section,adetaileddescription CORROSIONFUNDAMENTALS
Inthis oftheelectrochemical processesleading toand controllingcorrosionwillbeoutlined.Thefirstsectiondealswiththeelectrochemicalbasisfor corrosion,whichiscontrolledbythermodynamicprinciples,whiletheeffectofkinetic,orrate controlwillbeintroducedlaterinthesection.Itisimportanttounderstandtheroleofboth thermodynamicandkineticprocessesininitiatingandcontrollingcorrosioninordertodetermine corrosionrates,orreducecorrosionbydifferentmethods,suchassacrificialanodesoralloy developmentsuchasstainlesssteels. MetalDissolution In most corrosion processes, with the exception of cathodically controlled processes, metalwilldissolve.Anunderstandingoftheprinciplesbehindhowmetaldissolvesinasolution isanecessarypartofunderstandingcorrosion.ConsiderapieceofpuremetalMinasolution whichcontainsitsownions,Mz+.Inthiscasethemetalhassolubilityinthesolutioninwhichit isplaced.Anexamplewouldbeapieceofpurecopperinasolutionofcoppersulfate. Thegeneralequilibriumreactionforametalelectrodeinasolutionofitsownionsis: M=Mz++ze whereMrepresentsametalatom,Mz+isthemetalion,Zisthepossiblevalencesande representsanelectron. Atequilibriumthereisnonetcurrentflowandtheabovereactioncanbeseparatedinto twodifferentreactions.Onereactionproducesmetalionsinsolutionfrommetalatomsinthe solidwiththereaction: M>Mz++ze Againatequilibriumthereisnonetcurrent,sotheelectronsproducedinthereaction aboveareconsumedinareactionwherealltheelectronscombinewithmetalionsinsolutionto depositasatomsonthesolidmetalsurfaceincontactwithsolutioninthereaction: Mz++ze>M Theseindividualreactions arecalledhalfcell reactions astheyanotherreaction to eitherconsumeorproduceelectrons forthemtoproceed.Thesehalfcellreactions arealso oxidationandreductionreactions. Oxidationisareactionthatproduceselectrons.Theionizationofmetalatomstoionsis thereforeanoxidationhalfcellreaction. Reductionisareactionthat consumes electrons.Deionizationtoformametalatomis thereforeareductionhalfcellreaction. M=Mz++ze TheabovereactioniscalledaREDOXreactionbecauseitcontainsbothanoxidationanda reductionhalfcellreaction. Bothoxidationandreductionareimportantreactions.Theoxidationreactionisthebasis of corrosionasmetalisdissolvedwhilsttransformingfromatomstoionsinasolution.The reductionreactionisalsoimportantasitisthebasisofelectroplatingwhereionsinsolutionare depositedasatomsonasurface,forexamplechromeorgoldplating. 1
Duringcorrosion,thehalfcellreactionsdonotusuallyinvolvethesameelementsand ions in both the oxidation and reduction reactions. The electrons produced in the oxidation reaction arenot available forthereverse reduction reaction, whichwouldreplace the metal atomsbackonthemetalsurface.Instead,theelectronsfromtheoxidationreactionareconsumed byadifferentreductionreactionresultinginanewreactionproduct.Thisleadstoasecondary reactionbetweentheproductsoftheoxidationandreductionhalfcellstoformastablematerial. Thesecanfurtherreactwiththeenvironmenttoformstablecompoundssuchasrust.Theseare thenverydifficultreactionstoreversebacktotheatomicformofthemetalwhichisoneofthe majorreasonswhycorrosionisanextremelydeleteriousprocess. BasicCorrosionProcesses. Fournecessaryprocessesarerequiredtohavecorrosiontakeplace.Alltheseprocesses mustbeavailableforcorrosiontooccur.Ifanyoftheseareunavailablethencorrosionwillnot occur.Thisprovidesthefirstmeasuresofcorrosionprotectionbydisablinganyoneofthefour necessaryprocesses,thencorrosionwillbeblocked. Thisverysimpleapproachtocorrosion protectionisoftenoverlookedbutcanbeaninvaluabletoolfortheengineertoavoidcostly