Long-term deterioration of lubricant-infused nanoporous anodic aluminium oxide surface immersed in NaCl solution

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Delft University of Technology

Long-term deterioration of lubricant-infused nanoporous anodic aluminium oxide surface

immersed in NaCl solution

Wu, Dequan; Ma, Lingwei; Liu, Bei; Zhang, Dawei; Minhas, Badar; Qian, Hongchang; Terryn, Herman A.;

Mol, Johannes M.C.

DOI

10.1016/j.jmst.2019.12.008

Publication date

2021

Document Version

Final published version

Published in

Journal of Materials Science and Technology

Citation (APA)

Wu, D., Ma, L., Liu, B., Zhang, D., Minhas, B., Qian, H., Terryn, H. A., & Mol, J. M. C. (2021). Long-term

deterioration of lubricant-infused nanoporous anodic aluminium oxide surface immersed in NaCl solution.

Journal of Materials Science and Technology, 64, 57-65. https://doi.org/10.1016/j.jmst.2019.12.008

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JournalofMaterialsScience&Technology64(2021)57–65

ContentslistsavailableatScienceDirect

Journal

of

Materials

Science

&

Technology

jou rn a l h o m e p a g e :w w w . j ms t . o r g

Research

article

Long-term

deterioration

of

lubricant-infused

nanoporous

anodic

aluminium

oxide

surface

immersed

in

NaCl

solution

Dequan

Wu

a

_,

_Lingwei

_Ma

a

_,

_Bei

_Liu

a

_,

_Dawei

_Zhang

a,∗

_,

_Badar

_Minhas

a

_,

_Hongchang

_Qian

a

_,

Herman

A. Terryn

b,c

_,

_Johannes

_M.C.

_Mol

c

a_Beijing_Advanced_Innovation_Centre_for_Materials_Genome_Engineering,_Institute_for_Advanced_Materials_and_Technology,_University_of_Science_and

TechnologyBeijing,Beijing100083,China

b_Department_of_Materials_and_Chemistry,_Research_Group_{Electrochemical}_and_Surface_Engineering,_Vrije_Universiteit_Brussel,_Brussels,_Belgium

c_Department_of_Materials_Science_and_Engineering,_Delft_University_of_Technology,_Delft,_the_Netherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28October2019

Receivedinrevisedform

30November2019

Accepted3December2019

Availableonline8January2020

Keywords:

Lubricant-infusedsurface

Anodicaluminiumoxide

Deterioration EIS

a

b

s

t

r

a

c

t

Thisstudyinvestigatedthedeteriorationofalubricant-infusedanodicaluminiumoxidesurfaceina 1MNaClsolutionfor∼200days.Directobservationbycryo-SEMandquantitativeanalysesbyUV spec-troscopyandEISrevealedthatthelong-termdeteriorationofthelubricant-infusedsurfacewasdivided intotwostages:thesurface-adheredlubricantlayergraduallydissolvedataconstantrateuntilthe substratewasexposed;afterwardsthelubricantinfusedinthenanochannelsbegantodiffuseandwas depletedafter∼200days.TheEISresultsalsorevealedthatthedefectsreducedthecorrosionresistance ofthelubricant-infusedsurfaceconsiderably.

1. Introduction

Creatingdurablesurfacesforaluminiumanditsalloysthat with-standcorrosioninunderwaterenvironmentsisofgreatimportance formarineandenergy-relatedapplications[1–3].Surface anodiza-tionhasbeenwidelyusedtoimprovethecorrosionresistanceof aluminiumbyformingprotectiveanodicaluminiumoxides(AAO) consistingofathinandcompactbarrierlayerandathickporous layer[4,5].However, thehigh-aspect-rationanochannels ofthe porouslayerispronetoabsorband retaincorrosivemedia[6]. Tosolvethis problem,variousmaterialshavebeenusedtoseal theporesincludingchromates,nickelsalts,organicacidsand sol-gels[7,8]. Lubricantssuchashigh-viscosityoilsorgreases have alsobeenusedassealingmaterialstoimprovesurfacelubricity undertribologicalenvironment[9,10].Nevertheless,theeffectof lubricant-sealingonthecorrosionresistanceisrarelyreported.

Inspiredby Nepenthes pitcherplants, anew typeof surface calledthelubricant-infusedsurface(LIS)orslipperyliquid-infused poroussurface(SLIPS)hasbeeninventedbyinfusinglubricantwith lowsurfaceenergyintoporousmicro/nanostructures[11,12].The capillaryforceoftheroughnanostructurelocksthelubricantinthe

∗ Correspondingauthor.

E-mailaddress:dzhang@ustb.edu.cn(D.Zhang).

poroussubstrate,formingacontinuousandsmoothlubricant sur-face[13,14].Thelowsurfaceenergyoftheliquidlubricantenables thesurface a slipperyfeature usefulfor thereductionof water condensation,microbialfoulingandicenucleation[15,16]. Addi-tionally,theseamlessfeatureofthehydrophobiclubricantlayer mayalsoprotect thesubstrate frombeingattackedbyexternal corrosivemedia,makingLIS/SLIPSanemergingsurfacetechnology forwidepotentialapplicationsincorrosiveunderwaterconditions [17–20].Generally,thecriteriaofdesigning aLISareextremely rigorous:i)thelubricantmustfullywetthemicro-/nanoporous surfaceoftheLISsubstrate;ii)thelubricantisimmisciblewiththe ﬂuidsitcontacts;andiii)thecontactingﬂuidscannotpenetrate intotheLISsubstrate[21–23].

