Hexagonal Boron Nitride Impregnated Silane Composite Coating for Corrosion Resistance of

13 Pages·2017·4.58 MB·English

metals
Article
Hexagonal Boron Nitride Impregnated Silane
Composite Coating for Corrosion Resistance of
Magnesium Alloys for Temporary
Bioimplant Applications
SaadAl-Saadi1,ParamaChakrabortyBanerjee1,2,M.R.Anisur1andR.K.SinghRaman1,2,* ID
1 DepartmentofMechanicalandAerospaceEngineering,MonashUniversity,ClaytonVIC-3800,Australia;
[email protected](S.A.-S.);[email protected](P.C.B.);
[email protected](M.R.A.)
2 DepartmentofChemicalEngineering,MonashUniversity,ClaytonVIC-3800,Australia
* Correspondence:[email protected],Tel.:[phone]
Received:27September2017;Accepted:7November2017;Published:23November2017
Abstract:Magnesiumanditsalloysareattractivepotentialmaterialsforconstructionofbiodegradable
temporary implant devices. However, their rapid degradation in human body fluid before the
desired service life is reached necessitate the application of suitable coatings. To this end, WZ21
magnesiumalloysurfacewasmodifiedbyhexagonalboronnitride(hBN)-impregnatedsilanecoating.
ThecoatingwaschemicallycharacterisedbyRamanspectroscopy. Potentiodynamicpolarisation
andelectrochemicalimpedancespectroscopy(EIS)ofthecoatedalloyinHanks’solutionshowed
a five-fold improvement in the corrosion resistance of the alloy due to the composite coating.
Post-corrosionanalysescorroboratedtheelectrochemicaldataandprovidedamechanisticinsightof
theimprovementprovidedbythecompositecoating.
Keywords: magnesium; hexagonalboronnitride; silanecoating; Ramanspectroscopy; electrochemical
impedancespectroscopy
1. Introduction
Potentialuseofmagnesium(Mg)alloysastemporaryimplants(suchasplates,screws,pins,wires,
etc.) hasbeenattractingincreasinginterest[1–5],primarilybecauseuseofsuchalloyscouldcompletely
avoid the need for a second surgery. A second surgery is commonly required for removal of the
temporaryimplantsconstructedoutoftraditionalalloys,suchastitaniumalloys/stainlesssteels,after
theimplanthasaccomplisheditsfunctionandthetissueshavehealed. Thefundamentalpropertiesof
Mgarequitesuitablefortheirapplicationastemporaryimplants. Infact,Mgisalsoessentialtothe
humanmetabolism[6];furthermore,theMg2+generateduponbio-degradationduetohumanbody
fluidhasbeenreportedtoaidtissuegrowthandhealing[7]. Whilethebio-degradationproductsof
Mgarenotatalltoxictothehumanphysiology,anyexcessMg2+isharmlesslyexcretedthroughthe
renalsystem[7]. Mganditsalloysalsopossessthebestmechanicalcompatibilitywithhumanbones;
theirdensity(ρ=1.74–2.0g·cm−3)andelasticmodulus(E=41–45GPa)arebothsimilartothoseof
humanbones(ρ =1.8–2.1g·cm−3; E=3–20GPa, respectively)[7,8], whichconsiderablydecreases
theriskofstressshielding[8]. WZ21isoneoftheAl-freealloysthatweredesignedfortemporary
implantapplications[9]. Besidestheirsuitablemechanicalproperties,thesealloysalsoshowgood
cytocompatibility,andtheirhydrogengenerationrateisatacceptablelevelsforthispurpose[10,11].