corrosionproblems. OxidationHalfCellReaction. Thefirstprocessisanoxidation or anodic halfcell reaction.Theoxidationhalfcell reactionwasdescribedabove. Thecorrosionprocessisessentiallythisoxidationreactionas metalatomsaretransformedtotheirions.Solidmetalinasolutionistransformedintometalions insolution,resultinginweightlossorthinningofthesolidmaterial. M>Mz++ze ReductionHalfCellReaction. Thesecondprocessofacorrosioncellisareductionorcathodicreaction.Onereduction orcathodicreactionwasalsodescribedabovewhichistheplatingordepositionreaction. Mz++ze>M OthercathodicreactionsaredescribedbelowintermsoftheirREDOXreactions, althoughacathodicreactionwouldbeinthedirectionfromlefttoright: 2.Reductionofhydrogenionsinstrong,deaeratedacids: 2H++2e=H2 Forthecathodichalfcellreactionhydrogengasisproducedatthesametimeastheanodic reactionisprogressing.. 3.Reductionofoxygeninweakaeratedacidstoformwater. O2+4H++4e=2H2O 4.Changeinionicstate,e.g.,Cuprictocuprousion. 2
Cu2++e=Cu1+ 5.Reductionofoxygeninneutralorbasicsolutionstoformhydroxylions. O2+2H2O+4e=4(OH) For many practical service situations the last reaction is the most important. All the necessaryconstituentsforthereactiontoproceedarepresentinnormalenvironmentalexposure. Theoxygenispresentasdissolvedoxygeninwater.Alatersectionwilldiscussthereductionof oxygeninwateringreaterdetail.Theonlycomponentmissingarefourelectrons,whichduring corrosionwillbeproducedbytheoxidationoranodichalfcellreaction. CathodeSite. Animportantfeatureatthisstageistodistinguishthecathodichalfcellreactionfromthe physical cathode. The physical cathode is the location where the cathode half cell reaction occurs.Thecathodeonlyprovidestheelectricalconductivitynecessaryforelectrontransferfor thecathodehalfcellreaction.Anexamplecanbeobtainedfromthereductionofoxygeninwater reaction above,reaction5.Thisreactionwilloccuronasurfacewhichhasthecapabilityto conductelectrons.Thesurfaceonlyneedstobeaconductorofelectricity,itdoesnotneedtobe metallic, thereforenonmetallic materials thatareelectrically conductive canbecathodes. It should also be apparent that the cathode does not dissolve and can be termed as being cathodicallyprotected.Anothercathodereactionisreductionofhydrogenionstohydrogen, reaction 2. In this case hydrogen gas will be nucleated on a conducting surface as a correspondinganodicreactionproducestheelectronsforconsumption.Thisdistinctionbetween cathodereactionandthecathodematerialisimportantinlaterstageswhenlocalizedcorrosion formssuchaspittingandcrevicecorrosionarediscussedandlargeamountsofthemetalsurface exposedtoanenvironmentshownosignsofcorrosionwhileextensivecorrosionispresentin local regions on the surface. It is thought that the noncorroded regions are cathodically protected. IonTransport. The third process is ionic transport forwhich a conductive medium orelectrolyte is required.Inmanycasesofenvironmentalexposurethiscanbewater,seawater,acidicorbasic solutions.Thegreatertheionicflowcapabilityofthesolutionthegreaterthecorrosionrate whichcanbesupported.Thereforedistilledwaterwithahighresistanceandlowconductivity, usually supports a lower corrosion rate than seawater with a low resistance and high conductivity. ElectronTransport. Thefourthnecessaryprocessisamechanismforelectrontransportbetweentheanode andcathodesites. Aselectronsareproducedbytheoxidationanodichalfcellreactionsand consumed bythereduction cathodicreaction, electrons flow fromtheanodetothecathode. Withoutflowofelectronsbetweentheanodeandcathode,thesehalfcellreactionscannotoccur.
