Microbiologically influenced corrosion—more than just microorganisms

Abstract Microbiologically influenced corrosion (MIC) is a phenomenon of increasing concern that affects various materials and sectors of society. MIC describes the effects, often negative, that a material can experience due to the presence of microorganisms. Unfortunately, although several research groups and industrial actors worldwide have already addressed MIC, discussions are fragmented, while information sharing and willingness to reach out to other disciplines are limited. A truly interdisciplinary approach, which would be logical for this material/biology/chemistry-related challenge, is rarely taken. In this review, we highlight critical non-biological aspects of MIC that can sometimes be overlooked by microbiologists working on MIC but are highly relevant for an overall understanding of this phenomenon. Here, we identify gaps, methods, and approaches to help solve MIC-related challenges, with an emphasis on the MIC of metals. We also discuss the application of existing tools and approaches for managing MIC and propose ideas to promote an improved understanding of MIC. Furthermore, we highlight areas where the insights and expertise of microbiologists are needed to help progress this field.


Introduction
Engineered materials are essential to our society to ensure our curr ent pr osperity and sustainability in the future.A wide range of matters such as energy supply, food, transportation, housing, and various other of life's fundamentals r el y upon engineer ed materials.To support this, a variety of materials are used that can have limited lifetimes; thus, repair or replacement is inevitable in the long run.Very often, ho w e v er, the expected lifetimes of materials are not achieved.In many cases, damage occurs much earlier than expected, causing costly production downtime and repair expenses, and in some cases, e v en major environmental problems.More often than thought, microorganisms play a major role in this dama ge, and micr obiologists ar e k e y to help impr ov e our understanding of the associated problems as well as potential solutions.
It is scientifically proven that many metallic and non-metallic materials (e .g. concrete , wood, and plastic) can be deteriorated by micr oor ganisms, with detrimental (e.g.asset failure) or beneficial (e.g.biodegradation of plastic) consequences.Ho w e v er, biodeterioration of metals in our built environment can result in significant issues of production loss , en vironmental disasters , and/or asset safety (Jacobson 2007 ).While the topic of micr obiologicall y influenced corrosion (MIC) has been known and studied for decades, its understanding is limited, as are the methods for pr e v ention and monitoring, and it poses gr eat c hallenges in man y industrial settings.Fundamentall y, a collabor ativ e effort fr om v arious scientific and technical disciplines, including microbiology, material science, pr ocess and electr oc hemistry, bioc hemistry, corr osion engineering, and integrity management, is needed to help pr ogr ess the understanding of MIC.This combination of knowledge is critical to determine the root causes of a failure associated with MIC and to de v elop effectiv e long-term mitigation and monitoring str ategies specificall y ada pted to the associated system (Silva et al. 2021 , Ec kert and Sk ovhus 2022 ).
In recent decades, our understanding about the micr oor ganisms , the mechanisms , and the factors related to MIC has grown enormousl y, yet man y questions remain unanswered.In general, de v elopment in this field has been slo w ed due to poor communications between academia and industry, as well as amongst different disciplines within academia working on corrosion (abiotic/biotic).Unjustifiably, MIC is still considered a questionable mechanism in many industrial sectors, and in some cases, its existence is e v en denied, hindering important knowledge transfer and the de v elopment of envir onmentall y a ppr opriate solutions for this problem.In academia, some microbiologists working on MIC can be hampered by limited access to, or knowledge of, important aspects of abiotic corrosion or methods used by engineers and materials scientists .T he lac k of information exc hange between industry and academy means that scientists are not aware of the actual needs of the industry, thus academic r esearc h can lac k pr actical r ele v ance.
A k e y historical problem with MIC studies is the often-siloed nature of scientific disciplines working on the topic.While the number of MIC-related research articles has grown significantly in the past two decades, many of these articles only cover one or two aspects of this multidisciplinary topic.For example, Hashemi et al. ( 2017 ) demonstrated that a large proportion of the r esearc h published on MIC was siloed separ atel y within the corrosion/materials science and microbiology areas, despite the multidisciplinary nature of MIC.With such siloing of knowledge, valuable information can become isolated within one particular discipline instead of spreading amongst the wider MIC comm unity.This consequentl y delays pr ogr ess and innovation, despite the huge economic and environmental impact of MIC.
The focus of the current review is to directly tackle one aspect, the siloed nature of MIC resear ch, b y providing important bac kgr ound information on the non-micr obiological aspects of MIC to microbiologists.While microbiologists are experts in their fields, they typically have limited understanding of the corr osion/metallur gical and/or chemical aspects of MIC.This r e vie w, whic h has an emphasis on the degradation of metals, will provide information to help avoid mistakes that can be made when experts in one field start working on a m ultidisciplinary topic suc h as MIC.By pr oviding this information, we aim to encour a ge interdisciplinary and intersector al collabor ations to ease the entr ance of biological scientists , including microbiologists , into the complex field of MIC and, ultimatel y, to sha pe the next er a of MIC r esearc h and management.

MIC mechanisms and clarification of terminology
MIC has been defined by NACE and ASTM as "corrosion affected by the presence or activity, or both, of micr oor ganisms" (ASTM G193 2022 ), as adapted from Little and Lee ( 2007 ).Several terms are used to describe this phenomenon (micr obiall y influenced corr osion, MIC, biodeterior ation, and biocorr osion), with the r ange of different terms often resulting in confusion.Here we aim to clarify some of the k e y terminology used in relation to the phenomenon itself or to its various mechanisms.

MIC terminology
A broader term that defines the microbial degradation of metallic and non-metallic materials is biodeterioration , i.e. "any undesirable change in the properties of a material caused by the vital activities of or ganisms" (Huec k 1965 ).The fundamental cycling pr ocesses involved in the biodeterioration of stone and metal have recently been r e vie w ed b y Gaylar de and Little ( 2022 ).MIC is commonly associated with the biodeterioration of materials such as metals and concr ete.In Eur ope and in some international standards, the term corrosion is used only for metallic material (ISO 8044-2020 ), but the International Union of Pure and Applied Chemistry (IUPAC) ( 1997 ) pr ovides a br oader and widel y accepted definition, i.e. "corr osion is an irr e v ersible interfacial reaction of a material (metal, ceramic, or polymer) with its environment that results in consumption of the material or in the dissolution into the material of a component of the environment." The term biocorrosion is incr easingl y used as a synonym of MIC, although this term can create confusion as, in the US, it primaril y r efers to the corr osion of medical implants due to both biotic and abiotic processes (Little et al. 2020b ).It has been suggested that the term microbial corrosion hints that micr oor ganisms ar e the main cause of the corrosion (Gu 2012 ), while others use this term as a synonym for MIC.The ISO 8044 standard describes the term bacterial corrosion as a synonym for MIC if it is solely due to the activity of bacteria; ho w e v er, as arc haea or e v en fungi can be involved in the deterioration process, this term should only be used in unequivocal cases.
MIC is pr obabl y the most widely used term to describe the many ways in which microorganisms can affect corrosion processes.While some use the w or d induced instead of influenced, the presence/activity of certain microorganisms has also been known to reduce the rates of corrosion (Videla andHerrera 2009 , Kip andV an V een 2015 ) and so, the term induced is not as br oadl y a pplicable .T he adjectiv e "micr obiologicall y" in the term of MIC is gr ammaticall y incorr ect as it refers to corrosion caused by microbiology instead of micr oor ganisms; nonetheless, at the CORROSION/90 conference in Las Vegas, Ne v ada, NACE's Publication Committee supported its use for future NACE documents (Brooke 1990 ).Since then, many other associations and standards have adopted the term MIC , e.g.(GRI 1990, AMPP 2018 ); thus, we prefer to use this term in this r e vie w to align with these standar ds.Ho w e v er, r eaders ar e fr ee to decide their pr efer ence, as MIC can be the acr on ym for both microbiologicall y and micr obiall y influenced corr osion as well as for micr obial corr osion, and all thr ee terms ar e suitable to name the phenomenon.If one prefers, biocorrosion can also be used as long as it is pr operl y defined what users mean under the term.
Biofouling can lead to MIC, but the term cannot be used as a synonym for MIC.Biofouling is the accumulation and growth of various organisms, including microorganisms (microfouling), plants, and/or animals (e .g. algae , barnacles) (macrofouling), on a surface (AMPP 2023 ).While, in some cases, biofouling can be associated with corrosion, this is not always the case.Indeed, other problems associated with biofouling, including increased water resistance on ships, increased fuel consumption due to drag (Callo w and Callo w 2011 , Tulcidas et al. 2015 ), or the introduction of non-indigenous species (Li and Ning 2019 ), can be more of an issue .T hus , using biofouling as a dir ect synon ym for MIC is not recommended.

MIC mechanisms of metals
The term MIC does not describe a single mechanism for corrosion, it is rather a collective term for a variety of different mechanisms thr ough whic h micr oor ganisms alter the kinetics of corr osion r eactions by their presence or activity (Lee et al. 2022 ).For MIC to occur, the specific interplay of the "three M's" is required: microorganisms, media, and metals (Little et al. 2020b ).The combination of these interactions defines the various mechanisms that can change the rate of metal deterioration, either directly or indirectly.T here ha ve been many reviews specifically describing MIC mechanisms, but unfortunately, there are many inconsistencies with the terminologies used, making it difficult to navigate among them.
Here we aim to clarify the terminology used for MIC mechanisms without going very deep into details, with the goal of providing a common and easily understood language.

MIC due to surface deposition
Micr oor ganisms ar e involv ed in a r ange of pr ocesses that can lead to the formation of deposits on the surfaces of materials .T hey can form single or m ultispecies comm unities attac hed to a surface, known as biofilms, which are often embedded in a self-produced matrix of extracellular polymeric substances (EPS) (Flemming et al. 2016 ).Alternativ el y, metabolic pr ocesses due to some types of micr oor ganisms, suc h as metal-oxidizing bacteria, can lead to metal products being deposited on a surface (Lee and Little 2019 ).As discussed below, these deposits can affect and, in some cases, acceler ate corr osion.
MIC mec hanisms ar e often classified based on oxygen presence and/or availability in a given environment.Ho w ever, in real-life conditions , oxygen ma y intermittently be a vailable and/or consumed by micr oor ganisms that can react directly with the metal surface .T hus , instead of a strictl y aer obic or anaer obic envir onment, an oxygen gradient is often present that can v ary ov er time.This clashes somewhat with many laboratory-based MIC experiments, which aim to operate under strictly aerobic or anaerobic conditions.Indeed, there is potential scope for more experiments to be performed that look at the effect of alternating oxygen in MIC tests .In one example , Lee et al. ( 2004 ) sho w ed that SRB corr osion r ates incr eased by a factor of three if oxygen was intermittentl y pr esent, when compar ed to either strictl y aer obic or anaerobic conditions.
The growth of a biofilm itself can result in MIC in aerobic fluid environments when biofilms are formed in a patchy arrangement, cr eating o xygen concentr a tion or differential aer a tion cells between the anodic and the cathodic areas of the surface.Fundamentall y, the mec hanism of corr osion involv es electr on flow through the metal from the anode to the cathode, where (under aerobic conditions) oxygen is the electron acceptor (Hamilton 2003 ).The biofilm, which defines the anode, pr e v ents oxygen r eac hing the metal surface while the metabolism of aerobic bacteria uses up the oxygen present in the biofilm.The cathodic site ends up being the area uncovered by the biofilm that is exposed to oxygen.Roe et al. ( 1996 ) have shown that cell-free EPS alone can initiate corrosion.
Other surface deposits, such as metal oxides, can form oxygen concentration cells, resulting in under deposit corrosion or oxygen gradient corrosion .The two most studied groups of MICr elated aer obic metal-depositing bacteria ar e ir on-oxidizing bacteria (FeOB) and manganese-oxidizing bacteria (MnOB), which have been reviewed by Lee and Little ( 2019 ).For example , F eOB oxidize Fe 2 + into Fe 3 + in an oxygen-rich envir onment wher e the area underneath the accumulated iron oxides is depleted of oxygen and a small anode is formed r elativ e to the surr ounding lar ge, oxygen-satur ated cathode.The differ ence in dissolv ed oxygen concentr ation cr eates a potential differ ence r esulting in oxygen concentration cells or, alternatively, can cause a form of galv anic corr osion.These deposits can lead to pitting corr osion ["localized corr osion r esulting in pits , i.e .ca vities extending from the surface into the metal" (ISO 8044) (Table 1 )].These pits, or the complex geometries of the deposited metals that form, can create areas shielded from the bulk fluid/electrolyte.Subsequent hydrolysis of metal ions creates an acidic medium and attracts chargeneutr alizing ions suc h as c hloride and sulfate, r esulting in selfsustaining pitting that, if it occurs in cr e vices, is called crevice corrosion (Table 1 ).
Some materials, such as corrosion-resistant steels , ha ve enhanced resistance to oxygen corrosion as their passivated surface (thin metal-oxide layer; see the Materials and MIC section below) pr ovides pr otection.Ho w e v er, some biofilms can destr oy this pr o-tectiv e layer, r esulting in pitting corr osion (Yuan and Pehk onen 2007, Li et al. 2016, Dong et al. 2018, Cui et al. 2022 ).In addition to the formation of a biofilm, the r espir ation of aer obic bacteria within a biofilm can reduce the oxygen content, creating an anaer obic envir onment that can support the growth of anaerobes suc h as sulfate-r educing pr okaryotes (SRP) and nitr ate-r educing prokaryotes (NRP).