Recent studies have shown an increasing interest in devel-oping LIS/SLIPSonvariousmetalsubstratessuchasaluminium, magnesium, zinc and steels for enhanced corrosion protection [24–28].Forexample,Wangetal.havedevelopedaLISby infus-ingperﬂuoropolyether(PFPE)lubricantintoa superhydrophobic hydrothermally-treatedzincsurface[17].After10daysof immer-sionintheNaClsolution,theLISexhibitedamorestablebarrier propertythanthesuperhydrophobicsurfaceinwhichthe protec-tiveairlayercouldberapidlyreplacedbytheaqueoussolution. Morerecently,LISwaspreparedbyinfusingPFPEintothe layer-double hydroxide (LDH)ﬁlm grown onthe plasma electrolytic oxidisedAZ91Dmagnesiumalloy.Frompolarisationtests,the

cor-https://doi.org/10.1016/j.jmst.2019.12.008

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58 D.Wuetal./JournalofMaterialsScience&Technology64(2021)57–65

rosionrateoftheLIS-protectedAZ91Dwasﬁveordersofmagnitude lowerthanthebaremetal,and twoordersof magnitudelower thanthemetalprotectedbyasimilarlypreparedsuperhydrophobic surface[27].

Atpresentmoststudiesinthisfieldfocusonthefabricationof theLIS/SLIPSwithdelicatemicrostructuresorsuperiorfunctional performances,whereasthelong-termstabilityanddegradationof theLIS/SLIPSintheproposedserviceenvironmenthasrarelybeen studied.Forexample,theLISspreparedforcorrosionprotection [29,30]oranti-foulingapplications[23,31]havebeenstudiedonly fora short period oftime rangingfrom severaldays toweeks. However,becauseofthehighmobilityoftheliquidlubricant,the stabilityofLIS/SLIPSasaprotectiveorfunctionalsurfaceduring long-termenvironmentalexposureisquestionable[32–34].Partial dissolution,whichisacommonlyobservedphenomenonbetween organicandaqueousliquids,mayresultinaslowlubricantloss dur-inglong-termimmersion[35,36]Forexample,typicallubricants suchas Krytoxfor LISpreparation weremeasuredwitha solu-bilityrangingfromseveralnanogramstomilligramsperlitrein water[37–39].Weakfluctuationandanisotropicpressureofthe immersingsolutionmayfurtherpromotethediffusionofthe liq-uidlubricanteveninstaticunderwaterconditions[40].Inaddition, somesilicone-basedandKrytox-basedlubricants,whichhavehigh waterimmiscibilityandarewidelyusedinLISdevelopment,have beenalsofoundtocloakthedepositedwaterandberemovedas waterslidesofffromthesurface[41–43].Moreimportantly, quan-titativeandinsituanalyticaltechniquesbywhich theextentof LIS/SLIPSdeteriorationcanbemonitoredunderimmersed condi-tionshaveseldomlybeenreported.Duetotheliquidnatureofthe infusedlubricant,itisalsodifficulttodirectlyobservethelossof lubricantfromtheporoussubstrate[46].Althoughseveralstudies haveshownthatthesurfacehydrophobicitywoulddeterioratedue tothedepletionoftheinfusedlubricant[44],ithasbeendifficultto identifythequantityofthelostlubricantandthetransitionpoint fromwaterslidingtowaterpinningonthesurface[41].

WehaverecentlyreportedthedevelopmentofLISbyinfusing mineraloillubricantintothehigh-aspect-rationanochannels of AAOsurfaceviaavacuumimpregnationmethod[45]. Thework highlightedtheimportanceof rationaldesign in theLIS prepa-rationandthevacuumimpregnationmethodwhichwasproved tobemoreefﬁcientinﬁllingdeepporesthansimpleimmersion. The obtained surface demonstrated self-healing effects against mechanicaldamages(e.g.wearingandcracking)andanimproved corrosion resistanceover a relatively short exposure(85 days) underaqueousimmersion.

Inthepresentstudy,weaimedtounderstandthedegradation mechanismofthistypeofLISunderlong-termimmersionin1M NaClsolutionforover200daysbycarryingoutacombinationof microscopic,spectroscopicandelectrochemicalanalyses.During theimmersion,thechangeinthesurfacehydrophobicityoftheLIS wasevaluatedbywatercontactanglemeasurements.Thelubricant lossoftheLISwastrackedbydirectobservationusingcryogenic scanningelectronmicroscopy(cryo-SEM)andconﬁrmedby ultra-violet(UV)spectroscopy.Electrochemicalimpedancespectroscopy (EIS)wasemployedasaninsitumeasurementtechniqueto cor-relatethelossoflubricantwiththedegradationoftheprotective properties.

