Citation: Porcayo-Calderon,J.;Canto, J.;Martinez-de-la-Escalera,L.M.; Neri,A.SweetCorrosionInhibition byCO2 Capture. Molecules 2022, 27, 5209. https://doi.org/10.3390/ molecules27165209 AcademicEditor:IoanaDemetrescu Received:26June2022 Accepted:25July2022 Published:16August2022 Publisher’sNote: MDPIstaysneutral withregardtojurisdictionalclaimsin publishedmapsandinstitutionalaffiliations. Copyright: ©2022bytheauthors. LicenseeMDPI,Basel,Switzerland. Thisarticleisanopenaccessarticle distributedunderthetermsand conditionsoftheCreativeCommons Attribution(CCBY)license(https:// creativecommons.org/licenses/by/ 4.0/). molecules Article SweetCorrosionInhibitionbyCO2 Capture JesusPorcayo-Calderon 1,* ,JorgeCanto 2,L.M.Martinez-de-la-Escalera 2 andAdrianNeri 2 1 DepartmentofChemicalEngineeringandMetallurgy,UniversityofSonora,Hermosillo83000,Mexico 2 CorrosionyProteccion(CyP),Buffon46,MexicoCity11590,Mexico * Correspondence:jporcayoc@gmail.com Abstract: ThemostpracticalandeconomicalwaytocombattheproblemsderivedfromCO2 corrosion (sweetcorrosion)istheuseofcorrosioninhibitorsoforganicorigin.Itsmainprotectionmechanismis basedonitsabilitytoadsorbonthemetalsurface,formingabarrierbetweenthemetalsurfaceand theaggressivemedium.However,despiteitsexcellentperformance,itsinhibitionefficiencycanbe compromisedwiththeincreaseintemperatureaswellastheshearstresses.Inthisstudy,theuseof aninorganicinhibitorisproposedthathasnotbeenconsideredasaninhibitorofsweetcorrosion. ThereportedstudiesarebasedonusingLaCl3 asacorrosioninhibitor.Itsbehaviorwasevaluated on1018carbonsteelusingelectrochemicalmeasurements,suchaspotentiodynamicpolarization curves,open-circuitpotentialmeasurements,linearpolarizationresistancemeasurements,and electrochemicalimpedance.Theresultsshowedaninhibitionefficiencyofthesweetcorrosionprocess greaterthan95%,andthattheinhibitionmechanismwasdifferentfromtheclassiccorrosionprocess inCO2-freeelectrolytes.Inthiscase,itwasobservedthattheinhibitorycapacityoftheLa3+ cations isbasedonaCO2-captureprocessandtheprecipitationofabarrierlayeroflanthanumcarbonate (La2(CO3)3). Keywords: corrosion;CO2 capture;lanthanumchloride;lanthanumcarbonate;inhibitor 1.Introduction CO2 corrosionisoneofthemaincausesofthedegradationofmetallicmaterialsin theoilindustryandisthecauseofsignificantenvironmentaldamage,aswellaseconomic damageduetotheunavailabilityofequipment.Thisdegradationprocessisasurface processthatencouragedanin-depthstudyofthemechanismsofmetallicdissolution[1 3] aswellashowtomitigateit. ThedegradationofmaterialsbydissolvedCO2,alsoknownassweetcorrosion,isa processthatcausesdamagetotheinternalsurfaceofthepipelinesusedinthetransportation ofhydrocarbons.Thistypeofdamageisalsoknownasinternalcorrosion.Thepipelines usedtotransporthydrocarbonsaremadeofcarbonsteel,whereitsmainelement(Fe)hasa standardpotentialthatplacesitasahighlyactiveelementinacidconditions.Although carbonsteelcanprotectitselfduetotheprecipitationonitssurfaceofalayerofiron carbonate(FeCO3),theconditionsforthistooccurareenvironmentsofhightemperature, alkalinepH,andhighconcentrationofFe2+ cations[3 5],inadditiontothehighavailability ofdissolvedCO2.However,thehighconcentrationofFe2+ cationsimpliesanadvanced corrosionprocesssothatself-sealingcanoccur. Duetotheimpossibilityofcarryingoutacontinuousinspectiontodeterminethe internalsurfacestate,oneofthemainactionsusedtoinhibitthemetaldissolutionprocess hasbeentheinjectionofcorrosioninhibitorsintothecorrosivefluid.Thesecompounds canformprotectivelayersonthemetalsurface,eitherbyabsorptionorprecipitation.Most corrosioninhibitorsareorganiccompounds,andintheirstructure,theycontainnitrogen (N),sulfur(S),andoxygen(O)atoms,unsaturatedfunctionalgroups,aswellasaromatic ringsthatfavortheiradsorptiononthemetalsurface,duetoalonepairofelectrons[6 11]. Corrosioninhibitorscanreducemetaldissolutionbyatleast90%whentheyareadded Molecules 2022, 27,5209. https://doi.org/10.3390/molecules27165209 https://www.mdpi.com/journal/molecules
