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IntroductionTheAssociationofWaterTechnologies(AWT)CoolingCommittee’sCorrosionCouponTaskGrouprevisedthecorrosioncouponguidelinesin2010andearly2011tobetteroutlineacceptablecorrosioncouponsurveypracticesandtomoreproperlydefinecorrosioncouponresults.Theinformationpresentedisspecificforopenrecirculatingcoolingsystemsandclosedchilledwaterloops(itisnotintendedasareferenceforothersystemtypes).Thismaterialwascontributedbyanumberofindividuals,therebyprovidingaconsensusofopinionfromdifferingregions,perspectivesandoperations.Thecorrosionratesandcomparativegradingareseparatedintotwogeneralcategories:industrialprocesscoolingandcomfortcooling(HVAC)applications,soastoprovideamoreaccuratereflectionofperformancerelativetooperationalchallenges.Thedocumentprovides“RecommendationsandGuidelines”forthewatertreater.Thisisnotpresented,norisitproposed,asaStandard.Itisintendedtobeusedasanaid,inconjunctionwithothersurveymethodsanddata,todeterminehowwellthemetallurgiesofasystemareprotectedagainstcorrosionandtogaugetheimpactofchangesmadetoawatertreatmentprogram.Thisisa“livingdocument”andwillbeexpandedinscope,asmoreinformationisforwardedto,andreviewedby,theCouponTaskGroup.WarningsandDisclaimers:Thisdocumentisdesignedtoprovideinformationregardingthesubjectmatterpresented.ItisproducedwiththeunderstandingthatneitherAWTnorthecontributorsarerenderinglegal,medical,engineeringorotherprofessionalservices.NeitherAWTnorthecontributorsshallbeliablefordamages,inanyevent,forincidentalorconsequentialdamagescaused,orallegedtobecaused,directlyorindirectly,bytheuseofanyinformationdisclosedinthisdocument,includingtheuseofanyrecommendations,methods,products,services,instructionsorideas.

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AWTRecommendationsandGuidelinesforCorrosionCouponsinCoolingSystemsAcorrosioncouponsurveyshouldhaveadefinedobjectivepriortoinitiatingthestudy.Thishelpstopositiontheresultsandpursueanyneededcorrectiveaction(s).Corrosioncouponsareusefulindeterminingaveragelossratesovertheinstallationperiodandcanillustrateevidenceoflocalizedattack.Theresultsidentifytrendsthatreflectacceptablecorrosionprotection,gainsmadebychangestothetreatmentprogram,equipmentoroperationofthesystemortheneedforimprovementsthatshouldbeinvestigatedbyfurthersurveywork.Couponstudiescompriseonlyoneofanumberoftoolsthatareavailabletodeterminecorrosioncontrol.Eachresultneedstobeviewedasauniquevaluefortheoperatingconditionspresentduringthesurveyperiod.Thisvalueshouldbecomparedagainstotherresultstoidentifyimprovementsincontrolandprotection.Corrosioncouponresultsarenotanabsolutereflectionofactualsystemcorrosionrates–existingoxidesonolderwatersidesurfacestypicallyretardcorrosionmoreeffectivelythantheprotectivesurfacedevelopedonthecorrosioncouponduringthesurveyperiod.Apoorratedoesnotnecessarilyreflectasignificantproblem;however,itdoeshelptoidentifytheneedforcorrectiveaction.Thevaluesobtainedfromthegravimetricevaluationofcorrosionusingcoupons,dependsuponmanyfactors.Thesefactorsincludebutarenotlimitedto:1.Typeofindustry2.Exposuretimeinawatersystemcoupontestrack3.Couponrackdesignandcouponplacement4.Orientationofthecoupon5.Flowcharacteristicspassingoverthecoupon:watervelocity,downtime,etc.6.Temperatureofthewaterpassingthroughthecorrosionrack7.Waterchemistryandquality8.Processcontamination:organics,gases,acids,alkalis,metals,etc.9.Qualityandconcentrationofthewatertreatmentprogram1.AcceptablevaluesfromanHVACsystem(i.e.acoolingtoweroperatinginaninstitutionalorcommercialfacility)maynotbeattainableinarefinery,chemicalorotherindustrialmanufacturingplant.ThesamecanbesaidforHVACsystemsoperatingwithastressedwatersupply(i.e.make‐upwatershighintotaldissolvedsolids,sulfatesand/orchlorides,asexamples).Corrosionratesidentifiedinthesesystemsmaynotmatchthevaluesusedtoevaluateprotectioninunstressedsupplies.Itmightbemoreappropriatetoreferencetheguidelinesidentifiedforindustrialcoolingtowercircuitstogaugecorrosioncontrolwhenoperatingwiththesemoredifficultsources.

