research paper...a ship, through use of internal pad welding or underwater hull welding, or use of...

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Microbiologically Influenced Corrosion in Maritime Vessels

S.A.Wade1,P.L.Mart2,A.R.Trueman2

1IRIS,SwinburneUniversityofTechnology,Melbourne,Australia

2DSTO,Melbourne,Australia

Summary Theenvironmentalconditionsinmanylocationsonboardmaritimevesselscanfavourtheproliferationofmicro-organismsassociatedwithmicrobiologicallyinfluencedcorrosion(MIC).Thispaperwillbroadlyreviewtheinternalandexternalenvironmentsforshipsandsubmarines,andprovideseveralcasehistoriesofMICinmaritimevessels,bothmerchantmarineandnaval.ThemainfocuswillbeconsiderationofthevariouscomplementaryapproachesrequiredtounequivocallydiagnosethepresenceofMIC,possiblyinthepresenceofalternativecorrosionmechanisms,andthentocategoriseandquantifythemicrobiologicalspeciespresent.Particularconsiderationisgiventodiagnosisthatisapplicabletofieldtesting,aswellastechniquesthataremoresuitableforconfirmatorylaboratorybasedtesting.Quickbutaccuratediagnosisisessentialonmaritimevesselsandinshipyardswheremaintenanceactivitymustbetightlyscheduledandcontrolled,tocontrolcostsandtomaximiseavailability.ThepaperalsoconsidersdevelopmentsinMICsensors,formonitoringvulnerableregionsofmaritimevessels,andprovidingearlywarningoftheriskoronsetofMICsothatpreventativemaintenanceactivitycanbebetterscheduled.ThisispartofanindustrymovetowardsConditionBasedMaintenance,andisequallyapplicabletominimisingtheeffectsofMIConshore-basedinfrastructure.Finally,abriefsurveyofpossiblemitigationtechniquestoreducethepropensityorseverityofMICisdiscussed.Overallthepaperconsidersthediagnosis,measuringandmonitoringofMICinmaritimevesselsfromapragmatic,operator-basedviewpoint,allowingfutureintegrationwithmitigationstrategiesthatareaimedtominimisetheimpactonthrough-lifecostsofmaintenanceandrepair,whilemaximisingoperatoravailability.

1. Introduction Awiderangeofdifferentmaterialsareusedintheconstructionofmaritimevessels,inbothstructuralandsupportelements,andmanyofthesematerialtypeshavebeenshowntobesusceptibletomicrobiologicallyinfluencedcorrosion(MIC)[1-3].Thisincludesmanyofthemetalstypicallyusedinareassuchashulls,tanksandpipingsystems.Thedamageofthesestructurescannecessitateexpensiverepairsandtheaddedlossofearningsand/orassetavailabilitywhiletherepairsareundertaken,andinaworstcasescenariocanpotentiallyleadtostructuralfailure.Mostofthebetterknownformsofcorrosionthatattackmaterialsusedin

maritimevesselsarerelativelywellunderstoodandcanbeaccountedforinthedesignandmaintenanceprocesses[4].HowtoaccountforthepotentiallyrapidcorrosionratesanddifficultiesofpredictinganddiagnosingMIC,however,presentsamajorchallenge.

Oneofthedifficultieswithpredicting,identifyinganddealingwithMICisthewidevarietyofenvironmentalconditionsandmicroorganismsassociatedwiththisproblem[1,3,5].Whiletheenvironmentalconditionsinmanylocationsonboardmaritimevesselsandtheconditionsinwhichtheyoperatecanvarysignificantly(e.g.merchantshippingversusnavalships),thetemperatures,availabilityofnutrientsandoxygenlevelsareoftenconducivetothesurvivalofmanyMIC-relatedmicroorganisms[6,7].MIC-relatedmicroorganismshavebeendetectedforexampleinshipboardfueltanks[8],bilges[9-11],engineandothermechanicalrooms[12],andinthewatersofpollutedharboursandportswhichcouldpotentiallybeusedforballast[13,14].ThereisarangeofpossiblesourcesofnutrientsfortheMICmicroorganismsavailablefrombothonboardsources(e.g.cleaningproducts,fuelandlubricants)andexternalsources(e.g.pollutedwaterfromharboursandports).Theexistenceofstagnantwaterinbilges,tanksandpipes,especiallyifthevesselisdockedforextendedperiods,canalsobeconducivetotheestablishmentofanaerobicconditionsrequiredbysomeofthebetterknownMIC-relatedmicroorganisms,suchassulfatereducingbacteria(SRB).SomeoftheproblemswithMICinmaritimevesselshavepossiblybeenfurtherexacerbatedasaresultoftherestrictionsondischargeofbilgeandballastwater[15].

TherangeofMICrelatedmicroorganismsandinteractionsbetweenmicroorganismsinaconsortiummeansthatthecorrosioncanbeinfluencedinanumberofdifferentways.Someofthemanypossiblemechanismsbywhichthesemicrobescaninfluencecorrosionincludeassistingtheformationofoxygenconcentrationcells,theproductionofmetabolicby-productsthatincreasecorrosion(e.g.ironsulphide(FeS)),theproductionofacids(e.g.sulfuricacid)andthealterationofpassivatinglayersonthesurfacesofmetals[1,2].TherangeofpossiblecorrosiveeffectsincreasesthedifficultyinpositiveidentificationofMICasthecorrosivemechanisminvolved.WhiledefinitiveidentificationofMICisdifficultitisgenerallyacceptedthatacombinationofinformationincludingmetallurgical,chemicalandbacterialevidenceisrequired[3,16,17].Inadditiontheverynatureofmanymaritimestructurescanfurtherincreasedifficultiesindiagnosisandremediation.Forexamplethesizeanddesigncomplexityofvesselssuchasverylargecrudecarriers(VLCC),whichcanhaveballasttanksurfaceareas>200,000m2[18,19]meanthatdetailedinspectionsareextremelytimeconsumingifnotimpossible.Thisalsopresentsproblemsforaccessforinspectionandwhenattemptingtoapplyremediationstrategies.AnotherMICrelatedproblemformaritimevesselsisthechangestomodernvesseldesignthathavecomeaboutasaresult

Research PaperThispaperwasoriginallypublishedintheACA’s2011MicrobiologicallyInfluencedCorrosionSymposiumproceedings.}

ofattemptstoeitherreducebuildcostsand/ortoreduceproblemsduetospills.Newhighstrengthsteels(e.g.thoseproducedbyThermo-MechanicalControlledProcessing(TMCP))haveleadtobottomshellplatingbeingreducedtothicknessesof20mmcomparedto26-28mmusedinolderdesigns[20],whichcouldpotentiallybepenetratedinamuchshortertime.