However,despitetheseattractiveproperties,therehasbeenextremelylimitedsuccessintheuseof
Mg alloys for human implants, primarily due to their unacceptably high corrosion rate in human
Metals2017,7,518;doi:10.3390/met7120518 www.mdpi.com/journal/metals
Metals2017,7,518 2of13
bodyfluid[7,12]. Itisintriguingthatmagnesiumalloysareattractiveastemporaryimplantsprimarily
becausetheycanharmlesslycorrodeawayaftertheyhaveservedtheirpurpose,therebyavoiding
the need of a second surgery. However, they corrode rapidly in body fluid, which will seriously
compromise the mechanical integrity of the implant, and thereby compromise the tissue-healing
process. Therefore,itisessentialtofindasuitablecoatingthatcanprovideeffectiveprotectionagainst
corrosion in presence of body fluid, at least for the first few days, when the chances of secondary
inflammationsarehigh. Inselectingsuchcoatings,twocriteriaarecritical: (a)thecoatingmustbe
biocompatiblewiththehumanbody,and(b)thedurationforwhichprotectionagainstcorrosionis
requiredwillvarydependingonthenatureoftheuseoftheimplant. Toelaboratethesecondpoint,
differentimplants(e.g.,apinandaplate)mayberequiredtolastfordifferentdurations. Also,the
same implant may be required to last for different durations, depending on the age of the patient
(e.g.,animplantwillberequiredforamuchshorterdurationforachildthanforanelderlypatient).
Silane coatings have emerged as an attractive coating system, and can significantly improve
the corrosion resistance of various metals and alloys, including magnesium alloys [13–20].
Additionally, silane coatings are generally biocompatible [21]. The biocompatibility test of amine
silane-treatedbio-glassfibresshowednonegativeeffectonthebiologicalresponse[22]. Significant
increases in cell adhesion and proliferation were detected on stainless steel surfaces coated with
collagen and immobilised with an amino silane [23]. As a bioactive and biocompatible binder,
glycidoxypropyltrimethoxysilanewasusedasaprecursorforsynthesisofaporousgelatine-siloxane
hybridsforbonetissues[24,25].
Incorporation of suitable biocompatible additives can improve the mechanical integrity,
biocompatibilityandcorrosionresistanceofsilanecoatings. Differentbiocompatibleadditives(i.e.,
Ca(NO ) asasourceforCa2+,hydroxyapatite,andzinc-incorporatedhydroxyapatite)wereusedas
3 2
fillersincomposites,tofabricateahybridscaffoldtoenhancethebiocompatibilityand/ortoimprove
thecorrosionresistanceofimplantsinsimulatedbodyfluid[24,26–30]. Inthepresentresearch,boron
nitride(BN)wasimpregnatedintoasilanecoating.
Hexagonalboronnitride(BN)isalayeredmaterialconsistingoftwo-dimensional,atomicallythin
hexagonallayers. PropertiesofBNincludenon-toxicity,chemicalinertness,highthermalconductivity,
low thermal expansion, high electrical resistance, low dielectric constant and good thermal shock
resistance. Also, density (ρ = 1.9 g/cm3) and elastic moduli (E = 46.9 GPa in parallel direction
and73.8GPainperpendiculardirection)ofBNareclosetothecorrespondingpropertiesofhuman
bone. BN has been used as a filler in polymeric composite to improve thermal, mechanical and
otherproperties [31–37]. However, there islittlereportedontheincorporationof BNwithsilanes
forformationofacompositebiocompatiblecoatingtoimprovecorrosionresistanceofMgalloysin
physiological environments. In the present study, a composite coating of hexagonal boron nitride
(hBN)-impregnatedsilanewasdevelopedonWZ21alloywiththeaimofimprovingthecorrosion
resistanceofthisalloyinHanks’solution. Theelectrochemicaldegradationofthecompositecoatingof
BN-impregnatedsilaneonWZ21wasinvestigatedbyelectrochemicalimpedancespectroscopy(EIS)
andpotentiodynamicpolarisationtests. Additionally, post-corrosionmorphologicalandchemical
characterisationwereundertakentovalidatetheelectrochemicaldata.
