Corrosion control of dissimilar metals.docx

Corrosion control of dissimilar metals.docx

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Corrosioncontrolofdissimilarmetals

Corrosioncontrolofdissimilarmetals

Patent5445719IssuedonAugust29,1995.EstimatedExpirationDate:

 May25,2014.EstimatedExpirationDateiscalculatedbasedonsimpleUSPTOtermprovisions.Itdoesnotaccountforterminaldisclaimers,termadjustments,failuretopaymaintenancefees,orotherfactorswhichmightaffectthetermofapatent.

Abstract Claims Description FullText

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PatentReferences

1020480

3055813

3649492

CathodicprotectionmonitoringPatent#:

4351703

Issuedon:

09/28/1982

Inventor:

Winslow,Jr.

Inventor

∙Boiko,RobertS.

Application

No.249228filedon05/25/1994

USClasses:

205/776.5,Ofcoating,coatedsubstrate,orimbeddedobject204/404,Corrosion205/777Offerrousmetal

Examiners

Primary:

Tung,T.

Attorney,AgentorFirm

∙Emrich&Dithmar

InternationalClasses

G01N027/26

C23F013/00

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Description

BACKGROUNDOFTHEINVENTION

Thisinventionrelatestocorrosionprotectionsystems,andmoreparticularlytocorrosionprotectionsystemsforjoineddissimilarmetalswhichareemployedinanaqueousenvironment.

Potablewater,orsimilartypeelectrolytenormallycontainscertaincorrosioncausingcomponentssuchasdissolvedoxygen.Otheroxidizingcomponentsofmanyvarietiescanbepresentinthismedium,dependingontheparticularsampleofwater,thelocationfromwherethewaterwasobtainedasageologicalfunction,certainwatertreatmentssuchassoftening,andthemanyothervariableconditionstowhichwatercanbesubjected.

Itiscommonforpotablewaterandothertypesofwatersystemstocontainironorironbearingalloyswhichcanformagalvaniccorrosioncellbetweentheironbearingmaterialandeachoxidizingatomormoleculeinvolvedinalikereaction.Thesereactionscanreachfactorsof10.sup.（x）persquareinch.Therefore,ithasnotbeenpossibletorendereachseparatecorrosioncellinactivebydirectactiontoeachoxidizingatomormolecule.

Cathodicprotectionsystemshavebeendevelopedtoreducecorrosioncausedbyoxidizingcomponentsfounddistributedinthepotableorsimilartypewatercontainingthevulnerablemetal.Inacathodicprotectionsystem,thevulnerablemetaliselectricallychargedbyavariableorconstantvoltageinordertocausetheformationofamonatomichydrogenlayer.Thishydrogenlayer,whenundisturbed,formsaninterfacebetweentheoxidizingcomponentsandthevulnerablemetal.Inapassiveenvironmentwherelittleornoturbulenceexists,aswellasotherfactorswhichwouldhaveaneffectupontheintegrityofthemonatomichydrogenlayer,thisformofprotectioncanbeusefulinthereductionofcorrosioncellformationfromthewaterbornecorrosioninducingcomponents.Cathodicprotectionisevenmoreeffectivewhenthebasemetaliscoated,suchaswithglasslining,cementorepoxy,reducingthecontactareabetweenthevulnerablemetalandtheaqueousmedium.

However,impressedcathodicprotectionisnotaseffectiveandcanactuallyacceleratecorrosionincertainaggressiveenvironments.Examplesareanenvironmentwherethereisturbulentwaterflowagainstthevulnerablemetalremovingthehydrogenlayerandleavingachargedsurface,oranenvironmentcharacterizedbyelevatedtemperature,pHextremes,orcontactofthevulnerablemetalwithamorenobledissimilarmetal.

Fordissimilarmetaljunctions,itcanbeshownthatimpressedcathodicprotectionofthevulnerablemetalwillacceleratethecorrosionofthevulnerablemetalby20%to45%ormorewhenanysignificantturbulenceispresent.Naturallyoccurringcathodicprotectionthatoccuresbetweenjoineddissimilarmetalswillcauseonlyslightcorrosionratereductionsinalowtemperatureenvironment,about23°Centigradewhereverylittletonoturbulenceispresent,andthewatersampleisnon-aggressive,tapwater,untreatedatpointofuse.

Themajorwaterheatermanufacturershavelongrecognizedtheshortfallofcathodicprotectionagainstdissimilarmetaljunctionsandconsequentlyavoidtheuseofdissimilarmetaljunctionsincommercialunits.Manywaterheatertankshavedi-electricinletandoutletfactoryattachments.Thermostatcopperandbrassshanks,aswellasreliefvalveprobes,areplasticcoated.Drainvalvesareplastic,insteadofcopperorbrasswheneverpossible.Copperelectricheatingelementsaretincoated.Also,thenewclassofheatingelementsarenowcommonlyproduced.TheseheatingelementsarecomprisedofstainlesssteeloranironalloycalledIncolloywhichfurtherreducesdissimilarmetalcorrosion.Plumbingcodesalmostalwaysspecifythatdi-electricunionsmustbeusedwhencopperpipeisruntoandfromahotwaterheater.