2. Experimental

2.1. Materials

Rolledaluminiumfoilswithapurityof99.999wt.%were pur-chasedfromtheBeijingInstituteofNonferrousMetals(China).A mineraloillubricant(ShellAdvancedAX5,Shell)waspurchased

fromShellGlobal.Deionisedwaterwasusedinalltheexperiments. Otherchemicalreagentsemployedinthisworkwerepurchased fromSinopharmandwereofanalyticalgrade.

2.2. LISpreparation

AAOwasfabricatedbyatwo-stepanodisingmethod.Thefirst anodisationstepwasperformedin0.3Mphosphoricacidat195V via a DC source (N8741, Keysight, Ameirca) using an electro-polishedaluminiumfoilastheanodeandadegreasedaluminium foilasthecathode.After60min,thefirstanodicfilmwasimmersed inamixturesolution(6wt.%phosphoricacid,1.8wt.%chromicacid, anddeionisedwater)at60◦Cfor30min.Then,thesecond anodi-sationstepwasperformedunderthesameconditionsfor2h.The obtainedAAOhasaporediameterof∼200nmandaporedepthof ∼50␮masobservedbySEM.

TheLISwasfabricatedbyinfusingthelubricantintotheAAO nanochannelstructurethroughavacuumimpregnationmethod, asdescribedinourpreviousstudy[45].Thefabricationprocesscan bedescribedbrieﬂyasfollows.TheAAOspecimenswereplaced inavacuumchamber(∼3Pa)for5htocompletelyremovetheair inthecontainerandnanochannels.Subsequently,oilwasinjected intothecontainer,and theoilcoveredtheAAOspecimen.After soakinginvacuumconditionfor1h,thelubricantinfusedintothe nanochannelswithouttheresistanceofcompressedairtrappedin pores.Whenthevacuumchamberwasreﬁlledwithair,the atmo-sphericpressurefurtherpushedthelubricanttodeeplyinfuseinto thenanochannels.

2.3. Cryo-SEM

Cryo-SEM was carried out using a microscope (FEI Helios

NanoLabG3UC)toobservethesolid–liquidinterfaceoftheLIS directly.First,thespecimenwasﬁxedtoa copperholderusing conductiveglueandmountedonacryo-cellholder.Then,the cryo-cellholderwasplungedintoliquid nitrogenfor∼10stofreeze theinfusedoil.Thespecimenwasbentreciprocallybytweezers tobreakthespecimenandexposeafreshcrosssection.The cryo-cellholderwasthentransferredintothespecimenchamber,which waspre-frozento−140◦_C._Before_the_observation,_the_specimen

shouldbesublimatedat−90◦_C_for₂₅_min_to_remove_the_ice_crystals

formedduringliquidnitrogensubmersion.Afterthis,thecryo-SEM observationchamberwasmaintainedatapressureof∼1.02×10−4 Paandatemperatureof−140◦_C._Then,_the_specimen_was_sprayed

withgoldatacurrentstrengthof10mAfor60s.TheLISwasmilled byafocusedionbeam(FIB)atabeamenergyof30keVandacurrent strengthof2.5nAwithadwelltimeof100nsperpixel.Next,the specimenwassprayedwithgoldagain.SEMimageswereobtained byusingbackscatteredelectrons(2keVand60pA).

2.4. Surfacewettability

Thesessilewatercontactangle(WCA)and slidingangle(SA) weremeasuredusingacontactanglemeter(OCA20,Dataphysics) toevaluatethesurfacewettabilityofthespecimens.Thespecimens (∼1cmindiameter),whichwereimmersedinthe1MNaCl solu-tion,wereretrievedandairdriedfor30min.ThesessileWCAwas measuredbyplacingadeionisedwaterdroplet(∼5␮L)onthe spec-imensurface.TheSAwasrecordedbytiltingthesurfaceslowlyuntil a10␮Lwaterdropletcouldslideoff.Theresultswereobtainedby averagingﬁvemeasurementsondifferentareasofthespecimens. 2.5. Electrochemicaltests

EIS measurements were performed using a

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D.Wuetal./JournalofMaterialsScience&Technology64(2021)57–65 59

asaninsitumethodtotrackthelubricantlossunderimmersion in the 1M NaClsolution at 25◦C. The LIS specimen(∼1cm in diameter)wastheworkingelectrode.Thereferenceandcounter electrodeswereasaturatedcalomelelectrodeandPtfoil, respec-tively. The impedance spectra were measuredin the range of 10−2–105_Hz_with_an_amplitude_of₂₀_mV._To_simulate_the_inner

structureoftheLIS,dataﬁttingwascarriedoutusingZSimpWin software. Allthe tests were performedat least twice toverify reproducibility.

2.6. UVspectroscopy

UVspectroscopy wascarriedout usinga spectrophotometer (U3900H,Hitachi)tomeasurethequantityofoilremaininginthe LISspecimens.Dichloromethanewasusedasanextractionsolvent todissolvethelubricantintheLISandasareferenceblank solu-tiontomeasureabsorbance.ThespecimensimmersedintheNaCl solutionfordifferenttimeswereretrievedandthensubmergedin 5mLdichloromethaneunderultrasonicationforlubricant extrac-tion.Theextractionsolutionwastheninjectedintoaquartzdish andscannedunderthespectrophotometer.Toobtainthe relation-shipbetweentheoilconcentrationandpeakintensity,aseriesof standardoilsolutions(lubricantconcentration:0,0.4,0.8,1.2,1.6, 2,2.4,2.8,3.2,3.6,4mg/mL)werepreparedandtested.

