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Eprints ID : 14079
To link to this article : doi:
10.1016/j.electacta.2012.09.061
URL :
http://dx.doi.org/10.1016/j.electacta.2012.09.061
To cite this version : Amand, Sylvain and Musiani, Marco and
Orazem, Mark E. and Pébère, Nadine and Tribollet, Bernard and
Vivier, Vincent Constant-phase-element behavior caused by
inhomogeneous water uptake in anti-corrosion coatings. (2013)
Electrochimica Acta, vol. 87. pp. 693-700. ISSN 0013-4686
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Constant-phase-element
behavior
caused
by
inhomogeneous
water
uptake
in
anti-corrosion
coatings
Sylvain
Amand
a,
Marco
Musiani
b,∗,
Mark
E.
Orazem
c,
Nadine
Pébère
a,
Bernard
Tribollet
d,
Vincent
Vivier
daUniversitédeToulouse,CIRIMAT,UPS/INPT/CNRS,ENSIACET,4,alléeEmileMonso–BP44362,31030ToulouseCedex4,France bIstitutoperl’EnergeticaeleInterfasi,ConsiglioNazionaledelleRicerche,35127Padova,Italy
cDepartmentofChemicalEngineering,UniversityofFlorida,FL32611,USA
dLaboratoireInterfacesetSystèmesElectrochimiques,UPR15duCNRS,UniversitéPierreetMarieCurie,75252ParisCedex05,France
Keywords: CPE
Effectivemediumtheory Impedance
Porosity Resistivity
a
b
s
t
r
a
c
t
Theimpedanceofasubstrate/coating/electrolytesystemwascalculatedwiththeassumptions:(i)the coatinguptakeselectrolytetoanextentthatprogressivelydecreasesfromthecoating/electrolyte inter-facetothesubstrate/coatinginterfacewhereitbecomesnegligible;(ii)thevolumefractionof the electrolytevariesalongthecoatingthicknessaccordingtoapower-law;(iii)theresistivityand per-mittivityprofilesoftheelectrolyte-penetratedcoatingcanbecalculatedthroughaneffectivemedium theory(EMT)formulacorrespondingtoaparallelcombinationofthetwomedia(electrolyteand coat-ingmaterial);and(iv)someporesextendfromthecoating/electrolyteinterfacetothesubstrate/coating interface,providingalowresistancepath.Theimpedanceplotsthuscalculatedexhibitedaconstant phaseelement(CPE)behaviorinalargefrequencyrange.Someexperimentalresultsobtainedwith2024 aluminumalloy/hybridsol–gelcoatingsamplesimmersedinaNaClsolutionwereanalyzedwith refer-encetotheabovedescribedmodel.Theextensionoftherecentlydevelopedpower-lawCPEmodelto anti-corrosioncoatingsisshowntoyieldinsightintothedistributionofresistivityandassociatedwater uptake.Evaluationofmixingrulesforconductivitiesandpermittivitiesofthetwomedia(coatingand electrolyte)showedthatthelinearcombinationprovidedresultsthatwereconsistentwiththeobserved impedanceresponse;whereas,distributionsresultingfromaseriescombinationofthetwomedia,an EMTformulaproposedintheliterature,andtheMaxwellapproximationwereincompatiblewiththe observedCPEimpedanceresponse.
1. Introduction
Electrochemicalimpedancespectroscopyisapopulartechnique for the assessment of the corrosion protection performance of organiccoatings,whichhasbeen usedforsome decades[1–5]. Byfollowingtheevolutionoftheimpedanceresponse ofa sub-strate/coating/electrolytesystemoveranextendedperiodoftime, manyresearchgroupshavebeenabletomonitorthedegradationof thecoatingmaterialandtheonsetofcorrosionprocesses.The anal-ysisoftheimpedancedataisoftenbasedontheclassicalequivalent circuitproposedbyBeaunieretal.in1976[2]oritsvariants. Beau-nier’scircuitconsistsoftheuncompensatedelectrolyteresistance inserieswithaparallelconnectionof(i)thecoatingcapacitanceand (ii)abranchincludingtheresistanceoftheelectrolyte-penetrated coating,assumedtobeidenticaltotheresistanceoftheelectrolyte insideitspores,inserieswithaparallelconnectionofthedouble
∗ Correspondingauthor.