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2.Corrosionratesmeasuredbycorrosioncouponsaretheresultoftimeaveragingthelossofmetalfromthecoupon.However,therateoflossofmetalbycorrosionofaclean,unpassivatedcouponisnotlinear.Corrosionratesaremuchhigherintheinitialfewhoursordaysofexposure.Thehigherrateofcorrosionoccursduringanduntilthemetalsurfaceacquirespassivation.Therefore,arecommendedandwidelyacceptedexposuretimeforunpassivatedcorrosioncouponsinatestrackisaminimumof90days.Short‐termdurations(30‐daysorless)usuallywillresultinhigherobservedcorrosionrates.Thehigherratesaretheresultoftheshorttimeaveragingoverthehighercorrosionratespresentduringinitialpassivation.Theshorttermandresultinghighercorrosionratesmaybejustifiedwhenchangestotreatmentprograms,ornewtreatmentprogramsarebeingevaluated.Withlongertermduration,theinitialhighmetallossfromthecouponbecomesasmallerpercentageoftheoverallweightloss.Also,asthemetalcouponcorrodes,thecorrosionproductscanactasabarrierreducingtherateoffurtherattack.3.Thecorrosioncouponrackshouldbeconstructedofmaterialswhichwillneithercontributetocorrosionnorresultincorrosioninhibition.Optimalplacementoftherackwouldbeonthereturnheadertothecoolingtower,orthereturnlinefromachillerorheatexchanger(i.e.thesiteofferingthewarmestrecirculatingwater).Theeffluentfromtherackcanflowbacktothemainwatersourceinfreeflowtoatmosphericpressurewheretheflowratecanbemanuallymeasuredorreturntothepressurizedpipingsystemwhereaflowmeterisfirstusedtomonitorandcontroldesiredwaterflowrates.Thereisawiderangeofopinionsconcerningpreferredandacceptablematerialsofconstruction(M.O.C.)usedinthemanufactureofcouponracks.Thesevaryfromthemorecommonlyappliedmildsteel,tostainlesssteel,PVC,galvanizedandcoppermetallurgies.Whilesuccessfulapplicationsofeachofthecommonoptionshavebeennotedthereremainavarietyofphilosophiesonthepositiveandnegativeaspectsofeach.Someofthesephilosophiesareshownbelow.M.O.C. Pros Cons

CarbonSteel

• Suitableforhigherpressure/hightemperature

• Ruggedconstruction• Componentsarereadily

available• Representativeofthebulkof

thepipingmetallurgy(typically)

• Thewatersidesurfaceconditionscanbeusedasaqualitativereflectionofgeneralprotection,conceivablyforyears

• Undersevereoperatingconditionsthewatersidesurfaceswillcorrode(butthat,inandofitself,wouldidentifypoorcontrol)

• Ferrouscomponentsweighmorethanplasticcomponentsandmayrequireadditionalsupport.

StainlessSteel

• Suitableforhigherpressure/hightemperature

• Ruggedconstruction• Willnotcorrodereadily

• Moreexpensivethanotheroptions

• Nottypicallyrepresentativeofthebulkofthesystem

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• 316SSfittingsarereadilyavailable

piping• Fittingsforalloysotherthan

316maybenotbefoundeasily

PVC • Easytoconstruct• Relativelyinexpensive• Canbemanufacturedwith

transparentsegmentsallowingthecouponsurfacestobevisuallyinspectedthroughoutthesurveyperiod

• Schedule80isavailableinthreadedcomponents

• Limitedpressureapplications• Limitedtemperature

applications• Nottypicallyrepresentative

ofthebulkofthesystempiping

Galvanized • Suitableforhigherpressure/hightemperatureapplications

• Ruggedconstruction• Willcorrodeataslowerrate

thanwillcarbonsteel(althoughthismaymaskanobviouscorrosionconcern)

• Corrosionofthegalvanizedsurfacescouldindirectlyimprovethecorrosionratesofferrouscoupons

• Alkalineprogramsmayattackthegalvanizedpipe.