Electrical MIC (EMIC)
EPS secreted by microbial cells have many components with redox properties and electrochemical (EC) activity that play crucial r oles in micr obial r espir ation as well as in corrosion.For example, sessile cells in a biofilm can use metal, such as elemental iron, as an electron donor if thermodynamically more favorable electron donors ar e lac king (Philips et al. 2018 ).In anoxic en vironments , the terminal electron acceptor is an oxidizing agent such as sulfate or nitr ate.While the r eduction of the electr on acce ptor tak es place inside the cell, the oxidation of the electron donor happens outside the cell, so the extracellular electron from outside must enter the cell.This electr on tr ansport acr oss the cell wall is called extracellular electron transfer (EET), and the ov er all mec hanism thr ough which the associated corrosion of metal is achieved is called EMIC (Enning and Garrelfs 2014 ), alternative names include type I MIC (Gu 2012 ) and EET-MIC (EET MIC) (Jia et al. 2019 ).If the electron donor is an organic carbon source that can diffuse into the cell, there is no need for an electron transport mechanism because the electrons released are already located in the cytoplasm.Insoluble metals, such as elemental iron, cannot pass through the cell membr ane, and an extr acellular electr on tr ansport is r equir ed for it to be used as an electron source .T he electron can be transported into the cell in tw o w a ys , namely by direct or indirect mechanisms (Table 1 ).
In direct EMIC , direct extracellular electron transfer (DEET) (Lovley 2011 ) occurs, the cell can have direct contact with the metal surface and dir ectl y accept electr ons fr om the metal.Until r ecentl y, this mechanism had only been inferred, but Tang et al. ( 2019 ) provided evidence for iron as a direct electron donor.Electrons are taken up by cell surface enzymes , structures , or membrane redox pr oteins, suc h as c-type cytoc hr omes (P aquete et al. 2022 ), facilitating EMIC.Alternativ el y, the cell can attac h to the metal b y "nano wires ," e .g. electrically conductive pili in bacteria (Lovley 2017 ) or archaella in archaea (Walker et al. 2019 ), by which it can transfer electrons .T he exact mechanism for electron transfer thr ough electricall y conductiv e cell a ppenda ges is still debated, and Little et al. ( 2020a ) argue that any electronic transport via pili is unlikely to significantly contribute to MIC.T hus , further research is needed to resolve the role of electrically conductive pili in MIC.
From an EC standpoint, the term "direct electron transfer" is possibly not strictly correct as each mechanism described above r equir es a redox mediator.Blackwood ( 2018 ) has contested that true dir ect electr on tr ansport does not and cannot happen, as dir ect electr on tr ansfer between aqueous species cannot occur over distances of > 2 nm.This is a typical example of different disciplines using different language to describe a phenomenon, increasing the chance of misunderstanding and confusion.This confusion is e v en further incr eased by alternativ e terms and their abbr e viations for direct EMIC, including DET-MIC (direct electron tr ansport MIC) (Lekbac h et al. 2021 ) or DIMET (dir ect ir on-tomicr oor ganism electr on tr ansfer), if dir ect electr on tr ansport occurs fr om Fe 0 (Lekbac h et al. 2021 ), along with the other alternative terms for EMIC as indicated abo ve .Despite the controversies Table 1.Brief description of the main mechanisms associated with MIC of metals.

Under deposit corrosion, oxygen gradient corrosion
A type of "localized corrosion associated with, and taking place under, or immediatel y ar ound, a deposit of corr osion pr oducts or other substance" (ISO 2020), e.g.biofilm or metal de position by metal-o xidizing bacteria that is formed in a patchy arrangement.

Crevice corrosion
A type of "localized corrosion associated with, and taking place in, or immediatel y ar ound, a narr ow a pertur e or clear ance formed between the metal surface and another surface (metallic or non-metallic)."(ISO 8044) The accumulation of chloride and other aggressive anions in the pit acceler ates corr osion.

Electrical MIC (EMIC)
Corrosion caused by extracellular electron transfer by microorganisms.

Direct EMIC
Corrosion of metals achieved by extracellular electron transfer by micr oor ganisms in direct contact with the metal surface.Electrons are taken up by cell surface enzymes or membrane redox proteins.

Indirect EMIC
Corrosion of metals accelerated by soluble electron transfer mediators r eleased fr om micr oor ganisms that use the electr ons gained fr om the metal for r espir ation.

Metabolite MIC (MMIC)
Corrosion of metal achieved directly or indirectly by metabolites released by micr oor ganisms both in aer obic and anaer obic conditions.and mixed terminologies on the different categories of direct MIC, we use the umbrella term "direct EMIC" here when referring to micr oor ganisms ca pable of dir ectl y inter acting with the metal surface by one of the various proposed mechanisms.
In indirect EMIC, soluble electr on tr ansfer mediators (Huang et al. 2018, Tsurumaru et al. 2018 ) are released from the cell, oxidized at the anode, and r eturn bac k to the cell to be used in r espir ation (Kato 2016 ).Alternative abbreviations also exist for this mechanism [ MEET (mediated EET (Little et al. 2020a ); MET-MIC (mediated electr on tr ansport MIC) (Gu 2012 ); and SIMET (shuttle-mediated ir on-to-micr oor ganism electr on tr ansfer), if ir on is the electr on source (Lekbach et al. 2021 )].

Metabolite MIC (MMIC)
In metabolite-MIC, micr oor ganisms influence corr osion thr ough the creation of corrosive metabolites, such as protons, organic acids , or sulfur species .T hese metabolites ar e r educed on the metal surface, and a biocatalyst is not r equir ed for the process, as opposed to EMIC.At a sufficiently low pH, pr oton r eduction can be coupled with metal oxidation.This type of corrosion mechanism is also an EC pr ocess.Alternativ e terms for MMIC include type II MIC (Gu 2012 ) and chemical MIC (CMIC) (Enning and Garrelfs 2014 ).Li et al. ( 2018 ) argued that the term "metabolite-MIC" is preferable ov er "c hemical MIC," because c hemical corr osion is the dir ect r eaction of a metal with an oxidant, usually at high temperatures, with no separable oxidation and r eduction r eactions, as opposed to EC corrosion.

The historical cathodic depolarization theory
Historicall y, man y MIC mec hanistic studies in the absence of oxygen were reported in relation to sulfate-reducing bacteria (SRB), and the associated se v er e corr osion was often explained by the cathodic depolarization (CDP) theory first proposed by von Wolzogen Kühr and Van der Vlugt ( 1934) (translated into English in 1964).According to the CDP theory, the rate of iron corrosion by SRB is increased by the removal of H 2 from the cathode by hydrogenasecontaining SRB.To be precise, in the absence of oxygen, the electron acceptors for iron oxidation are protons derived from dissociated water, whereas in the cathodic reaction, the proton is reduced to H 2 .According to the theory, the H 2 formed on the metal surface is consumed by SRB and thereby further accelerating iron oxidation.
The CDP theory has been criticized for decades and been discr edited by man y (Hardy 1983, Cr olet 1992, Dinh et al. 2004, Mori et al. 2010 ), and r e vie wed in detail (Enning andGarr elfs 2014 , Blac kwood 2018 ).In short, the rate-limiting cathodic reaction in metal corrosion is the adsorption of protons to the metal, not the desorption (r emov al or dissolution) of H 2 fr om the surface , i.e . in abiotic cultur es, low corr osion r ates ar e due to the limited av ailability of protons and, thus, slo w H 2 formation on iron.It has been sho wn that the consumption of cathodic hydrogen by SRB did not significantl y incr ease ir on corr osion in the presence of iron as the sole electron donor (Venzlaff et al. 2013 ).This does not rule out the possibility that the utilization of hydrogen by microorganisms ma y still pla y a role in MIC, but not as it was proposed/intended by the CDP theory.Ov er all, it is recommended that in the future, the CDP theory should only be mentioned if needed for historical purposes, and if it is mentioned, the contr ov ersy and criticisms of this explanation for MIC should be acknowledged.
To summarize, MIC is not a single corr osion mec hanism.Instead, se v er al differ ent mec hanisms can contribute to MIC.Howe v er, the two main common features that are similar in all MIC cases are that (1) microorganisms play a role and (2) MIC is an EC process.

Siloed scientific fields and the need for interdisciplinary dialogue
While MIC by definition encompasses the fields of microbiology and corrosion, the involvement of other disciplines such as electr oc hemistry, pr oduction c hemistry, metallur gy and materials science, process engineering, fluid mechanics, and others is essential to getting a clear picture of the environments and operating conditions that support MIC.As early as 1934, while leading a group of scientists in the field study of a sphagnum bog, Baas Becking (Baas Becking and Nicolai 1934 ) observed that simply classifying the micr oor ganisms pr esent would yield "less satisfaction to the investigator" than it could have with additional scientific insights from geologists , geneticists , and ecologists .Decades later, a r e vie w of MIC state-of-the-art in 2005 by Videla and Herr er a ( 2005 ) noted that until the late 1970s there was poor transfer of knowledge between disciplines, including metallurgy, electr oc hemistry, micr obiology, and c hemical engineering, that prevented the study of MIC from going m uc h beyond a focus on SRB/SRP .Today , it is becoming mor e widel y understood that any inv estigation of MIC r equir es a m ultidisciplinary focus on m ultiple lines of evidence (MLOE), as reflected in Sharma et al. ( 2022 ), where data from molecular methods were analyzed and conclusions drawn by a multidisciplinary team.Yet, when industries today are attempting to understand the impact of MIC on their assets, many do not have experts from multiple disciplines on hand to guide their sampling, testing, and data integration to help them solve complex MIC issues.Further, while there are numerous standards available to guide specific types of testing, ther e ar e none that identify a trul y unified m ultidisciplinary a ppr oac h for combining MLOE to ultimately direct MIC management activities.

Diagnosing MIC requires MLOE
The diagnosis of MIC requires MLOE for a number of reasons, but perha ps for emost is the fact that there exists no singular test or assay that can conclusiv el y identify that MIC has occurred or is presently occurring, although some current works, e.g.Lahme et al. ( 2021 ), have suggested that [NiFe] hydrogenases in methanogenic archaea can be potential MIC biomarkers under specific conditions.Early MIC work in the oil and gas industry was heavily focused on the use of culturing methods (such as the most probable number-MPN-technique) and relating the likelihood of MIC to culturable cell counts in various types of media.Gr aduall y, and with the incr eased a pplication of molecular microbiological methods (MMM), asset owners discovered that while micr oor ganisms wer e pr esent nearl y e v erywher e, the cell counts poorl y corr elated with actual MIC dama ge (Zintel et al. 2003 ).
Another reason why MLOE are used for MIC diagnosis is the lack of a unifying model or equation to calculate corrosion rates due to MIC, which is made onl y mor e difficult by the fact that, and as explained in the pr e vious section, micr oor ganisms and biofilms can affect corrosion reactions in a variety of wa ys .In addition, MIC can at times be linked with corrosion caused by abiotic factors.The MLOE a ppr oac h is common to man y other scientific fields, including but not limited to the study of sediments, microbial fuel cells , and bioremediation.T he approach is a "systems" view of microbial ecology, including the roles of the chemical environment and the role of the material being degraded or deteriorated.Fig-ure 1 shows an example of the four MLOE categories often used for MIC in vestigations , including examples of parameters that are included in each category.The more pieces of the puzzle that can be provided, the better the picture/understanding of what is happening will be.
The best diagnosis of MIC requires MLOE from as many of the four categories shown in the puzzle as possible.Evidence from more categories provides increased confidence.At present, this is still a work in pr ogr ess, and ther e ar e no definitive guidelines on which tests or combination of tests provide the best evidence.According to Lee and Little ( 2017 ), the goal is to collect independent types of measurements that are consistent with a MIC mechanism.
To obtain MLOE, the investigation of corrosion in an asset would ideally be based upon proper characterization of (1) the conditions that ar e pr esent in the "bulk" envir onment (e.g.soil, water, process fluid); (2) any biofilm or other material at the metal/environment interface and associated physicochemical conditions, most likely to be involved in MIC; and the (3) the metal surface itself, both wher e corr osion has formed and wher e it has not.These thr ee environments can be quite different from one another, even though they are present at the same time in the same place.Wrangham and Summer ( 2013 ) sho w ed that the types and numbers of micr oor ganisms pr esent in a bulk fluid phase can be quite different than those located within a biofilm on a surface.Deposits on a metal surface can also vary in composition and physical pr operties thr oughout their thic kness and later all y, as demonstrated by Larsen et al. ( 2010 ), who showed significant differences in both corrosion product composition and microbiology in thick deposits inside of pipework on an offshore oil and gas production platform.
Obtaining as m uc h information as possible from a combination of the different environments present is important to get the most accurate understanding of the overall processes taking place.If only limited testing is a vailable , it is recommended the focus of testing should be on the metal interface, as this is where the k e y corr osion inter actions ar e likel y taking place.