Molecules 2022, 27,5209 2of18 totheelectrolyteindosesaslowasppm.However,ithasbeenreported[12 14]thatits inhibitionefficiencydecreaseswithincreasingtemperature. Inrecentdecades,therehasbeengreatinterestinusingrareearthelementstosynthesizecompoundswithcorrosioninhibitorycapacityinaggressiveenvironments[15 25]. Itsexcellentcorrosioninhibitingabilityhasbeenattributedtothehighreactivityofrare earth.Duringtheelectrochemicalcorrosionprocess,thecathodicreactionactivatesthe O2 reductionreaction,causinganincreaseinpHatthecathodicsites[22].Theseconditionsfavortheprecipitationofrareearthionsonthemetalsurface,formingaprotectivefilmthatdecreasesthecathodicreactionrateand,therefore,thecorrosioncurrent density[16,18,19,22,26,27]. Basedontheabove,theobjectiveofthisresearchworkistostudytheeffectofan inorganiccompound(LaCl3)onthecorrosionbehaviorofcarbonsteelinasolutionof NaCl(3.5%byweight)saturatedwithCO2 at60 ◦C.LaCl3 canbeconsideredasafecompoundsinceitstoxicityissimilartothatofNaCl[16,27,28],andtodate,ithasnotbeen consideredacorrosioninhibitorinenvironmentswithdissolvedCO2.Thetesttemperature waschosentoreferencetheexperimentalconditionsunderwhichtheperformanceof corrosioninhibitorswithpotentialuseinpipelinesusedtotransporthydrocarbonsisevaluated.Theaveragetemperatureofthetransportedfluidis50–60 ◦C[7 10].Theinhibitory capacityofLaCl3 wasdeterminedthroughelectrochemicalstudies,suchaspolarization curves,measurementsofopencircuitpotential,polarizationresistance,andelectrochemicalimpedance,aswellascomplementaryanalysesusingscanningelectronmicroscopy techniques(SEM-EDS)andX-raydiffraction. 2.ResultsandDiscussion 2.1.PotentiodynamicPolarizationCurves Figure 1 showsthepotentiodynamicpolarizationcurvesofcarbonsteel1018after50h ofimmersionina3.5%NaClsolutionsaturatedwithCO2 at60 ◦Cwithandwithoutthe additionofLaCl3 asacorrosioninhibitor. Figure1. Polarizationcurvesfor1018carbonsteelinCO2-saturatedbrineatdifferentconcentrations oflanthanumchlorideafter50hofimmersion. Intheabsenceofacorrosioninhibitor,theoxidationreactionofFedeterminesthe behavioroftheanodicbranch.Theobservedanodicbehaviorcorrespondstothatsug-
Molecules 2022, 27,5209 3of18 gestedbyincreasingthepolarizationoftheanodicbranch[3,29]—thatis,initiallyanactive dissolutionbehaviorfollowedbyatransitionbehavior,apre-passivebehavior,andfinally apassivestate.Theextensionofthesezonesdependsontheelectrolytecompositionwhere theFeisimmersed. Theadditionoftheinhibitorcausedthedisplacementofthepolarizationcurvestowardslowercorrosiondensitiesandslightlynoblerpotentials.Adecreaseinthecathodic currentdensityandthedevelopmentofapseudo-passivezoneintheanodicbranchare noticeable.ThesechangessuggestthatthepresenceofLa3+ ionscausedadecreaseinboth theanodicandcathodicreactionrates,possiblyduetothedevelopmentofaLa-basedprotectivefilmonthemetalsurface.Inthescientificliterature,thereislittleinformationonthe useoftrareearthortheircompoundsascorrosioninhibitorsinelectrolyteswithdissolved CO2.TheuseoforganicPrcompoundsascorrosioninhibitorshasbeenreported[30],and anincreaseincorrosionpotentialandaneffectonboththeanodicandcathodicbranches havealsobeenindicated.Theobservedbehaviorcorrespondstoamixed-typeinhibition; similarbehaviorhasbeenreportedwiththeadditionofcompoundsbasedonrareearthin environmentsrichinchlorides[16 18,23,24]. Table 1 showsthevaluesoftheelectrochemicalparametersobtainedfromtheTafel zonesofthepotentiodynamicpolarizationcurves(Figure 1). Table1. Electrochemicalparametersofthepolarizationcurves. LaCl3 EcorrbabcIcorr (mM)(mV)(mv/Dec)(mV/Dec)(mA/cm2) 0 740463790.158 0.1 7421123630.099 0.5 7441742040.052 1.0 7331762280.013 5.0 7221312440.031 