CopperTube

• Suitableforhigherpressure/highertemperatureapplications

• Sturdyconstruction

• Corrosionofthecouponrackcomponentscouldadverselyimpactthecorrosionresultsoflessnoblemetalcoupons(however,corrosionofthecopperrackwouldalsoimplythatcorrosionofthesystemcoppermetalwasoccurring).

Constructionmaterialscanbe¾”or1’inchdiameter,schedule40orschedule80piping.TemperaturesofthewaterandpressureinthesystemmustbeconsideredwhenselectingtheM.O.C.Allrequiredvalves,strainersandflowrestrictingdevicesshouldbemadeofsimilarmaterials.Couponrackdesignsareoftenpromotedwithhorizontalsectionsthatarenolessthantwelve(12)inchesinlengthandverticalsectionlengthsofthree(3)inches,ormore.Whenconsideringthatallflowsinexcessofone(1)gallonperminutewillbeturbulent,increasedhorizontalpipelengthsbeyondsix(6)inchesandincreasedverticalseparationsofthehorizontalsectionsareunlikelytoreduceturbulenteffects(seeTable1below).Comparativesurveysshouldalwaysbecompletedinthesamecouponrackorcouponrackdesignasthiswilllimitthevariablesassociatedtothedifferencesinlossrates.Itiscommonlyrecommendedthatcouponsshouldbeplacedintothecorrosioncouponrackfollowingthegalvanicseriesinseawater.Thisstipulatesthatthemost“active”orleast“noble”metaloralloymustbeplacedinthenumberoneposition,whichisthefirstcouponexposedtotheflowingwater.Othercouponswouldfollowthegalvanicseriesinseawater

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withthelastcouponbeingtheleast“active”ormost“noble”metaloralloy.Theprocedurepreventsthemore“noble”orleast“active”metaloralloyfromcathodicallydepositingontheactivemetaloralloy.Inreality,ifanappropriatewatertreatmentprogramisinplace,theactualcouponorderwillmakelittleornodifferencetotheresults.Thereisanargumentforlimitingthenumberoffactorstobeconsideredwhenanalyzingresultsandifthecouponsareinstalledaccordingtothegalvanicseries,anyhighlossrateswillnotbeblamedon,orassociatedto,galvanicdifferences(regardlessofwhetherthesecould,orcouldnot,impactresults).Asnotedpreviously,workingwithastandarddesignandaminimumofvariablesmakesitfareasiertoassessthepotentialissuesneedingredress.Havingaremovablecarbonsteelspoolpieceaspartofthecouponrackisaveryusefulmeansofvisuallymonitoringpipewallconditionsoverlonger‐termperiods.Ifthesesegmentsaretobeutilized,thecouponrackshouldinstalledinalocationthatallowsthesepiecestobeeasilyinspected(orremovedforinspection)withoutinterference.(SeeexampleinFigure2.)4.Couponsshouldbeplacedinthecouponracksuchthatwaterflowsfromthecouponholdertothetipofthecoupon.Thisorientationanddirectionofflowispromotedasthebestmeansofreducingthepotentialoferosionatthetipofthecoupon.However,atnormalflowratesandwithnoabrasivesuspendedsolidspresentinthewater,erosionisunlikelytobeanissue.Severalargumentshavebeenputforwardontheidealdirectionofflowtolimitsuspendeddebrisfromcollectingononeortheotherendofthecoupon(therebycausingunderdepositcorrosion).Somesuggestthattheflowshouldbefromtheholdertothecoupontip,othersfromthecouponendtotheholder.Bothconcernscanbeaddressed,atleastinpart,byutilizinganupstreamstraineraspartoftherackdesign(thiswillhelppreventdebrisfromcollectingoneitherthemountingscreworthecoupontip).Note:whereastrainerisincorporatedtoreducetheimpactofanyforeigndebris,particularattentionshouldbepaidtotheflowrateduringthecourseofthesurvey(thetrappedmaterialwithinthestrainermayrestrictflowandthechangeinflowratemayimpactresults).Wherepractical,theinstallationofatransparentplasticstrainerwillindicatewhetherthismightbecomeanissue.(SeePartsListFigure1.)Thecouponshouldbeorientedwiththebroadfaceinaverticalposition.Thisreducestheaccumulationofdebrisontothefaceofthecoupon,whichcanacceleratecorrosion.Whenremoved,thecouponshouldbeinspectedtoensureitisstillproperlyorientedandsecuredontheholder.Ifnot,thisinformationshouldbeincludedintheanalysisreport.Thecouponholdershouldbeequippedwithamarkidentifyingtheflatedgeofthecouponsothattheorientationcanbeconfirmedduringinstallation.Couponsshouldalsobeelectricallyisolatedfromtheholderusingsuitableplasticwashers,screwsandnutswherethegalvanicdifferencebetweenthecouponandthemountinghardwareisfelttobeaconcern.

