A Closer Look at Microbiologically Influenced Corrosion

Microbiologically influenced corrosion (MIC) refers to corrosion caused by the presence and activities of microorganisms—microalgae, bacteria, and fungi. While microorganisms do not produce unique types of corrosion, they can accelerate corrosion reactions or shift corrosion mechanisms. Microbial action has been identified as a contributor to rapid corrosion of metals and alloys exposed to soils; seawater, distilled water, and freshwater; crude oil, hydrocarbon fuels, and process chemicals; and sewage. Many industries and infrastructure are affected by MIC, including oil production, power generation, transportation, and water and waste water.¹

To better understand MIC and the corrosion threats it poses to pipelines, vessels, and structures, Materials Performance asked several NACE International members and others from industry, government, and academia to comment on the impact of MIC and challenges faced when identifying and mitigating MIC. Panelists are Richard Eckert and Torben Lund Skovhus with Det Norske Veritas (DNV); Gary Jenneman with ConocoPhillips; Sylvie Le Borgne with the Metropolitan Autonomous University at Mexico City; and Jason S. Lee and Brenda J. Little, FNACE, with the U.S. Naval Research Laboratory. (See their biographies in the sidebar, “Meet the Panelists.”)

MP: How does MIC impact structures, vessels, and pipelines?

Le Borgne: The first reports of MIC are from the nineteenth century. Most of the studies have been in relation to metallic materials. However, other materials such as concrete, plastics, and new materials or coatings increasingly used nowadays should be included. MIC affects a variety of structures, vessels, and pipelines by directly or indirectly influencing the overall corrosion process, and is usually estimated to account for 20% of the total cost of corrosion. Due to the complexity of systems involving microorganisms, it is generally difficult to precisely quantify the influence of MIC to the overall corrosion process.

Microbial ecology studies have clearly demonstrated that microbes can survive and be active in a wide variety of environments including many man-made structures and environments. Systems where MIC is especially important include hydrocarbon and fuel (gas and liquid) transmission and storage systems, as well as hazardous materials transport and storage structures. These systems provide adequate environmental conditions and substrates for microbial development, and the participation of microorganisms in corrosion has been clearly demonstrated and MIC failures documented. Utilities such as drinking water and sewer systems also provide adequate conditions for MIC development. However in such systems, MIC has often been underestimated, as has been corrosion in general.

Eckert and Skovhus: MIC typically manifests itself as localized (i.e., pitting) corrosion—with wide variation in rate, including rapid metal loss rates—both internally and externally on pipelines, vessels, tanks, and other fluid handling equipment. Despite advances in the understanding of MIC, it remains difficult to accurately predict where MIC will occur and estimate the rate of degradation. MIC can occur as an independent corrosion mechanism or in conjunction with other corrosion mechanisms. These characteristics present challenges to implementing effective corrosion management of engineered systems in which MIC is an applicable threat.

Jenneman: Although the techniques to identify MIC are nonstandard and subject to interpretation, the places where we suspect MIC to occur experience rapid pitting, usually at interfaces where solids such as scale, wax, and or other solids can settle out or precipitate. Areas downstream of welds, where cleaning pigs have difficulty removing deposits, as well as dead legs, low-velocity areas, and tank bottoms where solids and bacteria/biofilms can accumulate, are particularly susceptible to attack. Often this pitting is very isolated, with one hole surrounded by a number of shallower pits. Pitting rates range from a few mpy to >250 mpy.

Lee: MIC in itself is not a unique corrosion mechanism; rather it produces conditions that increase the susceptibility of materials to corrosion processes such as pitting, embrittlement, and under deposit corrosion (UDC). MIC can result in orders of magnitude increases in corrosion rates. The most devastating issue regarding MIC is its general lack of predictability—both spatially and temporally.

Little: In almost all cases MIC produces localized attack that reduces strength and/or results in loss of containment.

MP: What are the current techniques used to identify MIC?

Le Borgne: Current techniques to identify MIC after it has occurred or when it is suspected are based on detecting and identifying the (causative/present) microorganisms; examining the damaged material (pit morphologies), and analyzing the corrosion products in search of biogenic structures. Concerning the detection and identification of microorganisms, the traditionally used techniques generally involve culture techniques with already prepared media tests kits to detect the growth of specific microorganisms known to participate in MIC in specific environments, such as sulfate-reducing bacteria (SRB), acid-producing bacteria, nitrate-reducing bacteria, or iron-reducing bacteria.