Intheabsenceofaninhibitor,itwasobservedthattheanodicslopeisofthesame orderasthosereportedformetaldissolutionprocessesexclusively[4],anditsvalueisin goodagreementwiththatobtainedbytheexpression(46mVexp vs.44mVcalc)[3]: ba = 2.303RT (1 + β)F ,(1) where R isthegasconstant, T isthetemperature(K), β isthesymmetrycoefficient (β =0.5), and F istheFaradayconstant.Thissuggeststhatevenafter50hofimmersioninthe electrolyte,thecarbonsteelwasnotabletodevelopaprotectivelayeronitssurface.The decreaseincurrentdensityobservedintheanodicbranchbetween 685and 650mV maycorrespondtotheprecipitationofanamorphouscolloid(FeCO3/Fe(HCO3)2)dueto thesupersaturationofFe2+ ions,ashasbeensuggestedinotherstudies[4]. Itisobservedthatatinhibitorconcentrationslessthan1.0mM,thecorrosionpotential remainspracticallyconstant,andatconcentrationsgreaterthan0.5mM,thecorrosion potentialshiftsslightlytowardsthenobledirection.Similarly,inthepresenceofthe inhibitor,theslopeoftheanodicbranchincreases,whichindicatesadecreaseintherateof metaldissolution.Onthecontrary,ingeneral,theslopeofthecathodicbranchdecreased, possiblyduetoadecreaseinthecathodicsitesthatcausedadecreaseintherateofthe cathodicreaction.IthasbeensuggestedthatwhentheTafelslopechangesanddoesnot followatrendaccordingtotheconcentrationoftheaddedinhibitor,itsimplyactsasa barrierreducingthereactionarea[10].TheIcorrdecreased,uptoanorderofmagnitude, withincreasinginhibitorconcentrationupto1.0mM,andathigherconcentrations,ittends toincreaseslightly.TheincreaseinIcorrvaluesatconcentrationsgreaterthan1.0mM maybeduetotheincreaseinCl ionconcentrationduetotheadditionoftheinhibitor (LaCl3)[18,23,24].Thefewreportedstudiesindicatethatthedecreaseinthecorrosionrate
2.3.LinearPolarizationResistanceMeasurements
2 at60 ◦Cwithand
Molecules 2022, 27,5209 4of18 duetotheadditionofcompoundsbasedonrareearthisgreaterinenvironmentssaturated byCO2 thaninaeratedenvironments,possiblyduetotheformationofdifferentprotective films[30]. 2.2.OpenCircuitPotentialMeasurements Figure 2 showsthevariationinOCPvaluesfor1018carbonsteelduring50hof immersionina3.5%NaClsolutionsaturatedwithCO2 at60 ◦Cwithandwithoutthe additionofLaCl3 asacorrosioninhibitor. Figure2. EvolutionofOCPvaluesfor1018carbonsteelinCO2-saturatedbrineatdifferentconcentrationsoflanthanumchloride. Intheabsenceofaninhibitor,carbonsteelshowedanincreaseinitsOCPvaluesup tothefirst10hofimmersion,andsubsequently,itsvaluestendedtoincreaseslowlyuntil reachingasteadystateattheendofthetest.Inthiscase,theobservedtrendmaycorrespond totheformationandaccumulationofcorrosionproductsonsteelsurfaces.Ontheother hand,inthepresenceoftheinhibitor,inallcasesafteritsaddition,asuddenincreasein OCPvalueswasobserved,followedimmediatelybyaslightdecrease.Subsequently,aslow increaseinOCPvalueswasobserveduntilreachingthesteadystateafter10hofimmersion. Ingeneral,abruptchangesintheOCPvalueshavebeenassociatedwithareductionor increaseintherateoftheanodicand/orcathodicprocesses[16,17].Inparticular,theshifts tomoreactivepotentials,inadditiontoindicatinganincreaseinthemetaldissolutionrate, alsosuggestaprocessofinhibitionofthecathodicreaction[16],suchasthatobserved withtheuseofcompoundsbasedonrareearthascorrosioninhibitorsinsolutionsrichin chlorides[16,23,24,31].ThissuggeststhatinsolutionsrichinchloridessaturatedwithCO2, themechanismofinhibitionofrareearthisdifferentfromthatobservedintheabsence ofCO2 ThesuddenincreaseinOCPvaluesmaybeassociatedwithachangeinsolution chemistryand/ortheformationandpartialdissolutionofaprotectivefilmonthesteel surface.NotwithstandingtheevidentshiftinOCPvalues,themagnitudeoftheshiftisless than85mV,andthissuggeststhatlanthanumchlorideactsasamixedinhibitor[32,33].
Figure 3 showsthevariationinpolarizationresistancevaluesforcarbonsteel1018 during50hofimmersionina3.5%NaClsolutionsaturatedwithCO withouttheadditionofLaCl3 asacorrosioninhibitor.