1.1 Ship Maintenance - MIC implications Itisveryexpensivetowithdrawashipfromservicetoundergoperiodicmaintenanceindry-dockoronaslipwayorship-lift.Thelossofavailabilityofthevessel,crewdowntimeandpossiblerequirementtotemporarilyreplacethevesselthroughleaseofanothertomeetongoingcontracturalobligations(cargodelivery,fishing,maritimepatrolornavalcapability)canincurlargecostpenalties.Thisisinadditiontothesignificantdockingcosts.Suchmaintenanceactivityisthereforepreferablyscheduledwellinadvance,tominimizetheimpactoncostandavailability.Typicalmaintenanceactivityisscheduledonatimebasis,whichwillvarydependentontheclassofvesselandthenatureofitsoperationalservice.Bothcommercialandmaritimepatrol(includingnaval)operatorswillseektoextendtheintervalbetweendockingperiods,andtoschedulenecessarymaintenanceandrepairsconcurrently.Typicallyvesselsareservicedeveryfiveyears.

“Anounceofpreventionisworthapoundofcure”.Thisadageholdswellforcorrosionpreventionandcontrol,thusregularinspection,preventativemaintenanceandearlyinterventioncanpreventcorrosiongainingaholdandescalatingtothepointwhererepairactivityisrequired.However,withminimalcrewingofcommercialvessels,anddecreasingcrewsizesonmaritimepatrolandnavalvessels,theabilitytousecrewtoinspectforandperformregularmaintenanceactivityhasdiminished.USNavalpolicyhasmovedawayfromcrew“chippingandpainting”activity,inordertoofferamaritimelifestylemoreconducivetoattractingandretainingcrew[21].Thisputsanincreasedresponsibilityforcorrosioninspectionontocommercialmaintainersandhullsurveyors.Thecostsofemptying,opening,steamcleaningandinspectingballasttanks,cargotanks,grey-andblack-watertanks(alsoknownascollectionandholdingtanks-CHT)issignificant,andthesheernumbersoftanksonlargeUSNavalvesselsmitigatesagainstinspectionotherthanonaperiodicrotationalbasis.Thisisneithercostefficientwhensuchtanksarefoundtobecorrosionfree,norwhencorrodingtanksarenotinspecteduntilcorrosionhasreachedapointwheresignificantrepairactivityisrequired.

Regularhullsurveysthatidentifyareasofgeneralhullcorrosioninbilgesandtanksinamaritimevesselallowtimingofrepairofthecorrosiontobedeferreduntilscheduleddockingactivity.Hullplatingthicknessandscantlingdimensionsgenerallyincludeacorrosionallowance,althoughasmentionedearlierthisisdecreasingwithuseofnewhighstrengthsteelsandanti-corrosive

coatings.Weldfillermetal,inadditiontoitsmechanicalproperties,ischosentobemorecorrosionresistantthanshellplating,sothatgeneralcorrosionprimarilyaffectstheshellplatingorscantlingsratherthanthestructurallysignificantwelds.Knowledgeofgeneralcorrosionratesallowsrepairactivitytothenbesafelydeferredwithoutriskofhullplatingpenetrationorstructurallysignificantlossofmetal.However,MICcaninvolveverylocalizeddeeppittingcorrosionwithextremelyhighcorrosionrates,typicallyseveralmmperyear(seesection2).Weldscanalsobepreferentiallyattacked,especiallyinstainlesssteelsusedforexampleinpiping[2].Repairtechniquesareavailabletorepairsmallareasofhullplatingwithoutdry-dockingaship,throughuseofinternalpadweldingorunderwaterhullwelding,oruseofcofferdamstoperformsmallareacropandrenewrepairs.However,ifsignificantareasofhullplating,weldedorstructuralmembersareaffectedbyMIC,therepairswilllikelyrequiredrydocking.

ThedifficultyinpredictingtherateandaffectedareaofMICcausessignificantproblemsformaintenanceandrepairactivity,andalignmentwithscheduleddrydocking.

1.2 Condition Based Maintenance Toaddressthepreviouslymentionedinadequaciesoftime-basedinspectionandmaintenanceofballasttanks,thereisanincreasedmovetowardConditionBasedMaintenance(CBM),wheremaintenanceorrepairactivityisonlyscheduledwhenrequired.Forexample,theUSNavalResearchLaboratoryhasdevelopedasuiteofcorrosionsensors,includingaTankMonitoringSystem(TMS)basedoncathodicprotectionmeasurementsthatenablestheconditionofatanktobemonitoredandassessedwithouttheneedfortankopeningorsurvey[22].Thesavingshavebeenconsiderable[23].

WhilethisCBMapproachworkswellforgeneralareacorrosionintanks,itispotentiallydeficientfordetectingandmonitoringlocalizeddeeppittingasmayoccurwithMIC.ThereisarequirementforcorrosionsensorsthataresensitivetoMIC,andpreferablyrespondbeforeMICcausessignificantdamagetothehull,systemorstructure.However,themanypossiblemicro-organismsandcomplexmechanismsthatgiverisetoMICmeanthatthereisunlikelytobeasingleuniversalsensor,andthatanumberofdifferenttypesofsensorwillberequiredthatareresponsivetodifferentformsofMIC.Thisisdiscussedinafollowingsection.Also,theselectionofthemostappropriatesitesforsuchsensorsisdifficultwithoutdetailedknowledgeofthepredominantMICmechanismlikelytooccurinaparticularsituation,andifthesensorsarewronglylocatedtheymaynotdetectMICbeforesignificantdamagehasoccurred.AbetterapproachmaybetomonitortheenvironmentratherthanforMIC.AknowledgeofthekeyenvironmentalfactorsthatprovideconditionsconducivetoMICproliferationcanthenprovideearlywarningoftheneedtotakepreventativeactionbeforeMIChasinitiated.

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2. Case Histories of Mic in Maritime Vessels Beforelaunchingintodetailsofindividualcasehistoriesitisworthacknowledgingseveralofthemoregeneral/summaryreportsthathavebeenwrittenonMICinmaritimevesselsbybothindividuals(e.g.Stuart[6]andTowers[20])andby/inrelevantforums(e.g.[24]).

OneoftheearlierreportsontheproblemsofsuspectedMICinaship’sbilgewasmadebyCopenhagenin1966[25].Localiseddeeppittingincludingperforationof8mmmildsteelplatelocatednearthepropellershaftcasingintheship’ssternoccurredinlessthan2years.Thepresenceofferroussulphide,aby-productofthemetabolismofsulfatereducingbacteria[2],inbilgewateratthislocationwastakenasanotherindicationthatthecorrosionwasduetoMIC.