The roles of engineering design and operations in MIC assessment
It is valuable for microbiologists to have an understanding of the ov er all oper ation for assets wher e MIC is being assessed.Engineers , operators , maintenance personnel, production chemists, and c hemical v endors can pr ovide v aluable insights that r e v eal when and why environmental changes occur.A simple example is the oper ating temper atur e. Oper ations may r eport that the crude oil pr oduction temper atur e is 80 • C, leading micr obiologists to look for thermophiles as a possible cause of MIC; ho w e v er, it would also be important to know that the process only runs once a month for a day, then cools down to ambient temper atur e.An y factor in the design, operation, or maintenance of an asset that can affect the chemical and microbiological environment should be an ar ea of inter est, and micr obiologists may need to prompt other experts to obtain this type of information; it may not be volunteered otherwise.It is particularly important to understand the types and doses of various treatment chemicals that may be used, and pr oduction c hemists and corr osion engineers can gener all y provide this information.Changes to asset design, the fluids being r eceiv ed or pr ocessed, the corr osion mitigation measur es being a pplied, incr eases in ne wl y found corr osion, etc. can all pr ovide important insights to microbiologists working to solve a MIC issue.Leak and failur e histories, particularl y if r oot cause anal ysis has been performed, can also provide useful context when assessing MIC (Borenstein and Lindsay 2002, Eckert and Skovhus 2018, G ősi et al. 2022 ).
Engineering and design information is also valuable when assessing the potential for MIC and abiotic corrosion mechanisms.Such information includes the type and grade of materials used for construction, fabrication, and testing history; circuits and systems identified on engineering dra wings , process flow diagrams , and mass balance sheets; identification of dead legs (where flow infr equentl y occurs); clean-out ca pabilities for lar ge v essels; flow controls; and utilities supporting various processes in the assets.Often, MIC is found in piping and assets with no flow or stratified flo w, which allo ws solids and w ater to accum ulate and pr omote the growth of biofilms (Sharma et al. 2022 ).A r e vie w of oper ation and design parameters can help to identify such areas to guide inspection and mitigation acti vities.Ad ditionally, older assets may no longer conform to the operating conditions used as the basis for design, which affects the type and severity of likely corrosion threats (Wei et al. 2022 ).Table 2 provides some examples of operational and engineering information that can help support MIC threat assessments.

Chemistry: assessing the chemical environment
Non-micr obiologists gener all y do not have the same perspective on the chemical environment as microbiologists , e .g. the significance of different electron acceptors, energy sources, pH, redox potential, salinity, and other factors that affect microbial ecology (Skovhus et al. 2017 ).The concepts of exponential growth, the widespread diversity of microbiomes, the essential inputs to (and end products from) microbial metabolism, and the important roles of biofilms and EPS ar e some what for eign to experts in other disciplines (Wade et al. 2023 ).Microbiologists ma y ha ve the opportunity to help other disciplines to view information related to MIC through the "lens" of microbial ecology.Likewise, a dialog with c hemists, corr osion engineers, and oper ators can bring new insights to those focused on the microbiological environment (Hashemi et al. 2017 ).It is imper ativ e that all parties in the multidisciplinary con versation ha ve a clear understanding of the technical terms that are being used, as each discipline typically has its own technical vocabulary (Eckert and Skovhus 2018 ).
The chemical composition as well as the physical parameters of the environment in which MIC or abiotic corrosion occurs are very significant, in that the chemistry of the bulk phase environment and of surface films/deposits impacts both electr oc hemistry and micr obiological pr ocesses.While sampling and chemical/physical analysis of the bulk phase (e.g.aqueous) is fairly straightforw ar d, analysis of chemical conditions at the metal surface, particularly beneath solid particles and biofilms is considerabl y mor e complex (Phull andAbdullahi 2017 , Kromer et al. 2022 ).When anal yzing c hemical composition data, it is imper ativ e to k ee p this distinction in mind; e.g. the pH in the bulk phase may be consider abl y differ ent fr om the pH beneath a biofilm containing acid-pr oducing micr oor ganisms (Lee et al. 1993, Dexter and Chandr asekar an 2000, Phull and Abdullahi 2017 ).There are many commonl y used anal ytical methods for water composition, dissolv ed gas, and headspace gas analysis, and some examples of these are detailed here.

pH
pH is an essential measurement parameter in the aqueous phase of a system affected by MIC (Ibrahim et al. 2018 ).Changes in the pH may indicate the growth of acid-producing microorganisms,

Initially identified corrosion mitigation measures to be applied
Inspection and maintenance records, integrity assessment records Means of pre-commissioning testing; hydrostatic test records, pr ocedur es, actual test media used Pr ocess upsets, emer genc y shut do wn recor ds-Test such as acetogens, or partial pressure variations, such as dissolved O 2 and CO 2 concentrations (Lee et al. 1993, Mand et al. 2014, Kato 2016 ).In a given field environment, the local pH condition has a direct impact on the microbial community and activity; e.g. the corr osiv e acetogen Sporomusa sphaeroides thrives in pH ranging between 6.4 and 7.6 and can tolerate up to 8.7 (Philips et al. 2019 ), whereas some SRB can grow up to pH 9.5 but with an optimal pH r ange ar ound 7 (Ibr ahim et al. 2018 ).One k e y challenge is understanding the actual pH in a giv en envir onment, as pH is affected by temper atur e and pr essur e (Phull and Abdullahi 2017 ).For example, a sudden shift in system pr essur e, suc h as liquid withdr awal fr om a pr essurized system, will alter the pH measur ed (Ibrahim et al. 2018 ).According to the Henry's Law, gas solubility is dir ectl y pr oportional to the partial pr essur e, whic h is particu-larly important for CO 2 and H 2 S. Changes in their solubility will also influence the pH of the aqueous environment, that subsequently impact microbial growth and corrosion product formation (Ibrahim et al. 2018 ).Metal dissolution and corrosion product formation are intertwined with biofilm and influenced next to others by pH.For example, the formation of FeCO 3 (siderite) is more stable in higher pH, since the concentrations of HCO 3 − and CO 3 2 − are higher than the respective iron ions, thus favoring the crystallization of siderite (J oshi 2016 ).Furthermore , pH can act as an indicator for MIC mitigation.The topic of MIC mitigation is discussed below in the Corrosion Management section.

Concentr a tion of dissolved ions
Concentrations of cations and anions in the aqueous phase can also indicate possible microbial acti vity.For example, de pletions in the concentrations of electron acceptors such as nitrate and sulfate indicate the activities of NRP and SRP, r espectiv el y.The mass balance between the cations and anions of a given envir onment pr ovides useful e vidence for e v aluating the ov er all MIC process, including the metabolic process, corrosion product deposition, and metal dissolution.For example, the concentration of sulfur species in the aqueous phase, including S 2 O 3 2-, SO 4 2 − , HS − , SO 3 2 − , and S 0 , is closel y r elated to the oxygen concentration in a system and micr oor ganisms suc h as sulfur-oxidizing bacteria and SRB (Ibrahim et al. 2018 ).Correct dosages of biocide and nitrate injection to combat MIC also require close monitoring of cations/anions (Gieg et al. 2011, Ibrahim et al. 2018 ).For example, in the oil and gas industry, for nitrate injection to be successful, the concentration of NO 2 − needs to remain stable in the system to inhibit the activities of SRB, as the further reduced compounds of NO 2 − in the metabolic pathway of NRB, namely N 2 and ammonia, ar e ineffectiv e a gainst SRB.T hus , a close monitoring of the anions NO 3 − , NO 2 − , HS − , and SO 4 2 − will provide a detailed ov ervie w on the efficacy of nitrate injection on the activities of SRB.One k e y challenge is the timely measurement of the associated ions; e.g.HS − ions ar e highl y volatile and can quic kl y esca pe into the atmosphere post-sampling (Tangerman 2009 ).It is important to ensur e on-site r eadil y av ailable measur ements when conducting analyses of k e y ions.In ad dition, the differ ences in the le v els of cations and anions provide important evidence for the corrosion product formation process.For example, a decrease in the level of carbonate ion and Fe 2 + in the aqueous solution may indicate the formation of FeCO 3 , and the r espectiv e concentr ations of the ions are used for calculating the supersaturation index (SS) (Joshi 2016 ): where the K sp is the solubility product constant for FeCO 3 and an SS value of above 1 indicates that the solution is satur ated (Joshi 2016 ).Ov er all, the mass balance between the various cations and anion species is a strong indicator of the MIC process.

Gas production
Se v er al known micr oor ganisms associated with MIC pr oduce biogenic gas .For example , the corr osiv e methanogens pr oduce methane using the electrons from the metal surface (Beese-Vasbender et al. 2015, An et al. 2020, Tamisier et al. 2022 ), whereas SRB activities lead to the production of H 2 S. Gas chromatographs (GC) equipped with a thermal conductivity detector or flame ionization detector are typically used for gas analyses (Grob and Kaiser 1982 ).In the field of MIC, it is noteworthy that multiple bio-genic gases may need to be monitored, including CO 2 , CH 4 , H 2 S, H 2 , O 2 , N 2 , etc. Hydrogen is one of the k e y gases of special importance to MIC, as se v er al corr osiv e species ar e dependent on H 2 for their growth.In addition to GC, various handheld and in-line H 2 sensors are commercially available that allow field monitoring of H 2 and ar e particularl y useful during field sampling (Bosha gh and Rostami 2020 ).Ho w e v er, suc h de vices can be limited in r esolution, and their reliability can be affected due to contamination by other gases or wrong handling in the field.For extr emel y local envir onments, such as within the metal-biofilm interface, monitoring of the H 2 gradient can be performed using microsensors (Cai et al. 2020b ).While the current H 2 microsensor technologies are still e volving to r educe interfer ence fr om H 2 S and other compounds (Nielsen et al. 2015 ), local monitoring of H 2 remains an important line of evidence during MIC investigations and monitoring.

Microbiology: assessing microbiological composition and activity
The micr oor ganisms associated with corr osion ar e , of course , str ongl y linked to the chemical and physical environmental conditions present, but microbiological activities also affect the local environment in terms of organic or mineral acid production, sulfide production, or the formation of occluded areas and concentration cells on the metal surface.Little et al. ( 1996 ) demonstrated that microorganisms in biofilms can do both: create local anodic areas and are also "attracted" to existing anodic sites previously unaffected by microorganisms.Non-microbiologists can easily become lost in the complexity of interactions that could be occurring between various microorganisms in biofilms, their metabolic capabilities, and the kinetics that are driving reactions in one direction or the other.This is compounded by a lack of comprehension of the strengths and limitations of different micr obiological c har acterization methods/tec hnologies, the issues of interference, primer cov er a ge, biases, sensitivity, etc.It is often stated that microbiological conditions may be described in terms of di versity, en umeration, and acti vity.Engineers are generally not aware of the difficulty in determining the specific microbial activities that are occurring in a given en vironment, e .g. using RT-qPCR or metabolomics, and that these activities ar e dynamic, c hanging in parallel with environmental conditions .Microbiologists , with expertise on these and other associated topics , can pro vide essential insights on such matters to non-microbiologists.
Ov er the years, v arious tec hniques hav e e volv ed and been gr aduall y r eplaced by mor e adv anced tec hnologies to inv estigate MIC, all the way down to the molecular le v el (as r e vie wed in Little et al. 2006, Beale et al. 2016, Trif et al. 2018, Kotu et al. 2019 ).In support of this observation, Puentes-Cala et al. ( 2022) ov ervie wed the MIC liter atur e published in the last 12 years, which showed that a ppr oximatel y thr ee-quarters of the studies used molecular micr obiological a ppr oac hes to c har acterize micr obial comm unities in field samples.Table 3 summarizes some of the traditional as well as more advanced methods that can be used to obtain microbiological data, highlighting their pros and cons to aid decisionmaking during MIC in vestigations .All of these tec hniques ar e suitable to use for both field and laboratory studies if handling is done pr operl y, as described else wher e [e.g. in Eckert (2022g. in Eckert ( ) et al. ( 2022 ) ) or AMPP Standard TM21465 (under pr epar ation)].
It is imper ativ e to emphasize that the limitations of each microbiological method should always be considered, as , e .g. the detection of micr oor ganisms that have been associated with corrosion by itself is not diagnostic for MIC (Little et al. 2006 ).Also, the choice of methods should carefully be evaluated in light of the

