Corrosion&Materials

8/7/2019 Corrosion&Materials

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Corrosion& Materials24 Vol 34 No 4 August 2009 25

AStatisticsApproachforthePredictionofCO 2CorrosioninMixedAcidGases

YuliPancaAsmara 1,MokhtarCheIsmail2

1DepartmentofMechanicalEngineering,UniversityofMataram

JlMajapahit12,Mataram,NTB,Indonesia

2DepartmentofMechanicalEngineering,UniversitiTeknologiPETRONAS

BandarSeriIskandar,31750Tronoh,Perak,MALAYSIAEmail:[email protected], [email protected]

Abstract

PredictingCO2corrosionisanimportantelementinCorrosionDesignBasis(CDB)whichdeterminesmaterialselectionandcorrosioncontrolstrategies.SinceCO2corrosionisamultispeciescorrosionmechanism,therearenumerouscorrosionpredictionmodelsdevelopedwithdifferentparameters.However,mostofthesepredictionmodelsdonotconsiderthecombinedeffectofmixedgasescontainingCO2,H2Sandaceticacid(HAc)whichlimitsthecorrosionpredictionscope.ThisstudyanalyzestheeffectsofmixedgasesbyusingElectronicCorrosionEngineer(ECE)corrosionpredictionsoftwarecombined

withaResponseSurfaceMethodology(RSM)statisticaltechnique.ECEpredictionshowsthatsimultaneouseffectsofmixedgasesbehavedifferentlyascomparedtotheindividualeffects.ContrastbehaviorwasobservedforH2Sasanindividualspeciesandasamixedspecies;corrosionratedependsonconcentrationlevelofH2S.HAcspeciesshowedasharperincreaseincorrosioninamixturethanalone.Thispreliminaryanalysisofmixedgaspredictionshowedmultiplemechanisms.Thus,complexinteractionsofmixedgasesmustbestudiedinmoredetailtoobtainanaccurateprediction.

1.Introduction

Predictingcorrosionbehaviorisacrucialissueinoilandgasproduction.Corrosionpredictionisoneofthetoolstomakedecisionsinselectingmaterial,formulatingguidelineforscheduledinspection,andmaintainingofthepipelinestructurefacilities.Damagetothepipeline

duetoacorrosionreactionmustbedetectedforavoidinglossofproductionandinvestment.Therefore,corrosioncontrolandmonitoringmethodsareimportanttopreventtheprematurefailureofindustrialfacilitiesandtoincreasesafety.

Indesigningacorrosionmodel,themodelmustbeconcernedwithparametersaffectingCO2corrosionasintherealcondition.Predictingmodelsshouldalsobecapableofprovidingreasonablygoodagreementbetweentheexperimentalobservedvaluesandtherealcondition.ManyCO2corrosionpredictionmodelsareproposedin

theindustry.Eachmodelisdesignedbasedoncertainmechanismsundercertaincircumstances.So,inordertocalculatecorrosioninoilandgasproductionenvironmentsaccurately,itwillbenecessarytodenethechemistryofoilandgasenvironmentandthemechanismofthecorrosionreaction.

IncorporatingtheroleofH2Sgasincorrosioncalculations

isimportantforthepredictionofCO2corrosion.Manyoftheoileldsaroundtheworldcontainthisgas.NaturalgasproducedinnorthernVenezuela,forexample,hasaH 2Scontentof25-80ppm.InotherplaceslikeBraziloffshore,GulfofMexico,Norway,andDenmark,haveH2Sintherangeof10ppm30ppm[1-3].

BesidesH2S,aceticacidalsoissuspectedforcontributingtocorrosionfailure.Shellrecordedacorrosionfailureduetopresenceofthisorganicacidinconcentrationrangesfrom150to700ppm.Afailurewasreportedina10inchproductionowlineinWytchFarmoileld(UK)wasalsoreporteddueto100mg/lacetate[4].

TheroleofH 2Selementalisbelievedtoinuencethecorrosionrate.ItwasfoundbyBrown[5]thatsmallconcentrationsofH2S(lessthan30ppm)inCO2,increasedthecorrosionratecomparedtosolutionwithoutH2S.However,corrosionratedecreasedwhenthecondition;H 2S(500ppm),pH(


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Table 2:Randomizationofcodedexperimentsandresultpredictions.

No.run Var1 Var2 Var3 Predct.