Problemswithcontaminationoffuels,thecorrosionoffuelstoragetanksandequipmentwhichusesthefuel,andtheblockingofpurifiersandfiltershavebeenreportedbytheUSNavysincethe1960s[8,26-28].Insomecasesthishasresultedinconsiderablelossesinfleetefficiency,andhasrequiredexpensiveandtimeconsumingtreatmentmethodstoresolve.Theseproblemshavebeenlinkedtothepresenceofsulfatereducingbacteria,fungiandyeasts.Intestingcarriedouton80fueltanksof8navalships,viablemicroorganismswerefoundtobepresentinallsamplesexamined[8].ItisalsoworthmentioningtheobservationsofKlemme[28]whonotedthedifficultiesinmaintainingviableculturesofsuitablebacteriaandrecommendedtheuseofmorerealisticmixedmicrobialpopulationsasopposedtopureculturesinrelationtolaboratorystudies.

SimilarissueswithmicrobiologicalcontaminationofdistillatefuelhavebeenexperiencedbytheCanadianNavy[29],andincludedthedamageofseveralgasturbinesrequiringexpensiverepairs.Testsfromfueltankstakenatthefuelwaterinterfacefoundcontaminationwithbacteria,fungiandyeasts.Remediationrequiredthelengthyprocessofemptyingtanks,steamcleaning,wipingthetanksdry,inspection,refillingwithfreshfuelandtheadditionofabiocide.Otherfuelsystemcomponentswerealsocleanedandinspected.Duetoconcernswiththetoxicityofbiocidestobothcrewandtheenvironmenttheuseofbiocideshoweverwasnotrecommendedasaprimaryremediationprocess.

AsignificantamountofworkrelatedtoMICintheRoyalAustralianNavy(RAN)wasundertakenlargelytoinvestigateandpreventtheformationoftoxichydrogensulfidegas,whichisaby-productofthegrowthofsulfate-reducingbacteria[12,30].This,andcorrosionspecificstudiesundertakenatasimilartime[7],includedtestingforsulfatereducingbacteriaandthefactorswhichaffecttheirmetabolismandgrowthintheshipboardenvironment.TestingofbilgewatersintheengineroomandothermachinespacesofarelativelylargenumberofRANandforeignnavalvesselsfoundthepresenceofsulfate

reducing,aerobicandcoliformbacteriainthemajorityofthelocationsinspected.In2007Mart[31]reportedseveralexamplesofsuspectedMICinRANvesselsincludingonecaseinwhich10mmbilgeshellplatingwaspenetratedinlessthanayear.MorerecentlyWadeandfellowresearchers[10,11]undertookdetailedmicrobiological,chemicalandmetallurgicalteststodeterminethepotentialforMICcorrosionofvariousmetals,includinghullsteels,inthebilgewaterssampledfromRANvessels.

IntheRoyalNavy(UK)therehavebeenreportsoffailuresofgasturbineenginesduetoMIC,whichcausedsignificantissuesintermsofresourcesandplatformavailability[32].Thefailureswerefoundtobelinkedtocorrosionofcoolingsystemswhichuseseawaterasthecoolingmedium.Aninvestigationintothecauseshowedthatafterarefitthesesystemswerefloodedusingwatertakenfromanestuarinebasin,foundtobepollutedwithmarineorganismsandpotentialnutrients,andweretypicallyleftstagnantforperiodsofmorethanthreetofourweeksfollowingcommissioning.AnotherexampleofMICintheRoyalNavyistheseverepittingofcopper-nickelalloytubingusedinthecoolingsystemsofsubmarines[33].Thisproblemcoincidedwiththemovetotheuseofpotentiallypollutedseawatersourcedfromanon-tidalbasinasopposedtonominallycleanseawaterorfreshmainswater.Severepitting(e.g.2mm/yr)causedproblemswithoperationalavailabilityofplatforms.Trialsofcorrosioninhibitorsdesignedformacroandmicrofoulingwerefoundnottosolvetheissue.Oneoftheconfoundingresultsofsubsequentbacteriologicaltestswasthatsimilarpopulationprofiles,includingaerobicandanaerobic(includingSRBs)bacteria,weredetectedregardlessofwhetherthecoolerswereoperatedinnominallycleanornon-tidalbasinwaters.

Anotherexampleoftherapidcorrosionofsteelhullplateinaship’sbilgewasreportedbyCampbelletalinwhichpitdepthsof8mmin12monthswererecorded[34].Adetailedexaminationwasundertakenincludingmicrobiological,chemicalandmetallurgicalstudies.Variousmicroorganismsincludingaerobicandanaerobic(e.g.SRB)bacteria,fungiandyeastsweredetectedinthebilgewaterandsludge.Anumberofanalyticalmethodsuseddetectedthepresenceofsulfidesinthecorrosionproductormudsamplestaken,whichistypicalofMICcasesinvolvingSRB.

Clelanddiscussesacaseofacceleratedcorrosionintheballasttanksofashipincludingperforationofstringersinlessthan2years(i.e.corrosionrateof~6mm/yr)[15].Testingofcorrosionproductsfoundthataerobic,anaerobic(SRB)andmouldswerepresentinsamplestakenfromtheareaswithhighcorrosionrates.Inthepaperitisarguedthatcontrarytosomeopinions,thisformofcorrosioncanbemitigatedbykeepingthewateroxygenated,possiblyduetothefactthatSRBsareanaerobicbacteria,andthatthesituationofanaerobicandaerobiccyclesisthemostdangerous.Thiswouldagreewithlaboratorytests[35]whichshowedsignificantincreasesincorrosion

ratesofmildsteelsinanaerobicmediacontainingSRBafterspargingwithair.Itisalsoworthnotingthediscussioninreference15ofpossibleimplicationsforMICoftheInternationalMaritimeOrganisationGuidelinesforpreventingtheintroductionofunwantedaquaticorganismsandpathogensfromships'ballastwaterandsedimentdischarges.

ProblemswithMICinthecargotanksofbothsinglehullanddoublehullcrudeoiltankerswerereportedbyHuangetalin1997[36].Pittingofuncoatedbottomplatingwasreportedatratesofupto2mmperyear.TestingofanumberoftankersfoundMICbacteriaconsortia,includingSRBandacidproducingbacteria,insettledwaterandsludgeatthebottomofcargooiltanksandinwaterdropletsinthecrudeoilitself.ThecontrolofMICusingbiocidetreatmentswasfoundtobeimpracticalforcargooiltanks.

OneofthehighestcorrosionratesreportedthatwassuspectedtobeduetoMICwastheperforationof11mmhullinlessthan6months[37]Thesameauthorspresentedhistoricalresultsofmicrobiologicaltestsofbilgewatersof37vessels(includingferriesandtankers)whichshowedwidespreadcontaminationwithaerobicandanaerobic(includingSRB)bacteria,yeastsandmoulds[9].