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Blocking this electron flow is therefore another method of corrosion protection. The anode thereforehasanexcessofelectronsandisthereforethenegativepole.Thecathodeisthepositive pole. The necessary electron flow can be achieved by several mechanisms. One is a wire conductorwhichconnectstheanodeandcathode.Electronsflowalongthewirefromtheanode tothecathode.Agoodexampleofthisprocessisaflashlight.Thebatteryintheflashlighthasa cathodeandananode.Thezinccasingistheanodeandusuallythenegativepole.Thecenter postofcarbonisthepositivepole.Whentheswitchisplacedintheonpositionacircuitis completerbetweenthepositiveandnegativepolethroughthelightbulbfilamentwhichglows whitehotduetoresistiveheatingfromtheelectronflowfromtheanodetothecathode.The electronflowisthereforeprovidingbothavoltageandacurrent. Asecondandmuchmoreimportantmethodofelectricalconnectionforanengineeris controlledbythe microstructure ofthematerials.Theintimatecontactofphasesinametallic alloy can providethe necessaryelectrical contact to establish anode andcathode regions. It shouldberememberedthateachphasehasitsowndistinctcompositionsoeachphasewillreact differentlytotheenvironment.Thesedifferenceswillcauseanodesandcathodestoformwhich areinphysicalcontact.Impuritiesinmetalscanalsoactasanodesorcathodesduetochemical differences. Insummary,thefournecessaryprocessesforacorrosioncellareanodereaction,cathode reaction,electrolyteandelectrontransport.Unfortunatelyformanyengineeringmaterialssuchas steelsinwater,allthesefourprocessesarepresentandcorrosionwilloccur.Forothermaterials inthesameenvironment,nocorrosionwilloccur.Anexamplewouldbegoldinwater.Ifthereis nocorrosion,thenoneofthesefourprocessesisnotoccurring,Forgolditsohappensthatno anodehalfcellreactionisavailablewhichmakesitimmunetocorrosion. ExchangeCurrentDensity. TheREDOXhalfcellreactionsofoxidationandreductionbothinvolveelectrons.The oxidation,anodicreactionproduceselectronswhilethereduction,cathodicreactionconsumes them. M=Mz++ze AtequilibriumforaREDOXreactionthereisnonetionicorelectronflow,sothereisno corrosionweightlostbythemetalordepositionorweightincreaseofthemetal.Theweightof themetalMwillbethesameattheendasatthebeginningandtheconcentrationofM z+ in solutionwillbethesameattheendasthestartingconcentration.However,foranoxidation reaction,thereisacurrentflownumberofelectronsperunittimeastheyareproducedinthe anodic half cell reaction. There is also current flow for the reduction half cell reaction as electronsareconsumed. M>Mz++ze Mz++ze>M
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Atequilibriumthenumberofelectronsproducedbytheanodichalfcellreactionandthe number ofmeal ions transformedfrommetal atoms is thesameas thenumberofelectrons consumedbythecathodichalfcellreactionandalsothenumberofionstransformedbackto atoms,andsoboththehalfcellreactionsproceedatequalrates.Thecurrentflowsfortheanode and cathode half cell reactions are finite and identical, but in opposite directions. But the magnitudeofthecurrentflowsisidenticalforbothhalfcellreactions.Thismagnitudeofcurrent is called theExchange Current Density.The units usedare currentper unitarea ofsurface exposed, for example A/cm2, The reaction surface and ions involved controls the exchange current density. Some examples are given below for zinc when it is placed in a solution containingzincionsofthetypeindicatedinthetablebelow:
