Superhydrophobic fluoropolymer-modified copper surface via surface graft

Superhydrophobic fluoropolymer-modified copper surface via surface graft polymerisation for corrosion protectionShaojun Yuan a ,⇑,S.O.Pehkonen b ,Bin Liang a ,Y.P.Ting c ,K.G.Neoh c ,E.T.Kang caCollege of Chemical Engineering,Sichuan University,Chengdu 610065,ChinabChemical Engineering Program,Masdar Institute of Science and Technology,PO Box 54224,Abu Dhabi,United Arab Emirates cDepartment of Chemical and Biomolecular Engineering,National University of Singapore,Kent Ridge,Singapore 119260,Singaporea r t i c l e i n f o Article history:Received 18December 2010Accepted 2May 2011Available online 11May 2011Keywords:A.Copper A.Polymer B.XPSC.Polymer Coatings C.Superhydrophobicitya b s t r a c tWith the objective of developing materials with repellent surfaces by combining both low surface energy and rough structure,superhydrophobic fluoropolymer films were prepared via surface graft polymerisa-tion from copper substrates.A vinyl-terminated trimethoxysilane was firstly immobilised on the etched-copper surface to introduce active carbon–carbon double bonds.Subsequent graft polymerisation of 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA),in the presence of a polymerisation initiator 4,40-azobis-(4-cyanpentanoic acid),yielded the fluoropolymer films on the copper substrates.The resultant P(HFBA)-grafted surfaces not only exhibited desired superhydrophobic property with water contact angle above 150°,but substantially improved the corrosion resistance of copper substrates.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionCopper is a metal with a wide range of applications owing to its high electrical and thermal conductivity,mechanical workability,and malleability.However,it is an active metal that does not resist well to corrosion,particularly in the presence of aggressive chlo-ride anions [1].Various strategies have been developed to address the growing need for the inhibition of copper anic inhibitors containing polar groups [2],heterocyclic compounds (including N,S,O)with polar functional groups [3],and conjugated double bonds [4],have been extensively reported to protect copper from corrosion,due to their chelating action and the formation of an insoluble diffusion barrier on the substrate surfaces [5,6].The major disadvantage in using organic inhibitors is their inherent toxicity,which poses potential risks to environments and human health.Alternatively,self-assembled monolayers (SAMs)formed by reactions of alkyltrialkoxysilanes and alkanethios with copper substrates have recently been proposed corrosion inhibition [7–10].SAMs suffer from the drawback that the layers have limited stability and molecule-sized defects allow electrolytes to reach the underlying substrates [11].Another widely-used approach to inhibit corrosion is electropolymerisation of conducting polymer coatings,such as polypyrrole and polyaniline,on metal substrate surfaces [12,13].The major concerns of electrodeposited conduct-ing polymer coatings are water permeability and weak adhesion [13].The breakdown of weak bonds at the metal/polymer interface will result in structural or functional failures and ultimately leads to irreversible corrosion damage at the interface [14].It is therefore highly desirable to develop a novel,and yet general,approach for combating copper corrosion.Recently,considerable effort has been devoted to using super-hydrophobic films as corrosion inhibitors [15–20].Superhydropho-bic surfaces with a water contact angle above 150°display many remarkable physicochemical properties,such as water repellency,self-cleaning,lubricity and antifouling properties [21].The superhydrophobic films inhibit corrosion of metal substrates by providing an effective barrier to the penetrating electrolytes.It is well-known that the wettability of a solid surface is governed by both the surface chemical composition and geometrical micro-structure [22–24].Superhydrophobicity of a surface can thus be achieved by enhancing the surface roughness and lowering the surface energy [25].Different approaches have been reported for the fabrication of superhydrophobic surfaces.These approaches in-clude chemical vapour deposition [26],sol–gel processing [27],electrodeposition [28,29],solution immersion [24,30],electrospin-ning [31],layer-by-layer assembly [32],plasma polymerisation [33],and graft polymerisation [34].Among these approaches,graft polymerisation is a particularly good method due to its high effi-ciency in creating high density polymer brushes,better structural control of the resulting coatings,and good stability and durability of the substrate surfaces arising from robust covalent bonds.How-ever,relatively few studies on the fabrication of superhydrophobic surfaces via surface graft polymerisation have been documented0010-938X/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.corsci.2011.05.008Corresponding author.Tel./fax:+862885460557.E-mail address:yuanshaojun@ (S.Yuan).