1 -1 -1 -1 1.8

2 1 -1 -1 3.2

3 -1 1 -1 1.7

4 1 1 -1 2.4

5 -1 -1 1 2.25

6 1 -1 1 3.4

7 -1 1 1 2.1

8 1 1 1 3.2

9 1.7 0 0 3.1

10 -1.7 0 0 0.9

11 0 1.7 0 3.5

12 0 -1.7 0 3.7

13 0 0 1.7 3

14 0 0 -1.7 2.4

15 0 0 0 2.8

3.ResultandDiscussion

A. Analysis of regression model

ThematrixexperimentaldesigntocalculatecorrosionmodelissummarizedintheTable2andtheresultsoftheANOVAaregivenintheTable3.Table3presentsthevariantanalysisofthepolynomialmodelregressionascalculatedbyMinitab[21].ItwasusedtotcorrosionbehaviorcalculatedbytheECEprogramsoftware.Thecalculationshowsthecoefcientdeterminations.Itrepresented95.9%oftheoverallcalculationwhichmeansthatthemodelisinerrorbyabout4%.

Table 3:Analysisofvarianceformodelregression.

Source DF Seq SS Adj SS Adj MS F P

Regression 9 8 .3 66 52 8 .3 66 52 0 .9 29 61 2 6. 50 0 .0 00

Linear 3 5 .5 30 70 5 .5 30 70 1 .8 43 57 5 2. 55 0. 00 0

Square 3 2 .7 24 88 2 .7 24 88 0 .9 08 29 2 5. 89 0 .0 00Interaction 3 0 .1 10 94 0 .1 10 94 0 .0 36 98 1 .0 5 0 .4 11

Residual Error 10 0.35085 0.35085 0.03509

Lack-of-Fit 5 0.35085 0.35085 0.07017

Total198.71737

Corr.rate(mm/y)=2.83223+0.587083*CO2-

0.115385*H2S+0.208273*HAc-

0.338661*CO2*CO+0.214973*H 2S*H2S-

0.0964461*HAc*HAc-0.0937500*CO 2*

H2S+0.0187500*CO 2*HAc+0.0687500*H 2S*HAc

R-Sq=95.98%

TheanalysisofcorrosionregressionwascarriedoutatCO2concentrations(0-10%mol),H2Sconcentrations(0.002-0.1%mol),andHAcconcentrations(150470ppm).TodeterminesignicanceeffectsofthemodelaF-testwasused.TheF-testisformulatedbycomparingbetweenmeansumsofsquaresofpredictedmodelswiththemeansumsofsquaresoferrorexperiments[19].Fromthevariantanalysis(Table3),itcanbeseenthatregressionmodelshavesignicantvaluesfor95%ofcondentlevel(p-value


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differentcomparedtoitsindividualeffect.Intheindividualeffects,H 2Satconcentrationsfrom0.001%molto0.01%decreasedthecorrosionrate.But,inthecombinedeffects,thisdecreasewasonlyataconcentrationof0.006%mol.AswithCO2,H2Salsohasacapabilitytoformlmswhichcanactasabarriertoirondissolution[22].ButthelmformedbyH2Sgas,atdifferentconcentrationshasdifferentproperties[23,24,25].MoredenselmsareformedatthelowerH2Sconcentration[23].Withaconcentrationmorethan0.006%molH2S,irondissolutioncontrolsthereactionratecausingcorrosionratetoincrease.Figure4showstheeffectsofHAconcorrosionrate.ThecombinedeffectofHAcandCO2at0.006%H2Soncorrosionrateindicatedapolynomialmodel.EffectsofHAcshowedanincreaseofcorrosionrateintherangeof100ppmto400ppm.Bothcombinedandindividualeffectsindicatedthesametrends.Somestudiescarriedoutinthepastareconsistentwiththeseresults[4,13,26].Theyrelatedthecorrosionratewithanincreaseiontherateofthecathodicreactionandwiththepropertiesofironacetate,whichwassoluble,permittingsubsequentcorrosion.

4.Conclusion

AccordingtotheECEsoftware,thecombinedeffectsof

variableshavechangedthecorrosionbehaviorcomparedtotheindividualeffects,especiallyforH2Sgas.H2Sgasalonereducedthecorrosionrateconsistently.But,inamixtureofgases,H2Sreducedthecorrosionrateinitially.Afterthatitincreasedthecorrosionrate.HAcdidnotchangethetrendofcorrosionrateduringexposuretime.HAcalwaysincreasedthecorrosionrateeitheraloneorincombinationwithotherenvironments.