Molecules 2022, 27,5209 5of18 Figure3. Evolutionofpolarizationresistancevaluesfor1018carbonsteelinCO2-saturatedbrineat differentconcentrationsoflanthanumchloride. Intheabsenceofaninhibitor,carbonsteelshowedarapidincreaseinitsRpvaluesin thefirst10hofimmersion,andsubsequently,thetrendismaintainedbutataslowerrate untiltheendofthetest.Theobservedtrendmaycorrespondtoarapidcorrosionprocess andaccumulationofcorrosionproductsonthesteelsurface,whichlimitedtheavailable reactionarea.However,inthepresenceoftheinhibitor,asimilarbehaviorwasobserved regardlessofitsconcentration.Inallcases,immediatelyaftertheadditionoftheinhibitor, anabruptincreaseinRpvalueswasobserved,followedbyanabruptdecrease.Afterthis behavior,theRpvaluestendedtoincreaserapidlyuntil10hofimmersion,andthena constantincreasewasobserveduntiltheendofthetest.Theinitialbehaviorisconsistent withthatobservedwiththeOCPvalues(Figure 2). Thisnewevidencesuggeststhattheadditionoftheinhibitormayhavecausedan instantaneousdisturbanceinthechemicalcompositionofthesolution,therebycausinga temporarydecreaseinitscorrosivity.ThemagnitudeoftheRpvaluesisconsistentwiththe Icorrvaluesobservedwiththepolarizationcurves(Figure 1);thatis,theRpvaluesincrease withtheconcentrationoftheinhibitorupto1.0mM,andathigherconcentrations,they tendtodecrease,possiblyduetotheincreaseintheconcentrationofCl ions[18,23,24]. Figure 4 showsthevariationintheinhibitionefficiencyofLaCl3 asafunctionof itsconcentrationforcarbonsteelimmersedina3.5%NaClsolutionsaturatedwithCO2 for50h.Thecalculationsshowthattheinhibitionefficienciesaregreaterthan95%for concentrationsupto5mM.Athigherconcentrations,ittendstodecrease,possiblydue totheincreaseintheconcentrationofCl ions.Themaximuminhibitionefficiencywas 98%foraconcentrationof1mMofLaCl3.TheresultsshowthatLaCl3 isamoreefficient corrosioninhibitorinenvironmentsrichinchloridessaturatedbyCO2 thanintheabsence ofCO2 [23,24],possiblybecauseitsprotectiveactionisdifferent.
Molecules 2022, 27,5209 6of18 Figure4. EffectofLaCl3 concentrationoninhibitionefficiency. 2.4.ElectrochemicalImpedanceSpectroscopyMeasurements Figure 5 showstheNyquistandBodediagramsforcarbonsteelafter50hofimmersion ina3.5%NaClsolutionsaturatedwithCO2 at60 ◦Cwithandwithouttheadditionof LaCl3 asacorrosioninhibitor. Fromtheimpedancespectra,itisobservedthatallthespectrashowsimilarbehavior. TheNyquistdiagramshowstheapparentpresenceofasinglecapacitivesemicirclewhose maximumdiameterisobtainedforaninhibitorconcentrationof1mM.FromtheBode plotinitsimpedancemodulusformat,thepresenceofthehighandlow-frequencyplateau isobserved,aswellasalinearlogf-log|Z|relationshipintheintermediatefrequency region.Theevolutionofthelow-frequencyplateauisconsistentwiththemagnitudeof thediameterofthecapacitivesemicircles.FromtheBodeplotinitsphaseangleformat, thepresenceofonlyonetimeconstantisconfirmedinallcases.However,inthepresence ofaninhibitor,themagnitudeofthephaseangleisbigger,anditslocationisathigher frequenciesthanthatobservedintheabsenceofaninhibitor. Figure5. Cont.
Molecules 2022, 27,5209 7of18 Figure5. NyquistandBodediagramsfor1018carbonsteelinCO2-saturatedbrineatdifferent concentrationsoflanthanumchlorideafter50hofimmersion.(a)Nyquistdiagram;(b)Bodediagram initsimpedancemodulusformat;(c)Bodediagraminitsphaseangleformat. Sinceinthepresenceoftheinhibitor,theimpedancespectrashowedasimilarevolution,insubsequentanalyses,onlytheevolutionofthespectraintheabsence(Figure 6)and presenceoftheinhibitoratasingleconcentration,1mMLaCl3 (Figure 7)ispresented.
Molecules 2022, 27,5209 8of18 Figure6. EvolutionofNyquistandBodediagramsfor1018steelinCO2-saturatedbrineat60 ◦C. (a)Nyquistdiagram;(b)Bodediagraminitsimpedancemodulusformat;(c)Bodediagraminits phaseangleformat.
Molecules 2022, 27,5209 9of18 Figure7. EvolutionofNyquistandBodediagramsfor1018carbonsteelinCO2-saturatedbrineat 60 ◦Cand1mMofLaCl3.(a)Nyquistdiagram;(b)Bodediagraminitsimpedancemodulusformat; (c)Bodediagraminitsphaseangleformat.