Materials and MIC
Ther e ar e a fe w high-le v el points for micr obiologists to consider when thinking about corrosion mechanisms and electrochemistry.The first is that abiotic or non-biological corrosion reactions need to be considered in e v ery MIC e v aluation.Abiotic corr osion may be present separately from, or in conjunction with, MIC.Micr oor ganisms , e .g. could be forming biofilms that simpl y cr eate mor e cr e vices for differ ential aer ation corr osion cells, leading to localized pitting on passive materials such as stainless steel (SS) (Table 4 ).The second consideration is that microbial activity in a biofilm may simply enhance the effects of existing and wellknown abiotic metallurgical conditions that can promote localized corr osion, suc h as the effects of manganese sulfide inclusions forming microscopic anodic corrosion initiation sites or galvanic corr osion occurring wher e metals having differing native potentials are joined (e.g.carbon to SS).It is important for microbiologists to understand these abiotic contributors to corrosion when examining the role of micr oor ganisms in the corrosion of a given material, and metallurgists and materials scientists can readily explain these contributors.Finally, and probably one of the more elusiv e c hallenges, is de v eloping an understanding of how micr obiological metabolism facilitates or enhances the kinetics of anodic and cathodic corr osion r eactions that m ust be occurring for corrosion to take place.As one electr oc hemist r ecentl y stated in an MIC symposium, "I need to know wher e the electr ons ar e going!".This is an area where there is significant room for improvement in our understanding of MIC; ho w e v er, one that will r equir e a serious collabor ativ e effort between materials scientists and microbiologists to make significant progress.
In addition to the c hemical, micr obiological, and physical environment, the potential for MIC depends on the composition and metallurgical properties of the material being affected by these parameters.Carbon steel (CS) and concrete are two of the most predominant materials used in the construction of engineered assets, including pipelines, sewer and water lines, marine structures , ships , offshore energy generation, and infrastructur e, suc h as bridges and highways.Concrete and the CS reinforcing used within the concr ete ar e often subject to corrosion, although the percentage of this corrosion resulting from MIC other than in sewer lines (Wu et al. 2020 ) is not well understood.T here is , ho w ever, a long history of research and information published about the interaction of metals with biofilms.The aim of this section is to provide a general introduction to materials and, specifically, metal pr operties that ar e r ele v ant to MIC.Metals can gener all y be br oken do wn into tw o categories , i.e .passi ve and acti ve metals, de pending on the metal and the environment to which it is exposed.Passive metals , e .g. corrosionresistant alloys (CRA) like SS, form a protective metal oxide when exposed to aqueous environments containing o xygen.Acti ve metals, such as CS, do not form this protective layer when exposed to aer ated water.Typicall y, passiv e metals perform better in relation to corrosion; ho w ever, this is not always the case.Many metals used in industrial applications are alloys (a combination of elements), where small changes of compositions can make significant performance differences .T he processes used for manufacturing metals (e.g.temper atur es, mec hanical pr ocessing) can also affect the micr ostructur e of essentiall y the same alloy, which can affect corrosion.In addition, construction and fabrication processes such as welding can also adversely change the properties of metals to make them more susceptible to corr osion.Eac h of these factors can also affect the likelihood and magnitude of MIC that may occur.
Table 4 shows some examples of alloy categories and typical a pplications (Pierr e R. Rober ge 2012 ), along with r efer ences wher e MIC case studies of these materials can be found.
Since MIC most often results in localized corrosion (pitting), a metal's resistance to pitting is important to engineers and designers seeking to pr e v ent MIC.One indicator of pitting resistance in SSs is the pitting resistance equi valent n umber (PREN), which is based on a calculation using the amount of c hr omium, mol ybdenum, and nitr ogen pr esent in an alloy.PREN is used to compare the relative resistance of alloys to pitting corrosion in chloride-containing aqueous en vironments .Alloys with a PREN of 32 or greater are generally considered to be resistant to pitting corrosion in ambient-temperature seawater.A material's PREN value may also provide some level of insight in determining its r elativ e r esistance to MIC (Eckert andAmend 2017 et al. 2017 ); ho w e v er, car e needs to be taken not to overinter pr et this v alue (Cr aig 2020 ).A gener al r e vie w of the liter atur e in whic h MIC is cited as the cause of corrosion will show that as PREN increases, the frequency of MIC case studies decr eases.MIC is fr equentl y r eported for CSs and some what less fr equentl y for SS.For duplex (DSS) and super-DSS SSs, nickelbased and titanium alloys the incidence of reported MIC is fairly r ar e.
In laboratory studies using Desulfovibrio desulfuricans, 2205 DSS was r eported (Anton y et al. 2007 ) to experience etching, pitting, and cr e vice attac k after 40 days exposur e in a c hloride-containing medium.Another study (Machuca et al. 2012 ) of DSS in natural sea water showed cr e vice corr osion onl y occurr ed in samples that wer e electr oc hemicall y polarized (Mac huca et al. 2012 ).Nic kelc hr omium-mol ybdenum alloys and titanium have not been reported as being susceptible to MIC under field conditions, at least based on the liter atur e r e vie w performed her e .T here is , ho w e v er, one exception to this in environments containing oxygen.In surface waters and sediments containing oxygen, se v er al micr oor ganisms can oxidize dissolved manganese to form enric hed miner al-biopol ymer de posits.De posits of manganese o xides, when formed on SS and CRA, are highly cathodic and result in localized potential differences that can drive severe corrosion (Lew ando wski and Hamilton 2002 ).These deposits can be thin and brittle, resulting in fine cracks in the scale that act as crevices wher e corr osion is driv en by the lar ge corr osion potentials (E corr ) between manganese oxides and the exposed metal.Although the corrosion in this example is not directly caused by microorgan-isms, the mineral scales resulting from their activity resulted in localized corrosion by shifting E corr .
Copper-nickel and nickel-based alloys have been used successfully in flowing, aerated seawater service, although MIC has been reported in some cases, particularly where flow is stopped for extended periods of time (Javed et al. 2016a ).One study (Little et al. 1990 ) discussed se v er e corr osion of copper-nic kel (88.5% copper, 10% nickel, and 1.5% iron) piping after 1 year of service and nickel alloy (66.5% nickel, 31.5% copper, and 1.25% iron) after six months of service in stagnant estuarine water from the Gulf of Mexico.In both cases, localized corrosion was found under biofilms containing SRB.
The susceptibility of different materials to MIC has been investigated by many researchers under laboratory conditions .Ja ved et al. ( 2020) r e vie wed 26 pa pers wher e MIC pitting was claimed to hav e been observ ed in labor atory tests on SS alloys, including 304, 316, 2205, and other alloys .T he work concluded that the pits that formed as a result of the dissolution of inclusions (during cleaning) were comparable in shape , size , and depth to the pits that have been reported (possibly incorrectly) in the literature as indications that MIC attack had taken place on SSs.In another study, Javed et al. ( 2016b ) demonstrated that the chemical composition and micr ostructur e of differ ent gr ades of CS influenced initial bacterial attachment and subsequent corrosion in the presence of E. coli .The w ork sho w ed that the number of attached bacterial cells was different for different grades of CS and decreased with increasing pearlite phase content of the CS.
Another topic worth noting is the potential for metallurgical featur es suc h as inclusion content and surface roughness to affect biofilm establishment and corrosion rates.One industry study (Blythe and Gauger 2000 ) of welded CS found that: r No corr elations wer e found r egarding the effects of surface finish on the se v erity of MIC and the relationship between colonization versus the inclusion content and composition of the steels tested.
r Steels with lo w er inclusion content and few er sulfide inclu- sions consistently sho w ed lo w er corrosion rates in the testing, e v en though colonization was similar to other steels.
r Micr oor ganisms did NOT pr efer entiall y attac k MnS inclusions in the test.
r SRB wer e not r equir ed to cause MIC, although they increased the se v erity of the attac k.
Other w ork, ho w e v er, has indicated a link between the location of manganese sulfide inclusions in CS and localized pitting attack when samples were exposed to SRB (Avci et al. 2013, Avci et al. 2018 ).
While the use of CRA with a high resistance to localized pitting is a possible a ppr oac h to help avoid MIC, it is not economical in most cases.As a result, most oil and gas operations rely on CS as the primary material of construction.Some adv anta ge can be gained, ho w e v er, in selectiv el y a ppl ying CRA wher e the threat of MIC is the highest.It is fairly well established, e.g. that areas of dead legs in piping are more susceptible to MIC than pipeline sections that normally experience flow.A number of schemes for assessing and ranking the threat of MIC have been published (Wolodko et al. 2018 ).The threat of MIC in dead legs can be managed by material selection in the design sta ge, r etr ofitting CS with CRA, or eliminating the environment that promotes MIC.Produced w ater, seaw ater, and fir e water systems ar e also highl y susceptible to MIC.Non-metallic components (i.e.epoxy composite piping, etc.) can be considered where pressures , stresses , and fir e r esistance r equir ements allow alternativ es to metals .Abo veground piping for saltwater disposal systems , e .g. is sometimes constructed using fiber-reinforced plastic (FRP).
The application of CRA in equipment or piping that is highly susceptible to MIC can be made more economical using CRA-clad CS or limiting the use of CRA to only the most susceptible locations that cannot be tempor aril y isolated, cleaned, and c hemicall y treated.Limiting the extent of MIC-susceptible equipment that cannot be cleaned, flushed, and treated is another way to reduce the need for CRA.
Internal coatings and linings of CS equipment are other appr oac hes that can be used to avoid contact between the environment and material, at least for a finite period, i.e. the life of the coating.The use of high-density polyethylene liners in short sections of piping may also be a viable alternative .P otential issues with internal coatings and linings are damage from heat or rapid depr essurization, mec hanical dama ge during oper ation or maintenance, the absence of coating on tie-in welds, and a lack of insight for inspection site selection due to the presence of coating.

Welds and MIC
One of the well-documented failure modes for MIC is the r a pid attack of weld regions, with widespread reports of throughthickness pitting in the timescale of months .T here are many examples of such failures, which often manifest as small pinholes on the surface with a large cavity in the weld region underneath, e.g.(Kearns and Borenstein 1991, Borenstein 1991a, 1991b, Jenkins and Doman 1993, Kobrin et al. 1997, Borenstein and Lindsay 2002 ).While some early reports suggested that the associated surface morphology may have been unique to MIC and hence a way of diagnosing the failure cause, other work has shown that similar surface pitting can be observed for non-biological corrosion (Thomas and Chung 1999 ).Problems have been reported with welds of different metal types, including SS , CS , and alumin um (Walsh 1999a ).A n umber of causes have been attributed to the accelerated corrosion of welds, including associated mi-cr ostructur e (Walsh et al. 1993, Sreekumari et al. 2001 ) and composition (Walsh 1999b, Shi et al. 2020 ), while there is some debate about how/whether surface roughness might be involved (Walsh 1999a, Sreekumari et al. 2001, Amaya et al. 2002, Liduino et al. 2018 ).The micr oor ganisms most associated with weld MIC ar e metal-oxidizing bacteria and SRB (Licina and Cubicciotti 1989, Ray et al. 2010, Liduino et al. 2018 , Lee and Little 2019 ).There have been some reports that weld post treatment, including annealing and a voiding/remo val of heat-tinted scale (e.g.gas shielding during welding and pickling), can help to reduce these problems (Stein 1991, Borenstein 1991a, Pytlewski et al. 2001, Davis 2006, Ehrnstén et al. 2019 ).While these measures may help avoid MIC problems, it is important to note that there can be some practical difficulties in implementation (Hurh et al. 1999, Ehrnstén et al. 2019 ).
Lastl y, ther e ar e a number of important points to r emember in relation to metals when performing laboratory studies of MIC.As discussed abo ve , there are numerous factors that can affect the likelihood and extent of MIC for a particular metal type .T his includes (but is not limited to) surface finish, specific chemical composition, and microstructure.Researchers should be conscious of these factors and make sure that they design tests accordingly and provide detailed information on these aspects so that the tests can be compared appr opriatel y and ar e r epeatable.A list of examples of tec hniques that can be used to provide important information on metallurgicall y r ele v ant pr operties r elated to MIC studies is provided in Table 5 .
As discussed earlier, MLOE is r equir ed (micr obiological, metallur gical, and media c hemistry) to be able to distinguish between MIC and abiotic corr osion.Ther e ar e no specific rules about whic h exact analysis methods need to be used, and will likely depend upon what methods/instruments ar e av ailable, costs, and an y specific information needed that might be related to particular corr osion pr ocesses of inter est.In gener al, the use of m ultiple tec hniques to anal yze eac h of the micr obiological, metallur gical, and chemistry aspects can be beneficial; ho w ever, care and skill are needed to ensure that each test type is performed and anal yzed corr ectl y.Finall y, contr ol tests should be considered as a baseline comparison where possible .For example , it is critical to perform the same tests for a site with similar environmental conditions that has no signs of MIC as the location where MIC is suspected.

Silos-overcoming barriers to interdisciplinarity in MIC studies
Ther e ar e a n umber of barriers that mak e ac hie ving true interdisciplinarity in MIC studies a challenge.As described earlier, each discipline typically exists in a relatively siloed environment where other disciplines are acknowledged but with whom regular dialog is r elativ el y limited.Eac h discipline has its own unique langua ge and worldvie w, whic h complicates tr anslation between differ ent disciplines.Ev en differ ent sectors within a discipline may exist in silos , e .g. microbiology in human health vs. microbiology in industrial settings.For example, there has been very little translation of learnings from the biodeterioration of medical implants to microbial corrosion under non-medical conditions.Different disciplines and sectors also have different motivators and available resources that drive research and collaboration.On the industrial side , e .g. oilfield microbiological research around souring and corrosion has historically received much greater financial support than microbiological issues in, e.g. the pulp and A case study by Dubilier et al. ( 2015 ) discussed a global effort by scientists studying the Earth's micr obiome, wher e after ten years of work it was found that most of the data collected from different labs were not comparable because of differences in the test platforms used, the PCR primers selected, reporting formats , etc. T his demonstr ates that e v en for high-priority pr ojects with a great deal of potential to impr ov e human health, ther e is a great challenge to get all the various participants on the same page to achieve a successful conclusion.Ledford ( 2015 ) discussed one cause for the general lack of interdisciplinarity as being organizations' "un-derestimating the depth of commitment and personal relationships needed for a successful interdisciplinary project."It is likely that anyone who has experienced research projects that were run successfully and collaboratively can identify a core group of leaders in the project who promoted open technical exchange and w orked w ell together as a team because of their personal commitment and value placed on relationships.Advancing interdisciplinary collaboration in the area of MIC will be essential to futur e pr ogr ess in mana ging this integrity threat and increasing the sustainability of assets, particularly as used in r ene wable ener gy production.

Labor a tory models for microbial corrosion studies
MIC has been studied for over a century, with an explosion of publications emerging in the past 20 years (Lekbach et al. 2021 ).As micr oor ganisms ar e essentiall y e v erywher e, including associated with man-made infr astructur e, MIC has been studied across many sectors that include marine systems (e.g.shipping and marine infr astructur e), ener gy systems (e.g. oil and gas), and in both domestic and industrial water and w astew ater systems.As suc h, differ ent models hav e been used for studying MIC and its potential threat to infr astructur e (Fig. 2 ).It m ust be noted that the outcome of tests with such models will be influenced by multiple factors related to the test set-up and micr oor ganisms used; as indicated in se v er al sections abo ve , the micr oor ganisms , metal types , chemical en vironments , and operating conditions will affect whether MIC occurs .T he effects of experimental conditions have been discussed by a number of authors pr e viousl y (e.g.Wade et al. 2017, Salgar-Cha parr o et al. 2020a ,b ).The focus of this section, ho w e v er, is to r eview how the choice of microorganism(s) used may influence MIC tests.
By far, most laboratory-based MIC studies have used pure cultur es of micr oor ganisms (Lekbac h et al. 2021 ), but mor e and mor e studies are emerging wherein defined mixed cultures and complex field samples are also being studied to help ground-truth pur e cultur e studies (Salgar-Cha parr o et al. 2020a, Puentes-Cala et al. 2022, Sharma et al. 2022 ).Whether a pure culture, a defined mixed culture, or a complex model system is used to study MIC depends lar gel y on the goals of the study.It m ust be emphasized that all a ppr oac hes can yield v aluable information but ar e also associated with limitations that should always be k e pt in mind when making conclusions about MIC.