These kits are relatively easy to use although they need some basic laboratory expertise; the samples are inoculated directly in the field immediately after the sample has been collected. These kits also have the advantage of detecting only active bacteria, even in very low numbers. However, these kits can be rather unspecific and allow the growth of other types of microorganisms. Some years ago, genetic techniques had been proposed to allow a better detection and identification of microorganisms in MIC. These techniques need special expertise. Careful sampling is needed to avoid contaminations as these techniques are extremely sensitive and the samples must be transported and stored under special conditions to avoid degradation of the nucleic acids.

Following total DNA extraction from the samples, the total content and identity of virtually all the microorganisms present can be determined by different methods, from genetic fingerprints to pyrosequencing. When DNA is the starting material for these analyses, all the microorganisms, whether dead or alive, are detected. It cannot be determined which microorganisms were metabolically active when the sample was taken. RNA extraction from environmental samples is very challenging and is not a routine technique.

Lee: Advancements in molecular microbiology provide numerous methods to determine which ones are there, how many there are, and what they are doing. Metallurgical sectioning and microscopy provide information about material composition, corrosion morphology, and spatial relationships between microorganisms and sites of corrosion. Multiple techniques are used to determine the electrochemical properties of materials exposed to biologically active media. Surface science and crystallography provide the chemical and structural identity of corrosion products.

Jenneman: It is recommended when trying to justify MIC as a contributing or root cause of corrosion that the following lines of evidence be examined:

1. Biological: In this case we will chemically characterize the water for essential microbiological nutrients (e.g., organics, nitrogen, phosphorus) and perform microbiological testing, if possible, to determine if the environment can support growth and activity. We will use culture-based and molecular methods to determine the types/numbers of microorganism present if good samples are available. Other physical properties (temperature, pH, ionic strength) of the environment will also be checked and evaluated.

2. Chemical: In this case we work with corrosion engineers who will look at water chemistry, gas analyses, corrosion models, etc. to determine if abiotic mechanisms such as carbon dioxide (CO₂) corrosion can explain the corrosion.

3. Metallurgical: In this case both microbiologists and corrosion engineers will examine corrosion products (using x-ray fluorescence [XRF] and x-ray diffraction [XRD]) and pit locations/morphology, as well as determine maximum pit depth using surface profilometry to determine if parameters are consistent with MIC and/or other mechanisms

4. Operational: Many operational conditions and changes can influence the likelihood for MIC, e.g., low-velocity/stagnant conditions, pigging frequency, types of pigs, biocide usage, rapid failures, changes in temperature, introduction of oxygen, and upward trending of bacteria. All of these available lines of evidence and facts are then weighed to determine if MIC is the root cause or a contributing factor.

Eckert and Skovhus: MIC is identified by evaluating the physical conditions, chemical composition, microbiology, and metallurgy of the susceptible component or system. The integration of this data is what ultimately determines the extent to which MIC may be contributing to the observed corrosion. Therefore, the techniques used to identify MIC are varied and cross-disciplinary and require expertise in materials, corrosion, microbiology, chemical treatment, and asset operations. Although microbiological conditions are only one piece of the MIC puzzle, the counting of viable bacteria has historically received the most emphasis. Serial dilution using liquid culture media, despite its limitations, has been the predominant method used to identify viable bacteria.

The type (formulation) of the culture medium and incubation temperature determines the numbers and types of microorganisms that will grow. Since no culture medium can approximate the complexity of a natural environment, liquid culture provides favorable growth conditions for only about 1 to 10% of the natural microbiological population under ideal circumstances. Further, some microorganisms are incapable of growth in typical liquid media (e.g. some Archaea). While these factors bias culture-based results, serial dilution results are still useful for monitoring general trends of growth in some systems.

