets to minimize galvanic corrosion,
proper heat treatments, and post weld treatments. For
underground structures, providing ample drainage by backfilling
with gravel or sand will help prevent MIC. In some cases, a
change to an alternate material such as PVC piping has greatly
reduced underground pipeline corrosion problems. Coatings can
be formulated with biocides, though such coatings are not generally
used on the interior of systems. Smooth surface finishes
with minimized defects are preferred. Research into alternative
coatings that may deter MIC has shown polydimethylsiloxane
coated 4340 steel to have favorable results.[19] The silicone
compounds significantly reduced MIC of the steel in a 0.6M
NaCl solution over a two year period.
SUMMARY
The prevention and control of MIC may seem like a daunting
task. However, with knowledge of how and where MIC occurs,
as well as the prevention and control methods that may be used,
a majority of problems can be prevented. Maintaining the
cleanliness of systems is the best method to prevent MIC. Once
biofilms have established themselves, it is difficult to get rid of
the bacteria entirely. There is a need to implement a better
means of destroying biofilms and also to develop environmentally
friendly biocides. It is virtually impossible for
designers/maintainers to stay abreast of all the technologies and
methods used in corrosion prevention and control, so outside
professional assistance is usually required. To optimize MIC
prevention and control, subject matter experts should be consulted
when designing new systems where MIC has traditionally
been prominent, for setting up preventive maintenance procedures
for new systems, and for other related problems as they
arise. Ideally, all problems should be thoroughly documented
and entered in an information system for effective use in
designing future systems.
NOTES & REFERENCES | Download this Article
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