Molecules 2022, 27,5209 10of18 Theevolutionoftheimpedancespectraforthecaseofcarbonsteelintheabsenceof aninhibitorbasedontheNyquistdiagramshowsthepresenceofadepressedcapacitive semicircle,whosediameterincreasesasafunctionofimmersiontime.Datascatterinthe low-frequencyregionhasbeenassociatedwithinhomogeneityandmicroroughnessof theworkingelectrodesurface[11,34,35].FromtheBodeplotinitsimpedancemodulus format,|Z|,inthehigh-frequencyregion,thepresenceofthehigh-frequencyplateaufrom 1000Hzisobserved,aswellasthedevelopmentofalinearrelationship, log|Z|-logf, whichextendstothelow-frequencyregionasafunctionoftime.Inaddition,inthelowfrequencyregion,thepresenceofthelow-frequencyplateauisobserved,thelengthof whichdecreasesandmovestohigherimpedancemodulusvalueswithimmersiontime. FromtheBodeplotinitsphaseangleformat,thepresenceofatimeconstantisvisible, whichshiftstowardslowerfrequencies(80Hz → 5Hz)asafunctionoftheimmersion time,inaddition,itsmaximumphaseangleincreasesinthesameway(55◦ → 67◦).The characteristicsobservedarecongruentwithamaterialwhosesurfaceformsalayerof corrosionproductswithprotectivecharacteristics. However,afterthecorrosiontest,thesurfacecharacteristicsofthecarbonsteel (Figure 8)showedtheaccumulationofathinlayerofcorrosionproducts.Underit,aheterogeneousattackwiththepresenceofpittingwasobserved.Then,theincreaseinthediameter ofthecapacitivesemicircle,impedancemoduleandphaseanglemaybeassociatedwith theaccumulationofaporouslayerofcorrosionproductsthatdecreasedthereactionarea butnotthecorrosionprocess[19]. Figure8. Themorphologicalaspectof1018carbonsteelaftercorrosiontestinCO2-saturatedbrine solutionafter50h:(a)corrosionproductssurface;(b)cleansurface. Inthistypeofenvironment,themaincorrosionproductwithprotectedcharacteristics thatdeveloponthesurfaceofcarbonsteelisironcarbonate,FeCO3.However,theXRD analysisperformedonthesurfaceofthesamples(Figure 9)didnotdetecttheirpresence. Thisisbecauseitsformationisfavoredathighertemperaturesandlongerimmersiontimes thanthoseevaluatedhere[36].Inthiscase,themaincorrosionproductdetectedwasFe3O4
Molecules 2022, 27,5209 11of18 Figure9. X-raydiffractionpatternofthesurfaceof1018carbonsteel;(a)notcorroded,(b)corroded, (c)corrodedwiththeadditionofLaCl3 Ingeneral,thecorrosionprocessofcarbonsteelindeaeratedconditionsisduetothe followinganodicandcathodicreactions: Fe ↔ Fe2+ + 2e ,(2) 2H+ + 2e ↔ H2,(3) Duringtheactivedissolutionprocess,theFe2+ cationscanparticipateinsecondary reactionsthatfavortheformationofcorrosionproducts,whichcanactasabarriertothe freeaccessoftheelectrolyteand,withit,areductionintherateofmetaldissolution. Fe2+ + 2OH ↔ Fe(OH)2 + H2O,(4) 4Fe(OH)2 + O2 + 2H2O ↔ 4Fe(OH)3,(5) 4Fe(OH)2 + O2 ↔ 2Fe2O3 + 4H2O,(6) Fe(OH)3 + O2 ↔ FeOOH + H2O,(7) 4Fe(OH)2 + O2 ↔ 4FeOOH + 2H2O,(8) 2FeOOH + H2O ↔ Fe2O3 + H2O,(9) Ontheotherhand,theevolutionoftheimpedancespectrainthepresenceofthe inhibitor(intheentirerangeofconcentrations)presentedabehaviorlikethatshownin Figure 7.FromtheNyquistdiagram,thepresenceofacapacitivesemicirclewhosediameter increaseswithimmersiontimeisshown.Themagnitudeofthediameterincreaseisgreater thanthatobservedintheabsenceoftheinhibitor.FromtheBodeplotinitsimpedance modulusformat,|Z|,theformationofthehigh-frequencyplateauwasobservedfrom 1000Hz,andintheintermediatefrequencyregion,alinearrelationship,log|Z|-logf, withonlyoneslopewasobserved.Inthelow-frequencyregion,thepresenceofthelowfrequencyplateauwasobserved,whichmovestohigherimpedancemodulusvaluesas afunctionoftime.Themagnitudeofitsdisplacementisgreaterthanthatobservedin theabsenceoftheinhibitor.FromtheBodeplotinitsphaseangleformat,thepresence ofasingletimeconstantisvisible,whichshiftstowardslowerfrequencies(80 → 20Hz) andincreasesitsmaximumphaseangle(55◦ → 78◦)asafunctionoftime.Themagnitude
ofthisshiftwaslessthanintheabsenceofaninhibitor,andtheincreaseinphaseangle maximumwasgreater.Thesecharacteristicssuggesttheformationofathickerprotective layerwithhighercapacitivepropertiesthanthatformedintheabsenceoftheinhibitor.