E-mailaddress:m.musiani@ieni.cnr.it(M.Musiani).
layercapacityatthebaseofthepores(coating/substrateinterface) andanimpedancerepresentingthecorrosionprocess.Inthemain variantofBeaunier’scircuit,thecoatingcapacityisreplacedbya constantphaseelement(CPE);additionalmodificationsinvolvethe replacementofthedoublelayercapacitybyasecondCPEand/orthe inclusionofadditionalelements.SubstitutingCPEsforcapacities maygreatlyimprovethequalityofthefittingbetweenthe experi-mentaldata,whichoftenshowaCPEbehavior,andtheequivalent circuit,butcreatesambiguitiesinthephysicalinterpretationofthe results,sincetheCPEmoduluscannotbesimplyidentifiedwiththe coatingcapacity(orthedoublelayercapacity)andthecalculationof theeffectivecapacityfromtheCPEparametersrequiresadetailed knowledgeonthephysicalreasonsfortheCPEbehavior[6].Thus, thepricetopayforimprovingthefittingisthelossofinformation onthecoatingcapacity,aparameterthat,followingthepioneering workofBrasherandKingsbury[7],hasbeenusedfordecadesto calculatetheelectrolyteuptakeinthecoating.
The physical origin of the CPE behavior is still incom-pletely understood, but it is generally agreed that it must be ascribed to inhomogeneous physical properties of the system
= 1 Electrolyte Coa"ng Substrate Rpore
R
poreR
eZ
cCPE
(a)
(b)
Fig.1.(a)Schematicrepresentationofasubstrate/coating/electrolytesystem.(b)Equivalentcircuit;theupperboxcorrespondstoEq.(7)andthecompletecircuitistheone usedforregression.TheCPEinthelowerboxwasincludedtoaccountforanoxidelayerexistingonthealuminumalloyelectrode.
[8].With anoriginal experimentalapproach,Kittel et al. [9,10] demonstrated marked variations in the impedance of organic coatings along their thickness, and concluded that accurate models for substrate/coating/electrolyte systems should inte-grategradientsofpropertiesover thethicknessof thecoatings. However, those authors did not propose any specific resis-tivity or permittivity profiles. Inhomogeneities in the coating properties have been considered by Hinderliter et al. [11,12] who related these inhomogeneities with electrolyte uptake in the coating and, taking into account various geometries for the electrolyte-filled coating defects, developed physical mod-elsofsubstrate/coating/electrolytesystems. Rocheetal.[13,14] observed the formation of an interphase region, next to the substrate/coatinginterface,withpropertiessignificantlydifferent from those of the bulk coating material. Such an inter-phase affected the mechanical and adhesion properties of the system.
Ourgrouphasrecentlyshown thattheimpedanceofalayer exhibitsaCPEbehavioriftheresistivityofthematerialvariesalong thelayerthicknessaccordingtoa power-lawandits permittiv-ityiseitherindependentoftheposition[15]orvariesalongthe layerthicknessonlyinthelow-impedancepartsofthelayer[16]. This“power-lawmodel”hasbeenappliedtosystemsasdiverseas passiveoxidesandhumanskin[17].Inthepresentpapera possi-bleextensionofthepower-lawmodeltoanti-corrosioncoatingsis explored.Thehypothesesaremadethatinhomogeneitiesinthe coatingmorphologycauseaninhomogenouselectrolyte uptake, strongernearthecoating/electrolyteinterfaceand weakernear thesubstrate/coatinginterface,andthat thedependence ofthe electrolytevolumefractiononthepositionalongthecoating thick-nessobeysapower-law.Theprofilesofresistivityandpermittivity resultingfromtheelectrolyteuptakeprofilearecalculatedforsome effectivemediumtheory(EMT)formulas,andthen usedto cal-culatetheimpedance of thesystem.It is shown that only one oftheseformulasleadstoCPEbehavior. Finally,thetheoretical impedanceexpression is compared to experimentalimpedance dataobtainedwithhybridsol–gelcoatingsexposedtoa0.5MNaCl solution.