[34–36].Functional polymer coatings grafted from reactive groups on metal substrate surfaces have been shown to provide good cor-rosion resistance to metal substrates[37,38].Moreover,a thin poly(2,2,2-trifluoroethyl acrylate)film deposited on aluminium surfaces by admicellar polymerisation was reported to endow the substrate surface with a highly hydrophobic property,and to act as effective barrier coatings to keep away aggressive ions from reaching the metal substrates,thus inhibiting the occurrence of crevice corrosion[39].Herein,superhydrophobicfluoropolymerfilms were fabricated on copper substrates via surface-initiated free radical graft poly-merisation.The process of surface functionalisation is shown sche-matically in Fig.1.A vinyl-terminated trimethoxylsilane was coupled to the etched copper surface via self-assembly.Subse-quently,fluoropolymerfilms were synthesised on the copper sur-face by surface-graft polymerisation of2,2,3,4,4,4-hexafluorobutyl acrylate(HFBA)to produce superhydrophobic surfaces.The success in each functionalisation step was ascertained by X-ray photoelec-tron spectroscopy(XPS),scanning electron microscopy(SEM),and water contact angle measurements.The anticorrosion behaviour of the superhydrophobicfluoropolymerfilms in a3.5wt.%aqueous NaCl solution was evaluated by Tafel polarisation curves and elec-trochemical impedance spectroscopy(EIS).The stability of the superhydrophobic surfaces as a function of immersion time was also investigated.For the comparison purpose,bare copper coupons were used as controls under the same experimental conditions. 2.Material and methods2.1.Materials and chemicalsThe CDA110copper coupon(nominal composition:Cu P99.9%) was obtained from Metal Samples Company(Munford,AL,USA). 2,2,3,4,4,4-Hexafluorobutyl acrylate(HFBA)used for the graft poly-merisation was obtained from Sigma–Aldrich Chemical Co.(St. Louis,A)and was used as received.Solvents,such as N,N0-dimethylformide(DMF,99.8%)and toluene(99.8%)were dried with sodium.The polymerisation initiator,4,40-azobis(4-cyanenta-noic acid)(ACP),and the vinyl-terminated trimethoxylsilane cou-pling agent,3-(trimethoxysilyl)propyl methacrylate(TMSPMA, >98%),were obtained from Fluka Chem.Co.(Milwaukee,WI),and were used as received.Nitric acid(65%),hydrogen peroxide (37%),acetone,methanol,ethanol,sodium chloride and other chemicals were of reagent grade and were used as received.2.2.Copper surface preparation and introduction ofactive vinyl groups on copper surface by silanisation processingThe copper coupons were ground sequentially using a series of grit(180,500,800and1200)SiC paper to give a smooth surface. Disc coupons with a diameter of15mm and a thickness of3mm were used for surface characterisation and electrochemical studies. The newly polished coupons were washed with deionised water, acetone,ethanol,and deionised water,in that order,for5min each to degrease and clean the surface,and then were etched by immer-sion in a HNO3(65%)and H2O2(30%)mixed solution(v/v,1:1)for 2min at room temperature,using procedures described in detail previously[40].After etching,the substrates were immediately rinsed ultrasonically with deionised water and washed with copi-ous amounts of deionised water,followed by drying with a stream of purified N2and then stored in a vacuum desiccator.The vinyl-terminated trimethoxylsilane coupling agent,TMSPMA,was intro-duced onto the etched copper substrates via self-assembly.As illustrated in Fig.1,the etched copper coupons were immersed in10mL of1%ethanol solution of TMSPMA for6h at room temper-ature to produce the self-assembled silane monolayers(the Cu–TMSPMA surfaces).The TMSPMA-coupled surfaces were then washed with copious amounts of methanol and deionised