ThecasestudiespresentedaboveshowthatMICcorrosionandMIC-relatedmicroorganismshavebeenfoundinavarietyoflocationsonboardmaritimevesselsandthatthesubsequentcorrosionratescanbeextremelyhighcomparedtowhatwouldbeexpectedforgeneralseawatercorrosioninthesameconditions(i.e.intherangeof0.1mm/yr[38]).AsummaryofthemaindetailsofthecasestudiesisprovidedinTable1.

3. Diagnosing Mic in Maritime Vessels ItistypicallyrecommendedthatthedefinitiveidentificationofMICastheformofcorrosiveattackrequiresacombinationofevidence[2,3,16,17].Thisevidenceshouldincludechemical,biologicalandmetallurgicaltestssuchas:

Chemical–identificationofcorrosionby-productsrelatedtoMIC,

Biological–identificationofthepresenceofmicroorganismsrelatedtoMIC,and,

Metallurgical–observationofMIC-relatedcorrosionmorphology,andsignificantlyincreasedcorrosionrates.

Inadditionotherformsofcorrosion(e.g.galvaniccorrosion)shouldberuledoutasbeingapossiblecause.

AnumberoftestingstandardshavebeendevelopedrelatedtothediagnosisofMIC(e.g.reference39).WhilethereissomeusefulinformationandtechniquespresentedinthesedocumentstheyhavemainlybeendevelopedforoilandgaspipelineapplicationsandassuchhavelimitedapplicabilitytoMICtestinginmaritimevessels.

3.1 Onboard/Field Testing TherearearangeofteststhatcanpossiblybecarriedoutonboardmaritimevesselstoassistinthediagnosisofMIC.Unlikeland-basedinfrastructure,theabilitytoperformtestsonshipswillberestrictedbytheoperationalavailability,theneedtoadequatelyventtestlocationswhicharesubjecttogasbuildup,andtheabilitytousesometestkitsonboardtheship.Accesstolocationsfortestingcanalsopresentsomedifficultiesduetothedesignofstructures(seeFigure1),andinadditionthespaceandlightinginmanylocationsmaymaketestingdifficult.

Author [Ref.] Location MIC Evidence Reported CR (mm/yr)

μorg. By-prod. CR Morph.

Copenhagen[25] Bilgeplate ✔ ✔

USNavy[8,26-28] Fueltanksandrelatedequipment ✔ ✔ ✔

Haggart[29] Fueltanks ✔

Upsher[7,12,30] Bilgeandenginerooms ✔

Mart[31] Bilgeplating,sludgetanks,freshwatertanks ✔ ✔ ✔ >10

Wade[10,11] Bilges ✔ ✔ ✔ ✔

Bolwell[32] Enginecoolingsystem ✔ ✔

Nicklin[33] Seawatercoolingtubes ✔ ✔ ✔ ✔ 2

Campbell[34] Ship’shullplate,ballasttank ✔ ✔ ✔ ✔ 8

Cleland[15] Stringers,webframesinballasttanks ✔ ✔ ✔ 6

Huang[36] Bottomplating,longitudinalsandstringersofcargooiltanks,

✔ ✔ ✔ 2

Hill[9,37] Hull,bilges ✔ ✔ ✔ 22

Table 1.SummaryofcasestudiesofMICinmaritimevessels(μorg–MIC-relatedmicroorganismspresent,By-prod.–MICcorrosionby-products,CR–highcorrosionrates,Morph.–MIC-relatedcorrosionmorphology)

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Typicallythetimeconstraintsassociatedwithtestingmeanthatonlyalimitedsubsetofthedesiredlocationsfortestingcanbeexamined(e.g.the>200,000m2surfaceareaofVLCCballasttanks).AssuchtargetedtestingneedstobecarriedoutatspecificlocationsinwhichtheriskandlikelihoodofMICoccurringarehighest.TheriskofMICmayforexamplebehullpenetrationorareductioninstrengthatalocationinwhichsubsequentstructuralfailuremayoccur,oreventhecostassociatedintherepairofthedamagedarea.WhenworkingoutthemostlikelyplacesforMICtooccuritisworthkeepinginmindthelocationsinwhichpreviouscasesofMIChavebeenreported.Thesearetypicallyareaswhichhavestagnantwater,asourceofMIC-relatedmicroorganismsandnutrientsavailabletomaintainthemetabolismofthesemicrobes.

IntermsofspecifictestsforthediagnosisofMICinthefieldonecanperformmicroorganismtests,watersampling,coupontrials,physicalinspectionsandeventherelativelysimpletestforthepresenceofH2Sgas(rottenegg/sulphurodour).FieldtestkitstodeterminethepresenceandnumbersofvariousMIC-relatedmicroorganismsareavailablefromanumberofcommercialsuppliers.Thekitsrangefromrelativelysimpletouse,requiringminimaltraining,tothoseincludingsomewhatcomplexprocesses(e.g.multipleserialdilutions).Carehowevermustbetakenwhenusingandinterpretingtheresultsofthesekitsas;(a)onlyasmallfractionofmicroorganismswillgrowinartificialenrichmentmedia,(b)theexactnumbersoforganismsdoesnotusuallycorrelatewiththelevelofMIC,and(c)thelocationofsampling,andwhetherplanktonicorsessilesamplesaretaken,mayaffecttheresults[40,41].ItisalsowisetorememberthatSRBarenot

theonlyorganismsinvolvedinMIC.Watersamplingcanalsobeundertakenforusewithbacteriaidentification,determinationofwaterqualityparametersandthepresenceofMIC-relatednutrients.Whensamplingfluidscorrecttestproceduresshouldbedevelopedandfollowed,includingforexampletakingcarenottoaeratethesample,determiningthecorrecttypeofcontainerusedtostorethesample,keepingthesamplecoolwhentransportingandminimisingthetimebetweensamplingandsubsequentanalysis.SeeFigure1foranexampleofawatersamplingdevicedevelopedforonboardtesting.Coupontrials,whicharediscussedinmoredetailinsection4,arealsoanoptionforinvestigatingcorrosionratesandsamplingofbiofilms.Visualinspectionsofcorrosionpits,andmeasurementsofpitdepthsanddensity,providesomeofthemostusefulinformation.Wherepossiblephotosofcorrosionattackand/orcorrosionby-productsshouldbetaken,includingsomethingtoprovideanindicationofscale.Corrosionby-productsshouldbesampledforsubsequentlaboratoryanalysis.AsmentionedpreviouslyarangeofdifferenttesttypesshouldbeperformedasonepositivetestaloneisnotadequatetodiagnoseMIC.