Molecular microbiological methods (MMM), long used in health care and forensics, have gained popularity in the analysis of microbiological corrosion and are now included in a number of NACE standards and publications, including TM0194-2004,² 3T199,³ TM0212-2012,⁴ and the forthcoming revision of TM0106-2006.⁵ MMM require only a small amount of sample (liquid, biofilm, solid) with or without live microorganisms. After genetic materials are extracted from the sample, assays are specific and render a more accurate quantification of various types of microorganisms than culture tests. Molecular techniques that are finding increased use include quantitative polymerase chain reaction (qPCR), denaturing gradient gel electrophoresis (DGGE), and fluorescent in situ hybridization (FISH).

Little: Despite the limitations of liquid/solid culture techniques, it is my opinion that most industries use some form of culture to establish a most probable number (MPN) of viable organisms. Relating MPN to the likelihood of MIC is a questionable practice that can only be reliable in limited applications.  NACE TM0212-2012 describes microscopic analyses, chemical assays, and molecular methods for evaluating MIC. Most of the research in MIC testing is related to molecular techniques that identify and quantify microorganisms.  It is not clear that molecular techniques have provided a more accurate tool for predicting the likelihood of MIC. These techniques may provide a tool for assessing mitigation strategies. Microorganisms do produce mineralogical fingerprints that can be used to identify MIC. In many cases, MIC is assumed when there is no obvious cause of corrosion.

MP: What are the challenges faced when establishing MIC as the probable cause of corrosion?

Eckert and Skovhus: Since microorganisms are ubiquitous, and some are capable of life in even the most extreme environments, the greatest challenge is determining the degree to which MIC contributes to corrosion in conjunction with other relevant corrosion mechanisms. For example, biofilms that increase MIC susceptibility in pipelines often occur where the fluid velocity is continuously low enough to promote water accumulation and solid particle deposition. Deposit or sediment buildup may also allow UDC mechanisms, such as concentration cells, to occur.

Distinguishing the relative contributions of the biofilm and concentration cells, for example, may be difficult depending on the information available to the investigator. The second challenge is effectively collecting and integrating corrosion, microbiological, chemical, operational, design, mitigation, and metallurgical data to determine the predominant corrosion mechanisms that are present. Corrosion threat assessment for MIC should be conducted in view of all other applicable corrosion mechanisms for the asset. Identifying the predominant corrosion mechanisms supports the establishment of mitigation measures that are likely to have the greatest benefit.

Finally, establishing MIC as the probable cause of corrosion in a failed component may be particularly difficult since the failure event itself is likely to have altered the conditions that caused the corrosion damage. Careful sample preservation and field sample collection from representative undamaged areas can aid in forensic corrosion investigations. The identification of MIC as a damage mechanism should not be based solely on the presence, number, or type of microorganisms on a corroded component. 

Lee: MIC is a very subtle study. Rarely can a case of suspected MIC be confirmed without evidence from multiple analysis techniques and sciences. The presence of microbes alone does not prove the existence of MIC. Microorganisms exist throughout the environment. The greatest challenge is proving that microorganisms actually influenced the electrochemical properties of the system. In addition, higher numbers of microorganisms does not necessarily mean increased likelihood of MIC. Molecular techniques are required to detect the individual activities of each microbe species. A system baseline of normal operating conditions, where predictable corrosion occurs (e.g. uniform corrosion of carbon steel [CS] in freshwater), is required for comparison with suspected MIC cases.

Jenneman: There are really no definitive tests or accepted standardized methodologies that can be applied to directly implicate MIC as the probable cause. It is often determined through a process of deduction of the facts and elimination of other mechanisms. Therefore a challenge is to develop standardized tests and approaches that can be widely accepted by the industry. However, MIC is a complex problem involving various aspects of materials science, electrochemistry, and microbiology that necessitates the involvement of scientists and engineers from various disciplines to take on this challenge. Also, the potentially large number of microbial types and activities involved challenges us to develop better mechanistic understandings of how these microorganisms and activities influence corrosion processes.

Little: MIC does not produce a unique corrosion morphology, making it impossible to identify MIC without specific testing.

Le Borgne: Challenges include the nature of the collected samples and whether they are from biofilms or bulk water. Only microorganisms in biofilms influence the corrosion process, although these microorganisms proceed from the surrounding bulk liquid phase. The number of corrosive or potentially corrosive microorganisms detected in the bulk water is not related to the intensity of the attack. Live microorganisms may not be detected in the samples, but dead organisms that participated in the attack or influenced the corrosion process are present on the surface of the material and in the corrosion products.