TheappropriatenessofthemodelwasdiscussedandthemodelwasshowntopredictCO2corrosionprediction.ThepolynomialregressionmodelprovidedagoodagreementtothecorrosiondatacalculatedbyECE(96%condencelevelascalculatedbyRSM).Centralcompositedesignallowedamoreeconomicaldesignofexperimentsratherthantheuseofafullrandomizeddesign.Itreducednumberofexperimentssignicantly.Furthermore,theuseofRSMprovidedacontourplotwhichmakesiteasierto

analyzetheresultsoftwovariablessimultaneously.Achallengingrecommendationistobuildapredictionmodelinvolvingvariousmodicationsusingmoreparametersandusingotherregressionstatisticstechnique.However,suchapredictionmodelwouldstillneedtobeveriedandfurtherinterpreted.Actualdatafromrealserviceconditionsarerequiredforaccurateresults.

Acknowledgments

TheauthorsarethankfultoUniversitiTeknologiPETRONASforprovidinggrantandfacilitiesfortheresearch.

References

[1] E.Joseph,Shioya.N.,Costa.E.,Cynthia.D.A,Monica.O.,ACostEffectiveTreatmenttoMitigateBiogenicH2SonFPSO,NACECorrosionAnnualConf.,NACE International, Houston, Texas,Paperno.04751,2004.

[2] Egil.S.,H2SInhibitionbyNitrateInjectionontheGulfField,NACECorrosionAnnualConf.,NACEInternational, Houston, Texas,Paperno.04760,2004.

[3] Tebbal.S.,AnalysisandCorrosivityTestingofEightCrudeOilsNACECorrosionAnnualConf.,NACEInternational, Houston, Texas,Paperno.04636,2004.

[4] JamesA.Dougherty,AReviewoftheeffectoforganicacidsonCO2corrosion,NACE Corrosion

Annual Conf., NACE International, Houston, Texas,Paperno.04376,2004.

[5] Brown,B.,SrdjanNesic,CO2/H2Scorrosionunderscaleformingconditions,NACE Corrosion AnnualConf., NACE International, Houston, Texas,Paperno.05626,2005.

[6] Kun-LinJohnLeeandSrdjanNesic,TheeffectoftraceamountofH2SonCO2corrosioninvestigatedby

usingtheEIS,NACECorrosionAnnualConf.,NACEInternational, Houston, Texas,Paperno.05630,2005.

[7] SrinivasaninBrown,B.,Kun-LinLee,SrdjanNesic.CorrosioninmultiphaseowcontainingsmallamountsofH2S,NACE Corrosion Annual Conf., NACEInternational, Houston, Texas,Paperno.03341,2003.

[8] Hunnik,E.W.J.VanB.F.M.Pots,E.L.J.A.HendriksenKoninklijke.TheformationofprotectiveFeCO3corrosionproductlayersinCO2corrosion,NACECorrosion Annual Conf., NACE International, Houston,

Texas,Paperno.6,1996.

[9] CroletandBonnis,Carbondioxideinoilandgasproduction,Corrosion,Vol.59,pp.8,2003.

[10] GarsaniinOmkarA.Nafday,FilmformationandCO2corrosioninthepresenceofaceticacid,MasterThesisofFritz.j.,DoloresandH.RussCollegeof

EngineeringandTechnology,OhioUniversity,2004.

[11] JoosteninOmkarA.Nafday,FilmformationandCO2corrosioninthepresenceofaceticacid,MasterThesisofFritz.j.,DoloresandH.RussCollegeofEngineeringandTechnology,OhioUniversity,2004.

[12] OmkarA.Nafday,SrdjanNesic,IroncarbonatescaleformationandCO2corrosioninthepresenceofaceticacid,NACE Corrosion Annual Conf., NACE International,Houston, Texas,Paperno.05295,2005.

[13] Ismail.Che.Mokhtar,PredictionCO2corrosionwiththepresenceofaceticacid,PhDThesis,UMIST,UnitedKingdom,2005.

C. Combined Effect of CO2, HAc and H2S Calculated by ECE

EffectsofHAcandH2SadditiontotheCO2gasarepresentedinTable4below.ThecombinedeffectofHAcandH2Swastoincreasethecorrosionratefrom0.52mm/yto2.32mm/y.

Table 4:EffectofH 2SandhacinCO2corrosion.

CO2(%mol)

H2S(%mol)

HAc(ppm)

CR(mm/y)

5 0 0 0.52

5 0.006 0 0.5

5 0.006 300 2.22

5 0 300 2.32

D. Prediction of Corrosion rate calculated by RSM

Figures3and4belowaremodelsofthecombinationeffectofH2S,CO2,andHAconthecorrosionratecalculatedbyECE.ItshowsadifferenteffectcomparedtotheindividualeffectsasshowninFigure2.