2. MaterialsandMethods
2.1. CompositeCoatingPreparation
Silane coating solution was prepared by mixing glycidoxypropyltrimethoxysilane (GPTMS,
XIAMETER, Dow Corning, Auburn, MI, USA), propanol and deionised (DI) water in a ratio of
5:5:90(V/V).ThepHofthesolutionwasadjustedtobe~4.5byaddingaceticacid. Thesilanemixture
wasstirredfor1h,andthenheldstillfor3h. Forimpregnationofhexagonalboronnitride(hBN)into
silanecoating,100mgofhBN(procuredfromSigmaAldrich,Munich,Germany;1µm)wasdispersed
in90mLofDIwater(atpH∼4.5). Subsequently, thismixturewassonicatedatroomtemperature
Metals2017,7,518 3of13
for5h. TheextendedsonicationatroomtemperaturemaycauseexfoliationofhBNinwater[38].
TheexfoliatedhBNwasheldstillandsubsequently,5mlofeachofpropanolandGPTMSsilanewas
added. Thismixturewasstirredfor1hatroomtemperatureandthenheldstillfor3hforcompletion
ofthehydrolysisofGPTMS.
2.2. SamplePreparationandCoatingProcedure
The magnesium alloy WZ21 was used in extruded form, with an average grain size of 7 µm.
Thenominalchemicalcomposition(inwt%)isZn1%,Ca0.25%,Mn0.15%andY2%[39]. Couponsof
magnesiumalloyWZ21weregroundwithSiCpapersdownto2500gritsize,ultrasonicallycleaned
withacetoneandethanol(10minforeachstep),rinsedwithDIwater,anddriedusingcompressedair.
ThecoatingwascarriedoutbydippingtheWZ21couponsfor1heitherintheGPTMSsolutionorthe
hBN-impregnatedsilanesolution(preparationofwhichisdescribedintheprecedingsection).Thecoupons
werethentakenoutofthesolution,keptinairfor15min,anddriedinanovenfor1hat130◦C.
2.3. ElectrochemicalMeasurements
Potentiodynamicpolarisation(PDP)andelectrochemicalimpedancespectroscopy(EIS)werecarried
out using a Bio-Logic VMP3 multi-channel potentiostat and a three-electrode electrochemical cell.
Couponsofthebarealloy(WZ21),silane-coatedalloy(S_WZ21)andhBN-impregnatedsilane-coated
alloy(BNS_WZ21)wereusedastheworkingelectrodesinseparateexperiments,whereasasaturated
calomel electrode (SCE) was used as the reference electrode, and a platinum mesh as the counter
electrode. Theareaoftheworkingelectrodeexposedtotheelectrolytewas0.785cm2. Hanks’solution,
whichsimulateshumanbodyfluid,wasusedaselectrolyte. TheHanks’solutioncontained:D-Glucose
(5.551 mmol/L), KCl (5.365 mmol/L), MgSO ·7H O (0.811 mmol/L), KH PO (0.441 mmol/L),
4 2 2 4
Na HPO .2H O(0.337mmol/L)andNaCl(136.893mmol/L),CaCl (2.163mmol/L)andNaHCO
2 4 2 2 3
(4.169mmol/L).
Opencircuitpotential(OCP)wasmonitoredfor1.5h,i.e.,untilthepotentialattainedstability,
and electrochemical measurements were subsequently performed. A fluctuation of OCP within
10 mV for a period of 1000 s was considered to be a stable potential. Electrochemical impedance
spectroscopy(EIS)wasperformedafterdifferenttimesofimmersioninHanks’solution,byapplying
asinusoidalpotentialperturbation(amplitudeof10mV)atOCP.Theimpedancewasmeasuredat
frequenciesbetween1and10mHz,recording10pointsperdecadeoffrequencyusingaBio-Logic
VMP3multi-channelpotentiostat(BioLogicScienceInstruments,Seyssinet-Pariset,France).Impedance
analysiswascarriedoutusingEC-LabelectrochemistrypackageforWindows,generallyforfrequencies
between10kHzand50mHztopreventmisinterpretationofanyartefactsthatmaybepresentinthe
high-frequencyregion,orthescatterinthelow-frequencyregion. Potentiodynamicpolarisationwas
carriedoutatascanrateof0.5mV/s, startingatapotentialof250mVmorenegativetotheOCP.