Ion Perchlorate Sulfate Chloride
ECD(A/cm) 2 3x108 3x105 3x104
The exchange current density or ECD represents an equilibrium point. The metal in solutionofitsowniontypecanonlycorrodeorhaveionsdepositwhenthecurrentdensityisnot attheexchangecurrentdensityvalue. Asdescribedabove,REDOXreactionsarebothanodicandcathodichalfcellreactions Potentials. but balanced under equilibrium conditions. Clearly, some standard has to be set for this equilibriumandsostandardconditionsaredefinedasoneatmosphereofgaspressure,puresolid present, normal ionic solution strengths at 25oC temperature. Normalcy is the number of equivalentsperliterofsolution.Normalionicstrengthiswhenonegramequivalentisdissolved perliterofwater.Forhydrochloricacid,aonenormalsolutionwouldbethegrammolecular weight, or gms per liter of solution. For one normal sulfuric acid, H 2SO4, half the gram molecularweightinaliterofwaterwouldberequiredasithastwohydrogenionspresentanda normalsolutionrequiresonlyoneelectrontobeavailable.Forbasicsolutionsthenumberof hydroxylionsisusedinsteadofhydrogenionsorthenumberofelectronstransferredinthe reaction. WhatisunavailableatthisstageisascaletodifferentiatebetweenthedifferentREDOX reactionslistedforbothmetalsandnonmetals.Areferencescaleisrequired.Thisissometimes calledtheElectroMotiveForce(EMF)seriesorREDOXpotentialseries. Tomeasurepotentials(orvoltages)forcomparisonofREDOXreactions,thereference used is a Standard Hydrogen Electrode (SHE). This electrode is a constructed by bubbling gaseoushydrogenat1atm.ofpressureoverapieceofpureplatinuminasolutionofonenormal H+ions.Theredoxreactionistherefore: 2H++2e=H2g
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H 2g
Pt foil Permeable membrane 1 N H ions StandardHydrogenElectrode. Thepotentialforthisreactionbydefinitionis0.0V.ItisacompleteREDOXreaction and will maintain a standard, constant voltage unless the conditions change away from the definedstandards.Theoxidation,anodicreactionandthereduction,cathodichalfcellreactions aregivenbelow: H2>2H++2e 2H++2e>H2 Noelectronremovalisallowedandthehydrogenionstrengthmustbemaintained. BycouplingdifferentmetalsMtotheSHEtheirpotentialscanbemeasuredin voltsonavoltmeter.ThisgivesrisetothedatabelowforEMForREDOXpotentials.Notethat issomeareas,thescaleisoppositewithgoldforexampleat1.48V.Mostengineeringpractices usethescalebelow,butbecareful. Reaction Au=Au3++3e O2+4H++4e=2H2O Pt=Pt2++2e O2+2H2O+4e=4(OH) 2H++2e=H2 Fe=Fe2++2e Cr=Cr3++3e Zn=Zn2++2e Al=Al3++3e 6 Potentialatequilibrium(volts). 1.498 1.229 1.2 +0.401 0.0 0.440 0.744 0.763 1.662
In the figure below showing how a SHE is coupled to the REDOX reaction being measured,ahighresistancevoltmeterisusedtomeasurethevoltageforthisseriestoensurethat nocurrentflowsfromtheREOXcellsandsonocorrosionordepositionisoccurringasallthe reactionsareequalandoppositeinrate.TheREDOXreactionbeingmeasuredmustalsobeat standardcondition.Thisseriesprovidesthethermodynamicprobabilitythatapuremetalunder standardconditionswilleitherionizebyananodicreactionorplateoutinacathodicreaction.