Consideringsimultaneouseffectsofmixedgases,itseemsthatonlyH2Sgaveadifferenteffectcomparedtoitssoleeffect(Figure2).WhenH2SwasmixedwithHAcandCO2,H2Sshowedadifferentbehavior.Itreducedthecorrosionrate,initially,butataslightlyhigherconcentration(0.08%mol),itpromotedcorrosion.AlthoughthecombinedeffectsofH2SandHAcshowedasimilartrendcomparedwiththe

soleeffectofH 2S,.thecorrosionrateforthemixedgaseswaschanged.

AnalysisoftheinteractioneffectsofthosethreespeciesoncorrosionratearepresentedatFigure3.ThecalculationwasdoneusingtheECEsoftwaremodiedwithRSMstatisticsmethodology.TheprincipleimportanttoemphasizeisthatthepresenceofmixedspeciesinuencedcorrosionbehaviorasasshownbyFigure3,andCO2increasedcorrosionrateconsistently.However,atthehighconcentrationlevelofCO 2(10%mol),thecorrosionratewasdecreased.Comparedtotheindividualeffects(Figure2),therewasnosignicantdifference.Thus,itcanbeconcludedthatCO2wasadominantfactorindeterminingcorrosionbehavior[14,15,16].EffectsofH2Swereclearly

CathodicCorrosionProtectionofPre-CorrodedSteelinConcretewithGalvanicZincAnodes

H2S(%

mol)

C02 (% mol)

0.0112.5

C02 (% mol) = 7.51388H2S (% mol) = 0.0109649Corr rate (mm/y) = 3.41309

Corr rate(mm/y)

< 1.01.0 1.51.5 2.02.0 2.52.5 3.03.0 3.53.5 4.0

> 4.0Hold Values

HAc (ppm) 300

2.0 3.04.0

3.5

3.0

0.010

0.009

0.008

0.007

0.006

0.005

0.004

0.003

0.002

2 4 6 8 10

(a)

(a)

(b)

(b)

Figure 3:CombinationeffectsofCO2andH2ScalculatedbyECE presentedin2D(a)and3D(b)graphsshowcorrelationscorrosionratewithCO2,H2Sattemperature70C,HAc300ppmandtotalpressure1bar.(HACconcentrationatthemiddleofitslevel)

Figure 4:CombinationeffectsofHAcandCO2calculatedbyECE presentedin2D(a)and3D(b)graphshowcorrelationsofcorrosionratewithCO 2,attemperature70C,H 2S0.006%andtotalpressure1bar.(H2Sconcentrationatthemiddleofitslevel)

C02 (% mol)

H2S (% mol)

Corr rate (mm/y)4

3

2

1

04

812

0.000

0.004

0.008

0.012

C02 (% mol)

HAc (ppm)

Corr rate (mm/y)3

2

1

00

48

12100

200300

400

HA

c(ppm)

C02 (% mol)

450

400

350

300

250

200

150 2.51.5

C02 (% mol) = 8.21288H2S (% mol) = 436.880Corr rate (mm/y) = 3.35016

Corr rate(mm/y)

< 0.50.5 1.01.0 1.51.5 2.02.0 2.52.5 3.0

> 3.0

Hold ValuesH2S (% mol) 0.00615

2.0 3.0

1.0

2 4 6 8 10


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[14] GeorgeinOmkarA.Nafday,FilmformationandCO2corrosioninthepresenceofaceticacid,MasterThesisofFritz.j.,DoloresandH.RussCollegeofEngineeringandTechnology,OhioUniversity,2004.

[15] Heuer.J.K.,J.F.Stubbins,AnXPScharacterizationofFeC03lmsfromCO2corrosion,CorrosionScience,Vol.41,pp.1231-1243,1998

[16]Nesic.Srdjan.,Kun-Lin.,JohnLee.,AMechanisticmodelofironcarbonatelmgrowthandtheeffectonCO2corrosionofmildsteel,NACE Corrosion

Annual Conf.,

[17] http://www.intetech.com

[18] deWaard.C.,Lotz.U.,Dugstad.A.,"InuenceofliquidvelocityonCO2corrosion:asemi-empiricalmodel",NACE International, Houston, Texas,PaperNo.128,1995

[19] Box,G.E.P.,Draper.N.R.,EmpiricalModelBuildingandResponseSurfaces,"JohnWileyandSons,NewYork,1987.