3. Resultsanddiscussion

3.1. Cryo-SEMmorphology

TostudythedeteriorationoftheLIS,cryo-SEMwasemployed fordirectlyobserving thetopviewand crosssectionof theLIS duringthelong-termimmersion.Cryo-SEMallowstoimagethe liquidlubricantinitsfrozencondition,whichcannotberealized byconventionalSEMoropticalmicroscopy.AsshowninFig.1(a1) and(a2),theas-preparedLISspecimenconsistedoftheAAO sub-stratecoveredbyacontinuousandsmoothlubricantlayerwitha thicknessof∼7␮m.AfterimmersionintheNaClsolutionfor20 days(Fig.1(b1)and(b2)),thelubricantlayerdecreasedslightlyin thicknessandremainedlargelycontinuous,althoughlesslubricant coveragewasobservedinlocalspots.After60days,thenanoporous surfacewasexposedandsomeporesbecamevisibleinthetopview ofthesurface(Fig.1(c1)),suggestingthatthesurfacelubricantlayer wascompletelylost.However,thecross-sectionalview(Fig.1(c2)) indicatedthatthenanochannelswerestillalmostfullyﬁlledwith thelubricantatthisstage.

After90daysofimmersion,allporesontheAAOsurfacewere exposed (Fig. 2(a1)) and the cross section showed that a few micronsoftheinfusedlubricantwerereleasedfromthe nanochan-nels(Fig.2(a2)).After150daysofimmersion,aroughporousAAO surfacewasobserved(Fig.2(b1))andthecrosssection(Fig.2(b2)) showed that the lubricant had been released from pores with depthsof20–30_␮m.Thelubricantintheupperportion(Fig.2(b3)) oftheLISwaslargelydepleted,whereasthenanochannelinthe bot-tomportion(Fig.2(b4))wasstillﬁlledwiththelubricant.After210 days,thelubricanthadfurtherdepletedfromthenanochannels, manyofwhichbecamecompletelyempty(Fig.2(c)).

3.2. Surfacewettability

TheWCAreﬂectsthesurfacewettability,whichisdetermined bythechemicalcompositionandroughnessoftheunderlying sub-strate[46].AsshowninFig.3(a),whenasmoothlubricantlayer fullycoveredtheAAOsurface,theWCAontheinitialLISwas∼99.5◦_.

Whenthesurface-adheredlubricantwascompletelywipedoff,the waterdropletcontactedwithacompositesurfaceconsistingofthe exposedaluminiumoxidesurfaceandthelubricantﬁlledinthe

AAO nanochannels.ThecorrespondingWCAdecreased to∼76◦_.

Whenthelubricantwaspartiallyremovedfromthenanochannels, thewaterdropletwasessentiallyincontactwiththeporoussurface ofAAO,whichconsistedofaluminiumoxideandtheairtrapped insidethenanochannels.TheWCAinthiscasewas∼68◦_.

There-fore,thedeteriorationofthesurfaceconditionsoftheLIScanbe indirectlyestimateduptoacertainstagebythevariationofthe WCA.Fig.3(b)showstheWCAoftheLISafterimmersioninthe 1MNaClsolutionfordifferenttimes.After1 dayofimmersion, theWCAdecreasedfrom∼99◦_to₈₇◦_,_which_might_be_attributed_to

theresidualmoistureonthelubricantsurfaceafterimmersion.The WCAcontinuedtodecreaseslightlyinthefollowing60days.From the60thdaytothe90thdayofimmersion,theWCAdecreased remarkablyfrom78◦to69◦,whichwasattributedtotheexposure oftheAAOporoussubstrate.Itisnoteworthythatthetwodivided coloursinFig.3(c)indicatedthatthetransitionfromthedissolution ofsurface-adheredlubricantlayertotheexposureofAAOsubstrate tookplacesomewherebetweenthe60thdayand90thdayinthe immersion.TheWCAalmostlevelledoffat65◦after120days.The furtherreleaseofthelubricantstoredintheinteriornanochannels couldnotbedetectedbytheWCAbecausetheWCAreﬂectsonlythe surfacewettability.AsshowninFig.3(c),theSAoftheLISincreased from∼10◦_to_∼85◦_during_the_initial₆₀_days_of_immersion._On_the

90thday,thewaterdropletswerefoundtobepinnedontheLIS, evenwhentheLISwastiltedby90◦.Thetransitionfromdroplet slidingtodropletpinningindicatedtheexhaustionofthetop lubri-cantlayerandtheexposureoftheporousAAOsubstrate.Basedon thecombinationoftheWCAmeasurementandthedirect obser-vationbycryo-SEM,theLISdeteriorationprocesscanbedivided intotwostages,i.e.thedissolutionofthesurface-adheredlubricant layer,andtheleachingofthelubricantinfusedinthenanochannels duringthesubsequentimmersion.