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Couponmountingstems0.5inchesindiameterarerecommended.Non‐metallicholdersarethepreferredmaterialofconstructionbutstainlesssteelisacceptableinalkalineenvironments.TheholdermustpositionthecouponbeyondtheTeefittingthattheholderisthreadedinto.Itisgenerallyrecommendedthatthecouponholderstemlengthsarenolessthanthree(3)inchesinlength.5.Waterflowthroughacoupontestrackshouldbesettoemulatesystemequipmentflowrates.Inmanysystemsaconstantflowbetween3‐5feetpersecondisnormal.Highervelocities(i.e.8Ft./Sec.andabove)cancauseerosiononsoftalloyssuchascopper‐basedmaterials.Calculationofthewatervelocityinapiperequiresthreemeasurements,OD(OutsideDiameter),WT(WallThickness)andGPM(Gallonsperminuteflow).Thesevariablesareincorporatedintothefollowingformula:Velocity=(0.408Xgpm)/id2 whereidisinchesspecifictothepipeScheduleand

MOCLowerwatervelocitiescancauseparticulatemattertosettleontothecouponresultinginunderdepositattack;especiallyifabiofilmispresent.Aflowmeterorotherflowcontroldeviceshouldbeincorporatedtomaintainaconsistentandrepeatableflowrate.Thispracticefacilitatesaccuratecomparisonsofcouponratesfromonetimeframetoanother.Note:Flowrestrictingvalvesusuallyrequirea15psidroptooperateproperly,whileflowindicatorsrequireroughlya2psidifferential.

Table1:FlowRatesandReynoldsnumbersforSchedule80pipe

FlowGPM 3/4InFt/Sec 3/4In

Reynolds#1InFt/Sec 1InReynolds

#1 0.742 4258 .446 32982 1.484 8517 .892 65963 2.226 12776 1.338 98944 2.968 17034 1.784 131925 3.710 21293 2.230 164906 4.452 25551 2.676 197887 5.194 29810 3.122 230868 5.936 34068 3.569 263849 6.678 38327 4.015 2968210 7.420 42585 4.461 3298011 4.907 3627712 5.353 3957513 5.799 4287414 6.245 4617215 6.691 49470

Note:Inmanycoolingsystems(bothHVACandprocess),circulationmaybeinterrupted.Whereverpossible,determinethefrequencywithwhichthisoccursandincludeitinthe