2. Theory
Aschematicrepresentation ofa substrate/coating/electrolyte systemispresentedinFig.1a.Thecoatingthicknessisdenotedı
andthepositionalongthecoordinateperpendiculartothe sub-strate/coating and coating/electrolyte interfaces is expressedin dimensionlesscoordinatesas
=x
ı (1)
wherexisthedistancefromthesubstrate/coatinginterface. Thecoatingwasassumedtobepenetratedbytheelectrolyte whichfillsanumberofpores,thelateraldimensionofwhichmay bedistributed and not necessarily independentof . InFig. 1a, the poresare represented as being straight, although theyare obviouslytortuousinnature.Allporeshaveamouthatthe coat-ing/electrolyteinterfaceand,mostofthemarenotasdeepasthe coating.Thus,differentplanes paralleltotheinterfaces,located withinthecoatingatdifferentpositions,crossdifferentnumbers ofpores,anddifferentelementallayersofthecoating,dthick,have differentlocalelectrolytevolumefractions().Somepores(only oneisshownintherightsideofFig.1a)mayextendthroughout thecoatingandprovidealow-resistivitypathbetweenthe elec-trolyteandthesubstrate.Thus,theelectricalequivalentcircuitof thesubstrate/coating/electrolytesystemconsistsoftheelectrolyte resistanceinserieswiththeparallelcombinationoftwo compo-nents:a resistance(Rpore), representingthethrough pores,and
animpedance(Zc),representingtheelectrolyte-modifiedcoating.
SuchacircuitisrepresentedintheupperboxshowninFig.1b. Thelowerbox,containingaCPE,accountsfortheseriesimpedance responseoftheoxidelayeronthesubstratesurface.Thescheme inFig.1a,inwhichthelocalelectrolytevolumefractionincreases asincreases,issimilartotheoneproposedbyHinderliteretal. [11]who,however,clearlyseparatedanouterelectrolyte-affected coatingregionfromaninnerpristinecoatingregionandtherefore didnotconsiderporesthatreachthesubstrate.
Variouseffectivemediumtheory(EMT)formulashavebeen pro-posedthatmaybeusedtocalculateresistivityandpermittivity profilescausedbyadistributionof().Takingintoaccountthat, ineachelementallayerparalleltotheinterfaces,eachpositionis occupiedbyeitherthecoatingmaterialortheelectrolyte,thesetwo mediamaybeassumedtobeinparalleltoeachother.Therefore,the overallconductivity(=−1)andpermittivity(ε)maybeobtained
aslinearcombinationsoftheconductivitiesandpermittivitiesof thetwomedia(coatingandelectrolyte)withcoefficientsequalto therespectivevolumefractions[11],i.e.,
and
ε()=εw()+εc[1−()] (3)
wherethesubscriptswandcrefertotheelectrolyteandthecoating material,respectively.Sinceusually()≪1and−1
c ≪−1w,()−1
dependslinearlyon().
Itwasrecentlyshown[15]thataCPEbehaviorcouldbedueto avariationof()accordingtoapower-law.ForaCPEimpedance writtenas
ZCPE=
1
(jω)˛Q (4)
thepower-lawexponent andtheCPEexponent˛arelinkedby therelationship
˛= −1
(5)
Inthepresentwork,thehypothesisismadethat()changes from(0)=0at=0to(ı)at=1accordingtoapower-law,i.e., ()= (ı)
1+(ı)( −1) (6)
The condition (0)=/ 0 was not considered because Rpore
accountsforthecontributionofthroughpores,andthereforeonly theporesnotreachingthesubstrate/coatinginterfaceneedtobe consideredforcalculatingZc.Theelectrolytevolumefraction()
ispresentedasafunctionofpositioninFig.2forsometypical cases.ThesewerecalculatedfollowingEq.(6)fordifferent(ı) val-uesbetween0.001and0.1.Anexponent =5wasassumed,which correspondsto˛=0.8,atypicalvaluefoundwithsystems exhibit-ingamarkedCPEbehavior.Assumptionofadifferentvalueof wouldproducequalitativelysimilarresults.