Whencarryingoutonboardtestingthereareanumberofhealthandsafetyissuesthatmayariseandthereforeappropriateprecautionsneedtobetaken.Ballastandbilgewaterareoftencontaminatedwithitemssuchasdiesel,hydraulicoil,andchemicalsandcanpotentiallycontainharmfultoxinsandpathogens[42].Caremustbetakentoavoidskincontactthroughtheuseofappropriatepersonalprotectiveequipment.Allequipmentusedinsamplingandtestingshouldbecleaned/decontaminatedusingsuitableprocedures,suchasautoclavingorchemicaldisinfectionwithbleach.

3.2 Laboratory Testing InadditiontotestscarriedoutinthefieldthereareanumberoflaboratorybasedtestproceduresthatcanbeusedtohelpdiagnoseMICinmaritimevessels.Thesetypicallyhoweverrequirespecificallytrainedpersonnelandaccesstoexpensivelaboratoryequipment.Accesstotheseskillsandequipmentmaybepossibleviauniversities,researchinstitutionsorcommerciallaboratories.Abriefdescriptionofsomeofthepossibletestmethodsisprovidedbelow.

Therearearangeoflaboratorytechniquesthatcanbeusedforamoredetailedanalysisofthemicroorganisms.Thisincludesthemoretraditionalplatingandmicroscopymethods,andthemoremodernmicroscopyandgeneticcharacterisationtechniques(e.g.DAPI,FISH,qPCR,DGGE[40]).SuchtestingcanbeusedtoassistintheimportantstepofdeterminingthecompositionoftheconsortiaofmicroorganismspresentinasampleasopposedtosayindividualbacteriasuchasSRB.

TohelpdeterminethesusceptibilityofasystemtoMICattackcouponimmersiontrialscanbeundertakenusingeitherasolutiontakenfromthelocationofinterestoratestmediummadeupusingstrainsofMIC-relatedmicrobesculturedfromsamplesorsourceddirectlyfromanumberoforganisations.Reproducingtheconditionsandratesofcorrosionexperiencedinthefieldhoweverisnotatrivialtask.ThemostseriousformsofMIChavebeenreportedtotakeplacewhenaconsortiumofdifferenttypesofmicroorganismsispresent[3,5,16].Variouselectrochemicalmonitoring(seediscussioninSection4)andanalysistechniquescanbeusedincombinationwiththesetests.

TheidentificationofMIC-relatedcorrosionby-productsornutrientsisalsoatasktypicallyperformedinthelaboratory.Techniquessuchasx-raydiffractionandenergy-dispersivespectroscopycanlookatthepresenceforexampleofironsulfidetypicallyassociatedwithMICduetoSRB.Thesetestscanbeperformedbothonsamplestakenfromthefieldandonthosepreparedinthelab.

MetallurgicalanalysisisoneoftheotherkeystoMICdiagnosis.PittingmorphologyisoftenquotedasakeyidentifierofMIC,withhemisphericalorterracedpittingoftencharacteristicofthisformofcorrosion[17].TherearesomecaseshoweverwhensuchpittingshapescanbeformedduetoattackthatisnotMIC-based,andthereforethisevidenceshouldbeusedonlyincombinationwithotherMICidentifiers.Dependinguponthesizeofthepittingitcanbeexaminedeithervisuallyorusingoptical/scanningelectronmicroscopy.Measurementssuchasweightlossandpitdepthscanhelptoquantifytherateandextentofattack.Modernequipmentsuchas3Dopticalsurfaceprofilerscanhelpreducesomeofthetime-consumingeffortinvolvedwiththisanalysis.Figure2showsacomparisonusingascanningelectronmicroscope(SEM)ofthedifferentformsofcorrosiveattackofmarinegrademildsteelsafterimmersioninbilgewatercontainingSRB,bilgewaterwithoutSRBandcleanseawaterwithoutSRB.

Figure 1:Imagesofonboardsampling,including(a)ahandheldperistalticpumpwithextendablerod,(b)samplingbilgewaterexample,and(c)exampleoflimitedaccesssometimesavailablefortesting.

Figure 2:SEMimagesofmarinegradehullsteelafterimmersionin(a)bilgewatercontainingSRB,(b)bilgewaterwithoutSRBpresent,and(c)cleanseawaterwithoutSRBpresent.

a. b. c.

Microbiologically Influenced Corrosion in Maritime Vessels

a.

b.

c.

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4. Monitoring Mic ThegenerictermMICencompassesmanytypesofmicroorganismsandpossiblecorrosionmechanisms.ThereforethedevelopmentofasinglesensorthatcanmonitorallthevariousformsofMICisextremelyambitiousandpossiblyunrealistic.ManytechniqueshavebeenproposedfordetectingMIChowevermosttestinghasbeencarriedoutinalaboratoryenvironmentandfewhavebeendevelopedforuseinthefield.Ofthecommerciallyavailabledevicesmostsystemshavebeentargetedtowardspipingapplications,andtheauthorsareunawareofanyproductsthathavebeenspecificallydesignedformaritimeuse.

TechniquesformonitoringofMICcantargetarangeofdifferentprocessessuchasbiofilmformation,thedirecteffectsofcorrosiononmaterials(suchasweightlossandpitting),thepresence/numbersofmicroorganisms(asdiscussedpreviously)andelectrochemicalprocesses.Anotherpossiblemethodistomonitorrelevantenvironmentalparameters,suchasthepresenceofspecificnutrientsrelatedtorelevantbacteria,whichmayindicatethepotentialforMICtooccur.Theuseofacombinationoftheaforementionedtechniquesisalsoapossibilityandmayhelpforexampletocoverthespecificlimitationsofaparticularmeasurementmethod.Themonitoringtechnique(s)chosenwillbedrivenbyarangeoffactorsincludingcost,continuousorintermittentmonitoringrequired,personnelavailabilityandeaseofaccess.Selectionsofsomeofthesensingtechniquesthatmaybeapplicabletotestinginmaritimevesselsarediscussedbelow.FurtherinformationonMICandbiofilmmonitoringtechniquescanbefoundinreferences[1,2,43-45]

OneofthemoststraightforwardmethodsformonitoringMICistheuseoftraditionalweightlosscouponsormoresophisticatedsamplingdevices.Whiletheydon’tnecessarilyproviderealtimedataonthecorrosionrateatthelocationofinteresttheycanbeusedtoobtainimportantinformationoncorrosionproperties(e.g.weightloss,pitting,morphology,etc.)and/orbiofilmproperties(microorganismtypes,etc).Couponscanbemanufacturedfrommostmaterialsandwitharangeofsurfacefinishes,sotheycanbeakintothematerialusedinthestructureofinterest,andthereforetheformandrateofcorrosiveattackshouldbesimilar.Careneedstobetakentoensurethatcouponsareplacedinalocationandatanorientationthatmatchestheregiontobestudied.Basiccorrosioncouponsareavailablefromanumberofcommercialsuppliers.Themoresophisticatedsamplingdevices,suchasthemodifiedRobbinsDevicearedesignedtoallowtheformationofsessilebacteria/biofilmsonsmallcouponsthatcanberemovedforsubsequentstudy.Arangeofdevicesareavailable,manyofwhicharedesignedforfluidflowapplications(e.g.piping)thatcanbeusedoverawiderangeofpressures.Itshouldbenotedthatthemicrobiologicalandmetallurgicalanalysisofthecouponsafterremovalwillmostlikelyrequirespeciallytrainedpersonnel.