Variousmethodshavebeenproposedtopreventcorrosion.Forexample,L.Applegate,CathodicProtection,McGrawHill,February1960,pp1-28,disclosesamethodtostopthereactionbetweenseparatebarsofcopperandironwhichinvolvesapplicationofanimpressedcurrenttothedissimilarmetals.However,thismethodasdisclosedbyApplegateonlyhasalaboratoryapplication.Themetalsareneverdirectlyjoined.Furthermore,thepositivecurrentelectrodeisconnecteddirectlytothemorenoblecopperbarwhichcausesthecoppertobecomeveryactiveanddisintegrate.Moreover,Applegatedisclosesthattheacceptedconclusionisthatdissimilarmetalsshouldnotbeusedduetotheunpredictablenatureandwidespreadpotentialforsystemcorrosion.

SUMMARYOFTHEINVENTION

Thepresentinventionprovidesausefulmethodforcontrollingcorrosionofdissimilarmetalssuchasthosemetalsusedinpotablewaterorsimilartypeenvironments.Theinventionallowsthesemetalstobejoineddirectlywithouttheneedfordi-electricinsulationbyeffectingthecorrosioncurrentflowingfromthemorevulnerabletothelessvulnerablemetalinthissinglecorrosioncell.Insystemswheretwodissimilarmetalsarejoined,thereisonlyonecorrosioncellwhichcanbetreatedinadifferentmannerthanthe10.sup.（x）corrosioncellsformedpersquareinchbetweenavulnerablemetalandelectrolytecontainingoxidizingatomsormolecules,eachforminganindependentcorrosioncell.

Theinventionconsistsofcertainnovelfeaturesandstructuraldetailshereinafterfullydescribed,illustratedintheaccompanyingdrawings,andparticularlypointedoutintheappendedclaims,itbeingunderstoodthatvariouschangesinthedetailsmaybemadewithoutdepartingfromthespirit,orsacrificinganyoftheadvantagesofthepresentinvention.

DESCRIPTIONOFTHEDRAWINGS

Forthepurposeoffacilitatingandunderstandingtheinvention,thereisillustratedintheaccompanyingdrawingsapreferredembodimentthereof,fromaninspectionofwhich,whenconsideredinconnectionwiththefollowingdescription,theinvention,itsconstructionandoperation,andmanyofitsadvantageswillbereadilyunderstoodandappreciated.

FIG.1,whichislabeledPriorArt,isasimplifiedrepresentationofaknownarrangementforcontrollingelectrolyticcellactionusingimpressedcurrent;

FIG.2,isasimplifiedrepresentationofadissimilarmetalcorrosionprotectionarrangementappliedtojoineddissimilarmetalswhichareimmersedinanaqueoussolution;

FIG.3isanenlargedsectionalviewtakenalongthelines3--3ofFIG.2;

FIG.4isasimplifiedrepresentationofanarrangementforprovidingcorrosionprotectionofdissimilarmetalsinaccordancewiththepresentinvention.

FIG.5isanillustrationsimilartoFIG.4butwithoneoftheelectrodeslocatedremotefromthejunctureareaofthedissimilarmetalforprovidingcathodicprotectioninadditiontocorrosionprotection;

FIG.6isanelevationalviewinsectionofawaterheaterincorporatingthecorrosionprotectionarrangementofthepresentinvention;and

FIG.7isasectionalviewtakenalongthelines7--7ofFIG.6;and

FIG.8isanenlarged,fragmentaryviewillustratingthejunctionbetweenthedissimilarmetalsofwaterheater.

DESCRIPTIONOFTHEPREFERREDEMBODIMENTS

Theuseofimpressedcurrentforthepurposeofcorrosionprotectionofdissimilarmetalsisknownandhasbeendescribed,forexample,byL.Applegate,inCathodicProtection,McGrawHill,1960,pp1-28.AdiagrammaticrepresentationofthecontrolofcellactionasreportedbyApplegateisillustratedinFIG.1,whichislabeled"PriorArt".Thecell,illustratedinFIG.1,iscomposedofahalfcellofcopper11andahalfcellofiron12,wherethemorenobleelectrodeiscopperandthelessnobleelectrodeismadeofiron,whichisamoreactivemetal.Thetwoelectrodes11and12aredisposedinacontainmentvessel13spacedapartfromoneanother.Thevesselisfilledwithasuitableelectrolytesuchasimpurewater14.Abatteryofcells15,capableofsupplyingavoltagegreaterthanthe0.777voltsofthecopper-ironcell,isconnectedtoaslidewirevoltagedivider16andtothecopper-ironcell.Thepositivebatteryterminalisconnecteddirectlytothecopperelectrode11.Thenegativebatteryterminalisconnecteddirectlytotheironelectrode12.Iftheslide17isatpointA,thebattery15willhavenoeffectandthecellwillproceedinthenormalwaytodissolveiron.

Thereactionideallycanbestoppedbymovingtheslide17topointBwherethevoltage