Corrosion Protection of Cast Iron in Drinking Water withHarmlessTernary Inhibitor
M.Umarani1, T.Kasilingam2andC.Thangavelu3,*
1MunicipalHigher Secondary School,Karur,Tamilnadu,India.
2DepartmentofChemistry,TheivanaiAmmalCollegeforWomen,Villupuram,Tamilnadu,India.
3DepartmentofChemistry,Govt.ArtsCollegeforWomen,Nilakottai,Dindigul,Tamilnadu,India.*CorrespondingAuthorE-mail:thavankasi@gmail.com
ABSTRACT:
Theprotectionofcorrosionofcastironindrinkingwaterusingcurryleafextract(CLE),ZincionsandSodiumTartrate(ST)wasinvestigatedusingclassicalweight-lossmeasurement,TafelpolarizationandNyquistimpedancemethods.Inhibitionefficiencyofaboveternaryinhibitorreachesmaximumvaluesat85%.TheshapeofDCpolarizationprofilerevealedthenewternarysystemwhichactsasanodictypeinhibitor.ACimpedancespectroscopicstudiesindicatedtheformationofaprotectivefilmontothemetalsurfaceinthepresenceofinhibitors.AnalysesoftheprotectivefilmusingscanningelectronmicroscopeandenergydispersiveX-rayanalysis.
KEYWORDS:Curryleafextract,Tafelpolarization,Nyquistimpedancespectroscopy,classicalweight-losstechniquesandSEM-EDX.
INTRODUCTION:
Mechanicalpropertiesofcastironlikestrength,ductility,andmodulusofelasticitydependstronglyonstructureanddistributionofmicrostructuralconstituents.Physicalpropertiessuchasthermalconductivityanddampingcapacityarestronglyinfluencedbymicrostructure.Castironpopularitystemsfromanabilitytocastcomplexshapesatrelativelylowcostandthewiderangeofpropertiesthatcanbeachievedbycarefulcontrolovercompositionandcoolingratewithoutradicalchangesinproductionmethods.
Alloyingelementscanplayadominantroleinthesusceptibilityofcastirontocorrosionattack.Siliconisthemostimportantalloyingelementusedtoimprovethecorrosionresistanceofcastirons.
Siliconisgenerallynotconsideredanalloyingelementincastironsuntillevelsexceed3%.Siliconlevelsbetween3and14%offersomeincreaseincorrosionresistancetothealloy,butaboveabout14%Si,thecorrosionresistanceofthecastironincreasesdramatically[1].
Castironpipeshavebeenusedtotransportpotablewaterforover500years[2]andironpipecorrosionhasbeenaproblemforjustaslong.TheAmericanWaterWorksAssociation(AWWA)estimatesthatitwillcostUSwaterutilities$325billionoverthenext20yearstoupgradewaterdistributionsystems[3].ThisAWWAvalueisbuiltontheU.S.EnvironmentalProtectionAgency(USEPA)estimateof$77.2billionforserviceandreplacementoftransmissionanddistributionsystemlinesoverthenext20years[4].Themajorityofdistributionsystempipesarecomposedofironmaterial:castiron(38%),ductileiron(22%),andsteel(5%)[5].Moreover,a1997surveyofthe100largestAmericanWaterWorksAssociationResearchFoundation(AWWARF)memberutilitiesfoundthat“themostcommondistributionsystemproblemiscorrosionofcastironpipe”[6].
Corrosionisanaturalandthermodynamicallystableprocess.Theprocessofcorrosioncanbereducedbutitcannotbeprevented.Therearemanymethodsbywhichcorrosioncanbeinhibitedonesuchmethodistheuseofinhibitivemethod.Theseinhibitorswhenaddedissmallconcentration,decreasetherateofcorrosion.CorrosioninhibitorsusuallycontainpolargroupswithatomssuchasN,SandO.Correspondinglyinhibitorsincludeawidelistoforganicandinorganiccompounds[7].
Theobjectiveofthepresentstudyistoinvestigatetheinhibitoryeffectsofthecurryleafextract,sodiumtartrateandZn2+inthecorrosioninhibitionofcostironindrinkingwaterusingelectrochemicalmethodsandnon-electrochemicalmethod.Surfaceanalyticaltechniquewasalsousedtoinvestigatethenatureofthesurfacefilm.