10)afterthecorrosiontestshowed
v/v)for5–10s.Theremovalofthesuperficial
AnEDSanalysisandelementmapping(Figure
Molecules 2022, 27,5209 12of18
Thesurfacecharacteristicsofthecarbonsteel(Figure
11)revealedthattheformeddeposits areassociatedwithLa-O-Candthatovertheareaswherethebulkydepositsdidnotform, therewasalsothistypeofcompoundintheformofthinfilms. Figure10. Cont.
thepresenceofprecipitateswhoseappearancechangesdependingontheconcentration oftheaddedinhibitor.Atlowconcentrations(0.1mM),asurfacewithalayerofreaction productsofdenseappearancewasobserved;whenincreasingtheconcentration(1.0mM), theformationofstructuresthatresemblethechrysanthemumflowerwasobserved,and athigherconcentrations(10.0mM),theprecipitatesweredifferentinshapeandofhigher density.Toobservetheappearanceofthesurface,theprecipitatesformedwereremoved byimmersioninaHClsolution(10%
layersshowedthatatlowconcentrationsoftheinhibitor,amorehomogeneoussurface wasobtainedthanthatobservedintheabsenceoftheinhibitor,andwhenincreasingits concentration,scratchmarksofthesurfacepreparationwerestillobserved.However, intheevaluationofthehighestconcentrationoftheinhibitor,anirregularsurfacewiththe presenceoffirmlyadheredremnantdepositswasobserved,possiblyduetotheincreasein theconcentrationofCl ionsduetotheadditionoftheinhibitor.
Molecules 2022, 27,5209 13of18 Figure10. Themorphologicalaspectof1018carbonsteelaftercorrosiontestinCO2-saturatedbrine solutionatdifferentconcentrationsoftheaddedinhibitor.0.1mM;(a)corrosionproductssurface; (b)cleansurface.1.0mM;(c)corrosionproductssurface;(d)cleansurface.10.0mM;(e)corrosion productssurface;(f)cleansurface. XRDanalysisofthecarbonsteelsurface(Figure 9)showedthattheprecipitated La-O-Ccompoundcorrespondstolanthanumcarbonate(La2(CO3)3).Regardlessofthe concentrationoftheinhibitor,lanthanumcarbonatewasthemaincompounddetected inallcases.ItspresenceisassociatedwithdissolvedCO2 andLa3+ cations,theproduct ofinhibitordissociation.Then,theincreaseinthediameterofthecapacitivesemicircle, impedancemoduleandphaseanglecorrespondstotheprecipitationofthiscompound, whichactedasabarrierlayerbetweenthemetallicsurfaceandtheelectrolyte. Figure11. Cont.
Molecules 2022, 27,5209 14of18 Figure11. Approachtothesurfaceof1018carbonsteelevaluatedwiththeadditionof1.0mMof LaCl3.(a)secondaryelectronimage;(b)Femapping;(c)Omapping;(d)Cmapping;(e)Lamapping. 2.5.CO2-CaptureandProtectionMechanism UnlikethemultiplecorrosionstudiesinCO2-freeelectrolyteswhereithasbeenreportedthattheprotectionmechanismofrareearthisduetotheirprecipitation(asoxides andhydroxides)byhydrolysisreactionsoftheirmetalliccationswiththeOH ionsproducedinthecathodicsites[19,23 25,37 40],adifferentmechanismwasobservedinthis study,namely,CO2 captureandprecipitationoflanthanumcarbonate(La2(CO3)3)asa protectivebarrier. Recentstudies[41]indicatethatinenvironmentswithCO2,theformationofrareearth carbonates,Ln2(CO3)3,isthermodynamicallymorefavorablethanspeciessuchasLn(OH)3, Ln2O3,Ln(OH) 4, andevenLn3+.Thisisbecauseitsfreeenergyofformation(∆G°f)is considerablymorefavorable.Thisindicatesthatlanthanumcarbonatesaremorestable species,andthereforetheirprotectivecapacityisgreater.Inaddition,itisreportedthat theformationofrareearthcarbonatesismorefavorableinaqueoussystemswithavailable carbonateions(CO2 3)andthatthismechanismcorrespondstoaCO2 captureprocess, wheretheCO2 captureefficiencyismorethan95%[42]. Basedontheabove,thebehaviorobservedintheOCPandRPLmeasurementscanbe justified,thatis,theabruptincreaseinboththeopencircuitpotentialandthepolarization resistancewhenLaCl3 wasadded.ArapidcaptureprocessofthedissolvedCO2 presentin thevicinityofthemetalsurfaceoccurred,therebyreducingthecorrosivityoftheelectrolyte. Thesubsequentincreaseintheseparameters(OCP,LPR)wasaconsequenceoftheconstant bubblingofCO2,whichrestoreditsconcentration.BecausetheCO2 hydrationprocessisa slowstep[3,5],therestorationofOCPandRPLvaluesdidnotoccurinstantaneously.These suddenchangesshowthehighreactivityoftheLa3+ cations,whichoffersasignificant advantageoverorganicinhibitors.Underrealconditions,ithasbeenobservedthatthe maximumeffectivenessoforganicinhibitorsisdetecteduptotwomonthsaftercontinuous injection.