The microorganisms may act as consortia and not as isolated organisms, which may complicate the diagnosis and interpretation of the data. Different techniques are available for studying and diagnosing MIC. These analyses are generally performed in parallel and a multidisciplinary approach is necessary and might not always be easy to manage. There must be a link between the microbiological studies, the pit morphologies, and the composition of the corrosion products in order to clearly establish MIC as a corrosion mechanism, which may contribute from 0 to 100% in a corrosion process.

MP: Are current identification technologies adequate or is additional research necessary to develop more effective methods to identify MIC?

Little: The identification tools that can be used to determine that MIC has taken place appear to be adequate. There are recent refinements in sample preparation and fixation for more accurate molecular analyses. However, there are few tools/technologies for predicting MIC before it occurs.

Eckert and Skovhus: Current technologies, when used in combination with each other, can usually provide adequate information to assess and characterize MIC. Since MIC must typically be diagnosed using a combination of data (chemical, microbiological, metallurgical, operational, etc.), no single technology or tool can reliably identify MIC in all cases. Many operators have used extended coupon analysis to collect chemical, microbiological, and corrosion data from one sample point with much success.

The integration of results from MMM with other corrosion information is one area where additional research is needed to take advantage of the vast amount of information provided by genetic technologies. Researchers and asset owners are both continuing to find new insights resulting from collaboration between corrosion/materials professionals and microbiologists. Distinguishing the effect of MIC in combination with other abiotic external corrosion mechanisms on buried metallic structures and the influence of cathodic protection (CP) potentials more negative than -850 mV are other areas that deserve further attention and additional research—the pipeline industry would benefit from additional engineering guidance in this area.

Lee: Additional research is needed in development of a link between biological activity and corrosion rate. Real-time monitoring of corrosion rate and microbiology currently is not available. Lab-on-chip devices being developed are promising for use in microbiological monitoring programs, but academic disagreements still exist on which microbial markers are most important. Corrosion sensors have also become more sophisticated, but still lack the ability to be used in prediction of long-term corrosion susceptibility.

Le Borgne: Many identification technologies are available to provide a complete description of systems where MIC might have occurred. Some of these techniques require specific expertise and do not give an immediate response. However, more research is required in order to develop portable devices or online/remote sensors to detect MIC. The development of international standards and actualized protocols and programs that take the peculiarities of each system into account and allow the determination of risk factors is also needed to prevent MIC before it occurs in different facilities.

Jenneman: Better methods are definitely required to identify MIC. The traditional culture testing is very slow and does not give a very complete picture of the microbial communities involved in the corrosion. The newer molecular methods (e.g., DGGE, qPCR, and metagenomic sequencing) are gaining more widespread use and may eventually replace culture testing as costs decrease and availability of these technologies to oilfield end users increases. They do have the advantage of providing a faster and more complete picture of the microbial communities, but they currently require highly skilled professionals to perform the testing and interpret the results.

There are currently no accepted standards by which these tests are performed and no accepted models to help the end user interpret the results. These tests are typically outsourced to specialized laboratories and require the end user to understand the potential pitfalls of sampling, preservation, procedural nuances, and interpretation of results. There are currently industry-sponsored programs aimed at applying genomic technologies to better understand and identify MIC.

MP: When MIC is established as the corrosion mechanism, what are the mitigation and monitoring strategies typically used? Are these strategies effective?

Eckert and Skovhus: Common strategies for internal MIC mitigation in oil and gas pipelines include maintenance pigging and chemical treatment. Depending upon the pigging frequency and pig design, maintenance pigging can be effective in removing deposits/biofilm that promote MIC. A further benefit of removing deposits is increasing the effectiveness of chemical treatment by allowing the chemical to reach the exposed metal surface. Chemical treatment is typically performed using corrosion inhibitors (some with the added benefit of a biocidal tendency), biocides, and combinations of these chemicals. External MIC on buried structures and pipelines is more challenging to diagnose and mitigate properly, since nearly all soils are naturally rich with microbiological activity.