Single species models
A list of ∼50 different pure microorganisms associated with metal corr osion (primaril y using CS or SS) was r ecentl y tabulated (Lekbach et al. 2021 ), and while many more are likely to be identified, it gives an indication of the diversity of taxa (both aerobic and anaerobic) that can be involved in MIC.For example, under aerobic conditions, Pseudomonas sp. has been studied the most fr equentl y, while under anaerobic conditions, strains of sulfate-reducing micr oor ganisms suc h as Desulfovibrio sp. have been the most widely used (Lekbach et al. 2021 ).While the major limitation in using pur e cultur es to study MIC is that they ar e not necessaril y r eflective of, nor participants in, r eal-world corr osion scenarios, studying MIC using pur e or ganisms allows for highl y contr olled studies to better understand the behaviors and mechanisms of MIC.For example, experimental systems of any type (e.g. using EC techniques , bioreactors , weight loss experiments, etc.) can be estab-lished in the presence and absence of the pure culture of interest, and differences in metabolic indicators (such as electron donors and acceptors), EC signals, corr osion pr oducts, surface anal yses, etc. can be determined between the live and control incubations (e.g.Tsurumaru et al. 2018, Tang et al. 2019, Lekbach et al. 2021 and r efer ences ther ein).
As discussed earlier, obtaining MLOE e v en in pure culture MIC studies helps to provide the strongest case of whether microorganisms contributed to a corrosion scenario.Notably, pure culture studies also allow for the simplest inter pr etations of any MMM that may be used to tr ac k micr obial metabolism in a corrosion case, such as through transcriptomic , proteomic , or metabolomic a ppr oac hes, a gain compar ed to a non-corr osion scenario.These types of a ppr oac hes can potentially help to elucidate a target gene, protein, or metabolite that may be indicative of MIC.For example, if specific genes are upregulated during a corrosion versus a non-corrosion scenario, the expression of these genes may be important for MIC to occur.Ultimatel y, cr eating m utants wher ein these genes are deleted and corrosion no longer occurs is a strategy that might be able to be used for linking specific genes/gene expression to MIC (Lekbach et al. 2021 ).For example, a gene deletion a ppr oac h was used to help pr ovide e vidence that a corr osiv e methanogen ( Methanococcus maripaludis strain OS7) uses an extracellular [NiFe] hydrogenase in MIC (Tsurumaru et al. 2018 ).Subsequently, a qPCR assay was developed to quantify this gene ( micH ), which could be detected in corrosive but not in non-corrosive biofilms established from oil field samples (Lahme et al. 2021 ).A gene deletion a ppr oac h was also used to help pinpoint that Geobacter sulfurreducens could corrode Fe 0 by using it as a sole electron donor (Tang et al. 2019 ), as well as to suggest that Shewanella oneidensis strain MR-1 can corrode CS both directly and through hydr ogen-mediated electr on tr ansfer (Hernández-Santana et al.

Defined mixed species
In the r eal world, micr oor ganisms exist in most environments in the form of complex multispecies consortia.MIC can occur due to both planktonic and surface-attached microorganisms and their metabolic by-products .T he compositions of the microbial consortia in a particular location will be affected by a variety of biotic and abiotic par ameters (e.g.temper atur e and other physicoc hemical pr operties, nutrient suppl y, fluid mixing, etc.) (Fuhrman et al. 2015 , Dang andLovell 2016 ).In relation to the attached/biofilm versions of microbial consortia, it is generally acknowledged that the micr oor ganisms attac h and form a biofilm in a sequence and that the creation of a biofilm can offer overall benefits to the community, such as enhanced resistance to stress and disinfectants (Bridier et al. 2011, Schwering et al. 2013, Burmølle et al. 2014 ).The presence of different microbial species in a consortium can lead to interspecies cooper ation wher e , e .g. certain species ma y pro vide nutrients or create habitats that are essential for other species.In relation to MIC, aerobic biofilm formers may attach early and cr eate anaer obic nic hes that ar e suitable for anaer obic species (such as Desulfovibrio sp.) that have been implicated in accelerated corrosion.Multispecies models have been developed to simulate envir onments suc h as or al biofilms (Kommer ein et al. 2018 ); ho w e v er, ther e ar e man y c hallenges involv ed, suc h as determining whic h species/c har acteristics should be included, the order of inoculation and nutrients, and other environmental conditions (e.g.flow and redox poising) (Foster and Kolenbrander 2004, Røder et al. 2016, Tan et al. 2017, Olsen et al. 2019 ).There has been some work performed on defined multispecies models for MIC studies, but aside from a few cases, it has typically been limited to combinations of two bacterial species (Phan et al. 2021 , and r efer ences therein).This is an understudied area with potential for much futur e r esearc h to better understand the fundamental processes involved in MIC when more than one microorganism is present and to pr oduce m ulti-species models that better sim ulate the r ates and types of acceler ated micr obial corr osion observ ed in the field.

Real-world consortia
The final type of model system that can be used to study MIC is one that uses samples tak en, or contin uousl y sampled, fr om the field as the test medium or as inoculant for the test system.This a ppr oac h can pr ovide conditions most closel y r epr esenting the real world.An example of this is the work of Lee et al. ( 2004 ), wher e natur al seaw ater w as used as the test medium for an MIC study.Changing the test conditions in this example system (creating a sta gnant anaer obic solution) resulted in increased numbers of SRB present and led to more aggressive corrosion.In another example, Marty et al. ( 2014 ) reported a corrosion test reactor system that utilized natural marine microbial consortia, was capable of simulating tidal changes, and was able to supply a continuous flow through the test system of natural seawater.Changes in test conditions (e .g. pro viding an initial pulse of organic matter) w ere sho wn to lead to incr eases in localized corr osion r ates, and the identification of similar bacterial populations to those identified in accelerated lo w-w ater corrosion suggests that the system can be used to simulate real-world marine conditions.In another example, Wade and Blackall ( 2018 ) used samples of accelerated lo w-w ater corr osion pr oducts as the micr obial inoculum in corrosion tests and varied the testing conditions .T he results obtained sho w ed ho w changing the specific test conditions (e.g.b y ad ding n utrients) can affect both the magnitude of corrosion that takes place and the microbial community that develops.A k e y issue from these types of studies is that taking the microbial samples out of the field changes the environmental conditions and hence affects the test outcome in some way.Salgar-Cha parr o et al. ( 2020b ) sho w ed for tests using microbial consortia sampled from floating production storage and offloading facilities that changes in supplied nutrients affected biofilm properties and subsequent corrosion.Studies using real-world consortia with minimal alter-ation have the least control over the specific microbial species present and suffer from increased difficulties in terms of reproducibility.Additional studies that minimally alter the conditions of the samples being tested (e .g. by a voiding nutrient additions or changing the water-to-solids/biofilm ratio) are also needed to help better understand MIC under r ealistic, r eal-world conditions in multiple environments (Wade et al. 2017 ).
Laboratory models are an integral part of the overall efforts to tackle the challenges associated with MIC.They can provide k e y information on critical aspects such as the fundamental processes and microorganisms involved, the performance of materials and mitigation methods, as well as a means for MIC dia gnosis.Micr obiologists ar e well placed to offer leadership and guidance on many facets of future MIC laboratory model de v elopment.

Field (meta)data collection and standardization
To date, there has been limited success on predicting MIC problems and e v aluating potential mitigation str ategies.Significantl y more work will be required to achieve effective and tailored anti-MIC measures.An example of one of the k e y challenges that remains to be addressed and overcome is the lack of r eadil y av ailable k e y data re positories , i.e .field (meta)data collections r ele v ant to industrial a pplications, suc h as biobanks of biofilm samples and MIC samples (e.g.materials with MIC, environmental, and metallurgical data).This requires the development of standard data collection and assessment protocols to ensure consistency and allow a ppr opriate anal ysis and comparisons to be made.Suc h r epositories ar e critical for incr easing knowledge and pr omoting ne w adv ances in the field, which potentially may be enhanced by integrating artificial intelligence and machine learning techniques (Goodswen et al. 2021 ).Suc h tools ar e essential for modeling and predicting MIC scenarios , disco vering MIC markers and biosensors, and de v eloping standards.
In this context, de v eloping standardization of measur ement pr ocedur es, r ele v ant pr otocols (e.g.sample pr eserv ation), v alidation tests, and methodologies is an essential step to w ar ds impr ov ed MIC mitigation.Standardization helps to ensure that MICrelated assessment tests are accurately cataloged, allowing them to become comparable or able to be correlated, thus leading to a more comprehensive understanding of MIC and MIC control str ategies.Unfortunatel y, ga ps in the field continue to delay the de v elopment of universal standards.For example, there is still a significant lack of translation of small-scale r esearc h labor atory experiments to a field scale .Likewise , there ha ve been only limited efforts to de v elop well-v alidated models (physical and theor etical) that sim ulate the complex r eal-world conditions .T hese efforts are essential tools for standardization and the development and assessment of mitigation solutions, which could save time and resources before the final validation stage.Further work is also r equir ed to de v elop standards r ele v ant to or adopted by legislation or regulatory assessment (e.g.standards to assess the efficiency and effectiveness of biocidal mitigation strategies) that mor e closel y matc h r eal-world conditions .Efforts ha ve been made in specific fields to overcome this gap (Skovhus 2014, Silva et al. 2021 ), particularly with the introduction of MMM (Skovhus 2014 ).Ev en so, most MIC r esearc hers use pr otocols or methodologies ada pted fr om inaccessible or expensiv e or ganizational standards (e.g.ISO, ASTM, and NACE) to e v aluate their a ppr oac hes or tec hnologies, whereas industry uses the available organizational standards or de v elops its own (Skovhus et al. 2017, Silva et al. 2019, Wade et al. 2023 ).

Corrosion management
MIC is regarded as a difficult-to-treat industrial "cancer" (World Corr osion Or ganization (WCO) Shen yang Declar ation, 2019 ), resulting in se v er e economic losses and underestimating long-term environmental and societal impacts (Usher et al. 2014, Conley et al. 2016, Di Pippo et al. 2018, Jia et al. 2019, Stamps et al. 2020, Little et al. 2020b, Lou et al. 2021 ).It has undoubtedly become vital to not only understand the MIC phenomenon but also how to control it effectively.To date, a range of methodologies and technologies have been designed, developed, and implemented to control microbial activity and thus reduce the threat of MIC (Fig. 3 ).The c har acteristics of eac h system and field envir onment will dictate the selection of a specific countermeasure, whether based on removal and/or preventive strategies.
In industry, the corrosion control process typically consists of three primary activities: (1) identifying the r ele v ant corr osion threats; (2) identifying preventive and mitigative measures to address those threats; and (3) monitoring the effectiveness of the response .T he cycle of activities is continuous, with each of the three activities providing input to the subsequent activity.Information about a system's microbiology is typically needed in each of the thr ee corr osion contr ol activities, and MMM is incr easingly being used to provide that information.Ho w ever, corrosion engineers also need a way to corr elate micr obiological information with other r ele v ant information, suc h as data fr om corr osion monitoring (e .g. coupons , probes , and inspection), operating conditions (e.g.pr essur e, temper atur e, and fluid velocity), fluid composition and chemistry, mitigation measures, etc.Such an a ppr oac h is consistent with the use of MLOE, as described earlier in this r e vie w.The following section briefly describes each of the thr ee corr osion mana gement activities that ar e emplo y ed to manage internal corrosion on various types of assets in different sectors.

T hrea t assessment
During the corrosion threat assessment stage, the potential for each plausible corrosion threat mechanism is evaluated.Corrosion engineers typicall y r e vie w data about the asset design and ov er all pr ocess, oper ation, c hemical tr eatment, corr osion moni-toring data, and leak/failure history data to help identify corrosion threats .T he potential damage rate of some threats, such as corrosion caused by acid gases, can be estimated using mathematical models; ho w e v er, ther e ar e pr esentl y no widel y accepted corr osion rate models for MIC since microorganisms can influence corrosion in man y differ ent wa ys .In assessing the potential for MIC, the corr osion engineer typicall y looks for a r elationship between the microbiological and chemical conditions and any observed corrosion information.Data produced using MMM are used in this step to c har acterize baseline microbiological conditions in the asset and to look for associations between biofilm community distribution, chemical composition, and the frequency, distribution, and severity of localized corrosion.The threat assessment may also seek to relate biofilm and corrosion characteristics to operating conditions, such as changes in flow (e.g.periods of no flow), temperature, or fluid composition (e.g.increases in nutrients or electron acceptors).Significant operating condition changes may affect the initiation and/or pr opa gation of MIC.A number of in vestigators , such as Skovhus et al. ( 2010 ), Eckert et al. ( 2012 ), and Larsen and Hilbert ( 2014 ), have demonstrated the utility of MMM in forensic corr osion inv estigation, wher e methods suc h as next-gener ation sequencing and metagenomics could provide insights.