TheZeroResistanceAmmetry(ZRA)orgalvanicmethodofcorrosionmeasurementisbasedonthecurrentgeneratedwhentwoelectrodesofdifferentmetalsareimmersedinanaqueousliquidandareelectricallyconnected.Thisisduetothefactthatdifferentmetalswillreachdifferentpotentialswhenimmersed.Themagnitudeofthecurrentgeneratedcanberelatedtotherateofcorrosionoccurringatthemoreactiveofthetwometals.Acommercialdeviceusinggalvanicmeasurementshasbeendesignedtomonitorbiofilmformationwhichusesaseriesofdisksmadefromstainlesssteel(seeforexample[46,47]).Thissystemtakestwosetsofmeasurements,thefirstinwhichavoltageisappliedtoonesetofdiskssothattheyarepolarisedrelativetotheotherset(performedonceadayforashortperiod)andthecurrentrequiredtoreachthedesiredpotentialismeasured.Fortheremainderofthetimetheappliedpotentialisturnedoffandthecurrentgeneratedbetweentheelectrodesetsismonitored.

OnecorrosionmeasurementmethodwhichcanmonitorinstantaneouscorrosionratesinaconductingfluidistheLinearPolarisationResistance(LPR)method[48].Thistechniqueusesasensorwith2or3electrodeprobeswhichareelectricallyisolated.Asmallpotential(~20mV)isappliedtotheelectrodesandtheresultingcurrentismeasured.Theslopeofthevoltageversuscurrentcurveisthepolarisationresistancewhichinturnisinverselyproportionaltothecorrosionrate.CommercialdeviceswhichusethistechniquearewidelyavailableandithasbeenusedinmanystudiesofMIC(e.g.[49,50]).Whenusedforlocalisedcorrosion,typicalofMIC,itissuggestedthatLPRisusedasaqualitativeindicationthatrapidcorrosionisoccurring,ratherthanforanindicationofexactcorrosionrates[1].

TheElectricalResistance(ER)ofametalsampleisinverselyproportionaltoitscross-sectionalarea,thereforewhencorrosionoccursandthecross-sectiondecreases,theresistanceincreases.Theaveragecorrosionrateoveraspecificperiodcanbecalculatedusingtheresistancereadingsobtainedatthestartandendoftheperiod.Thisrelativelysimpleprincipleisusedasthebasisofarangeofcommerciallyavailablecorrosionmeasurementdevices.Thesensitivityofthesensingprobecanbeoptimisedforaparticularapplicationbychangingtheinitialdimensions,wherethinnerprobesaremoresensitivebuthavereducedlifetimes.TheERmethodhasbeenusedbyarangeofauthorstostudyMIC(e.g.[49,51,52]).WhenusedforMICstudiesfoulingbyelectricallyconductingsulfidefilms,aby-productforexampleofthemetabolismofsulfatereducingbacteria,canleadtoerroneousresistancereadings.Thelocalisedcorrosiveattack,whichiscommonlyobservedwithMIC,canalsocausedifficultiesinthecalculationofcorrosionrates.Figure3showsexamplesofacorrosionsensorboardwith5stripsofmildsteeldesignedforERmeasurementsbeforeandafterimmersionfor9daysinnaturalseawatercontainingaerobicbacteria.Thereisclearevidenceoflocalisedcorrosioninthesamplewhichhadbeenexposed.

AswhendiagnosingMIC,careneedstobetakenwiththelocationofanysensorsusedformonitoringMICtoensurethattheyaresubjectedtotheconditionsinwhichMICmayoccur.Likewisethematerialschosenforuseinsensorsshouldtypicallybesimilartothestructurethatisbeingmonitored.FinallyitissuggestedthattheuseofacombinationofsensingtechniquesmayprovidethemostdependablewayofpickingupifandwhenMICmayoccur.

EnvironmentalsensorsthatmonitorlevelsofnutrientsthatpromotegrowthofmicroorgansimsresponsibleforMIC,orbiosensorsthatmonitormetabolicby-productsofsuchmicroorganisms,arepotentialfuturedevelopmentsthatwillsupplementMICsensors.TogethertheywilloffertheopportunityforConditionBasedMaintenanceofMICinmaritimevessels.Suchsensorsarethesubjectoflaboratoryresearchanddevelopment[53-55]butarenotyetsufficientlyadvancedorruggedforuseinthedemandingenvironmentaboardmaritimevessels.

5. Potential Mitigation Strategies Anumberofauthorshavereviewedstrategiesforprevention,controlandmitigationofMIC[1,44,56,57].Goodengineeringdesign,selectionofappropriatematerials,goodmaintenanceandoperationalprocedures,andavoidingtheriskofmicrobialcontaminationofthesystemareallhighlyrelevantcriteria.Howevertherearepracticalconstraintsinmaritimevesselsthatoftenpreventtherealisationofthesegoals.Shipsaredesignedandbuiltforstructuralandengineeringconsiderations,andcorrosiondesignisoftensecondary.Difficultyofaccessandtightlypackedauxiliarysystemspresentconstraintsinmaritimevesselsthatcanimpactanumberofpreventionandmitigationstrategies.Materialsthataresusceptibletocrevicecorrosionorunder-depositcorrosionappeartobesusceptibletobiocorrosion[56].Materialsareselectedprimarilyfortheirmechanicalpropertiesinordertomeetmarinestructuralandengineeringdesignrequirements,withcostanadditionalfactor,butcorrosionresistanceshouldalsobeconsideredandmaterialselectionoptimisedwherepossible.