Furthermore, CP and an external coating are essentially the only mitigation options for external corrosion (including MIC) on direct buried pipe. Pipeline industry guidelines often call for applied potentials more negative than -850 mV when MIC is suspected; however, additional research is needed in this area to validate the effectiveness of more negative potentials in consideration of other parameters that influence external corrosion of buried structures. Regardless of the type of system, monitoring the effectiveness of MIC mitigation measures must include corrosion monitoring in addition to any microbiological monitoring that is performed, since ultimately the goal of mitigation is to control corrosion. Often MIC mitigation programs are focused on measuring microbial numbers, types or activity, which can be helpful in optimizing mitigation but is not a replacement for corrosion monitoring.

Little: Accelerated low water corrosion (ALWC) of CS in saline waters is a form of MIC most often attributed to microorganisms in the sulfur cycle (i.e., SRB and sulfur-oxidizing bacteria). Both CP and coatings have been effective in preventing ALWC.

Jenneman: Biocides are still the chemicals of choice when mitigating MIC; however, biocides usually need to be combined with a mechanical or chemical cleaning program to enhance their effectiveness, especially if the biofilms and corrosion are already firmly established. Biocides are comprised of both oxidizing and non-oxidizing chemicals. Both can be effective, but the environment and metallurgy will often dictate the choice. Other strategies are possible, including the injection of biostats or inhibitors. We have found that some low-toxicity film-forming corrosion inhibitors can inhibit MIC development in model laboratory flow cells.

Other tactics include developing new chemicals and surfaces (e.g., nanomaterials) that will not allow bacteria to attach and form biofilms, or destroy microorganisms on contact. In addition, application of natural chemicals can interfere with the quorum sensing capacity that microbial communities rely on to form mature biofilms, potentially rendering them less corrosive. Unfortunately, much of the testing to evaluate these techniques is targeted at controlling the microbes themselves and not the corrosion.

Testing that simply addresses the reduction of microbial populations without addressing the changes in corrosiveness is insufficient. To determine the effectiveness of these strategies, it is necessary to have effective monitoring and inspection strategies. Monitoring can be used to examine effectiveness of the mitigation strategy to deliver the chemicals, control microbiological growth, and reduce corrosiveness of the environment; however, monitoring is only as good as the locations selected and samples collected, as well as the analyses performed.

Le Borgne: The main problem associated with the use of chemicals is the adaptation capacity of microorganisms that allow them to develop resistance mechanisms and, in some cases, the ability to biodegrade these products. Constant injection of chemical products is necessary. Recently, the injection of nitrate in oilfields has been described as an effective technique to control MIC by SRB; however, the long-term effects of this manipulation of the environment have not been evaluated. Strategies based on the use of bacteriophage to control specific bacterial populations have also been proposed. These strategies, as well as their long-term effects, have to be tested.

MP: When selecting materials for new construction and/or predicting material lifetime, is MIC a consideration?

Lee: In my experience, often times MIC is not a consideration in materials selection. Certain materials have been shown to not be susceptible to MIC (e.g. titanium and high Ni-Cr alloys), but these alloys are often cost prohibitive.  In the last 20 years, MIC has gained traction in industrial, commercial, and military sectors. The result of unexpected failures due to MIC has increased the attention of MIC and its consideration in material selection.  While many sectors are hiring corrosion scientists and engineers to deal with increased failure concerns, MIC still lags behind in consideration in the field of corrosion.

Le Borgne: To my knowledge, it is rarely considered, at least in the systems I have been involved in. MIC is not usually taken into account until it occurs and few reports deal with prevention and the assessment of risk factors associated with MIC. If such information could be systematized and proper documentation of MIC failures cases organized, then MIC could be taken into account in materials selection. Standardized protocols and test methods are also needed to test for MIC of materials under laboratory conditions and norms must be established.

Jenneman: Yes. In some cases, particularly where the risks (e.g., dead legs and low-velocity sections) and consequences are high (e.g. oil and gas lines), we have changed from CS to corrosion resistant alloys (typically duplex stainless steels [SS]) as a means to mitigate the impact of MIC. I cannot say this will be effective in all cases, but we have seen good results in some instances thus far. Also, the application of fusion-bonded epoxies to tank bottoms and the use of non-metals (e.g., glass-reinforced epoxy [GRE] or high-density polyethylene [HDPE]) for low-pressure water lines can be effective strategies to combat MIC.