Allthemeasurementswererepeatedatleastthricetoexaminereproducibility.
2.4. SurfaceMorphologyandCoatingCharacterisation
2.4.1. RamanSpectroscopy
RamanspectrumoftheBNS_WZ21wasobtainedusingRenishawInviaRamanspectrometerequipped
with514nmwavelengthgreenlaser(10%oflaserpower)and1µmspotsizeundera50×objective.
2.4.2. ScanningElectronMicroscopy(SEM)andEnergyDispersiveX-raySpectroscopy(EDS)
ThemorphologyandelementalanalysisoftheWZ21andBNS_WZ21specimensbeforeandafter
exposuretoHanks’solutionwereexaminedusingJEOLJSM-7001FFEGSEM(JEOLLtd.,Tokyo,Japan)
equippedwithanEDSanalyseratanacceleratingvoltageof15kV.
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3. ResultsandDiscussion
3.1. ChemicalandMorphologicalCharacterisation
The Raman spectroscopy of BNS_WZ21 in Figure 1 has the G-band peak at ∼1368 cm−1 that
confirmsthepresenceofhBNinthecoating[40]. The1256cm−1 bandisduetothesymmetricring
stretchoftheepoxygroupinGPTMSsilane[41]. Thepeaksat∼3000cm−1and3070cm−1aredueto
CH andCHstretchingmodesconnectedtoepoxygroups[41]. Thetwopeaksat2924and2928cm−1
2
areassignedtothehydroxylstretching[42].
Figure1.RamanspectroscopyofthehBNimpregnatedsilane-coatedWZ21(BNS_WZ21)specimen.
Figure2a,bpresentstheSEMmicrographsoftheWZ21andBNS_WZ21specimens. Onlyscratch
marksfromthegrindingprocesswerevisibleontheWZ21specimens,whereasauniformlycoated
surfacewithcluster-likeformationwasobservedinthecaseofBNS_WZ21. Theaveragediameterof
theseclusters(markedwithdottedline)was~100µm. TheEDSanalysisofBNS_WZ21(Figure2d)
confirmsthepresenceofSi,O,BandN,whereasonlyMgandYttrium(Y)peakswereprominentin
caseoftheWZ21specimen(Figure2c).
Figure2.Cont.
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Figure2. (a,b)ScanningElectronMicroscopy(SEM)imagesofWZ21andBNS_WZ21(cluster-like
formation of hBN is marked with red circles), (c,d) Energy Dispersive X-ray (EDS) of bare and
BNS_WZ21,(e,f)cross-sectionalthicknessofsilane-andhexagonalboronnitride-impregnatedsilane
compositecoatings.
Figure 2e,f reveal the cross-sectional thickness of the developed coatings (i.e., silane and the
hexagonalboronnitrideimpregnatedsilanecomposite)onWZ21. Whiletheaveragethicknessofthe
silanecoatingwas~1.61µm,thehexagonalboronnitride-impregnatedsilanecompositecoatingwas
relativelythicker(~1.9µm).
3.2. ElectrochemicalCharacterisation
Figure3showsthepotentiodynamicpolarisationcurvesofWZ21,S_WZ21andBNS_WZ21in
Hanks’ solution after pre-exposure to the same Hanks’ solution for 1.5 h. Although the corrosion
potential(E )forthecoatedspecimens(i.e.,S_WZ21andBNS_WZ21)shiftedslightlytothenegative
corr
direction,indicatingalittleincreaseintheirsusceptibilitytocorrosion,thecorrosioncurrentdensities
(i ) of both of these specimens were lower than the i for the WZ21. The i of WZ21 was
corr corr corr
4 times higher than that of BNS_WZ21, and twice that of the S_WZ21. These results suggest that
thehBN-impregnatedsilanecoatingsignificantlyimprovedthecorrosionresistanceofWZ21alloy
in Hanks’ solution by retarding the cathodic reaction. Such behaviour could be attributed to the
formationofaprotectivelayeronthecathodicactivesitesonWZ21surface. TheE andthei data
corr corr
asgeneratedfromthepotentiodynamicpolarisationtestsarepresentedinTable1.