Coatingsareamajorpreventativemeasureforcorrosionprotection,whetherthecorrosioniscausedbyabioticcorrosionmechanismsorMIC.Ifthecoatingformsanadherentandimperviousfilm,freefromholidaysordefects,whichdoesnotdegradeinthepresenceofmicrobiologicalorganisms,thentherewillbereducedopportunityfortheunderlyingmetaltocorrode.However,theconstraintsofsurfacepreparation,applicationandshipboardoperationcausethecoatingtocontaindefectsortodegradein-service,allowingopportunityformicrobialattack.

Cathodicprotectioncanberegardedasbothapreventionandmitigationstrategy[56],andinthispaperisdiscussedunderthelatterheadingforconvenience.

5.1 Cleanliness and Physical-Mechanical treatments Theforemostrequirementisto“keepthesystemclean”,andwhereverpossiblethedesignofavesselshouldaddressthisasapreventionstrategy.Ifthisisnotpossible,orMICisdiagnosedinthesystem,thenactivecleaningofthesystemisnecessaryasamitigationstrategy.Thisincludesmechanicalremovalofbiofilms,andwatertreatmentstodecreasethenumbersandtypesoforganismsbymakingtheenvironmentlessconducivetotheirgrowthandsurvival.

Inpipingsystems,hydrostatictestwatercanbeasourceofmicrobialcontamination,sotheuseofacleanwatersourceanddraininganddryingimmediatelyaftertestingisessential.Ifitisnotpossibletoremovetrapsforstagnantwater,thenthedesignshouldallowforperiodiccleaningorflushing,togetherwithfiltrationsystemstoremovesuspendedsolidsfrommake-upwater[58].Designchangestoincreasefluidvelocitiesinpipingsystems,soastoreducebacteriaresidencetime,eliminationofcrevices,andstagnantareas,arealleffectivecountermeasuresforavoidingbiocorrosion[44].However,fluidvelocitiesmustbekeptsufficientlylowthaterosioncorrosiondoesnotresult.

Intanksandbilgesinmaritimevesselsmechanicalremovalofbiofilmismoredifficultthaninpipelines,wherespongeballs,brushes,piggingandhighfluidvelocitycanbeused.Difficultaccesstosuchareasalsopreventstotalremovalofbiofilm,whichmayre-establish.

Somemitigationstrategiesinvolvetechniqueswhichcombinephysical-mechanicaltreatmentwithkillingofmicro-organisms.Theseincludeultrasonictreatmentandthermalmethods.

Ultrasonictreatment[59]orsonication[60]producescollapsingcavitationbubblesandmicro-jetsthatcandisruptlargercellsandbacteria.Ultrasonictreatmentisnotpracticalforlargetanksinmarinevessels.Possiblyballasttankwatercouldbepumpedthroughacentralizedultrasonicunitduringloading,butthiswouldaffectonlyplanktonicbacteria,andnotsessilebacteriaandbiofilmsthatestablishontanksurfaces.

Microbiologically Influenced Corrosion in Maritime Vessels

Figure 3: Photosofanelectricalresistancesensorboard,110×105mm,(a)beforeand(b)after9daysimmersioninnaturalseawatercontainingaerobicbacteria.Localisedcorrosion,whichincludedtubercles,isclearlyvisibleintheimmersedsample.

a. b.

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Heattreatmentofballastwaterhasbeenproposedforkillingintroducedmarinespeciesinballasttanks,butthereareenergy,structuralandenvironmentalconcernswithheating,holdingforextendedperiodsoftime,andthendischargingthelargevolumesofwaterinballasttanksonships[61].Highpressuresteamcleaninghasbeenusedforkillinganddislodgingbiofilms,alsopriortoabrasiveblastingofballasttanksforrecoating.However,accessforpersonnelandhosesisdifficult,andtheprocedureislabourintensive.

5.2 Treatments targeting the bioorganisms Otherwatertreatmentsaimtodecreasethenumbersandtypesoforganismsbyuseofphysicalorchemicaltreatmentsthataremorefocussedondisruptingtheorganismsthanjustontheirremoval.Biocidesareanti-microbialchemicalsthateitherkilltheorganismsorinhibittheirgrowthandreproductivecycle[57,62].Theycanbeeitheroxidizing(e.gchlorine,bromine.ozone)ornonoxidizing(e.g.glutaraldehyde,carbamates,guanides,isothiazolines,quaternaryammoniumcompounds).Itismoredifficulttokillbacteriainbiofilmsthanitistokillthesameorganismssuspendedinaliquidmedium(i.e.planktoniccells),duetotheinabilityofthebiocidetopenetratethebiofilm.Thereforebiocidesworkbestincleansystems.Therearemanycriteria[58]thatmustbeconsideredwhenselectingandapplyingabiocide,includingcompatibilitywithequipment,solubility,doselevel,dosefrequency,chemicalcompatibility,safety,persistence,toxicity,andcost.Bacteriacandevelopresistancetoasinglebiocide,soitisnecessarytoperiodicallychangethebiocide.Biocidescanonlybeusedinenclosedsystems,andincreasingOccupationalHealthandSafetyandenvironmentalconcernslimittheirapplicability.Considerationmustbegiventotheirsafeuse,dischargeordisposal,withoutdeleteriousconsequencesforhumans,marinelifeortheenvironment.

Exposuretoultravioletlightat254nmisanestablishedmethodfordisinfectingmicrobiologylaboratoryapparatus.Itisalsousedtodisinfectdomesticpotablewatersuppliesandinwastewatertreatmentonanindustrialscale.Unlikebiocides,therearenotoxicresidues.UVexposureisonlyeffectiveinrelativelyclearwaters,notturbidwaterscontainingahighproportionofsuspendedparticles.Fortanksandbilgescontaininglargevolumesofwater,orcomplexstructureswhichcauseshadowing,itisunlikelythatsurfaceswillreceivesufficientlyhighdosestokillbiofilms.Otherconsiderationsincludeelectricalpowerrequirements,cabling,fragilequartzUVtubes,andsedimentationandfoulingofthetubesreducingtheirUVoutputandrequiringperiodiccleaning[63].Openbilgesinattendedcompartmentswillrequirecrewshielding..IftankorbilgecontentsarepumpedthroughacentralUVunit,lowflowrates,longexposurepathlength,andhighUVintensityarerequiredtoachievesufficientexposure.However,onlyplanktonicbacteriaandothersuspendedorganismswill

beexposed,whilesessilebacteriaandbiofilmsontanksurfaceswillnot.

SRBaregenerallyobligateanaerobes,whichflourishinoxygendepletedenvironments,andusesulphateasaterminalelectronacceptor.ItwasthereforeoncethoughtthataerationofasystemwouldpreventcorrosionarisingfromSRBbyreducingorkillingthepopulationofSRB.However,asdiscussedbyLittleandLee[64],thishassincebeendisprovedasSRBexistinconsortwithotherorganismsonwhichtheydependforremovalofoxygenandproductionofnutrientswhichtheycanmetabolise.Thereforetheycansurviveinaeratedsystems.