MATERIALSANDMETHODS:
Materials:
Thespecimensofsize1.0cm×4.0cm×0.1cmwerepresscutfromthecastironsheet,weremachinedandabradedwithaseriesofemerypapers.Thiswasfollowedbyrinsinginacetoneandbidistilledwaterandfinallydriedinair.Beforeanyexperiment,thesubstratesweretreatedasdescribedandfreshlyusedwithnofurtherstorage.Theinhibitorcurryleafextractwasusedascollected.Astocksolutionof1000ppmofcurryleafwaspreparedinbidistilledwaterandthedesiredconcentrationwasobtainedbyappropriatedilution.Thestudywascarriedoutatroomtemperature.
Alltheweighingofthecastironspecimens,beforeandafterimmersion,weredoneusingaDenverbalance,TP214model,withareadabilityof0.1mgin210grange.Thisbalancehasreproducibility(standarddeviation)of0.1mgin210grangeandwassuppliedbyDenverinstruments,Germany.
Electrochemicalstudies:
Boththepotentiodynamicpolarizationstudiesandelectrochemicalimpedancespectroscopic(EIS)studieswerecarriedoutusingtheelectrochemicalworkstationmodelCHI-660AandCHI-760dandtheexperimentaldatawereanalysedbyusingtheelectrochemicalsoftware(Version:12.22.0.0).Themeasurementswereconductedinaconventionalthreeelectrodecylindricalglasscellwithplatinumelectrodeasauxiliaryelectrodeandsaturatedcalomelelectrodeasreferenceelectrode.
Theworkingelectrodewascastironembeddedinepoxyresinofpolytetrafluoroethylenesothattheflatsurfaceof1cm2wastheonlysurfaceexposedtotheelectrolyte.Thethreeelectrodessetupwasimmersedincontrolsolutionofvolume100mlbothintheabsenceandpresenceoftheinhibitorsformulationsandallowedtoattainastableopencircuitpotential(OCP).
Polarizationcurveswererecordedinthepotentialrangeof-750to-150mVwitharesolutionof2mV.Thecurveswererecordedinthedynamicscanmodewithascanrateof2mVS-1inthecurrentrangeof-20mAto+20mA.TheOhmicdropcompensationhasbeenmadeduringthestudies.Thecorrosionpotential(Ecorr),corrosioncurrent(Icorr),anodicTafelslope(βa)andcathodicTafelslope(βc)wereobtainedbyextrapolationofanodicandcathodicregionsoftheTafelplots.Theprotectionefficiency(IEp)valueswerecalculatedfromtheIcorrvaluesusingtheequation.
Where
IcorrandI’corrarethecorrosioncurrentdensitiesincaseofcontrolandinhibitedsolutionsrespectively.
ElectrochemicalimpedancespectraintheformofNyquistplotswererecordedatOCPinthefrequencyrangefrom60KHzto10MHzwith4to10stepsperdecade.Asinewave,with10mVamplitude,wasusedtoperturbthesystem.Theimpedanceparametersviz.,chargetransferresistance(Rct),doublelayercapacitance(Cdl)wereobtainedfromtheNyquistplots.Theprotectionefficiencies(IEim)werecalculatedusingtheequation,
Where
RctandR’ctarethechargetransferresistancevaluesintheabsenceandpresenceoftheinhibitorrespectively.
Scanningelectronmicroscope(SEM):
Thecastironwasimmersedintheabsenceofinhibitorandinthepresenceoftheinhibitor.After7days,thespecimensweretakenout,washedwithtripledistilledwaterandairdried.TheSEMimagesofthesurfacesofthespecimenswereobtainedusingVEGA3TESCANmodelintheCentralInstrumentFacility,NationalCollege,Trichy.
