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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
Rome, NY

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.

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.

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.

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.

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.

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.

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.

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