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evaluationofresults(stagnantflowconditionsinterferewithfilmformationandmaintenanceandmayresultinhighercalculatedcorrosionrates).Incorporationofacheckvalvetopreventsystemdrainagehelpsmaintainmoreconsistentresults.6.Normally,thecorrosioncouponrackisconnectedtothereturnrisertothecoolingtower,becausethewateriswarmerthanthecoldwellsupplyandpotentiallymoreaggressive.Itshouldbenotedthatcouponresultsdonotreflectcorrosionratesofheatexchangerwatersides,asthereisnoheattransferacrosscouponsurfaces.7.Waterchemistryandqualityhasastrongimpactoncorrosion.Temperature,pH,conductivity,bacteriaandbiofilm,dissolvedandsuspendedsolidswillinfluencetherateofattackonthecoupons.Uncontrolledmicrobialpopulationsand/orsuspendedsolidscontaminationcanresultinsurfaceaccumulationsthatmayharboursulfatereducingbacteria.This,inturn,cancauselocalizedmetallossresultinginfalsehighcorrosionvalues.Note:Thequalityofthewatertreatmentprogramwillimpacthowwellthesystemmetallurgiescanbeprotectedfrommostcorrosionmechanisms.Surfacepassivationandcontrolovermicrobiologicalpopulations(andcontroloversuspendedmaterialthatmightharbourSRB’s)willimpactresults.8.Processcontaminationscanaccelerateattack.Organics,gases,acids,alkalisandmetalscanimpactcorrosionrates.Sourhydrocarbonleakswillincreasecorrosionduetosulfideattackoncopperorcarbonsteel.AmmonialeakscandecreaseattackoncarbonsteelduetohigherpH.However,ammoniawillaccelerateattackoncopper.Acidcontaminationwillacceleratecorrosiononsteelandcopper.Alkalicontaminationwillaccelerateattackoncopper,brassandzincgalvanizedcomponents.9.Thewaterchemistryandqualityofchemicaltreatmentwillinfluencecorrosionratesofcoupons.Usingpre‐passivatedcouponspriortoinstallationintherackisnotrecommendedasthisprocessmayresultinafalse,lowcorrosionrate.Typically,un‐passivatedcouponsareusedinarackwithcontinuouswaterflowfora90daytestperiodwithapropermaintenancelevelofthesystem’schemicaltreatment.Usingun‐passivatedcouponsforashorttestperiodofonly30dayswillyieldahigherinitialcorrosionratecomparedtousingpassivatedcoupons.Fora90daytestperiod,un‐passivatedcouponsaresuccessfullyusedifthechemicaltreatmentisataleveltopassivatethesystem.Followingtheaboveguidelines,acceptablecorrosionratesforcarbonsteelandcopperalloysinopenrecirculatingcoolingwatersystemsarelistedinTable2.ThecorrosionratedataanddescriptionspresentedintheTable2barenotnecessarilyapplicabletoallcoolingtowersystems.Itisimportanttonotethatsomewatersupplieswillpresentadditionalchallengestothewatertreatmentprofessional.Corrosioncouponsurveysconductedinthese“stressed”waters(i.e.thosewithhighTDS,sulfatesand/orchlorides,forexample)mayneedtobeevaluatedrelativetoTable2aoragainstahybridoftheTables2aand2bvalues.Whereamake‐upwatersupplyisnotstressed,theTable2bvaluesshouldbeconsideredforHVACsystems.CorrosionratesforclosedcoolingwatersystemsarelistedinTable3.

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Table2a:QuantitativeClassificationofCorrosionRatesforOpenRecirculating

CoolingWaterSystems–IndustrialCoolingSystemsandHVACSystemsOperatingWithHighStress/HighTDSMakeupWaters

CorrosionRates(mpy)

Description CarbonSteel CopperAlloysExcellent Lessthanorequalto1 Lessthanorequalto0.1VeryGood 1to3 0.1to0.25Good 3to5 0.25to0.35

ModeratetoFair 5to8 0.35to0.5Poor 8to10 0.5to1

VeryPoortoSevere >10 >1

Table2b:QuantitativeClassificationofCorrosionRatesforOpenRecirculatingCoolingWaterSystems–HVACforInstitutionalandCommercialFacilities.

CorrosionRates(mpy)

Description CarbonSteel CopperAlloysExcellent Less0.5 Lessthanorequalto0.1VeryGood 0.5to1 0.1to0.2Good 1to2 0.2to0.3

ModeratetoFair 2to3 0.3to0.5Poor 3to5 0.5to1

VeryPoortoSevere >5 >1

Table3:QuantitativeClassificationofCorrosionRatesforClosedRecirculating

CoolingWaterSystemsCorrosionRates(mpy)

Description CarbonSteel CopperAlloysExcellent Lessthanorequalto0.2 Lessthanorequalto0.1VeryGood 0.2to0.3 0.10to0.15Good 0.3to0.5 0.15to0.20

ModeratetoFair 0.5to0.8 0.2to0.3Poor 0.8to1 0.3to0.5

VeryPoortoSevere Greaterthanorequalto1 Greaterthanorequalto0.5

NotesforTables2and3:• Theseratesassumethatthemetallossisuniformwithnopitting,localizedattack,

gouging,etching,microbialorcreviceattackevident.Theseconditionsarenotacceptable,andifanyarenoted,thecauseshouldbeaddressed,near‐term,withfollow‐upsurveystoconfirmimprovement.