3.3. LubricantlossdeterminedbyUVspectroscopy

UVspectroscopywasusedtoprovideamoreaccurate quan-tiﬁcation ofthelubricantloss duringthelong-termimmersion. BecausetheUVabsorptionisdirectlyproportionaltothesolution concentrationaccordingtotheBeer–Lambertlaw[47],a quantita-tiverelationshipcanbeestablishedbetweentheUVabsorbance and lubricantconcentration to quantify the lubricant in pores.

Fig. 4(a)shows the normalisedUV spectraof theLIS with dif-ferent concentrations of the lubricant extracted from the AAO poresindichloromethane.Thetypicalabsorptionwavelengthsof the lubricantwere in therange of 220−400nm, and the char-acteristic peakwaschosentobe282nm.Thelinear correlation betweentheabsorbanceandlubricantconcentrationisshownin

Fig.4(b). Fig.4(c) showsthevariation in theUVspectraof the lubricantextractedfromtheLISafterdifferentimmersiontimes. Basedonthesespectra,theamountoflubricantremainingintheLIS nanoporeswasestimated.Thequantityofthelubricantwasfound todecreaseataconstantrateintheﬁrst60daysofimmersion (Fig.4(d)).Associatedwiththevariationofthesurfacewettability (Fig.3),themasslossduringthisperiodcouldbeattributedtothe dissolutionofthesurface-adheredlubricantlayer.After60days, therateofmasslossofthelubricantintheLISspecimenslowed down.From180–210days,only∼2.7mgoflubricantresiduewas leftinthenanochannels.

3.4. EISmeasurements

EIShasbeenfrequentlyusedtodetectinformationpertainingto thesurface/interfaceofbareandcoatedmetals[48].Inthisstudy, EISwasusedtounderstandthedeteriorationprocessoftheLIS dur-inglong-termimmersion.UnlikeUVspectroscopy,whichreﬂects thelubricantlossintheentirespecimen,EIScandetectthe

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dete-60 D.Wuetal./JournalofMaterialsScience&Technology64(2021)57–65

Fig.1. Cryo-SEMimagesshowing(1)topviewand(2)crosssectionofLISduringimmersionin1MNaClsolutionfor(a)0,(b)20,and(c)60days.

Fig.2. Cryo-SEMimageofLISduringimmersionin1MNaClsolutionfor90(a),150(b)and210(c)days.(a1)Topview,(a2)uppercrosssection.(b1)Topview,(b2)entire

crosssection,(b3)uppercrosssection,and(b4)bottomcrosssection.(c1)Uppercrosssection,(c2)bottomcrosssection.

Fig.3.(a)WCAonsurfacecoveredbyoillayer(a1),solidandoilcompositesurface(a2),andsolidandaircompositesurface(a3).WCA(b)andSA(c)ofLISduringdifferent

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Fig.4.ChangeinlubricantmassofLISduringlong-termimmersion.(a)UVabsorptionspectraofstandardlubricantsolutionswithdifferentlubricantconcentrations(0.4,

0.8,1.2,1.6,2.0,2.4,2.8,3.2,3.6,and4.0␮L/mL)indichloromethane,(b)linearcorrelationbetweenUVabsorptionintensityandlubricantconcentration,(c)UVabsorption

spectraofextractedlubricantindichloromethane,(d)changeinlubricantmassinLISduringdeteriorationprocess.

Fig.5. (a)Impedancemodulusplotsand(b)phaseangleplotsforLISwithcontrolledtoplayerlubricantthicknesses(0,1,3,5and7␮m),bareAAOimmersedin1MNaCl

solutionfor0day(theNaClsolutionwasjustinfusedintothenanochannel),andbareAAOimmersedin1MNaClsolutionfor20days.(c)Impedancemodulusplotsand(d)

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riorationoftheprotectivepropertyoftheLISeveninaverysmall area.Tomoreclearlyidentifythedeteriorationstages,EISwasﬁrst carriedoutontheLISwithexcesssurfacelubricantofdifferent thicknesses(0−7␮m)tosimulatethegraduallossofsurface lubri-cant.Additionally,theimmersionsolution(1MNaClsolution)was infusedintothenanochannelsofbareAAOtosimulatethesituation inwhichtheimpregnatedlubricanthadcompletelydiffusedoutof thenanochannels,andthebareAAOwasimmersedinthe1MNaCl solutionfor20daystosimulatethesituationinwhichthebarrier layerwasattacked.TheEISresultsforthesestandardspecimens areshown inFig.5(a)and (b).Accordingtopreviousstudieson sealedAAOsuchasthosebychromatesorhydrothermaltreatment, thelow-frequencyrangeoftheimpedancespectramainlyreﬂected theinnerbarrierlayerofAAO,whereasthehigh-frequencyrange correspondedtotheouterporouslayer[49,50].In Fig.5(a),the impedancemodulivalueinthehigh-frequencyregiondeclinedas thethicknessofthesurfaceoillayerdecreased[51].InFig.5(b),the phaseangleplotsoftheLISshowtwotimeconstants:thecrestin thehighfrequenciesreferstothelubricantlayer,andthecrestin thelowfrequenciesreferstothebarrierlayer[52].Whenthepores wereinfusedwiththeNaClsolution,theimpedancemodulusplot (Fig.5(a))becameastraightlineandonlyonetimeconstantcould beobservedovertheentiremeasuredfrequencies[53,54].After AAOwasimmersedinthe1MNaClsolutionfor20days,the mod-ulusvalueinthelowfrequenciesdecreasedremarkably,indicating corrosionofthebarrierlayer[55].BasedontheEISresults,the dete-riorationoftheprotectivepropertyoftheLIScouldbedividedinto threestages:(I)thedissolutionofthesurface-adheredlubricant untilthesubstratewasexposed,(II)thediffusionoftheinfused lubricantfromtheAAOnanochannelsuntilthebarrierlayerwas exposed,and(III)thecorrosionattackofthebarrierlayer.