Theresistivity()anddielectricconstantε()profiles, calcu-latedaccordingtoEqs.(2)and(3)forthesamew,c,εwandεc
valuesasthoseconsideredinRef.[11],areshowninFig.3.With (ı)=0.1(correspondingtoameanelectrolytepartialvolume<2%), theresistivityvariationacrossthecoatingisca.8ordersof magni-tude.Evenwith(ı)aslowas0.001,(ı)is5ordersofmagnitude lowerthan(0).Incontrast,thepermittivityvariationsare mod-erate:for(ı)=0.1,ε(ı)islargerthanε(0)byonlyafactorofca. 2.Atlowvalues,i.e.,closetothemetal/coatinginterface,Fig.3a showsahighresistivityregioncompatiblewiththeassumptionof aninterphasewithpropertiessignificantlydifferentfromthoseof thebulkcoating[13,14].
Undertheassumptionthateachelementallayermaybe rep-resentedbyaparallel combinationofa resistance()d and a
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.02
0.04
0.06
0.08
0.10
= 0.1
= 0.05
= 0.01
= 0.001
( ) /dimensionless
/ dimensionless
Fig.2. Wateruptakeprofileswith(ı)asaparameter,calculatedaccordingtoEq.
(6)with(0)=0and =5. 0.0 0.2 0.4 0.6 0.8 1.0 103 106 109 1012 = 0.1 = 0.05 = 0.01 = 0.001
(
)
/
cm
/ dimensionless
a
0.0 0.2 0.4 0.6 0.8 1.0 8 10 12 14 16 = 0.1 = 0.05 = 0.01 = 0.001)
/
d
im
e
n
s
io
n
le
s
s
/ dimensionless
b
Fig. 3.Resistivity (a)and permittivity (b) profileswith (ı)as aparameter, calculatedaccording to Eqs. (2) and (3) respectively,with c=2×1011cm, w=330cm,εc=8,εw=82,(0)=0,and =5.
capacitanceε()ε0/d,theimpedanceoftheelectrolyte-penetrated
coating(Zc)iscalculatedas Zc(ω)=ı 1
Z
0 1 ()−1+jωε()ε0 d (7)Plotscorrespondingtofourdifferent(ı)valuesareshownin Fig.4,bothinBodecoordinates,currentlyusedinthecoating com-munity,andinNyquistcoordinates.Clearly,thecoatingimpedance becomessmallerwhen(ı)increases(andthemeanelectrolyte partialvolumeincreases).However,theshapeoftheplotsis insen-sitivetothevalueof(ı).Theplotofthephaseangleclearlyshows aCPEbehaviorinahigh-frequencyrange,approximatelybetween 100Hzand 10kHz. TheCPEbehavioris alsovisiblein thehigh frequencybranchoftheNyquistplots.
Thefrequencydependenceofthedlog|Z′′
c|/dlogfderivativeis
showninFig.5.Inafrequencyrangewherethesystemexhibits aCPEbehavior,dlog|Zc′′|/dlogf =−˛[18].Theplots
correspond-ingtofourdifferent(ı)valuescompletelyoverlapwithavalueof −0.8(correspondingto˛=0.8)inthehighfrequencyrange. Corre-spondingly,theconstant-phase-anglevalueinFig.4bisgivenby 0.8×90◦=72◦.
To obtain the overall impedance Z of the sub-strate/coating/electrolyte system, the pore resistance Rpore in
parallel with Zc and the electrolyte resistance in series with
the parallel combination of Rpore and Zc should be considered.
10-2 10-1 100 101 102 103 104 105 103 104 105 106 107 108 = 0.1 = 0.05 = 0.01 = 0.001
|Z
c
|
/
cm 2f
/ Hz
a
10-2 10-1 100 101 102 103 104 105 0 30 60 90 = 0.1 = 0.05 = 0.01 = 0.001/
d
e
g
re
e
f / Hz
b
0 1x107 2x107 3x107 0 1x107 2x107 3x107= 0.1
= 0.05
= 0.01
= 0.001
-Z
c"
/
cm
2Zc' /
cm
2c
1.259 Hz
Fig.4.Impedanceplotswith(ı)asaparameter,inBodecoordinates(a)and(b) andNyquistcoordinates(c),calculatedaccordingtoEq.(7)withı=20mmandthe conductivityandpermittivityprofilesshowninFig.3,i.e.,with(0)=0and =5.