• Localizedattackatthecouponholdermaybeignoredifthetreatmentisunabletointeractwiththecouponinthisareaandnootherabnormalitiesarenoted.

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• Laboratoryanalysisshouldexpresscouponweightstothenearest0.1mg,readingsaretypicallyreportedasmils(or1/1000ths)ofaninchperyear.

• Couponsshouldbestoredinsuitablecorrosionresistantenvelopestopreventcorrosionbothpriortoandfollowingthesurveyperiod.Topreventanyalterationtothecouponsurfacebeforeinstallation,donottouchcouponsurfacewithfingersorcontaminatesurfaces

• Uponremoval,couponsshouldbegentlyblotteddryusingonlyacleanpapertowel.• Couponsshouldbestoredinsuitablecorrosionresistantenvelopestoprevent

corrosionbothpriortoandfollowingthesurveyperiod.Figure1

PartsforCorrosionCouponRack

1InchSchedule80PVC

LetterDesignation

Description NumberRequired

SpearsNumber

A CompactBallValveSocket 2122‐010B Schedule80Nipple6"Trimmed 864‐060C SSDoleControlValve1"X10GPM 1 DOLED1 Schedule80Nipple18" 884‐180D2 Schedule80Nipple12" 4 884‐120D3 Schedule80Nipple9" 884‐090E TEEFiptxFiptxFipt 805‐010F CorrosionCouponHolder 4 MS MetalSamplesRC13QC0100663"Teflon

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StemG Schedule80Nipple2" 884‐020H 90oELLFiptxFipt 808‐010I Schedule80Nipple18"Trimmed 1 884‐180J Schedule80PVCPipe PipeSupplyK TEESocxSocxSoc 810‐010L ReducingBushingSpigxFipt(3/8") 1 838‐129M Schedule80Nipple3/8"x1.5" 1 881‐015N LabBallValve3/8" 1 1521‐003O UnionSocXSoc 857‐010P 90oELLSocxSoc 806‐010Q SRThreadedY‐Strainer,Clear YS31S8‐

010CLSRExampleofthecomponentsusedina4positionPVCcorrosioncouponrack.Notetheuseofaflowrestrictingdevice(itemC).Figure2

Exampleofamildsteelcorrosioncouponrackshowinganupstreammountedstrainer,upwardflow,aflowindicator,flowfromthecouponholderthreadedendtowardthe

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coupon,unionandremovablespoolpiece,dedicatedcirculationpumpandisolationvalves.Thepumpandflowindicatorareusedtomaintainrepresentativeflowconditions.NOTE:Thismildsteelpiperackinstallationutilizessomeplasticcomponents–itismountedinalocationwheretemperatureandpressurearenotaconcern.Itisequippedwitha6inchsteelpipespoolinthelowerleftcornerandisequippedwithanin‐linewaterboosterpumptoassureoperationwithadequatewaterflow.AcknowledgementsThisdocumenthasbeenproducedbytheAWTCoolingSubcommitteeCorrosionCouponMonitoringTaskGroupbythefollowingindividuals:TaskGroupChair:KenSansom,CWT–SolutechWaterManagement,Inc.‐Calgary,Alberta,CanadaTaskGroupContributors:ArthurJ.Freedman,Ph.D.–ArthurFreedmanAssociates,Inc.–Naperville,Illinois,USATimothyE.Keister,CWT‐ProChemTechInternational,Inc.–Brockway,Pennsylvania,USAK.AnthonySelby–WaterTechnologyConsultants,Inc.‐Evergreen,Colorado,USAJamesScott,CWT–SanJoaquinChemicals–Fresno,California,USAW.John(Jack)Soost,CWT–ArthurFreedmanAssociates,Inc.–Lancaster,Pennsylvania,USA