Mitigation and prevention
Based on the threat assessment, the preventive and mitigative measures needed to manage the applicable corrosion threats are selected.Options for internal corrosion mitigation in pipelines include the use of biocides, corrosion inhibitors, or oxygen scavengers; v elocity contr ol; mec hanical cleaning (e.g.pigging or flushing); ultr aviolet r adiation; fluid pr ocess v essels (e .g. filters , separ ators, etc.); or contr ol of fluid quality (or sources) to the extent possible.Larsen et al. ( 2010 ) demonstrated ho w MMM w ere beneficial for e v aluating the effectiv eness of ne w c hemical tr eatments when corrosion incidence rate and severity are linked with observations about the types , numbers , and activities of microorganisms after the treatment is applied.One of the most significant challenges to this process is the collection of biofilm samples from the asset being treated and the processing/analyzing the samples in a timely manner so that genetic information is not lost.Another significant challenge is the current lack of standards to assess the efficiency and effectiveness of MIC mitigation strategies based on micr oor ganisms in biofilms or MIC diagnosis and monitoring methodologies, as the conditions promoting MIC may be quite different from system to system.
In terms of MIC mitigation, most conventional strategies comprise physical and/or chemical methods.Mechanical removal or cleaning of surfaces is the most straightforw ar d physical appr oac h, comprising an y method able to r emov e the biofilm attached on a surface , in volving those using mechanical forces (e.g.pigging, flushing, ultr asonic tr eatment).Ho w e v er, this is not the optimal a ppr oac h for MIC contr ol as it does not pr e v ent further biofilm formation, demanding costly ongoing maintenance and r etr ofitting measur es .For example , once a surface is in contact with seawater, a biofilm can form in minutes and pr ogr ess to macrofouling in just a few da ys , which would require frequent maintenance, rendering it an unsustainable mitigation strategy (Omar et al. 2021, Silva et al. 2021, Yazdi et al. 2022 ).
The most effective countermeasures currently adopted to control biofilm development and minimize MIC on industrial surfaces r el y on a c hemical str ategy that comprises the dir ect or contr olled r elease of biocides onto the contaminated surface .T heir use is promoted on the basis that disinfection, or killing microbial cells, will solve the problem.Ho w ever, inefficient cleaning of organic matter remaining on the surface and inadequate monitoring strategies, allied to a lack of skilled MIC professionals, can actuall y pr omote an incr ease of MIC pr oblems .T hus , chemical strategies are generally integrated with other methods, such as pr otectiv e pol ymeric coatings, cathodic pr otection (CP), UV irr adiation, mec hanical cleaning, or ultr asonic tr eatment.Among those, antifouling coatings containing active agents , i.e .biocides and corrosion inhibitors are one of the most well-established prev entiv e measur es (Abdolahi et al. 2014, Cai et al. 2020a, Chen et al. 2022, Lamin et al. 2022, Wen and Li 2022 ).A significant disadv anta ge of these coatings, ho w e v er, is the continuous release of toxic and persistent c hemicals, r esulting in shorter protection periods and potential ecological problems (Rosenberg et al. 2019  Other greener or less to xic alternati ves with enhanced effects have also emerged.From coating strategies based on the development of polymer structures to create or improve properties such as hydrophilicity , amphiphilicity , surface topogr a phy, non-biocide-r elease mec hanisms and/or the incor por ation of bioactive nanoparticles to generate nanocomposite coatings (Selim et al. 2020 ;Gu et al. 2020 ;Kumar et al. 2021 ;Sousa-Cardoso et al. 2022 ), to the search for natur e-inspir ed biomimetic and synthetic agents to natural bioactive compounds or extracts (e.g.metabolites from marine organisms, molecules of microbial origin, plants) (Vilas-Boas et al. 2021 ; Lav an y a 2021 ).Ho w e v er, the full exploitation of these gr eener a gents is limited by long synthesis processes, low yields, the scarce availability of some natur al sources, the lac k of pr oof of concept in real-world conditions, the absence of an environmental impact assessment, as well as the need for significant funding and time for a ppr ov al by regulatory agencies (Qian et al. 2009, Brinch et al. 2016, Pai et al. 2022 ).
Another of the commonly discussed methods for MIC mitigation, used for a range of structures such as buried and submerged pipelines, stor a ge tanks, and sheet piling, is the application of CP (Wilson and Jac k 2017, Ac kland and Dylejko 2019, Angst 2019 ).This technique involves the application of a direct current (via a galvanic or impressed current system) to lo w er and maintain the potential of the metal sufficientl y negativ e with r espect to the environment.CP is a w ell-kno wn and widel y a pplied method for abiotic corrosion, and it is often discussed that a further lo w ering of the protection potential from that used for abiotic corrosion ma y pro vide protection against MIC.While field tests and anecdotal reports indicate CP may be capable of pr e v enting acceler ated corrosion due to microorganisms, the conclusions of laboratory studies ar e m uc h less certain, and ther e is r oom for m uc h mor e work on this topic to understand the mec hanisms involv ed and how to optimize its use for avoiding/minimizing MIC (Thompson et al. 2022 ).

MIC mitigation challenges
Similar to MIC r esearc h in gener al, mitigation str ategy de v elopment is also gr eatl y affected from the initial design stage to the final implementation by the siloed nature of this field.Figure 4 summarizes and highlights some of the most important challenges that need to be overcome in order to allow successful MIC mitigation str ategy de v elopment and implementation.To ac hie v e this, the following k e y questions need to be answered: (i) What are the challenges/knowledge gaps to control MIC? (ii) What are the current needs for the de v elopment/implementation of anti-MIC strategies?(iii) What tests and metrics are appropriate to evaluate the effectiveness of an anti-MIC strategy?

Understanding the biofilm community interactions with the environment and surfaces
Biofilm and subsequent MIC are driven by environmental conditions, either natural or under industrial operating conditions, involving ecological and engineering factors.Understanding and identifying the role of micr oor ganisms in MIC is a big challenge, as the composition of the biofilm matrix and its dynamic structur e will v ary depending on those conditions, and contact with differ ent infr astructur e materials, r esulting in metabolic ada ptations in response to their long-term survival under external stress conditions (Jia et al. 2019 ).Cells incor por ated within a biofilm, e.g.show a high tolerance to treatment compared to planktonic cells.
In an extreme scenario, this can result in an increase of resistance to antimicrobial agents of 1000 times (Mah et al. 2003 ).It is also recognized that the response of the biofilm community to environmental conditions cannot be predicted by studying free-living bacteria or single-species biofilms alone (Flemming et al. 2016 ).
For the initial step of designing a mitigation strategy, these studies are nevertheless useful for a screening task, but multi-species studies are even more critical to improve the design.This fundamental understanding of the complex properties of biofilm communities and their interaction/development with different environments, including biota and conditions fluctuations from static and quasi-static to dynamic flow conditions (Toyofuku et al. 2016 ), remains limited, and further advances in the design of effective mitigation countermeasur es ar e desir ed.This pr ogr ess is hampered further by a lack of understanding of surface-biofilm interactions and their heter ogeneity, whic h pr omote localized gr adients and micr oenvir onments acr oss the surface (Ren et al. 2018 ) and may involve multiple microbial mechanisms.
Understanding how surface and biofilm structures and their physicoc hemical pr operties inter act, e.g.pr ovide answers on which factors contribute to biofilm structure and composition and how the multi-species system interacts is critical for developing a better strategy against MIC and avoiding the implementation of mitigation actions when they are not needed (Skovhus et al. 2022 ).

Limited and fragmented knowledge on mitiga tion str a tegies
Problems due to the resistance of biofilms to treatment can hamper the effectiveness of mitigation strategies, particularly those involving the release of active agents such as corrosion inhibitors and bioactiv e a gents (e.g.biocides and biocide-r elease coatings).Biofilm resistance is related to the complex three-dimensional functional structure biofilms, which limits the penetration of bioactiv e a gents and pr e v ents them fr om inter acting with other cells, particularl y for matur e biofilms (Bas et al. 2017 , Merc hel Piov esan Per eir a et al. 2021 ).This is complicated by the complex processes by which bioactive agents interact with biofilms , in volving biological and physicochemical factors, and the exact degree, fr equency, and mec hanisms that give rise to r esistance ar e still unclear.
Bioactiv e a gents ar e commonl y selected based on the following criteria: the spectrum of action/efficacy , toxicity , biodegradability , cost-effectiveness , en vironment safety, and compatibility with the system, i.e. allow the maintenance of fluids and materials under oper ational conditions.Furthermor e, the mode of action depends on the type and dose of bioactive agent used (Sharma et al. 2018, Capita et al. 2019 ).Ho w ever, their long-term use can promote the r esistance of micr oor ganisms, leading to an ineffectiv e inhibition effect.
The use of corrosion inhibitors is another simple and potentially efficient mitigation strategy.A diverse range of chemical molecules acting as corrosion inhibitors has been exploited, mainly including surfactants and heterocyclic organic compounds containing electricall y ric h heter oatoms (N, O, and S) or gr oups with π -shar ed electr ons (Feng et al. 2022 ).Those primarily inhibit corrosion by adsorbing on metal surfaces by physical adsorption (V ander W aals force adsorption) or c hemisor ption (c hemical bonding), creating a physical or chemical barrier between the surface and the corr osiv e media, hence inhibiting cell adhesion and subsequent biofilm formation (Migahed and Al-Sabagh 2009, Kokalj 2022, Ma et al. 2022 ).
Combining corrosion inhibitors with bioactive agents has also been a common strategy to find synergistic effects (Greene et al. 2006, Pinnock et al. 2018, Anandkumar et al. 2023 ).Ho w e v er, it can sometimes lead to interferences affecting agents' performance, suc h as c hemical incompatibility (e.g.c hemical and physical inter actions, pH r ange of action) and competitive function (e.g.adsorption for the same metal sites), reducing their primary function and resulting in inadequate control of MIC (Maruthamuthu et al. 2000, Xiong et al. 2015, Rahmani et al. 2016 ).To avoid interfer ences, these a gents need to be car efull y selected, not onl y considering standard criteria like the ability to oxidize the metal, the presence of a particular functional group, the capacity to cover a wide ar ea, cost-effectiv eness , solubility, and en vironmental safety (Lav an ya 2021 ), but also the conditions present throughout the entire system.
Certain corrosion inhibitors can also interact with biofilms and impair their structure and functionality.For example, positively c har ged heter ocyclic quaternary ammonium salt surfactant can selectiv el y adsorb on the negativ el y c har ged SRB biofilm surface and penetrate the cell membrane, disrupting their selective permeability and genetic system, thus leading to the inhibition of SRB activity or death (Feng et al. 2022 ).This shows the ability of synthetic corrosion inhibitors to also provide antimicrobial effects .T his multifunctional ability has been r eported particularl y for cationic surfactants, including Gemini and poly (quaternary ammonium) salt surfactants (Badawi et al. 2010, Labena et al. 2020, Feng et al. 2022 ).The effectiveness of these mechanisms, ho w e v er, depends on the specific system's conditions and the micr oor ganisms involv ed.In some cases, they may e v en become ineffecti ve (Dari va andGalio 2014 , Mand andEnning 2021 ) or act as a source of nutrients for bacterial growth (Edwards andMcNeill 2002 , Fang et al. 2009 ) Ther efor e, it is crucial to fill the knowledge ga p r egarding the mec hanisms of action of corrosion inhibitors and bioactive agents, as well as their effects on the development and resistance of biofilms (Bridier et al. 2011 , Bas et al. 2017 , Kimbell et al. 2020, Tuck et al. 2022 ).Despite progress over recent y ears, kno wledge is still scar ce and fr a gmented (Ar aújo et al. 2014, Huang et al. 2020, Silva et al. 2021, Lima et al. 2022 ).
Furthermor e, similarl y to bioactive agents, the long-term use and toxic c har acteristics of synthetic corr osion inhibitors calls for more work on sustainable and environmentally friendly agents deriv ed primaril y fr om natur al sources (Lav an ya 2021 , Verma et al. 2021, Al Jahdaly et al. 2022, Fazal et al. 2022, Wang et al. 2023a ,b ).
The increasing discovery of greener and natural agents, including corrosion inhibitors and bioactive agents, with new chemical structures and functionalities is likely to uncover additional modes of action (Lav an ya 2021 , Barba-Ostria et al. 2022 ).This is likely to further improve our understanding of the mode of action of bioactive agents and how they interact with biofilms, which is essential for de v eloping mor e effectiv e mitigation str ategies.Artificial intelligence has been proposed as a potential tool to accelerate the identification of targets for novel active agents (Paul et al. 2021 ).
The gr owing cr oss-sector al awar eness of the economic importance of microbial biofilms has helped to accelerate the development of mitigation a ppr oac hes and tec hnologies as well as our understanding related to biofilm-bioactive agent interaction.Ho w e v er, some sectors possess more advanced knowledge than others .For example , the incr easing pr oblem of antibiotic r esistance is well known in the healthcare sector, while the marine sector has been a ppl ying anti-biofouling a ppr oac hes for some time.Regr ettabl y, anti-biofouling, anti-MIC, or anti-corr osion a ppr oac hes ar e r ar el y r elated, although a fe w r ecent publications have started to emphasize their similarities (Li and Ning 2019 ).This lack of sectoral and multidisciplinary knowledge shar-ing undoubtedly limits the potential for advances in mitigation strategies.
Finally, it is worth noting that the complete eradication of biofilms in most industrial situations is highl y unlikel y, as micr oor ganisms will al ways be pr esent.T hus , the economics and efforts r equir ed to meet suc h a stringent target need to be car efull y questioned.A mor e r ealistic goal is to learn how to coexist with and manage the presence of biofilms, minimizing unwanted interferences in the most efficient, benign, and long-term manner possible .Furthermore , MIC management extends beyond single solutions.Rather, integr ated a ppr oac hes should be used, le v er a ging multidisciplinary teams and cross-sectoral knowledge sharing.