EnergydispersiveX-rayanalysis(EDX):
EDAX(Model:BRUKERNanoGermany)systemattachedwithScanningElectronMicroscopewasusedforelementalanalysisorchemicalcharacterizationofthefilmformedonthecarbonsteelsurface.Asatypeofspectroscopy,itreliesontheinvestigationofsamplethroughinteractionbetweenelectromagneticradiationandthematter.Sothat,adetectorwasusedtoconvertX-rayenergyintovoltagesignals.Thisinformationissenttoapulseprocessor,whichmeasuresthesignalsandpassedthemintoananalyzerfordatadisplayontheanalysis.
RESULTSANDDISCUSSION:
ClassicalWeight-lossmethod:
Thevaluesofinhibitionefficiencyandcorrosionrate(mmy-1)obtainedfromweightlossmethodatdifferentconcentrationaresummarizedinTable1.Itisobservedfromthatinhibitionefficiencyincreaseswithincreasingtheprotectorconcentrationindrinkingwaterandshowsasharpincreaseintheprotection,whichreacheditsmaximumvalueatconcentrationof5ppmZn2++100ppmST+250ppmCLEandfurtherincreaseintheprotectorconcentrationdoesnotshowanyappreciablechangeintheinhibitionefficiency.Thisindicatesthattheprotectiveeffectofinhibitorisnotsolelyduetotheirreactivitywiththedrinkingwater.Theprotectorybehaviouroftheternaryinhibitoragainstcorrosionofcastironcanbeattributedtotheadsorptionofinhibitoronthecastironsurface,whichlimitsthedissolutionofthecastironbyblockingofitscorrosionsitesandhencedecreasingthecorrosionratefrom139.2to22.2mmy-1.
Table1.Non-electrochemicalparametersofcastironindrinkingwaterintheabsenceandpresenceofCurryleafextract+Zn2++ST
|
Concentration(ppm) |
Corrosionrate(mmy-1) |
Inhibitionefficiency(%) |
||
|
CLE |
Zn2+ |
ST |
||
|
Control |
- |
100 |
139.2 |
- |
|
10 |
5 |
100 |
82.1 |
41 |
|
50 |
5 |
100 |
72.3 |
48 |
|
100 |
5 |
100 |
64.0 |
54 |
|
150 |
5 |
100 |
45.9 |
67 |
|
200 |
5 |
100 |
34.8 |
75 |
|
250 |
5 |
100 |
22.2 |
84 |
Impedancespectra:
Nyquistimpedance(EIS)spectraofcastironunderourstudyareshowninFig.1.Nyquistparameters,namelythechargetransferresistance(Rct)andthedoublelayercapacitance(Cdl)aregiveninTable2.Whencastironisimmersedindrinkingwater,theRctisfoundtobe213.1Ωcm2andtheCdlvalueis7.8693×10-6F/cm2.Whenpresenceofternaryinhibitor(5ppmZn2++100ppmST+250ppmCLE),theRctvalueshasincreasedtremendouslyfrom213.1Ωcm2to1467.6Ωcm2andCdlvaluesdecreases7.8693×10-6F/cm2to0.1507×10-6F/cm2.TheincreasedRctanddecreasedCdlvaluesobtainedfromimpedancestudiesjustifythegoodperformanceofacompoundasaninhibitorindrinkingwater[8].Thisbehaviourshowsthatthefilmobtainedactsasabarriertothecorrosionprocessthatobviouslyprovestheexistenceandformationoftheinhibitivefilm.
Figure1.Nyquistplotsofcastironintheabsenceandpresenceofternaryinhibitor
Table2.ElectrochemicalNyquistparametersforthecastironindrinkingwaterinthepresenceandabsenceofinhibitor
|
Conc.ofCLE(ppm) |
Conc.ofST(ppm) |
Conc.ofZn2+(ppm) |
Rctohm.cm2 |
CdlF.cm-2×10-6 |
%IEim |
|
Blank |
- |
- |
213.1 |
7.8693 |
- |
|
250 |
100 |
5 |
1467.6 |
0.1507 |
85 |
PolarizationTechnique:
TheTafelparametersobtainedfromelectrochemicalmeasurementssuchascorrosionpotential(Ecorr),corrosioncurrent(Icorr),anodicandcathodicTafelslopes(βaandβc)andinhibitionefficiencyaregiveninTable3.