Fig.5(c)and(d)showtheEISresultsfortheLISpreparedinthis studyover200daysofimmersioninthe1MNaClsolution.Inthe ﬁrst60days,theimpedancemoduliinthehigh-frequencyregion (Fig.5(c))declinedclearly,whichwasattributedtothedissolution ofthetoplayerlubricantcoveringtheLIS[52].Thisresultagreed wellwiththecryo-SEMobservationinFig.1.From90–210days,the impedancemoduliinthehigh-frequencyregion(Fig.5(c))declined gradually,whichwasattributedtothecontinuouslossofthe lubri-cantfromthepores.ThecorrespondingphaseangleplotsinFig.5(d) showthatthecrestinthehighfrequenciesﬂattenedasafunction of immersion time, which also indicated the loss of the lubri-cantlayer.However,theimpedancemoduliinthehigh-frequency regiondidnotdisappearcompletely.Theexistingtimeconstantsin thehigh-frequencyregion(Fig.5(d))indicatedthatsomelubricant stillremainedinthenanochannels.Theseresultsagreedwiththose obtainedbyUVspectroscopy.AsshowninFig.5(c),theimpedance modulusplotsinthelow-frequencyregionoverlappedwiththe |Z|0.01Hzvaluesremainedalmostunchangedandthephaseanglein

thelow-frequencyregionremainedatahighvalueofaround∼80◦

duringtheentiredurationof120days,indicatingthatthebarrier layerwaswellprotectedbytheinfusedlubricantintheinitialtime. After150days,amajordecreasewasobservedinthe|Z|0.01Hzvalue

andthephaseangleinthelow-frequencyregiondecreasedto∼50◦_,

suggestingthatthebarrierlayeroftheAAOﬁlmwasattacked[55]. Noticeably,thefactthat|Z|0.01Hzvaluesbegantodecreaseafter

150daysofimmersiondidnotagreewiththeobservationsfrom cryo-SEMorUVspectroscopy,whichindicatedthatthe nanochan-nelsbecameemptyafter210daysofimmersion,beforewhichthe barrierlayerofAAOmighthavebeenprotectedbythelubricant leftinthenanochannels.Wepostulatedthatthisdiscrepancywas causedbythedefectsintheAAOsurface[55].Duringfabricationor storage,theformationofporedefectsorcracksintheAAOnanopore arrayisinevitable.Thesurfacemayalsohavecracksfromwhichthe lubricantcouldmorerapidlyleachout[5].Byusingcryo-SEM,we foundthatintheinitialstageofimmersion(Fig.6(a)and(b)),the

nanochannelsofAAOandthedefectscouldallbefilledbytheliquid lubricant,makingitdifficulttodifferentiatethembyEIS.Inthelater stageofimmersionperiod,asthetoplubricantlayerdisappeared, theoilcoulddepletefromthecracksmorerapidlybecausethesize ofthecrackswasconsiderablylargerthanthatofthe nanochan-nels(Fig.6(c)and(d)),makingthedamagedaluminiumsubstrate exposetotheNaClsolutionfinally.

Toconfirmthatthedecreased corrosionresistanceafter150 daysofimmersionwasindeedtheresultofthesurface defects, additionalEIStestswereperformedonLISspecimensthatwere artificiallycrackedbybendingthemaroundacylinderwitha diam-eterof5mm.Tosimulatethedifferentstagesofthedeterioration, thedepletionofthelubricantfromthecrackedLISwas acceler-atedbyplacingitunderflowingwaterfor10minorbycarrying outultrasonicationfor 10min.For comparison,we alsoinfused the1MNaClsolutionintoacrackedemptyAAOsurface(without employingsurfaceprotectionorimpregnatingthesurfacewitha lubricant).Asshown inFig.7(a),thecrackedLISexhibitedclear impedancemoduliinthehigh-frequencyregion,whichindicates thecoverageofthesurfacebythelubricantlayer.Afterthe speci-menwaswashedorultrasonicallytreated,thelubricantstoredin thecracksreleasedquicklyandthepartiallubricantlockedinthe nanochannelsstillremained,whichresultedinthediminishingof theimpedancemoduli.Whenthelubricantwaslostcompletely andreplacedbytheNaClsolution(asshownintheplotlabelled as‘AAO-solutioninfusion’inFig.7),thehigh-frequencyregionof theImpedancemodulusplotalmostbecameastraightline.Inthe low-frequencyregion,theImpedancemodulusplotofthecracked LISoverlappedwiththatoftheLISbecausethecrackswere cov-eredbythelubricantflowingintothem[4◦]_._After_the_cracked_LIS

waswashedorultrasonicallytreated,thecracksordamaged bar-rierlayerwereexposedtothesolution,leadingtoamajordecrease inthe|Z|0.01Hzvaluecomparedtothecorrespondingvalueforthe

undamagedAAO.Theseresultsshowedthattheexistenceofcracks couldbedetectedbyEISevenwhentheLIShadnotcompletely deteriorated.