Monitoring
The third core activity in the corrosion control process is measuring the system performance and effectiveness of the methods used to reduce the likelihood and/or severity of corrosion.Various corrosion monitoring techniques and inspection methods can provide information about the rate of metal loss due to corrosion (Bardal 2004, Dawson et al. 2010, N ACE 2012 ); ho w e v er, man y of these methods do not identify the mechanism of the corrosion or the effects of mitigation activities on the cause of the corrosion, biofilms for MIC.Again, the MLOE a ppr oac h is useful for e v aluating the effectiveness of mitigation measures and optimizing these measures as system operating conditions and corrosion threats c hange ov er time (see Fig. 1 ).For MIC mitigation, monitoring measur es ideall y need to be able to identify both changes in corrosion rates and microbiological changes in associated biofilms (Fig. 5 ).
Corr osion engineers typicall y attempt to integr ate MLOE, operating data, corrosion monitoring data, c hemical/micr obiological fluid and deposit analysis results, in-line inspection (ILI) and other inspection data, and flow/corr osion r ate model outputs to ascertain the short-term and long-term effectiveness of mitigation measures.Short-term effectiveness (i.e .o ver hours , da ys , or weeks) may be e v aluated thr ough differ ent par ameters or measurements than those used to monitor long-term (i.e.monthly or annual) effectiveness.For example, short-term effectiveness monitoring could focus on controlling microbial populations in biofilms, wher eas long-term effectiv eness would focus mor e on contr olling corr osion dama ge that r esulted fr om those biofilms.One significant MIC management challenge faced by industry is the lack of an established, widely accepted processes (or standards) for integrating MLOE into monitoring pr ogr ams.Often, micr obiological data ar e incor por ated into decision-making by inferring the activities and roles of micr oor ganisms in corr osion mec hanisms, and mitigation measures are adjusted based on empirical observations.
A k e y challenge in providing timely and effective anti-MIC measures is the establishment of early biofilm-specific detection systems suitable for in-situ and point-of-use industrial contexts (Xu et al. 2020 ).This could potentially include surface monitoring, regular chemical and microbiological analyses, and the use of probes, sensors , and MIC markers .Earl y pr ediction is critical during the initial and validation stages of mitigation strategy development (Fig. 4 ), as it allows the identification of specific locations with MIC threats as well as the e v aluation, tailoring, and implementation of a ppr opriate anti-MIC strategies.
Finally, one of the major problems with MIC prediction capacity is that the entire contextual story is not always reported or considered.For example, material engineers (e.g.metallurgists) may tend to ignore biological data, whereas microbiologists may not a ppr opriatel y consider materials/metallur gical aspects.Collecting r ele v ant data fr om all aspects (media conditions , fluids , micr oor ganisms, and materials) r equir es m ultidisciplinary teams composed of oper ators, micr obiologists, corr osion engineers , chemists , and materials engineers .Furthermore , for some, there has been a major stigma associated with r e v ealing MIC cases .Hence , data on MIC failures in industry is largely inaccessible to the broader R + D community, while MIC mitigation business agents often protect customers through commercial confidentiality a gr eements, significantl y limiting the av ailability of cases and the transfer of potential mitigation technologies between industry and academia.De v elopment of forums for the sharing of MIC case histories with the a ppr opriate le v el of detail so as to maintain the identities of those providing the information is one way to impr ov e knowledge sharing.

EC techniques used to study MIC
As noted earlier in this r e vie w, MIC is a m ulti-disciplinary field that r equir es expertise encompassing significantl y differ ent fields of study.The corrosion of metals (including MIC) is inherently an EC pr ocess wher e one or mor e c hemical species under go c hanges in o xidati v e state.Numer ous EC tec hniques hav e been de v eloped to mec hanisticall y study fundamental corrosion mechanisms in the laboratory, in addition to monitoring corrosion behavior in field conditions.As most of these methods are out of the scope of expertise of many microbiologists, we decided to dedicate a separate section to provide some basic background information about EC methods and the main techniques used in MIC assessment.Nonetheless, the authors highly encourage the collaboration of microbiologists with subject matter experts in corrosion and electrochemistry.All EC techniques have limitations in application and interpretation; thus, the choice of technique and the interpretation of results need to be car efull y weighted.
In the following sections, we aim to provide a more detailed ov ervie w about specific EC techniques that are beneficial during MIC studies .T he EC techniques ha v e been gr ouped based on the amount of external signal (e.g.applied potential or current) r equir ed during measurement.In general, the larger the exter-nal signal, the more information about the system can be obtained (Fig. 6 ); ho w e v er, a ppl ying a lar ger signal can also r esult in alterations to attached biofilm or substrate surface chemistry.The traditional EC cell is a three-electrode system containing: (1) the metal of interest (working), (2) a stable (i.e.non-polarizable) electr ode (r efer ence), and (3) a corr osion-r esistant metal used to complete the electrical circuit for external signal application (counter).Modifications on the number and types of electrodes are dependent on the EC technique being applied.The following is not an extensive review of EC techniques, rather those that are most commonly used in the design and monitoring of MIC experiments.

Techniques requiring no external signal
The following techniques do not apply current or potential signals to the working electrode .T hese techniques are used to monitor corrosion behavior, but ov erinter pr etation of the measurements is cautioned.

Corrosion potential, E corr
The simplest EC technique is the potential measur ement acr oss a two-electrode system immersed in an electrolyte; where one electrode is the material of interest and the other is a stable r efer ence electrode .T he potential is measured across a high-impedance voltmeter that pr e v ents curr ent flow between the electrodes .T his potential is called the E corr but is also r eferr ed to as the open circuit potential.Regardless of nomenclatur e, E corr measur ement is a passive monitoring method that does not disturb an attached biofilm.P assiv e metals suc h as titanium and gold exhibit higher E corr compar ed to mor e activ e metals such as zinc and aluminum.Biofilms can also affect E corr and make the inter pr etation of results difficult (Little and Wagner 2001 ).The most common utilization of E corr measurements has been the study of potential ennoblement.Ennoblement is the incr ease (i.e.mor e electr opositiv e) of E corr due to the formation of a biofilm on a metal surface (Little et al. 2013 ).Ennoblement of passive alloys exposed in marine environments due to biofilm formation has been extensively documented (Mollica and Tr e vis 1976, Johnsen and Bardal 1985, Scotto et al. 1985 ). Theor eticall y, E corr ennoblement should incr ease the probability for pitting and crevice corrosion initiation and propagation for those passive alloys where E corr is within a few hundred millivolts of the pitting potential (E pit ) (Fig. 6 ).Little et al. ( 2008 ) r e vie wed mec hanistic inter pr etations of ennoblement in marine waters.Ennoblement has also been shown to occur in fresh and estuarine waters through microbial manganese oxidation and deposition on the metal surface (Dickinson and Lew ando wski 1996, Dickinson et al. 1996, Dexter et al. 2003 ).While the ennoblement phenomenon has been observ ed thr ough the world under different water conditions, a unifying mechanistic explanation for all observations does not exist.The main dr awbac k of E corr measur ement is the inability to inter pr et whether ennobled E corr (or other changes in E corr ) are due to thermodynamic effects, kinetic effects, or both.In addition, E corr measurement alone cannot be used to determine changes in corrosion rates o ver time .Unfortunately, ho w e v er, ov erinter pr etation of E corr with respect to corrosion rates is commonly found throughout the literature.

Dual cell technique
The dual cell uses two similar EC cells that are separated by a semipermeable membr ane.Eac h cell contains the same electr ol yte and nominally similar working electrodes .T he two working electrodes are connected electrically to a zero resistance am-meter (ZRA), and the semipermeable membr ane pr ovides ionic conduction to complete the circuit.One cell is maintained under sterile conditions.Micr oor ganisms ar e added to the other, and the sign and magnitude of the resulting current through the ZRA are monitored to determine the details of the corr osiv e action of the bacteria.The dual cell technique does not provide a means to calculate corrosion rates, but rather changes due to the presence of a biofilm.Dexter and LaFontaine ( 1998 ) used a dual cell configuration to monitor the corrosion of copper, steel, 3003 aluminum, and zinc samples coupled to panels of highly allo y ed SS.Natural marine microbial biofilms were allo w ed to form on the SS surface.On the control tests, the action of the biofilm was pr e v ented.Corrosion of copper, steel, and aluminum anodes was significantly higher when connected to cathodes on which biofilms were allo w ed to grow naturally.

Electrochemical noise analysis (ENA)
EC noise has conv entionall y been a pplied to two electr odes of the same material.ENA data can be obtained with applied signal (i.e.fluctuations of potential at an applied current or vice versa).In addition, ENA can also be operated with no applied signal, where small fluctuations of E corr are recorded as a function of time.For MIC studies, the no-signal mode provides a monitoring technique that has a clear adv anta ge ov er the a pplied signal mode that may influence biofilm properties.Under controlled laboratory studies, it is possible to measure potential and current fluctuations simultaneousl y.Sim ultaneous collection of potential and current data allows analysis in time and frequency domains .T her e ar e numer ous par ameters that can be determined through data analysis with EC noise resistance (R n ) being the most commonly interpreted.To this day, the interpretation of R n to a quantifiable corr osion r ate is debated.Bertocci et al. (1997a ,b ) described methods for data analysis.Little et al. ( 1999 ) sho w ed an example of using ENA to examine the influence of marine bacteria on localized corrosion of a coated steel.Samples with intentional defects in the coatings exposing bare metal were immersed in artificial and natural seawater with and without attached zinc coupons , pro viding sacrificial CP to the exposed ar eas.R n incr eased with time for all cathodicall y pr otected samples due to the formation of calcareous deposits in the defects.Surface analysis sho w ed that very few bacteria were present in the defects of the cathodically protected samples, while large amounts of bacteria were found in the rust layers of the fr eel y corr oding samples.

Techniques requiring a small external signal
The following techniques require an external signal to be applied to the working electrode .T here are , ho w ever, no standar ds in regards to the magnitude of the signal a pplied, whic h is most commonly an applied potential.The majority of applications of these techniques in the literature apply between + /-5 and 10 mV to the working electrode.

Polarization resistance technique
The polarization resistance (R p ) technique is a direct current (i.e.no frequency dependence) method that can be used to continuously monitor the instantaneous corrosion rate of a metal, as detailed in ASTM G59-97 ( 2014 ) and r e vie w ed b y Scully ( 2000 ).Mansfeld ( 1976 ) described the use of the R p technique for the measurement of corrosion currents.
A simplification of the R p technique is the linear polarization tec hnique, in whic h it is assumed that the relationship between E and i is linear (i.e.resistance is a scalar value) in a narrow r ange ( + / −5 mV) ar ound E corr .The potential is scanned fr om -5 mV vs. E corr to + 5 mV vs. E corr at specified intervals (e.g. 1 mV) and scan rate (e.g. 1 mV/sec).The selection of these measurement parameters is dependent upon the electrolyte/metal system being examined (Mansfeld 1976, Scully 2000 ).The slope of the curve pro vides R p '. T he in v erse of R p (R p −1 ) is pr oportional to the corr osion r ate i corr .This a ppr oac h is used in field tests and forms the basis of commercial corrosion rate monitors (ASTM G96-90, 2018 ).Applications of R p techniques have been reported by King et al. ( 1986 ) in a study of the corrosion behavior of iron pipes in environments containing SRB.In a similar study, Kasahara and Kajiyama ( 1986 ) used R p measurements with a compensation of R s and r eported r esults for activ e and inactiv e SRB. Lee et al. ( 2004 ) used linear polarization measurements to demonstrate that corrosion of CS was more aggressive in stagnant anaerobic seawater than in stagnant aerobic seawater over a 396-day exposure.In general, instantaneous corrosion rates for the anaerobic condition were two orders of magnitude higher than the aerobic condition.
Significant errors in the calculation of corrosion rates can occur for electr ol ytes of low conductivity or systems with very high corr osion r ates (low R p ) if a correction for R s is not a pplied.Corr osion rates will be underestimated in these cases.Additional problems can arise from the effects of the sweep rate used to determine R p according to equation ( 1 ).If the sweep rate is too high, the experimental value of R p will be too low, and the calculated corrosion rate will be too high.For localized corrosion, experimental R p data should be used as a qualitative indication that r a pid corr osion is occurring.Large fluctuations of R p with time are often observed for systems undergoing pitting or cr e vice corr osion.R p data ar e meaningful for general or uniform corrosion but less so for localized corrosion, including MIC.Additionally, the use of Stern-Geary theory, wher e corr osion r ate is inv ersel y pr oportional to R p at potentials close to E corr , is valid for conditions controlled by electron transfer but not for diffusion-controlled systems as frequently found in MIC.R p and E corr tec hniques ar e often performed simultaneously during monitoring as the two techniques provide complementary EC data.Measurement of the time-dependent R p /E corr trend is one of the most commonly used corrosion monitoring techniques in field conditions.