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Figure3.PotentidynamicpolarisationplotsofWZ21,S_WZ21andBNS_WZ21pre-immersedfor1.5h
inHank’ssolution.
Table1.Corrosionpotential(Ecorr)andcorrosioncurrentdensity(icorr)dataforWZ21,S_WZ21and
BNS_WZ21couponsexposedtoHanks’solutionfor1.5h.
Sample Ecorr(VSCE) icorr(µA/cm2)
WZ21 −1.525(±0.0083) 1.217(±0.269)
S_WZ21 −1.546(±0.009) 0.538(±0.059)
BNS_WZ21 −1.594(±0.007) 0.292(±0.006)
TheEISdatafortheWZ21,S_WZ21andBNS_WZ21inHanks’solutionwithpre-exposuretothe
sameHanks’solutionfor1.5hareshowninNyquistandBodephaseplotsinFigure4a,b. Consistent
withpolarisationresults(Figure3),thesilanecoating(S_WZ21)improvedthecorrosionresistanceof
thealloyby2.5times,assuggestedbytheincreaseinimpedanceatalowestfrequencyof11kΩ·cm2
forthebarealloyto27kΩ·cm2 forthesilane-coatedalloy. However,theimpregnationofthesilane
coatingwithhBN(BNS_WZ21)furtherimprovedthecorrosionresistance(by>5times),assuggested
bytheimpedance(57kΩ·cm2).
Figure4.(a)Nyquistand(b)phaseplotsforWZ21,S_WZ21andBNS_WZ21pre-immersedfor1.5hin
Hanksolution.
For S_WZ21 and BNS_WZ21, two distinct time constants can be observed in Figure 4b.
Thehigh-frequencyregimetimeconstantisgenerallyattributedtotheelectrochemicalresponseatthe
coatings/electrolyteinterface[19,20,43–49],whereasthetimeconstantinthelow-frequencyregimeis
commonlyattributedtotheelectrochemicalresponseatthemetal/electrolyteinterface[19,20,45–51].
Thehigh-frequencyregimetimeconstantismuchprominentinthecaseofBNS_WZ21. TheWZ21
showedasingletimeconstant,i.e.,atlowfrequencies.
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AclassicRandlescircuit(Figure5a)couldexplaintheelectrodekineticsoftheWZ21exposedto
theHanks’solution,whereR representstheelectrolyteresistance. R representsthechargetransfer
s ct
resistance and Q is the constant phase element of the alloy/electrolyte interface. However, an
dl
electricalequivalentcircuit(EEC)withanadditionaltimeconstant(R Q )wasrequiredtoexplainthe
C C
kineticsofS_WZ21andBNS_WZ21(Figure5b)duringimmersioninHank’ssolution. Thesimulated
impedancedatawereingoodagreementwiththeexperimentaldata,asshowninFigure5c. Inthe
EECs,inFigure5a,b,aconstantphaseelement(Q)isusedinsteadofacapacitor(C)inordertoaccount
for the surface reactivity, heterogeneity, roughness, electrode porosity and current and potential
distributionsassociatedwiththeelectrodegeometry[43]. Table2presentsthevaluesofEISparameters
oftheWZ21,S_WZ21andBNS_WZ21after1.5hofimmersioninHanks’solution. R wasconsiderably
c
higherforBNS_WZ21thanthatforS_WZ21,whereastheQ washigherforS_WZ21alone,which
c
indicatesthehigherporeresistanceandlowernumberofconductivepathwaysthroughtheBNS_WZ21.
Additionally,theBNS_WZ21showsconsiderablyhigherR (~59kΩ·cm2)thantheWZ21(~11kΩcm2)
ct
andS_WZ21(~30kΩ·cm2),whichfurthersupportsthattheadditionofhBNdecreasesthecorrosion
rate of WZ21 alloy. Further, the lowest value of Q for BNS_WZ21 indicates a lesser area at the
dl
coating/metalinterfacegettingincontactwiththeelectrolytethanothersamples.
