Microbiological Corrosion
MIC is actually 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, occurring in fuel and lubrication systems as well. Table 18 lists applications where MIC has been found to be prominent while Figure 24 shows one such location.
Types of Microorganisms
The types of microorganisms with species attributable to MIC include algae, fungi, and bacteria. Algae produce oxygen in the presence of light (photosynthesis) and consume oxygen in darkness. They can be found in most any aquatic environment ranging from freshwater to concentrated salt water. 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 – 104ºF 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, but 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 metabolize while anaerobic species require a lack of oxygen to do the same. 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 25 is an example of rod-shaped bacteria observed using transmission electron microscopy.
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 promote hydrogen damage in metals. Most microorganisms form an extracellular membrane which protects the organism from toxic chemicals and allows nutrients to filter through. 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 listed below. Following these recognized forms; Table 19 lists some specific microorganisms within these categories, along with their characteristics.
Sulfate Reducing Bacteria (SRB)
Sulfate reducing bacteria 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 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 – 35ºC, although there are some that can function well at temperatures of 60ºC. They can be detected through the presence of black precipitates in the liquid media or deposited on surfaces, as well as a characteristic hydrogen sulfide smell.
Sulfur/Sulfide Oxidizing Bacteria (SOB)
Sulfide oxidizing bacteria are an aerobic species which oxidize sulfide or elemental sulfur into sulfates. Some species oxidize sulfur into sulfuric acid leading to a highly acidic 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 is 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 over corrosion pits on steels. Some species are known to accumulate iron or manganese compounds resulting from the oxidation process. The higher concentration of manganese in biofilms has been attributed to 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 26).
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. Like slime formers, they can create environments suitable for anaerobic species. These organisms have been attributed to the widespread corrosion problems observed in aluminum fuel tanks in aircraft.
Metals Affected by MIC
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, and thus exposed to microbial activity and the potential of MIC, 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 all have shown effects of 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 one of the most readily corroded metals. Mild steels are normally coated for corrosion protection, while cathodic 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. Numerous cases have also been documented where microorganisms caused debonding of coatings from the underlying metal. Underneath the delaminated coating in turn, creates an ideal environment for further microbial growth.
Poor quality water systems and components with areas that accumulate stagnant water/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. 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 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 in the same type of scenarios as mild steels – primarily in locations where water accumulates. There are two notable problems that have surfaced with stainless steel MIC. One is an accelerated corrosion rate, primarily through pitting or crevice corrosion that occurs in low lying areas, joints, and corner locations. Stainless steel tanks and piping systems are sometimes hydrotested subsequent to manufacture and prior to field use. Several cases of severe MIC have been documented whereby hydrotesting using well water was performed, and the product was then stored for a period of time before being placed into service. The tanks and piping were not adequately dried, nor was a biocide used to deter biofilm growth. In one particular case, a 304 stainless steel pipeline for freshwater service, failed 15 months after being hydrotested. A second MIC problem discovered with stainless steels occurs adjacent to weldments. Microorganisms readily attack areas around welds due to the inhomogeneous nature of the region. In one case, perforation occurred in a 0.2 inch diameter 316L stainless steel pipe adjacent to a welded seam after four months in service under intermittent flow conditions. Stainless steels containing 6% molybdenum or greater, have been found to be virtually resistant to MIC.
Aluminum Alloys
The major applications where MIC has attacked aluminum alloys have been in fuel storage tanks and aircraft fuel tanks.29 MIC problems exist in the low-lying areas of 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. Cladosporium resinae, a fungus, has widely been attributed to corrosion of aircraft fuel tanks. Its presence decreases the pH to around 3-4, which can attack the protective coatings and underlying metal. Pseudomonas aeruginosa and Candida species are also likely to be found in conjunction with MIC of aluminum fuel tanks.
Additionally, heavy fungal growth on interior surfaces of helicopters has occurred subsequent to depot maintenance and prior to returned field use. Fungal growth had been reported in passenger areas of the H-53 helicopter and was therefore slated for cleaning during refurbishment. Fungi could be found on virtually all interior surfaces of the helicopter. The surfaces were cleaned with 100% isopropanol, treated with a biocide, 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 CO2, H2S, NH3, organic and inorganic acids, and sulfides. 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 used in high velocity water environments including evaporators, heat exchangers, pumps, valves, and turbines blades, as they generally have a higher resistance to erosive wear than copper alloys. However, some nickel alloys are susceptible to pitting and crevice attack under stagnant water conditions, so that downtime and unused 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 virtually 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 as appropriate. 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 monitor temperature, pH, conductivity, and total dissolved solids directly from the operating system, while taking samples for portable or laboratory testing methods to evaluate dissolved gases, bacteria counts, and for bacteria identification. Bacteria counting, via cultured growth, may be helpful, but strict conditions must be followed 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 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 27). 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 28. 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 a distinct hydrogen sulfide odor. 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 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 through weight loss and possibly to collect microorganisms from biofilms 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. Utilizing experience 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. These are compounds (dyes) that change 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 metal alloys and forms of corrosion. Monitoring and detection of microorganisms will effectively guide preventive maintenance procedures.
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 should be monitored and removed when the content becomes too high. All fluids should be monitored for solid particles and filtered to prevent particle contamination. Contaminants increase the likelihood of biofilms through their use as nutrients. Bacterial counts and biosensing help to adjust the level of biocides introduced to the system. Biocides are widely used and are effective at killing planktonic microorganisms. The cost of biocides is significant however, and along with their toxicity, effective management of biocide use can reduce costs and damaging effects on the environment. Cleanliness also includes scheduled cleaning of exterior components where any debris accumulation has occurred. Non-abrasive cleaning methods are preferred as to not damage coatings. Inspection/cleaning should also be performed on normally inaccessible components that are exposed during maintenance/repair procedures. Designing systems that minimize MIC prone areas and providing accessibility for maintenance as appropriate 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, possible 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.38 In laboratory tests, the silicone compounds significantly reduced MIC of the steel in a 0.6M NaCl solution over a two year period.