V olts H 2g
Pt foil Metal
1 N H ions
1 N M ions
Permeable membrane CellfordeterminationofEMFseriesorRedoxpotentials. ThemorenegativethepotentialmeasuredintheEMForREDOXseries,themorelikely thematerialistooxidizeinananodichalfcellreaction.Therefore,gold,Auistermedverynoble asithasahighpositiveEMFvalue.ZincisveryanodicasithasalownegativevalueofEMF. Thereforeconnectingahalfcellofgoldtoahalfcellofzincwillinduceanodicreactionofthe zinctoproduceelectrons,i.e.: Zn>Zn2++2e Concurrently, the cathodic half cell reaction will be favored in the half cell with the more positivepotentialandgoldionswillbereducedtogoldby: Au3++3e>Au. Thezincwillhavecorrodedasweightlosswilloccur.Thisprocessisessentiallygalvanic corrosion.Aswillbediscussedlaterthisisausefulcorrosionprocessasprotectiontechniques
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sometimesrelyondissolvingacheap,easilyreplacedmetalratherthantheexpensivestructural metals.Twoexamplesofthisaregalvanizingandsacrificialanodes. SomeimportantfeaturesfortheSHEandEMFseries. Thestandardhydrogenelectrodeisadifficultpieceofapparatustouse.Hydrogengasis required along with a reasonably strong acid solution. It is not practical for everyday use. SecondlytheEMFseriesonlyappliestopuremetalsinhighlyspecifiedconditions,whichare notoftenseen,suchasonenormalionicsolutions.Itisagoodguideonlyfortheseconditions. Thirdly,someelementsareverylowintheseriessuchaschromium,yetthiselementisaddedto irontomakesteels"stainless". Atthispointthereissomediscrepancybetweentheoryandpractice,whichisansweredby thesofarundescribedroleofkineticsoncorrosion.Kineticsinvolvestherateofareaction; thermodynamicsconsiderswhethera reactionisfavorableornot.Forareactiontoproceedit mustbethermodynamicallyfavorable.Soreactionscanbethermodynamicallyfavorable,but kineticsdeterminestherateatwhichthereactionproceeds.Fortunatelyincorrosion,thekinetics ofareactionwilloftenaidinreducingtheexpectedrate.Exampleswouldbestainlesssteelsand titaniumanditsalloys. Reference Electrodes. The Standard Hydrogen reference electrode has already been described in earlier notes. It is an impractical electrode with a strong acid and highly flammable hydrogen gas required. A simpler reference electrode is required for practical measurements and several exist. The most common ones are the Calomel electrode, the copper/copper sulfate electrode and the silver/silver chloride electrode. The requirements of a reference electrode is that are a constant reference against the standard hydrogen electrode over a wide range of conditions. This usually means that a self contained half cell reaction occurs that cannot be change by the environment in which the reference electrode is placed. A good example is the Saturated Calomel Electrode or SCE. A diagram of the electrode is shown below.
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To Voltmeter Platinum Wire
Filling Hole
Glass or Plasic Body
Saturated KCl
Porous Tip Saturated Calomel Reference Electrode. This is a very common electrode. It is based on the mercurous to mercury half cell reaction of:Hg2Cl2 = Hg2++ + 2ClHg2++ + 2e- = 2Hg Overall Reaction:Hg2Cl2 + 2e- = 2Hg + 2ClNernst Equation gives:ECal = Eo - (RT/zF) ln (aCl-)2 = Eo - (2.303RT/F) log aClECal = +0.2677 - 0.059 log aClEcal = +0.242 V against SHE for Hg/Hg2Cl2 in saturated KCL.
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In the SCE the Calomel paste of mercurous chloride is mixed with mercury and and the filling solution is saturated potassium chloride. The saturated solution has a fixed chloride ion activity and the mercurous chloride is only slightly insoluble in water. A porous tip enables ionic transport to complete the necessary components for measuring voltage along with connection to a high resistance voltmeter.The electrode maintains its chemical conditions and provides a reference potential against the standard hydrogen electrode of +0.242 V. Other common reference electrodes are the Copper/Cupric Sulfate electrode where a copper bar is placed in a saturated cupric sulfate solution. E Cu/CuSO4 = +0.34 V(SHE) for saturated copper sulfate solution