Figure5.(a)Electricalequivalentcircuit(EEC)thatfitsdataforWZ21,and(b)EECthatfitsdatafor
S_WZ21andBNS_WZ21,pre-immersedfor1.5hinHanks’solution.(c)Theagreementbetweenthe
experimentalandfitteddataofEISmeasurements(linesrepresenttheexperimentaldataandsymbols
representthefitteddata).
Table2.Electrochemicalimpedancespectroscopy(EIS)resultsforthecorrosionofWZ21,S_WZ21and
BNS_WZ21couponsexposedtoHanks’solutionfor1.5h.
Sample Qc(S·sn·cm−2) nc Rc(Ω·cm2) Qdl(Ssncm−2) ndl Rct(Ω·cm2)
WZ21 - - - 2.639×10−5 0.7 11,048
S_WZ21 2.164×10−6 0.6 213 1.106×10−5 0.9 29,736
BNS_WZ21 5.169×10−8 0.9 403 8.735×10−6 0.9 59,364
In order to investigate the durability of the barrier properties of the hBN-impregnated silane
coating, EIS was performed on the BNS_WZ21 alloy after different durations of immersion
in Hanks’ solution. Figure 6a compares the Bode plots of the WZ21 at 1.5 h of immersion
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with the BNS_WZ21 after different durations of immersion (i.e., 5–96 h) in Hanks’ solution.
After 5 h of immersion, the corrosion resistance of the BNS_WZ21 specimen (~57 kΩ·cm2) was
nearly5timeshigherthanthatoftheWZ21specimen(~11kΩ·cm2). However,thecorrosionresistance
oftheBNS_WZ21graduallydecreasedwithincreasingimmersiontime. Nevertheless,thecorrosion
resistance of the BNS_WZ21 even after 96 h (~16 kΩ·cm2) was 1.5 times higher than that of the
WZ21specimen(~11kΩ·cm2)immersedjustfor1.5h. Figure6bshowsthephaseangleplotsofthe
BNS_WZ21afterdifferentdurationsofimmersioninHanks’solution. Smallchangesinthemagnitude,
aswellasthenature,ofboththelow-andhigh-frequencytimeconstantswithincreasingimmersion
timesuggestaslowdegradationrateforthecoating.
Figure6.(a)Bodeand(b)phaseplotsoftheWZ21pre-immersedfor1.5hinHank’ssolutionandthe
BNS_WZ21pre-immersedupto96h.
The impedance data of the BNS_WZ21 immersed in Hanks’ solution for different durations
(5–96 h) were fitted with the simulation data generated using the equivalent electrical circuit
(ECC)inFigure5b. Evolutionoftheelectrochemicalparametersatthecoating/electrolyteandthe
metal/electrolyteinterfacesduring96himmersionareshowninFigure7. Thecoatingresistance(R )
c
graduallydecreasedinthefirst50handthencontinuouslyincreased. Thedecreaseinporeresistance
duringthefirst50hsuggestsanincreaseintheconductivepathwaysinthecoating[19,20],whichis
supportedbytheincreaseintheconstantphaseelement(Q )thatresultsfromincreasedactivityatthe
c
coating/electrolyteinterface(Figure7b). Theinitialdecreaseinporeresistanceandthecorresponding
increaseinnumberofconductivepathwaysthroughthecoatingexplainthedecreaseinchargetransfer
resistance(R )atthemetal/electrolyteinterface(Figure7c),aswellastheincreaseinthedoublelayer
ct
capacitance(Q ). Thisisconsistentwiththedeteriorationinsilanecoatingwithincreasingdurationof
dl
immersioninthecorrosivesolution,asreportedintheliterature[19,20].
Figure7.Cont.
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Figure7. Evolutionof(a,b)thecoating/electrolyteand(c,d)themetal/electrolyteinterfacesofthe
hBN-impregnatedsilane-coatedWZ21(BNS_WZ21)pre-immersedupto96h.
