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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.
Source:https://blogs.nace.org/the-challenges-of-diagnosing-mic?