negligible, and henceforth will be disregarded. Thus Zmay be simplycalculatedas
Z(ω)=
Zc(ω)−1+R−1pore −1(8) Fig.6 showsimpedanceplotscalculated accordingtoEq. (8) for variousRpore values and for theresistivity and permittivity
10-1 101 103 105 -1.0 -0.5 0.0 0.5 1.0 = 0.1 = 0.05 = 0.01 = 0.001
d
lo
g
|Z
c"|
/d
lo
g
(f
)
f / Hz
= 0.8Fig.5.Frequencydependenceofdlog|Zc"|/dlogf,calculatedwiththesame param-etersasforFig.4.
profilescorrespondingto(0)=0,(ı)=0.1and =5.Aslongas Rpore>Zc(0),whereZc(0)=ı
1
Z
0
()disthezerofrequencylimitof Zc,ithasonlyaslighteffectontheoverallimpedance.WhenRpore
becomesprogressivelysmaller,theimpedancemodulusdecreases andtendstoapproachRpore,asshown inFig.6a;thetransition
fromaCPEtoaresistivebehaviorisshiftedtohigherfrequency, asshowninFig.6b;andtheshapeoftheNyquistplotsbecomes moresymmetricalwithrespecttothemaximumofZ”,asshownin Fig.6c.
Hinderliteretal.[11]consideredothereffectivemedium theo-ries,inadditiontoEqs.(2)and(3).Assuminganelectrolyteuptake describedbyEq.(6),(0)=0, (ı)=0.1and =5,resistivityand permittivityprofileswerecalculatedcorrespondingto
(i) aseriescombinationofthetwomedia[11],i.e.,
()=wϕ()+c[1−ϕ()] (9) ε()=
1−() εc + () εw −1 (10) (ii)anEMTformulaproposedbyBrasherandKingsbury[7],i.e.,log[()−1]=() log[−1
w]+[1−()] log[c−1] (11)
log ε()=ϕ()logεw+[1−ϕ()] logεc (12)
and
(iii)theMaxwellapproximation[19],i.e.,
()−1=−1c −1 w +2−1c +2ϕ()(w−1−−1c ) w−1+2c−1−ϕ()(−1w −c−1) (13) ε()=εcεw +2εc+2ϕ()(εw−εc) εw+2εc−ϕ()(εw−εc) (14) TheresultsareshowninFig.7.Eqs.(9),(11)and(13)giveriseto muchsmootherresistivityprofilesthandoesEq.(3).The(0)/(ı) ratioiscloseto10fortheBrasherandKingsburyformula,andbelow 2forboththeseriescombinationandtheMaxwellapproximation, inagreementwithHinderliteretal.[11].Thepermittivityprofiles computedaccordingtoEqs.(10),(12)and(14)arealsosignificantly smootherthanthatcalculatedaccordingtoEq.(3).Theprofiles cal-culatedaccordingtoEqs.(9)–(14)wereusedtoobtaintheNyquist plotsshowninFig.8a,togetherwithaplotcorrespondingtoa sim-pleRCparallelcombinationwithR=cı,andC=εcε0/ı. Allplots
haveanalmostperfectsemicircularshape,i.e.,thereisnoevidence ofaCPEbehavior.Thisresultisconfirmedbythelogarithmicplots
10-2 10-1 100 101 102 103 104 105 102 103 104 105 106 107 108 Infinite Rpore Rpore = 108 cm2 Rpore = 107 cm2 Rpore = 106 cm2
|
Z
| /
cm 2f / Hz
a
10-2 10-1 100 101 102 103 104 105 0 30 60 90 Infinite Rpore Rpore = 108 cm2 Rpore = 107 cm2 Rpore = 106 cm2/ degree
f / Hz
b
0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00Infinite Rpore
Rpore = 108 cm2 Rpore = 107 cm2 Rpore = 106 cm2-Z
"/
R
p
/
dimensionless
Z'/Rp / dimensionless
c
25.12 Hz 3.162 Hz 1.585 Hz 1.259 HzFig.6.ImpedanceplotswithRporeasaparameter,inBodecoordinates(a)and(b) andNyquistcoordinates(c),calculatedaccordingtoEq.(8)with(0)=0,(ı)=0.1, and =5.Tofacilitatethecomparisonoftheshapeofthediagrams,theimpedance isnormalizedbythepolarizationresistanceRp.