3.3. Post-CorrosionMorphologyofBareWZ21andBNS_WZ21
Figure8showsthepost-corrosionmorphologiesandsurfacechemicalanalysisoftheWZ21and
BNS_WZ21 specimens pre-exposed to Hanks’ solution for 96 h. The entire surface of WZ21 was
coveredwiththickcorrosionproducts(Figure8a),whereasconsiderablylessamountsofcorrosion
productscouldbeobservedontheBNS_WZ21(Figure8b). Thecracksinbothsamplesresultedfrom
theirexposuretothehighvacuumintheSEMchamber,whichiswellknowntorapidlydehydrate
thesurfacefilms,therebycausingcracks.
ItiswellknownthatwhenamagnesiumalloyisexposedtoHanks’solution,calciumphosphate
depositionoccursattheexpenseofmagnesiumoxideformation[52]. TherelativequantitiesofCa
and Si to Mg on the WZ21 and the BNS_WZ21 specimens after exposure to Hanks’ solution were
characterisedbyEDSanalysis. Asexpected,theCa-to-Mgratioofthedepositedcorrosionproductsfor
WZ21washigherthanthatforBNS_WZ21(Figure8c). Additionally,theSi-to-Mgratiowasprominent
inthecaseofBNS_WZ21(Figure8c),suggestingthatthesilane-containingcompositecoatingwas
present on the alloy even after 96 h exposure to simulated human body fluid (Hanks’ solution).
Figure8dshowsthecross-sectionofthehexagonalboronnitride-impregnatedsilanecompositecoating
developed on WZ21 alloy after exposure to Hanks’ solution for 96 h. Despite scattered corrosion
products with average thickness (~1.16 µm) being visible on the top of the developed composite
coatinglayer,nocorrosionproductsordelaminatedcoatingwerenoticedatthecoating/metalinterface.
Theaveragethicknessofthecompositecoatingafter96hofexposuretoHanks’solutionwasfoundto
be~1.793µm,whichissmallerthantheaveragethicknessoftheintactcompositecoating(~1.9µm)
priortoexposuretoHanks’solution.
Figure8.Cont.
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Figure8.SEMmicrographsandchemicalelementratiosofEDSofWZ21andBNS_WZ21specimens
afterexposuretoHanks’solution:(a,b)SEMimagesofWZ21andBNS_WZ21;(c)Theratiosofchemical
elementsobtainedbyEDSanalysisafterexposuretoHanks’solution;(d)Thecross-sectionalthickness
ofthehexagonalboronnitride-impregnatedsilanecompositecoatingonWZ21alloypre-exposedto
Hank’ssolution.
4. Conclusions
Hexagonalboronnitride(hBN)wasimpregnatedsuccessfullyintosilaneforthedevelopmentof
atwo-dimensionalbiocompatiblecompositecoatingonWZ21Mgalloy,whichsignificantlyimproved
thecorrosionresistanceanddurabilityofthealloyinaphysiologicalenvironment. Potentiodynamic
polarisationandelectrochemicalimpedancespectroscopy(EIS)analysesconfirmednearlyfive-fold
improvementincorrosionresistanceoftheMgalloy(WZ21)insimulatedhumanbodyfluidduring
thefirsthoursofimmersion,duetothecoating. Post-corrosionmorphologiesshowtheformation
ofathickcorrosionproductontheWZ21surface, butconsiderablylesscorrosionproductsonthe
BNS_WZ21surfaceevenafter96himmersioninHanks’solution.
AuthorContributions:SaadAl-Saadididthesynthesisofsalineandhexagonalboronnitride(hBN)-impregnated
silane coatings. He carried out the Mg alloy sample preparation, coating and electrochemical tests at the
corrosionlaboratoryintheChemicalEngineeringDepartment,MonashUniversity.M.R.Anisurcarriedoutthe
characterisationofthecoatedMgalloybyRamanspectroscopy.Scanningelectronmicroscopywascarriedout
attheMonashCentreforElectronMicroscopybySaadAl-Saadi.Al-Saadipreparedthemanuscriptunderthe
guidanceofR.K.SinghRamanandParamaChakrabortyBanerjee.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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