Figure2.Tafelcurvesofcastironintheabsenceandpresenceofternaryinhibitor
TheobservedEcorrvaluesdolargechangeinaregularmannerfromtheblankvalue.Thisindicatesthattheinhibitorworksthroughanodicmodeofinhibition.ItisobviousfromFigure2,thatTafelcurvesareshiftedmarkedlytolowercorrosioncurrentdensityinthepresenceofinhibitor(5ppmZn2++100ppmST+250ppmCLE).TheIcorrvaluedecreasefromtheblankvalue(7.450to1.314),thisdecreaseinIcorrisanindicationofincreasedinhibitionefficiency(82).
Table3.ElectrochemicalTafelparametersforthecastironindrinkingwaterinthepresenceandabsenceofinhibitor
|
Conc.ofCLE(ppm) |
Conc.ofST(ppm) |
Conc.ofZn2+(ppm) |
Tafelparameters |
IEp (%) |
|||
|
EcorrmVvsSCE |
IcorrAµ/cm2 |
βamV/decade |
βcmV/decade |
||||
|
Blank |
|
- |
-741 |
7.450 |
151.4 |
122.6 |
- |
|
250 |
100 |
5 |
-575 |
1.314 |
210.1 |
182.7 |
82 |
Scanning Electron Microscope:
Figure 3. SEM micrographs of cast iron a) Polished surface b) Absence of inhibitor c) Presence of ternary inhibitor
SEMmicrographsforcastironinabsenceandpresenceofinhibitorsystemhavebeenpresentedinFig.3.Themorphologyofthecastironspecimenintheabsenceofinhibitorisveryroughandthesurfacewasdamagedduetometaldissolution.Polishedcastironsurface(Fig.3a)isgoodsurfacepropertiesascomparedtotheblankmaterial(Fig.3b).At5ppmZn2++100ppmST+250ppmCLE,ithasbeenfoundthatthesmoothnessofthesurfaceimprovedremarkablyandthemetalsurfaceisalmostfreefromcorrosionduetothebettercoverageofthemetalsurface(Fig.3c).
EnergyDispersivex-rayAnalysis:
ThecompositionofprotectivefilmformedonthecastironsurfacewasanalyzedusingEDXasshowninFig.4.(a-c).TheEDXspectrumofpolishedcastironsampleinFig.4ashowsaunityofsurfacecompositionproperties,whilethespectrumincaseofcastironsampleimmersedinabsenceofinhibitormoleculeswasfailedbecauseitisseverelyweakenedduetothecorrosionasshowninFig.4b.byadding5ppmZn2++100ppmST+250ppmCLEthedecreaseofironpeakandappearanceofcarbon,heteroatomandzincpeakwasobservedduetotheformationofastrongprotectivefilmoftheinhibitormoleculesonthesurfaceofcastironsampleshowninFig.4c.Theactionofinhibitorisrelatedtoadsorptionandformationofabarrierfilmontheelectrodesurface.TheformationofsuchabarrierfilmisconfirmedbySEMandEDXexaminationofcastironsurface.
Figure4.EDXspectraofcastirona)Polishedsurfaceb)Drinkingwater(blank)c)Presenceofternaryinhibitor
CONCLUSION:
Thefollowingmainconclusionsaredrawnfromthepresentinvestigation.
1 Ternaryinhibitorformulationhasgoodinhibitionefficiencyforthecorrosionofcastironindrinkingwater.
2 Tafelcurvesshowedthattheinvestigatedternaryinhibitorformulationactsasananodictypeinhibitoroncastironindrinkingwater.
3 Allresultsobtainedfromweightlosstechnique,polarizationtechniqueandelectrochemicalimpedancespectroscopyareinreasonablygoodagreementandshowsincreasedinhibitionefficiencywithincreasinginhibitorconcentration.
4 Surfaceanalysisusingscanningelectronmicroscope(SEM)andEnergydispersivex-rayanalysisshowedasignificantmorphologicalimprovementonthecastironsurface.
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Receivedon04.01.2018Modifiedon28.02.2018
Acceptedon20.04.2018©AJRCAllrightreserved