ofZ”asafunctionoffrequency,showninFig.8b.Owingtothe mod-eratevariationofresistivityacrossthecoating,theimpedancesare comparabletothatofacoatingthathasuptakennoelectrolyte, andaremorethan10timeslargerthanthatshowninFig.4for thesameıand(ı)value.Thus,thedistributionsresultingfrom aseriescombinationofthetwomedia[11],describedbyEqs.(9) and(10);theEMTformulaproposedbyBrasherandKingsbury[7], describedbyEqs.(11)and(12);andtheMaxwellapproximation [19],described byEqs.(13)and(14)areincompatible withthe observedCPEimpedanceresponse.
In the next two sections a preliminary test of the theory describedaboveispresented.
0.0 0.2 0.4 0.6 0.8 1.0 103 105 107 109 1011
/
cm
/ dimensionless
Eq. 2, parallel Eq. 9, series Eq. 11, Brasher-Kingsbury Eq. 13, Maxwella
0.0 0.2 0.4 0.6 0.8 1.0 8 10 12 14 16b
Eq. 3, parallel Eq. 10, series Eq. 12, Brasher-Kingsbury Eq. 14, Maxwell/ dimensionless
/ dimensionless
Fig.7.Resistivity(a)andpermittivity(b)profilescalculatedfor(0)=0,(ı)=0.1, and =5,accordingtotheequationsindicatedonthefigure.
3. Experimental
Thesol–gelcoatingwaspreparedbyusingapolyaminoamide (PAA), 3-glycidoxypropylethoxysilane (GLYEO), tetraethylortho-silicate (TEOS), butanol and an additional epoxy resin. The compoundswereintroducedwithaGLYEO/TEOS/PAA/Epoxyratio of 1:1:10:8. The complete chemical structures of the various components cannot be reported herebecause they are partof proprietaryformulations.Thesol–gelfilmdidnotcontainany pig-mentsorfillers.
Thecoatingsweredepositedontoa2024T3aluminum alloy currentlyusedintheaerospaceindustry.Thespecimensconsisted of125mm×80mm×1.6mmplatesmachinedfromrolledplate. Beforepainting,thesamplesweredegreasedinanalkalinebath at 60◦C (pH=9) for 15min, rinsed twice with distilled water,
then etched in an acid bath at 52◦C for 10min, and rinsed
again with distilled water. The liquid paints were applied by air spraying and cured at 100◦C for 1h. The coating
thick-ness was measured at several locations from the observation of the cross-sections of the samples by SEM and found to be 20±1mm.
Athree-electrodeelectrochemicalcell,filledwith0.5MNaCl, wasusedinelectrochemicalimpedancemeasurements.Acoated specimenwasusedasworkingelectrode.AcylindricalPlexiglas tubewasassembledontopofthecoatedsample(exposedsurface areaof24cm2).Thiscellconfiguration(recessedsystem)allowed
eliminationofthefrequencydispersionduetocurrentand poten-tialdistributionsatthediskelectrode[20].Alargeplatinumsheet andasaturatedcalomelelectrodewereusedascounterand ref-erenceelectrodes,respectively.Theelectrochemicalcellwaskept atroomtemperatureandopentoair.Electrochemicalimpedance measurementswerecarriedoutusingaBiologic VSPapparatus. Theimpedancediagramswereobtainedunderpotentiostatic con-ditionsatthecorrosionpotentialoverafrequencyrangeof60kHz
0
1x108 2x108 3x108 4x1080
1x108 2x108 3x108 4x108-Z
"
/
cm
2Z' /
cm
2Eq. 9 and 10, series
Eq. 11 and 12, Brasher-Kingsbury Eq. 13 and 14, Maxwell
No electrolyte uptake
a
1.259 Hz 10-2 10-1 100 101 102 103 104 105 103 104 105 106 107 108Eq. 9 and 10, series
Eq. 11 and 12, Brasher-Kingsbury
Eq. 13 and 14, Maxwell
No electrolyte uptake
-Z
"
/
cm 2f / Hz
b
Fig.8.Impedanceplots,inNyquist(a)andlogZ”vs.logf(b)coordinates,calculated accordingtoEq.(7),fortheresistivityandpermittivityprofilesshowninFig.7.The solidlineisthecoatingimpedanceintheabsenceofelectrolyteuptake.
to10mHzwith6pointsperdecade,usinga20mVpeak-to-peak sinusoidalpotential.