Cryo-SEM, UV spectroscopy, and EIS have different advan-tages/limitationsinthestudyofLISdeterioration.Thecryo-SEM methodcanprovidesurfaceandcross-sectionalinspectionofthe lubricantinfusedinsmallarea(eveninindividualnanochannels), butthedetectionofdefectshiddeninthenanoporearrayisnoteasy. UVspectroscopymeasurestheamountoflubricantintheentire ﬁlmincludingthesurfaceadheredlubricantlayerandtheamount oflubricanttrappedinthenanochannelsorcracks.Incontrast,EIS candetectsmalldefects intheLISowingtothelow-frequency impedance,which isoftenusedtoreﬂecttheonsetofcorrosion andthedeteriorationoftheprotectivepropertyoftheLIS.

EIScanalsobeusedtoquantitativelyassessthedegradationof theLISbyﬁttingtheEISresultsintheformofequivalentcircuits, asshowninFig.8.Becauseofthepresenceofinhomogeneityinthe barrierandporouslayers,theircapacitivebehaviourswerebetter simulatedbyconstantphaseelements(Q)thanbysimple capaci-tances(C).TherelationshipbetweenQandCisdescribedinEq.1

[56,57],

C=Q1/n_R(1−n)/n ₍₁₎

wherevaluesofnareadispersionindexdescribingthenon-ideal behaviourofcapacitance(n=1,Q referstoa realhomogeneous capacitance;n=0,Qreferstoaresistance)[53].

Itiswellknownthatananodicoxideﬁlmconsistsofathininner barrierlayerandathickouterporouslayer.Theporouslayeris com-posedofporesandwallsintheshapeofhexagonalcells.Therefore, theoxideﬁlmcanbemodelledbytheequivalentcircuitshownin

Fig.8(a).ThiscircuithasbeenpreviouslyusedforAAOwithsealed poresobtainedbyhydrothermaltreatmentorchemicaldeposition [58–60].Inthiscircuit,Rsisthesolutionresistance.RwandQware

(9)

Fig.6. DeteriorationprocessofcrackedLIS.(a,b)CracksandnanochannelsinLISstillﬁlledwithlubricantafterimmersionin1MNaClsolutionfor60days.(c,d)lubricant

dissolvedfromnanochannelspartiallybutfromcrackspredominantlyafterimmersioninNaClsolutionfor150days.

Fig.7. (a)Impedancemodulusplotsand(b)phaseangleplotsofstandardspecimens(intactandcracked)tosimulatedeteriorationprocessofdefectiveLIS.

Fig.8.EquivalentcircuitsformodellingimpedancebehaviourofLISindifferentdeteriorationstages.(a)Nanochannelsinfusedwithlubricant,(b)simpliﬁedcircuitof(a),

(10)

Table1

ElectrochemicalparametersﬁttedfromEISresultsofLISimmersedin1MNaClsolutionfrom0to120days.

Time(d) Qp (×10−10_S_sn cm−2₎ n Cp (×10−10_S_sn cm−2₎ Rp (×103_cm2₎ Qb (×10−10_S_sn cm−2₎ n Cb (×10−10_S_sn cm−2₎ Rb (×109_cm2₎ 2 0 7.38 0.89 3.08 1175 242 0.90 377 2.24 4.5×10−3 20 17.7 0.98 15.3 474.3 308 0.84 675 1.99 2.1×10−3 40 139 0.82 34.7 128.6 369 0.92 549 2.63 6.2×10−4 60 904 0.89 431 27.47 409 0.95 503 1.26 3.3×10−4 90 752 0.98 658 18.7 443 0.92 659 2.16 3.2×10−3 120 2674 0.95 2018 17.9 447 0.94 610 2.91 7.8×10−3 Table2

ElectrochemicalparametersﬁttedfromEISresultsofLISimmersedin1MNaClsolutionfrom150to210days.

Time(d) Qp (×10−10_S_sn cm−2₎ n Cp (×10−10_S_sn cm−2₎ Rp/ (×103_cm2₎ Qb (×10−10_S_sn cm−2₎ n Cb (×10−10_S sn_cm−2₎ Rb/(×109cm2) 150 2566 0.80 778 33.0 685 0.95 814 0.39 80 18410 0.67 4640 33.1 738 0.87 1297 0.59 210 12460 0.85 6187 15.2 1650 0.64 14251 0.28 (1-)Qdl(×10−10Ssncm−2) n (1-)Cd (l×10−6Ssn cm−2₎ Rct/(1-) (×106_cm2₎ 2 284 0.72 126 6110 2.2×10−4 404 0.97 5.02 288 4.3×10−4 416 0.97 4.55 4.55 3.1×10−4