[43] ThecaptureprocessofCO2 anditsprecipitationaslanthanumcarbonatecanbe explainedaccordingtothefollowingreactions[41]: LaCl3(s) ↔ La3+ (aq) + 3Cl(aq),(10) CO2(g) + H2O(l) ↔ H2CO3(aq) ,(11) H2CO3(aq) ↔ HCO3(aq) + H+ (aq),(12) HCO3(aq) ↔ CO2 3(aq) + H+ (aq),(13) 2La3+ (aq) + 3CO2 3(aq) ↔ La2(CO3)3(s) ,(14)
Molecules 2022, 27,5209 15of18 Duringthemetaldissolutionprocess,thedissociationreactionofcarbonicacid (H2CO3),responsiblefortheformationoftheelectrochemicallyactivespecies(H+)that bufferstheconcentrationofhydrogenionsonthemetalsurface[1,2],favorstheformation ofcarbonateionsandwithittheprecipitationreactionoflanthanumcarbonate.However, becausecarbonicacidisaweakdiproticacid,ithasbeensuggestedthatthefirstdissociationstepisafastprocess,andthesecondisaslowprocessfavoredaroundneutralpH. Therefore,itispossiblethattheprecipitationoflanthanumcarbonatecanalsooccurina similarwaytotheprecipitationreactionofironcarbonateinacidenvironments[5]: 2La3+ (aq) + 3HCO3(aq) ↔ La2(HCO3)3(s) ,(15) La2(HCO3)3(s) ↔ La2(CO3)3(s) + 3H+,(16) ThehydrationanddissociationreactionsofthedissolvedCO2 arereactionsinherentin theclassicmechanismofthesweetcorrosionprocess[1 3],andinthiscase,thepresenceof theLa3+ cationsinterruptthisroutebycapturingthedissolvedCO2,therebyreducingthe corrosivityoftheelectrolyte,andincreasesthecorrosionresistanceofthesteelbyforming aprotectivelayerofLa2(CO3)3 onitssurface. Thesourceofcationsfortheformationofthesecarbonatesdoesnotdependon theFe2+ cationsgeneratedduringthecorrosionprocess,sotheirformationisfavoredat lowertemperatures.Althoughtemperatureinfluencesinhibitionefficiency[12 15],inthis case,adecreaseinsolutiontemperaturewillincreasetheamountofdissolvedCO2 [44] availabletoreactwithLa3+ cations;therefore,itsinhibitionefficiencywillnotbeaffected inasignificantway.Inaddition,foreachLa2(CO3)3 moleculeformed,threemolesof Carecaptured,unliketheformationofFeCO3,whoseratiois1:1.Theformationand precipitationoflanthanumcarbonateincreasesboththeadsorptionandhydrationofCO2, thusfavoringaCO2-captureprocessandtransformingitintoamorestableproductwith lowsolubility[45].Thisreducesthecorrosivenessoftheelectrolyte,favoringtheformation ofabarrierandtherebyreducingthecorrosionrateofcarbonsteel.WhenCO2 capture occursbyitsdissolutioninwater,itisalsocalledaCO2 mineralizationprocess[41,46]. Furthermore,lanthanumcarbonatehasahigherfreeenergyofformationandgreater E–pHstabilityrangethanironcarbonate[47].Theformationofironcarbonateisfavored attemperaturesandtimesgreaterthanthoseevaluatedhere[4].Inthisway,theformationandprecipitationofabarrierlayeroflanthanumcarbonateguaranteesagreater protectivecapacity. 3.MaterialsandMethods Asastudymaterial,1018carbonsteelspecimenswithdimensions10 × 10 × 5mm wereused(1cm2 reactionarea).Usingthespot-weldingtechnique,acopperconductor wirewasweldedtooneofthefacesofthespecimens,andinthiscondition,theywere encapsulatedinepoxyresin.Theencapsulatedsampleswereroughenedwithabrasive paperfromgrade120tograde600.Subsequently,theywerewashedwithdistilledwater andethanol,dried,andusedimmediatelyinthecorrosiontests. Ascorrosivemedium,asolutionofNaCl(3.5%byweight)saturatedwithCO2 at atemperatureof60 ◦Cwasused.ThesolutionwassaturatedwithCO2 