The impedance data analysis was performed using a non-commercialsoftware developed at the LISECNRS, Paris, which allowedregressionofamodelconsistingofacombinationof pas-sivecircuitelementsandanalyticalexpressions.Thissoftwaredid notprovideconfidenceintervals.
4. Resultsanddiscussion
Theimpedance of a 2024T3 aluminum alloy/hybridsol–gel coating/0.5MNaClsolutionsystemwasmeasuredat the corro-sionpotentialasafunctionoftheimmersiontime.Asanexample, theimpedancerecordedafter72hispresentedinFig.9(points)in BodeandNyquistformats.Thehigh-frequencyloopwasattributed tothecoating,andthelow-frequencyquasi-verticalstraightline wasattributedtoanoxidefilmpresentatthesurfaceofthealloy [21,22].Theexperimentaldatawereanalyzedwithreferencetoa modelconsistingofaseriescombinationofanimpedancedescribed byEq.(8),correspondingtothecoating,andaCPE,corresponding totheoxidefilm.Anequivalentcircuitforsuchamodelisshown inFig.1b(allcircuitelements).Theoxideresistancewasassumed tobeverylarge,inagreementwiththequasi-capacitiveblocking behaviorobservedatlowfrequencyinallexperiments,therefore, noresistancewasconsideredinparallelwiththeCPEassociated withtheoxidelayershowninFig.1b.Suchabehaviorsuggests that,fortheimmersiontimesexploredinthepresentwork,the
10-2 10-1 100 101 102 103 104 104 105 106
Experiment
Fit
|Z
|
/
Ω
cm 2f / Hz
a
10-2 10-1 100 101 102 103 104 0 30 60 90Experiment
Fit
θ
/
d
e
g
re
e
f / Hz
b
0 1x105 2x105 3x105 0 1x105 2x105 3x105Experiment
Fit
Z
"
/
cm 2
Z'
/
cm2c
260.4 Hz 5.208 Hz 0.1045 HzFig.9.Impedanceplotsrecordedwith2024aluminumalloy/hybridsol–gelcoating, aftera72-himmersionin0.5MNaClsolution.Theexperimentaldata(points)are comparedwiththeregressionresults(solidline).
coating-oxideensembleeffectivelyprotectstheunderlyingmetal fromcorrosion,andcircuitelementsaccountingforcorrosion reac-tionsneednotbeconsidered.Duetotheresistivityoftheelectrolyte (22cm)andtheelectrodesize(24cm2),theelectrolyteresistance
couldbeneglected.
Intheregression procedure, , (ı)andRporeweretheonly
adjustableparametersforthecoating;whereas,w,c,εwandεc
weregivenfixedvalues(w=2.22×101cm,
Table1
Dependenceof(ı), andRporeonthedurationoftheimmersionofthe2024aluminumalloy/hybridsol–gelcoatingsamplein0.5MNaCl.
Immersiontimein0.5MNaCl(h) (ı)(dimensionless) (dimensionless) Rpore(cm2)
24 1.46×10−5 3.5 1.04×106 48 2.24×10−5 3.4 3.12×105 72 3.21×10−5 3.1 2.11×105 168 1.16×10−4 3.0 5.15×104 0.0 0.2 0.4 0.6 0.8 1.0 105 106 107 108 109 1010 1011 1012 24 h 72 h 168 h
/
cm / dimensionless
Fig.10.Resistivityprofilesinanelectrolyte-penetratedhybridsol–gelcoating, cal-culatedaccordingtoEqs.(2)and(3),withthebestfitted and()valuesobtained forthethreeimmersiontimesreportedonthefigure.