[20] Devore,Peck.Statistics,TheExplorationandAnalysisData,fthedition,ThomsonLearning,Belmont,USA,2005.

[21] MinitabInc.MinitabStatisticalSoftware,Release15forWindows,StateCollege,Pennsylvania,2006.

[22] HouyiMaa,Theinuenceofhydrogensuldeoncorrosionironunderdifferentconditions,CorrosionScience,Vol.42,pp.1669-1683,2000.

[23] RudolfH.Hausler,ContributiontotheunderstandingofH2S,NACECorrosionAnnualConf.,NACEInternational, Houston, Texas,Paperno.4732,2004.

[24] Prakala,Shilpha,EISinvestigationofcarbondioxideandhydrogensuldecorrosionunderlmformingcondition,MasterTheses,OhioUniversity,2005.

[25] Agrawal,C.Durr,AndG.H.Koch,Suldelmsandcorrosionratesofaisi1018carbonsteelinsalinesolutionsinthepresenceofH2SandCO2,NACECorrosion Annual Conf., NACE International, Houston,

Texas,Paperno.04388,2004.

[26] Veloz,M.A.,Gonza.l.,ElectrochemicalstudyofcarbonsteelcorrosioninbufferedaceticacidsolutionswithchloridesandH2S,ElectrochemicaActa,Vol.48,pp.135-144,2002.

CathodicCorrosionProtectionofPre-CorrodedSteelinConcretewithGalvanicZincAnodes

TheUseofAtomicForceMicroscopyinCorrosionResearch

B.Kinsella 1&T.Becker2

1WesternAustralianCorrosionResearchGroup,DepartmentofAppliedChemistry,

CurtinUniversityofTechnology,Australia

2NanochemistryResearchInstitute(NRI),CurtinUniversityofTechnology,Australia

Summary:Atomicforcemicroscopy(AFM),inventedin1986,hasfoundwidespreaduseinscience.Thetechniqueenablesimagingofmaterialatthemolecularlevelforthersttime.Unlikemostothermicroscopytechniques,theimagingcanbecarriedoutinsituwithoutfearofdestroyingtheintegrityoftheinter-phaseandtheprocessthatisbeingmeasured.TheapplicationofAFMtocorrosionscienceisshownbytwoexamples.Therstexampleconcernsthemechanismofadsorptionofcarbondioxidecorrosioninhibitors(surfactantmolecules)onsteel.Thesecondexampleinvolvesaninvestigationofthemechanismof

stresscorrosioncrackingofweldable13chromesteel.1.Introduction

AtomicForceMicroscopy(AFM)wasinventedin1986andcommercialinstrumentsbecamereadilyavailableintheearlynineties.Developmentofimprovedtechnologyandfriendlieruserapproachhasseenthetechniqueourishinmanyareasofscienticinvestigation.ThispaperprovidesadescriptionoftheAFMtechniqueandvariousmodesofoperationanditsapplicationtocorrosionresearch.Twoexamplesaregivenaninvestigationofthemechanismofadsorptionofcarbondioxidecorrosioninhibitorsandthemechanismofstresscorrosioncrackingofstainlesssteel.

2.ScanningProbeMicroscopy(SPM)

ScanningProbeMicroscopesallowresearcherstoinvestigatethetopographyofaspecimeningreatdetailwitharesolutiondowntotheatomiclevel.Thebasicprincipleof

thesemicroscopesisasharpprobethatispreciselyscannedacrossthesurfaceofthesample.Theprobeismovedusingascannertubemadeofpiezoelectricmaterialthatallowsaccuratepositioning.Typically,thescannertubeconsistsoftwoparts,oneforthelateralandonefortheverticalmovements.AnimportantadvantageofallSPMtechniquesistheabilitytobeusedinuidenvironments,enablingresearcherstostudysamplesin-situ,forexamplebiologicalspecimenortheformationofsurfactantlayersonasample.

2.1 Scanning Probe Microscopy (SPM)

In1981GerdBinnigandHeinrichRohreratIBM,Zurich,inventedthersttypeofanewkindofmicroscope,theScanningTunnelingMicroscope[STM](1).Fiveyearslater,in1986,theyreceivedtheNobelPrizefortheirdesignof

theScanningTunnelingMicroscope.TheSTMutilisesthequantum-mechanicaltunnelingeffect,whereasharptip(i.e.anetchedtungstenwire)isbroughtinclosecontactwithanelectricallyconductiveorsemi-conductivesample.Abiasvoltageisappliedbetweentipandsampleandforatip-sample(probe-sample)separationof

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