Electrochemical impedance spectroscopy (EIS)
EIS tec hniques r ecord impedance data as a function of the fr equency of an applied signal at a fixed potential.For comparison, EIS is an AC (alternating current) frequency-dependent technique compared with the DC (direct current) polarization technique described abo ve .A large frequency range (mHz to kHz) must be investigated to obtain a complete impedance spectrum.Dowling et al. ( 1988 ) andFranklin et al. ( 1991 ) demonstrated that the small signals r equir ed for EIS do not adv ersel y affect the numbers, viability, and activity of micr oor ganisms within a biofilm.EIS data may be used to determine R p , the inverse of corrosion rate.EIS is commonly used for steady-state conditions (uniform corrosion); ho w e v er, sophisticated models hav e been de v eloped for localized corrosion (Mansfeld et al. 1982, Kendig et al. 1983 ).
Se v er al r eports hav e been published in whic h EIS has been used to study the role of SRB in the corrosion of buried pipes (Kasahara and Kajiyama 1986, King et al. 1986, 1991 ).Formation of biofilms and calcareous deposits on three SSs and titanium during exposur e to natur al seaw ater w as monitored using EIS and surface analysis (Mansfeld et al. 1990 ). Dowling et al. ( 1988 ) used EIS to study the corrosion behavior of CSs affected by bacteria and attempted to determine R p from the EIS data.
EIS is also useful for studying the MIC of metals with pr otectiv e coatings .J ones-Meehan et al. ( 1991 ) used EIS to determine the effects of se v er al mixed micr obiological comm unities on the pr otectiv e pr operties of e po xy top coatings over zinc-primed steel.Spectra for the control remained capacitive, indicating intact coatings, while spectra for five of six samples exposed to mixed cultures of bacteria indicated corrosion and delamination.
While EIS can provide useful information for MIC studies, it potentiall y r equir es an incr eased le v el of understanding compared to some of the other EC methods in order to be able to corr ectl y inter pr et the r esults.One of the k e y issues is the determination of the equivalent electrical circuits used for modeling of the solid/electr ol yte interface.

Medium and large signal polarization
Medium and large signal polarization tec hniques r equir e potential scans ranging from several tens of mV to se v er al V (see Fig. 6 ), with the exact range applied depending on the information that the test is aiming to obtain.An external signal is applied to obtain potentiostatic or potentiodynamic polarization curves as well as pitting scans.Medium signal polarization is often capped at + / −200 mV vs. E corr .Medium signal polarization curves can be used to determine i corr by Tafel extr a polation as well as specific electr on tr ansfer r eactions.With lar ge signal polarization, mass tr ansport-r elated phenomena can be e v aluated based on the limiting current density (i L ).For metal/electrolyte systems for which an acti ve-passi ve transition occurs, the passive properties can be e v aluated based on the passive current density (i pass ) .Pitting scans are used to determine (E pit ).
A disadv anta ge of lar ge-signal polarizations is the destructiv e natur e , i.e .the irr e v ersible c hanges of surface properties due to the application of large anodic or cathodic potentials.Choice of scan rate is important in MIC studies to reduce effects on biofilm structure and character.The faster the scan rate, the less the impact on microbial activities.Recording polarization curves provides an ov ervie w of reactions for a given corrosion system, e.g.c har ge tr ansfer or diffusion-contr olled r eactions, passivity, transpassivity, and localized corrosion phenomena.Due to the irr e v ersible c hanges to biological and chemical surface c har acteristics, large-scale polarization experimental design should include enough samples for separate anodic and cathodic polarization scans , i.e .lar ger-scale polarization should not be a pplied to one sample through the full cathodic and anodic potential ranges.Due to the destructive nature of large-signal polarizations (e.g.inducing pitting), care also needs to be taken not to confuse such changes to the surface of a test sample as being due to corrosion that has taken place prior to the EC testing.
Numer ous inv estigators hav e used polarization curves to determine the effects of micr oor ganisms on the EC properties of metal surfaces and the resulting corrosion behavior.In most of these studies, comparisons have been made between polarization curves in sterile media with those obtained in the presence of bacteria and fungi.K er esztes et al. ( 1998 ) used measurements of E corr , R p , and potentiostatic polarization measurements to obtain corr osion r ates.Cultur e media containing sulfide of both biogenic and chemical origin were used to determine the effects of metalsulfide la yers .Biocides were used to inhibit bacterial metabolic activity.An atomic force microscope was used to image the topogr a phy of sulfide la yers .T hey concluded that SRB produced continuous and localized sulfide, r egener ating anodic sites and-in the case of ir on-activ ating cathodic sites in the vicinity of the anodes.
In gener al, EC tec hniques ar e v aluable methods for mec hanistic investigations and monitoring of MIC.Like all specialized tec hniques, selection, a pplication, and data inter pr etation for eac h EC tec hnique ar e the k e y learning curv e for an y scientist wishing to utilize them.The most common mistake r esearc hers make using these techniques is ov erinter pr etation of data.In addition, all of the described techniques are highly dependent upon the specific environment of application, as well as changes in the en vironment (e .g. temperature , pressure , and flow rate) o ver time .For example , temper atur e fluctuations can dir ectl y affect EC behavior.Finally, as MIC is a wide interdisciplinary phenomenon, EC should be just one part of the ov er all experimental design in addition to other chemical and biological measurement techniques.

Future perspectives
MIC r esearc h is a truly interdisciplinary field requiring various expertise.In addition to basic laboratory skills, microbiologists have a wealth of advanced knowledge and expertise that could be applied to help solve some of the k e y issues in this important topic.For example, microbial ecology studies can provide important nutrient utilization rate data r equir ed for pr edictiv e model de v elopment (Okabe and Char ac klis 1992 ). Modeling ecological networks and functional interactions of community members of the microbial consortia involved in corrosion may further our understanding of k e y players in and factors influencing MIC.Evolutionary studies can offer important information on the differences between field isolates and culture collection species typically used in laboratory studies.Work could be performed to produce targeted detection that finds specific gene markers related to corrosion rather than just general phenotypes (e.g.sulfate reduction), genetic, or metabolite markers that may serve as leading indicators of MIC for helping to optimize treatments or de v eloping ne w mitigation strategies .T he tr ansfer of knowledge on and the a pplication of the latest analytical methods (e.g.microscopy, isolation, species identification/sequencing, metabolomics, transcriptomics , proteomics , etc.) to MIC r esearc h ar e likel y to pr ovide further useful insights.Microbiologists are also needed to provide insights/knowledge to the de v elopment of laboratory and fieldtesting standards and best-practice guides relevant to MIC (e.g.field testing, biocide selection, sample handling/pr eserv ation for genomics analysis, etc.).
As discussed throughout this review, reliable diagnosis of MIC r equir es MLOE, wher e in gener al, an incr eased number of measur ements of differ ent types (e.g.micr obiological, c hemical, metallur gical) impr ov es confidence in the conclusions that can be dra wn.T here are at present, ho w ever, no guidelines as to how many types of measurements are needed or if specific measurements are better than others.While efforts on this are underway (e.g. in updated NACE/AMPP and other standards), it is an area that could definitely benefit from additional work and collaborations that have the potential for major impact.
Close collaboration among disciplines involved in MIC research is truly the k e y to efficiently tackling current challenges .T he association for material protection and performance (AMPP), the international biodeterioration and biodegradation society (IBBS), and the EUROCORR, among others, aim to bring together r epr esentatives of various fields as well as provide platforms for continuous communication between industry and academy.A frequent exchange between various societies would be also beneficial for the understanding of MIC.
Ther e ar e numer ous c hallenges that can affect interdisciplinary collaboration in MIC research and its application in industry such as differences in research priorities, communication barriers , funding constraints , and e v en the format of guidelines (Wade et al. 2023 ).Ho w e v er, as we hav e seen in the oil and gas industry ( geno-MIC Pr oject ), ne w de v elopments stem fr om close cooperation among industrial operators and stakeholders as well as r esearc hers fr om academia.Suc h collabor ations can gr eatl y impr ov e our understanding of MIC mec hanisms, de v elop ne w monitoring techniques and green mitigation measures .T he Euro-MIC COST Action ( eur o-mic.org ), launc hed in the fall of 2021, has similar aims , in v olving MIC resear chers across the globe from various disciplines as well as stakeholders from various industrial sectors.Many of these groups/organizations are open to new members, ar e a gr eat way to de v elop one's knowledge of other disciplines and make further contacts with experts in the field of MIC.
International networks encour a ge debate on ur gent global challenges, as well as international collaborations, particularly within the Higher Education Sector, allowing graduates and early car eer r esearc hers to acquir e knowledge in a div erse and pr ofessional environment, as well as new perspectives on their research, pr e v enting stigmas and paradigms and promoting unity of efforts by bringing together stakeholders with specialized and complementary expertise to address critical industry-led scientific challenges, and enhancing educational and r esearc h outcomes.
Corr osion tr aining and education pr ogr ams av ailable to date are often limited to a specific area without giving a broader ov ervie w of MIC.The cooperation of different panels, knowledge exc hange between differ ent disciplines as w ell as betw een different generations of scientists needs to be enhanced, and MIC training and education pr ogr ams should be de v eloped.Dissemination is k e y to quic kl y and efficientl y deal with MIC issues.

Concluding remarks
The paper has provided an ov ervie w of some of the k e y aspects of MIC to provide background to microbiologists, as well as those new to the field, with a focus on non-microbiological aspects of MIC.A major aim of the work is to help break down some of the siloed knowledge and r esearc h being undertaken on this topic and encour a ge m ultidisciplinary collabor ations.
Some of the k e y messages from the paper include: r MIC does not describe a single mechanism for corrosion, and there is still much more work to do to clarify some of the mec hanisms involv ed.
r Corr ect dia gnosis of MIC r equir es MLOE (micr obiological, metallur gical, and c hemical), as well as information on engineering design and operations.
r A wide range of materials can suffer degradation due to MIC, and various examples have been discussed with an emphasis on metal alloys, including methods for analyzing metallurgical aspects of MIC.
r Models for studying MIC vary from single strain through to real-world consortia with each type allowing different aspects to be studied.There is significant potential for de v eloping and testing models that mor e accur atel y mimic the 'real world'.
r Mana gement of MIC involv es the k e y aspects of threat assess- ment, mitigation/pr e v ention and monitoring.Again, many of these r equir e MLOE to pr ovide accur ate and useful information.
r EC tec hniques can pr ovide critical information for MIC stud- ies in the laboratory and in the field; ho w e v er, ther e ar e numer ous limitations, so car e needs to be taken when designing, performing, and inter pr eting r esults fr om these methods.
MIC is a field growing at an exponential rate, with huge potential for new scientific disco veries .MIC researchers and specialists with multidisciplinary background are critical to drive this field forw ar d.Microbiologists with an inter disciplinary mindset will have an important role in shaping the future of MIC r esearc h.

Figure 1 .
Figure 1.MLOE used in the MIC assessment .Puzzle pieces r epr esent the four main categories of evidence with typical types of measurement.To solve the puzzle, evidence from most or all four categories is needed.

Figure 2 .
Figure 2. Depiction of the potential increasing complexity of different combinations of microorganisms in model systems that can be used to study MIC.Note that the depiction of the EPS and metal surface changes is not intended to indicate how the different model microbial systems affect corrosion outcomes.

Figure 3 .
Figure 3. Examples of different strategies used for MIC control.
, Mansor and Tay 2020 , Machate et al. 2021 , de Campos et al. 2022 ).Their use is now strictly regulated in certain areas [e.g.EU Biocides Regulation 528/ 2012 (98/8/EC)], prompting the search for effective and environmentally friendly long-term solutions (Loureiro et al. 2018 , Ferr eir a et al. 2020 , Vilas-Boas et al. 2020 , Ferr eir a et al. 2021 ).Significant advances have been achieved in relevant coating technologies, with work on a range of different properties such as the group of targeted organisms, mechanism of action, bioactiv e a gents, the pol ymeric matrix, surface structur e, envir onmental surr ounding, or e v en fundamental working principle being conducted (Ferr eir a et al. 2020 , Bhoj et al. 2021 , Silva et al. 2021 ).

Figur e 4 .
Figur e 4. T he main steps involved in developing an MIC mitigation strategy, along with associated challenges to attain effective solutions.

Figure 5 .
Figure 5. MIC monitoring r equir es integr ativ e data anal yses using a compr ehensiv e r ange of tools.

Table 3 .
Example of methods used to obtain microbiological data for MIC studies.
Larger sample size is needed to perform analyses as it is more difficult to isolate RNA than DNA.Samples are more sensitive to degradation.Can be costly and a specialized laboratory is needed.Skilled personnel are needed for data inter pr etation.(Krohnetal.2021 )

Table 3 .
Continued (Dockens et al. 2017)-Test asset affected, the main questions to be answered, the availability of trained personnel to perform the sampling, and/or access to r ele v ant labor atories .T he methods presented abo ve could be used individually or in various combinations to provide the best possible line of evidence to assess the involvement of microorganisms in corr osion.Furthermor e , these methods , either used alone or in combination, are not sufficient to support the involvement of micr oor ganisms in a corr osion pr ocess; collecting other lines of evidence , e .g. chemical, metallurgical, and operational information, is critical during MIC diagnosis/studies.

Table 4 .
Examples of engineering materials and reports of their susceptibility to MIC [Note: where possible, references showing examples of MIC of materials in the field or r e vie ws hav e been included].

Table 5 .
Examples of analytical techniques for the study of metal surfaces and corrosion by-products.
their mitigation pr ogr ams.Like wise, academia's motiv ations for participation in r esearc h include the continued need for peerr e vie wed publication, r equir ements for bringing funding to a department, and the support of work for graduate students .T hese are examples of only some of the silos that challenge multidisciplinary work.The need for multiple disciplines to understand MIC is where the roles of chemistry , microbiology , metallurgy , physics, electr oc hemistry, genetics, and other sciences are essen-