Corrosion Essentials – Posted 4/24/2020
The complexities of determining microbiologically induced corrosion as a cause of component or asset failure is explained.
Microbiologically influenced corrosion (MIC) refers to corrosion caused by the presence and activities of microorganisms. While microalgae, bacteria, and fungi 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.
The following excerpts from articles appearing in past issues of Materials Performance provide expert insights into the impact of MIC and the challenges faced with identifying and mitigating its threat.
How does MIC impact structures, vessels, and pipelines?
Sylvie Le Borgne: (Professor researcher in the Department of Process and Technology at the Metropolitan Autonomous University at Mexico City, Mexico) 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. 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.
Richard Eckert (Principal engineer, corrosion management at DNV GL in Dublin, Ohio, USA) and Torben Lund Skovhus (Project manager at DNV GL in the Corrosion Management & Technical Advisory Group in Bergen, Norway):
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 it 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 systems in which MIC is an applicable threat.
Gary Jenneman (Principal scientist within the Global Production Excellence group of ConocoPhillips in Bartlesville, Oklahoma, USA): 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 shallower pits.
Jason S. Lee (Materials engineer at the U.S. Naval Research Laboratory, Stennis Space Center, Mississippi, USA): MIC by 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.
Brenda J. Little, FNACE (Senior scientist for marine molecular processes at the Naval Research Laboratory, Stennis Space Center, Mississippi, USA): In almost all cases, MIC produces localized attack that reduces strength and/or results in loss of containment.
What techniques are 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 microorganisms, examining the damaged material, and analyzing the corrosion products in search of biogenic structures. Concerning the detection and identification of microorganisms, the traditionally-used techniques generally involve cultures 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 acid-producing bacteria, nitrate-reducing bacteria, or iron-reducing bacteria.
These kits are relatively easy to use; 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, they can be rather unspecific and allow the growth of other types of microorganisms. Genetic techniques, which need special expertise, have been proposed to allow better detection and identification of microorganisms in MIC. 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 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.
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: When trying to justify MIC as a contributing or root cause of corrosion it is recommended that biological, chemical, metallurgical, and operational lines of evidence all need to be examined.
Eckert and Skovhus: The integration of all MIC evidence (data) is what ultimately determines the extent to which it may be contributing to corrosion. Therefore, the techniques used to identify MIC are varied and cross-disciplinary and require expertise from various fields of study. 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. MMM require only a small amount of sample 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.
Little: Despite the limitations of liquid/solid culture techniques, 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. A NACE standard 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/quantify microorganisms and may provide a tool for assessing mitigation strategies.
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. 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. 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 influenced the electrochemical properties of the system. In addition, higher numbers of microorganisms do 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 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. 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 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.
The Future of MIC testing
In a NACE industry expert roundtable regarding the future of the corrosion industry, Eckert, shared his prediction regarding MIC noting that advances in the field of genomics may offer a straightforward diagnostic test that provides actionable results. “Metagenomics, proteomics, and metabolomics” produce information that needs to be translated and integrated with other information about the chemical environment and physical conditions in which the collective of microorganisms live in order to understand “who” is there and “what” they are doing.
And while no singular data element found that is diagnostic for MIC, a successful future test method would likely need to integrate numerous chemical and microbiological factors using a model and some form of machine learning, based on a large and reliable data set.
With accurate and reliable MIC diagnosis, prevention and mitigation measures could be more effectively applied, resulting in improved asset integrity, longevity, and sustainability.
Need a roadmap to corrosion management and asset sustainability? Learn about IMPACT PLUS, from the NACE International Institute.
“A Closer Look at Microbiologically Influenced Corrosion,” by Kathy Riggs Larsen.
“Roundtable on the Future of Corrosion Control: Part 1, by Gretchen Jacobson.
Both originally appeared on materialsperformance.com