εw=82 and εc=8). The adjustable parameters for the oxide, Q
and ˛, are not discussed here as theyare not relevant tothe modelpresentedaboveforthecoating.Theregressionresultsare shown in Fig.9 as solid lines.The agreementwiththe experi-mentaldata wasgoodover theentirefrequency range,both in NyquistandBodecoordinates.The(ı) andRporevalues
mea-suredfor fourimmersiontimes arereportedin Table1.Asthe coating degradation proceeded, the electrolyte uptake became progressivelystronger,althoughit remainedatlow levels((ı) justexceeded1×10−4after168h),andR
porebecamesmaller.In
Fig.9,thelow-frequencylimitofthecoatingimpedanceisclose toRpore,i.e.,thepseudo-diameterofthehigh-frequencyloopmay
beidentifiedwiththeporeresistanceasfor theclassical Beau-nier’scircuit[2],and,therefore,thehigh-frequencyloopisalmost symmetrical with respect to the Z” maximum (compare with Fig.6c).
The()–dependencecalculatedwiththebestfitted and (ı)valuesisshowninFig.10forthreedifferentimmersiontimes. Sharp resistivityprofiles wereobtained, even for thesmall fit-tedvalues of (ı).The ε()–dependence wasalso calculated; however, the permittivity underwent only negligible variation (between 8 and 8.009, for 168h) and, therefore, no plot is shown.
Themodel didnotfit aswell forsomeotherhybrid sol–gel coatingswith different formulations, tested in electrolytes less aggressivethan0.5MNaCl.Presumably,amoresophisticated inter-pretationisneededforthosesystems,andworkisunderwayto refinethemodel.Theaboveresults,however,providea prelimi-naryvalidationofthemodelproposedinthepresentmanuscript. Theresultsconfirmtheassumptionthatthevalueofεinthe power-lawmodelcanbeconsideredasaconstantparameter.Theuseofthe power-lawmodelshowsthatthewateruptakethroughthecoating inducesaCPEbehaviorwithoutsignificantvariationofthe per-mittivity.EISdatarelevanttoothersubstrate/coating/electrolyte systemsarebeingexaminedandwillbedescribedinafollowing paper.
5. Conclusions
Observation of a CPE behavior in the impedance of a substrate/coating/electrolytesystemmaybeexplainedbyan inho-mogeneouselectrolyteuptakegivingrisetoapower-law()– profile,ifthelocalresistivityandpermittivityvaluesateach posi-tionalongthecoatingthickness,()andε(),arecalculatedbyan EMTformulacorrespondingtotheparallelcombinationofthe coat-ingmaterialandtheelectrolyte.Sincetheindividualresistivitiesof thetwomedia,candw,typicallydifferbyseveralordersof
mag-nitude,theelectrolyteuptakecausesmarkedresistivityvariations acrossthecoating.Incontrast,thepermittivityundergoesonlya mildvariationsincetheindividualpermittivities,εcandεw,differ
byafactorofabout10.Theseresultsallowtheextensionofa pre-viouslyderivedmodel[15,16],appliedsofartopassiveoxidesand humanskin[17],toanti-corrosioncoatings.
Whenexperimental EIS datarecorded with2024 aluminum alloycoatedwithhybridsol–gelcoatings,immersedin aqueous NaClsolution,wereanalyzedwithreferencetothemodeldescribed in thepresent work,goodqualityfitting wasobtainedusingas adjustableparametersforthecoatingonly ,(ı)andRpore.The
fitted(ı)valueswerequitelow(oftheorderof10−4 orlower),
butsufficienttocauseresistivityvariationsoveratleast5orders ofmagnitude(andvirtuallynovariationofpermittivity).The coat-ingdegradationwascausedbythecombinedeffectofincreasing (ı)andincreasingthenumberand/orcross-sectionofthethrough pores,witnessedbythedecreasingRporevalues.
Acknowledgments
Theexperimentaldatawereobtainedintheframeworkofthe SMILEproject(SurfaceMonoInnovativeLayerforEnvironment), withthefinancial supportof theDGCIS(Direction Généralede laCompétitivité,del’IndustrieetdesServices).S.AmandandN. Pébère thanks the partners of the project: Mapaero(Pamiers), Rescoll(Bordeaux),Airbus(Toulouse),Dassault(Paris),EADSIW (Suresnes),l’Electrolyse(Bordeaux),andthelaboratoriesLCPOand US2B(UniversitédeBordeaux).M.Musianiacknowledgesfinancial supportfromtheCNRShortTermMobilityprogram.M.E.Orazem acknowledgesfinancialsupportfromtheCharlesA.Stokes Profes-sorship.
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