More research is needed on the effect of MIC in non-austenitic, high-alloyed SS and non-metallic coatings to qualify them for use in various MIC environments. Unfortunately, to my knowledge, there are currently no reliable mechanistic MIC models that can be used to predict material lifetimes in CS or SS.

Little: Certainly, reports of ALWC as a global problem in saline waters has forced design engineers and insurers to question the probability of MIC in specific locations and to plan accordingly.

Eckert and Skovhus: The threat of MIC needs to be considered in the design of new projects to enable monitoring and mitigation for managing MIC during the operational stage of the asset. More importantly, designing to reduce the potential for conditions that would promote MIC (e.g. dead legs, low velocity) should be part of the development process. Materials selection should be based upon the anticipated operating conditions through the life of the asset and the intended design life.

Few metallic materials commonly used for engineered structures exhibit complete resistance to MIC, therefore material selection is usually based primarily on other engineering requirements for the project. While a number of models have been proposed to rank the susceptibility of a system to MIC, widely accepted models for reliable prediction of MIC corrosion rates have yet to be developed, and in fact may remain elusive due to the vast range of conditions under which MIC can occur.

MP: Recent research has demonstrated new MIC-based corrosion mechanisms. Has this new information changed the approach to managing MIC?

Lee: The traditional understanding of MIC involves the formation of a biofilm that provides a niche for corrosive microorganisms to proliferate. Recent research has demonstrated that metal surfaces alone can produce redox, oxygen, and nutrient gradients without an established biofilm. Many mitigation and monitoring strategies operate under the assumption of a substantial biofilm presence and treat accordingly.

Little: The list of microorganisms that can influence corrosion and the causative mechanisms is constantly growing. Recent research has, in general, demonstrated the metabolic flexibility of causative organisms. Most recently it has been demonstrated that some bacteria can accept electrons for iron (iron is the electron donor). However, it is not clear that increased understanding has translated into increased predictability.

Eckert and Skovhus: Research continues to confirm that MIC does not occur by any single, exclusive mechanism, and that various microbial consortia in different environments have established novel ways to use the energy sources available to them. The increased knowledge of microorganisms in industrial systems brought about by application of genetic methods has resulted in new understanding, and at the same time raised new questions about how the activities of specific microorganisms contribute to corrosion. Increased knowledge of the ways in which microorganisms influence corrosion through both biotic and abiotic processes will ultimately lead to improved mitigation and monitoring strategies and technologies. However, even with improved understanding of MIC mechanisms, development and implementation of innovative MIC management technologies will take time.

Jenneman: The recent revelations of the ability of certain SRB and methanogens to directly use electrons from metallic iron prior to the formation of molecular hydrogen is indeed opening our eyes to the different ways in which microorganisms can influence corrosion and to the need to expand our approaches and methods when looking for these causative agents of MIC. We need to better understand how these microorganisms accomplish this and how to detect their presence and control their activity. Their presence and potential activity can also impact how we currently manage and formulate the risks to our pipelines and facilities.

Le Borgne: To my knowledge it has not changed the approach yet, at least in the systems I have been involved in. It will probably take some time until this new knowledge is incorporated and taken into account in the field.

References
1 B.J. Little, J.S. Lee, Microbiologically Influenced Corrosion (Hoboken, NJ: John Wiley & Sons, 2007).
2 NACE Standard TM0194-2004, “Field Monitoring of Bacterial Growth in Oilfield Systems” (Houston, TX: NACE International, 2004).
3 NACE Publication 3T199, “Techniques for Monitoring Corrosion and Related Parameters in Field Applications” (Houston, TX: NACE, 2013).
4 NACE Standard TM0212-2012, “Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines” (Houston, TX: NACE, 2012).
5 NACE Standard TM0106-2006, “Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion (MIC) on External Surfaces of Buried Pipelines” (Houston, TX: NACE, 2006).

Bibliography
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Borenstein, S.W. Microbiologically Influenced Corrosion Handbook. Cambridge, U.K.: Woodhead Publishing Ltd, 1994.

Videla, H.A. Manual of Biocorrosion. Boca Raton, FL: CRC Press, 1996.