CorrosionRates,PolarizationandFaradaysLaw Sofar,muchhasbeenexaminedregardingequilibriumconditions.However,corrosion anddepositiondonotoccurunderequilibriumconditions.Themeasurementofthecorrosionrate ofmaterialintheenvironmentisvitaltothecorrectselectionofamaterial.Onemethodtoplace thematerialintheenvironmentandmonitoranyweightchange.Anotherisconductalaboratory studyandconductmeasurements.Theissueiswhatshouldbemeasured.Inthissection,the parameters which provide information on corrosion rates and which can be measured in a laboratorywillbedetermined. InconsideringthegeneralREDOXreactionatametalelectrodeM: M=Mz++ze AttheRedoxpotentialunderstandardconditionsusedtodeterminetheRedoxseries, thereisnonetcurrentflowastheanodereactionrateisthesameasthecathodereactionrate. ThisvoltagewillbecalledErforRedoxpotential.Whenthepotentialmovesawayfromthis equilibriumvaluesoitnolongerequalsEr,thenthereactionratesbecomesunbalancedandone orotherofthehalfcellreactiondominates,eithercathodicoranodic.Atthisnewpotentialeither anetfluxofelectronswillbeproduced,iftheanodichalfcelldominates,orelectronswillneed tobeprovidedtoallowthecathodichalfcellreactiontoproceed.Sobycontrollingthepotential thereactioncanbeeitheranodicorcathodic.ThisisalsocalledPolarization,themovement awayfromtheRedoxpotentialvalue. PolarizationofanElectrode. Polarizationofanelectrodeisthemovementawayfromtheequilibriumstatewherethe anodereactionrateisequalandoppositetothecathodicrateatstandardconditions.Asimplified modelofthesituationwillbepresentedinthissectiontodeterminewhathappenswhenthe conditions arealtered awayfromthestandardones.Theparticular casewillbewhen some electronsproducedbytheanodichalfcellreactionareremovedandnotavailableforusebythe cathodichalfcellreaction. Considerapuremetalatequilibriuminitsownions.Atomssitinlowenergypositionson thesurface.Anactivationenergybarrierexistsfortheseatomstobecomeionsandproduce electrons.Aschematicdiagramofthesituationisshowninthefirstfigurebelow. Atequilibriumtheionexchangecurrentdensityisavailableofatomstransformingtoions (anodereaction)andatanequalratetheionsfromsolutionaretransformingtoatoms(cathodic reaction)attheredoxpotentialofEr
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AtequilibriumtheactivationenergybarrierisGe Ge=BF Theenergybarier,Ge,isthesameindependentofthereactiondirection,anodicorcathodic. Inthesecondfigurebelow,theenergyoftheatominthesurfacewasincreased,oritwas AnodicallyPolarisedtomakeitmorefavorablefortheatomstoformions. AssumeBE=FG LetBC/AC= ThenAB/AC=1
Ge
Atom in Surface
Ion in Solution
G Ga F
D Atom in Surface E Ion in Solution
C
B
A
Bysimilartriangles,CD/EB=AC/AB=1/(1) EB=CD(1) CD=Changeinanodicenergyappliedbyanexternalvoltage
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CD=z aF whereznumberofelectronsinvolvedinthehalfcellreaction, aoverpotentialorvoltagemovementfromErvalue,FFaradaysconstant(96500coulombs). EB=(1a)z aF Thenthenewactivationenergybarrierforanatomtoiontransfer,releasingelectronsis: Ga= GeCD+FG Ga=GeCD+EB Ga=Gez aF+(1)z aF Ga=Gez aF Also,astheprocessisactivationcontrolleditfollowsanArheniusratelawof: Rate=Ke( G/RT) o RBoltzmannsconstantof1.96cal/oK,Ttemperaturein K Therateinthiscaseisproportionaltothecurrentflowing. Then: i=Ke( G/RT) i =Ke( Ga/RT) a i =Ke(( Gez aF)/RT) a AtequilibriumioistheExchangeCurrentDensityatRedoxpotential,Er i =Ke( Ge/RT) o Expandingtheequationforanodicdissolution: ia=Kexp(Ge/RT)exp(z aF/RT) ia=ioexp(z aF/RT) Takinglogs lnia=lnio+z aF/RT Rearranging: a=(lnialnio)(RT/zF) Forcathodicpolarization. c=(lniolnic)((RT/(1)zF) ThesearetheTafelequationsforpolarizationofanelectrode.Theyfollowthegeneralformofa straightlineifisplottedasafunctionoflogi. y=c+mx =a+/blogi TheequationaboveistheTAFELequationwhichrelatespotentialtocurrentdensityforan electrochemicalreaction. Theconstantsfortheequationarelistedbelow.Theconstant"b"iscalledtheTafelconstant. Anodic a=lnio(RT/zF) b=RT/zF
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Cathodic a=lnio(RT/(1)zF) b=(RT/(1)zF) Overall,changingtheconditionsfromthe halfcellreactionsthereforeproducespolarization resultinginacurrent.
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