foratleastone hourbeforestartingthetests.Tomaintainsaturation,CO2 bubblingwasmaintainedfor thedurationofthetestsperformed.Lanthanumchloride(LaCl3)wasusedasacorrosion inhibitoratconcentrationsof0.0001M,0.0005M,0.001M,0.005M,and0.01M.Inall cases,theinhibitorwasaddedtothecorrosivemediumonehouraftertheimmersionof theworkingelectrode.Theinhibitorwasaddedinitspurestate,anditshomogenizationin theelectrolytewascarriedoutwithstirringfor5–10s. Theelectrochemicaltestswerecarriedoutinathree-electrodecellwheretheencapsulatedspecimensweretheworkingelectrodes,thereferenceelectrodewasasaturated calomelelectrode,andagraphitebarwasusedasacounterelectrode.Theelectrochemical testscarriedoutwerepotentiodynamicpolarizationcurves,opencircuitpotential(OCP)
Molecules 2022, 27,5209 16of18 measurements,linearpolarizationresistance(RPL),andelectrochemicalimpedancespectroscopy(EIS).Beforeeachtest,theworkingelectrodewasimmersedintheelectrolytefor 15mintoachieveasteadystate.Electrochemicalmeasurementswereperformedfor50h withaGAMRYpotentiostat/galvanostat(model1000). Thepotentiodynamicpolarizationcurveswereobtainedbypolarizingtheworking electrode ±300mVwithrespecttothecorrosionpotentialatascanningspeedof1mV/s. OCPmeasurementsweremadeathourlyintervals.RPLmeasurementsweremadeby polarizingtheworkingelectrode ±10mVwithrespecttoitsopen-circuitpotentialat hourlyintervals.EISmeasurementswereobtainedbyapplyingtotheworkingelectrode anamplitudeperturbationof10mV(AC)withrespecttotheopen-circuitpotentialovera frequencyrangeof100kHzto0.01Hz. Theinhibitionefficiencywascalculatedaccordingtothefollowingexpression (Rpi =resistance topolarizationinthepresenceofaninhibitor,Rpb =resistancetopolarizationintheabsenceofaninhibitor): E(%)= Rpi Rpb Rpi × 100,(17) ThesurfacesofthesampleswereanalyzedbySEM-EDSwithanaccelerationvoltage of20kV,andbyX-raydiffractionintherangeof10◦ ≤ 2θ ≤ 90◦ withastepof0.003◦ anda timeof320sperstep. 4.Conclusions Electrochemicalstudiescarriedoutshowthatlanthanumchlorideshowsahighsweet corrosioninhibitioncapacityatconcentrationsaslowas1mM.Itsinhibitionefficiencyis greaterthan95%andisgreaterthanthatobservedinCO2-freeelectrolytes.Itsaddition totheelectrolytecausedarapidincreaseintheOCPandRPLvaluesduetoadecreasein theelectrolytecorrosivityduetothecaptureofdissolvedCO2.Athigherconcentrations, itsinhibitionefficiencytendstodecreaseduetotheincreaseintheconcentrationofCl ions.Electrochemicalimpedancestudiesshowedthattheprecipitationanddeposition oflanthanumcarbonatecausesanincreaseinthecapacitiveresponseofthecarbonsteel surfaceandtherefore,anincreaseinthechargetransferresistance. Unliketheclassicalinhibitionmechanisms,wheretheinhibitormoleculesareadsorbed onthemetalsurfaceorprecipitateonitbyreactingwiththeOH ionsgeneratedatthe cathodicsites,inthisstudy,itwasfoundthattheLa3+ cationsreactwithCO2 dissolved formingabarrierlayeronthemetalsurface.La3+ cationsactbyaCO2 captureprocessthat interruptstheclassicrouteofthesweetcorrosionmechanism,unlikeorganicinhibitors, whoseinhibitionmechanismisbasedontheiradsorptioncapacityonthemetalsurface. AuthorContributions: Conceptualization,J.P.-C.;formalanalysis,J.P.-C.,J.C.,L.M.M.-d.-l.-E.and A.N.;investigation,J.P.-C.,J.C.,L.M.M.-d.-l.-E.andA.N.;methodology,J.C.,L.M.M.-d.-l.-E.andA.N.; supervision,J.P.-C.;writing—reviewandediting,J.P.-C.Allauthorshavereadandagreedtothe publishedversionofthemanuscript. Funding: Thisresearchreceivednoexternalfunding. InstitutionalReviewBoardStatement: Notapplicable. InformedConsentStatement: Notapplicable. DataAvailabilityStatement: Notapplicable. ConflictsofInterest: Theauthorsdeclarenoconflictofinterest.
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