Pumpingofotherwisestagnantwatersmaybeeffectiveindisruptingthegradientsofoxygenconcentration,nutrients,pH,organismsandtheirmetabolicbyproductswhichmightotherwisedevelopinastagnantbodyofwater,togetherwithdisruptingtheformationofbiofilms.Therefore,ifpumpingoftanksisundertakenregularly,thismayhaveabeneficialeffectonpreventingtheinitiationofMIC.

De-oxygenationornitrogenpurgingofseawaterballasttankshasbeenusedtopreventcorrosionofheadspacesandtolimitthespreadofintroducedmarinespecies.However,asdiscussedbyLittleandLee[1,64,65],corrosioninanaerobicseawaterismoreaggressivethaninaerobicseawaterastotallyanaerobicconditionsrapidlyformthatpromoteSRBwithresultantcorrosionofexposedsteel.Inevitablysealsandgasketsfailandoxygenleaksintothetank,andthisgivesrisetohighercorrosionratesincarbonsteelthandoesconsistentlyaerobicordeoxygenatedseawater.

5.3 Topical Issues with MIC Mitigation LittleandLee[1]addressseveralstrategiestomitigatetheeffectsofMIC,includingalteringpotentialelectronacceptorstoinhibitspecificgroupsofbacteria,andusingselectedbacteriatoinhibitcorrosion.

AdditionofnitratecausesashiftinthemicrobialpopulationfromSRBtonitrate-reducingbacteria(NRB).LittleandLee[1,58,65]discusstheseveralpossiblemechanismsforthisobservation,andnotethatnitrate-nitritesupplementationiseffectivefordecreasingsulphideconcentrations,butfurtherresearchisrequiredtooptimisethisforwaterswithdifferenthydrocarbonconcentrations.

LittleandLee[58,65]reviewedlaboratoryandfieldtrialsofcorrosioninhibitionduetobiofilms,butconcludedthatwhileithasbeendemonstratedinthelaboratoryforseveralmicroorganismsonseveralmetalsandalloys,ithasneverbeendemonstratedinafieldapplication.Theyhighlightedthestochasticnatureofbiofilms,theissueofcontaminationandnaturalcompetition,theinfluenceofnutrientsonelectrochemicalmeasurements,andonthecorrosionmechanism.

5.4 Cathodic Protection Cathodicprotection(CP)involvesapplicationofprotectivecurrenttothemetaltobeprotectedbyuseofsacrificialanodesorimpressedcurrentanodes.Cathodicprotectionappearedtobeeffectiveininhibitingthegrowthofbiofilmsformedbyaerobicbacteriaonsurfacesofmildsteelstructuressubmergedinseawater[44].Theoppositeeffectwasreportedforanaerobicbiofilmsofsulphate-reducingbacteria.ThecombineduseofCPandprotectivecoatingscanbeveryeffectiveincontrollingbiocorrosionofpipesandstructuresexposedtoseawater,suchasshipbilgesandballasttanks.However,CPmustbeusedfromtheoutsetinconjunctionwithgoodcoatings,andislesseffectivewhenretrofittedtomitigatebiocorrosioninstructureswherecorrosionhasalreadytakenhold.[66]

AreviewofCPefficiencyinthepresenceofSRBconfirmsthatthecriterionof-0.900VvsAg/AgClisnotenoughtoprotectcarbonsteelfromMICbySRB[67].FurtherresearchisrequiredtodeterminetheeffectivenessofCPinengineeringapplications[63],andthisincludesbilgesandballasttanksonmarinevessels.

6. Conclusions Microbiologicallyinfluencedcorrosionhasbeenfoundtocauseseriousproblemsinarangeoflocationsonboardmaritimevessels.IndeedtestinghasfoundthewidespreadpresenceofmicrobesrelatedtoMICinmanydifferentareasonboardshipsandboats.Someoftheproblemswerereportedtohaveoccurredfollowingtakingonwatersthatweremostlikelypollutedwithboththesemicroorganismsandthenutrientsthattheyrequire.Thissituationshouldobviouslybeavoidedwhereverpossible.

TherearemanyfieldandlaboratorytechniquesavailableforthediagnosisofMIC.Theuseofarangeoftestmethodsincludingacombinationofthosewhichprovidechemical,biologicalandmetallurgicalevidenceisrecommendedasisthecarefuldocumentationofanyevidencefound.LikewisemonitoringofMICinamaritimevesselcouldpotentiallybecarriedoutwithanumberofdifferentcommercialdevices,whichusevariousdetectionmethods.Knowingthelimitationsofaparticularmonitoringtechniqueandhavingaclearunderstandingofhowtointerpretthesensoroutputarecriticalwhenusinganysensor.AswasthecaseforMICdiagnosisitisexpectedthatnosinglemonitoringtechniquewillprovideadefinitivesolutionforMICanditisexpectedthatacombinationoftechniquesisprobablybest.ThereisaneedforruggedenvironmentalsensorsthatmonitorthelevelofnutrientsorbyproductsthatarespecifictoparticularMIC-causingmicroorganisms,tocomplementMICsensorsandelectrochemicalbiosensors,andtoalloweventualdevelopmentofConditionBasedMaintenanceforMIC.

Arangeofpreventionandmitigationstrategiesareavailable,withtheprimaryonebeingtokeepthesystemcleanfromtheoutset.Again,nosingletreatmentwillworkforallsituations,andcombinationsoftreatmentsmayberequireddependentuponthespecificmaterials,environmentandmicroorganismspresent.Foroperatorsofmaritimevessels,thereisoftenalimitedsuiteofavailableoptions,dictatedbycost,complexityandregulatoryconstraints.Furtherresearchisrequiredtodeterminetheon-boardefficacyofthesimpler,environmentallybenignandmorereadilyavailabletechniques,suchascathodicprotectionandsomephysical-mechanicaltreatments,whilecontinuingtomonitorresearchdevelopmentsofstrategiestargetingspecificmicroorganisms,butstillatlaboratoryscale.

Thepotentialrewardsformaintenanceandrepaircostavoidanceandincreasedavailabilityofmaritimevesselsishuge.

7. Acknowledgments TheauthorswouldliketothankstafffromtheDefenceScienceandTechnologyOrganisation,ASCPtyLtd,MonashUniversityandmembersoftheRoyalAustralianNavywhoassistedinsomeoftheMICworkpresented.FundingfromtheCRCforIntegratedEngineeringAssetManagementandDefenceMaterialsTechnologyCentreisgratefullyacknowledged.

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Microbiologically Influenced Corrosion in Maritime Vessels