Fig.9. Fittingresultsofparameters(R(orR)andC(orC))of(a)lubricantinporouslayerand(b)barrierlayerduringimmersionin1MNaClsolutionfor210days.

theresistanceandcapacitanceofthehexagonalporewalls,

respec-tively.RbandQbaretheresistanceandcapacitanceofthebarrier

layer,respectively.RpandQparetheresistanceandcapacitance

ofthelubricantintheporouslayer,respectively.RwandQware

usuallyomittedbecauseRwisveryhighandQwislowfortheAAO

porewalls[61,62].Therefore,theequivalentcircuitcanbe

simpli-ﬁedasillustratedinFig.8(b).For acompletelydeterioratedLIS, thelubricantinthenanochannelwasentirelyreplacedbytheNaCl solution.TheporouslayercouldnotbedetectedbytheEISbecause ofthehighconductivityoftheNaClsolutioninsidethe nanochan-nels.Therefore,theequivalentcircuitelementthatreferredtothe porouslayerwasignored[58],asshowninFig.8(c).Pitting corro-sionorcrackscouldoccuronthebarrierlayeratthebottomof thenanochannels. Suchsurfaceinhomogeneitiescouldbe mod-elledbyintroducingaparametertorepresentthefractionofthe surfacecoveredbytheintactbarrierlayerandaparameter(1-)to representthefractionofthedefectiveorcorrodedsurface[54,55] (Fig.8(d)).Inthiscase,thedouble-layercapacitanceQdlandcharge

transferresistanceRctshouldbetakenintoaccountfortheactive

corrosiononthedamagedbarrierlayer.When=1,thebarrier

lay-ersarenotdamaged.Thealuminiumsurfaceisentirelycovered bytheanodicﬁlm,soRct andQdl donot exist.When isclose

to0,theporousandbarrierlayersarecompletelyremovedand theequivalentcircuitsrepresenttheexposureofthealuminium substrate.

Tables1and2summarizedtheelectrochemicalparametersfor theLISbyﬁttingtheEISresultsusingtheequivalentcircuitsin

Fig.8(b)and(d),respectively.Thesmallvalueof2_indicated_that

theEISresultswerewellﬁtted.Thevaluesofnvariedfrom∼0.7 to∼1.Fig.9(a)and(b)illustratedtheﬁttingparametersfromthe EISresultsasafunction oftime.In Fig.9(a),thecapacitanceof thelubricant(CporCp)increasedandtheresistance(RporRp)

decreasedatarelativelyconstantrateduringtheinitial60days ofimmersion;thiscorrespondedtothecontinuousdeterioration ofthesurfacelubricantlayer.Theseresultsagreedwellwiththe cryo-SEMandUVspectroscopyresults.After60days,thechange inthecapacitanceandresistanceofthelubricantsloweddown. Fortheinnerbarrierlayerstructure(Fig.9(b)),thecapacitance(Cb)

andresistance(Rb)remainedalmostunchangeduntil120daysof

(11)

After120days,theresistance(RborRb)decreasedandthe

capaci-tance(CborCb)increasedclearly,whichindicatedthatthebarrier

layerwasprogressivelyattacked.

4. Conclusion

Inthisstudy,thedeteriorationprocessofatypicalLIS,prepared byinfusingthenanoporesofAAOwithmineraloil,was investi-gatedunderlong-termimmersionin 1MNaClsolutionforover 200days,usingacombinedmicroscopic,spectroscopicand electro-chemicalapproachinvolvingsurfacewettabilityanalysis,cryo-SEM observations,UVspectroscopymeasurements,andEIS measure-ments.Thelossofthelubricantfromthesurfaceandnanochannels wasquantitativelystudied.Theresultsshowedthatthe deteriora-tionoftheLIScouldbedividedintothefollowingstages:inthe ﬁrststage,thesmoothsurfacelubricantlayergraduallydissolved ata relativelyconstantrateuntiltheporousAAOsubstrate got exposed.Inthesecondstage,thediminishingof theimpedance moduliinthehigh-frequencyregionoftheEIS resultsindicated thecontinuouslydissolutionoflubricantinfusedinthe nanochan-nels.Thedecreaseinthe|Z|0.01Hzinthisstageimpliedthatdefects

suchascracksontheLIS,whichwereinitiallymaskedinthe elec-trochemicalresponsebysufficient lubricantfillingofthedefect areas,wereexposed and penetratedbytheNaClsolution. Con-sequently,thebarrierlayerunderthedefectswaspreferentially attackedmuchearlierthanthecompletedepletionofthe lubri-cantfromthenanochannels (∼210days).Theresultsfrom this studyprovidedcomprehensiveinsightsofhowatypicalLIScould deteriorateunderlong-termimmersion.Aroot-causeanalysiswas conductedtohighlightthesignificantimpactofAAOdefectson thecorrosionresistanceoftheLIS.Theseunderstandingswouldbe highlyusefulinlightofthegrowinginterestinLISandparticularly theirapplicationsforunderwatercorrosionprotection.

Acknowledgements

This work was supported by the National Key Research

andDevelopment ProgramofChina(No.2016YFE0203600),the NationalNaturalScienceFoundationofChina(No.51771029),the BeijingNovaProgram(Z171100001117076)andthe111Project (B17003).

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