water, respectively,and dried in an oven at80°C for60min.2.3.Graft polymerisation of HFBA from the vinyl-containing copper surfacesFor the synthesis of P(HFBA)films from the Cu–TMSPMA sur-faces,1.75mL(1.0mmol)of HFBA was introduced into5mL of DMF in a PyrexÒtube.The reaction mixture was degassed by bub-bling Argon for30min.The Cu–TMSPMA coupon was introduced into the degassed monomer solution with the addition of 0.02mmol(11.21mg)of ACP.The reaction mixture was kept in a 75°C oil bath for2and6h,to give rise to the Cu-g-P(HFBA)1 and Cu-g-P(HFBA)2surfaces,respectively.After the reaction,the resulting surfaces were washed thoroughly with copious amounts of ethanol to remove the residual monomers and adsorbed homopolymers.2.4.Surface characterisationThe surface composition of the functionalised coupons was determined by X-ray photoelectron spectroscopy(XPS).The XPS measurements were performed with a Kratos AXIS HSi XPS spectrometer(Kratos Analytical Ltd.,UK)with an Al K a X-ray source(1486.6eV photons),using procedures described pre-viously[41].All binding energies(BEs)were referred to the C1s peak(284.6eV)arising from surface hydrocarbons(or adventi-tious hydrocarbon).The contact angles(CA)measurements were performed on a VCA optima surface analysis system(AST Prod-ucts Inc.,Billerica,MA,USA)using the sessile drop method with a3l L deionised water droplet.After dropping a water droplet onto the sample surface,contact angles were automatically determined by the analysis system.The average CA value was obtained from measurements atfive or more different locations on the same samples.The accuracy of CA value was±2°.Scan-ning electron microscope(SEM)images were obtained on a JEOL JSM-5600(Tokyo,Japan)SEM to reveal the morphology of sub-strate surfaces.The thickness of the grafted polymerfilms on the copper substrates was determined by ellipsometry.The mea-surements were carried out on a variable angle spectroscopic ellipsometer(model VASE,J.A.Woollam,Inc.,Lincoln,NE)at incident angles of70°and75°in the wavelength range250–1000nm.The refractive index of the driedfilms at all wave-lengths was assumed to be1.5in the Cauchyfilm modelused S.Yuan et al./Corrosion Science53(2011)2738–27472739for the simulation offilm thickness.All measurements were con-ducted in dry air at room temperature.For each sample,thick-ness measurements were made on at least three different surface locations.Data were recorded and processed using the WVASE32software package.2.5.Electrochemical characterisationIn all cases,the corrosion experiments were carried out in a 3.5wt.%aqueous NaCl solution under aerated conditions.The anti-corrosion behaviour of the superhydrophobicfilms on the copper surface was investigated by the measurements of Tafel polarisation curves and EIS spectra on an Autolab PGSTAT30electrochemical workstation(Ecochemie Co.,The Netherlands).A conventional three-electrode glass corrosion cell with a capacity of500mL was used.The copper coupons were mounted on a PVDF holder, leaving a circular area of0.785cm2in contact with the solution, to serve as the working electrode.An Ag/AgCl electrode was used as the reference electrode,and a platinum rod as the counter elec-trode.Tafel polarisation curves were obtained at a scan rate of 2mV/s in the range of±250mV vs.the open circuit potential (OCP).The impedance spectra were recorded under OCP using a 10mV amplitude sinusoidal signal in the frequency range of 0.005–100,000Hz.2.6.Stability and durability of the superhydrophobicfluoropolymer filmsThe changes in superhydrophobicity of thefluoropolymer-grafted surface as a function of exposure time were investigated by measuring the CA value after a predetermined immersion time in a3.5wt.%aqueous NaCl solution.SEM images of the coupon sur-face were also captured after21days of exposure in a3.5wt.% aqueous NaCl solution to evaluate the stability of the superhydro-phobic P(HFBA)films.3.Results and discussionTo achieve a superhydrophobic surface,surface roughness has the amplification effect on the hydrophobic properties[24,40].In the present study,a nitric acid and hydrogen peroxide oxidising mixture(v/v,1:1)was used as the etching reagent to generate the rough surface,with the simultaneously removal of carbon con-taminants from the substrate surface.The etched copper surface was characterised by X-ray photoelectron spectroscopy(XPS). The presence of hydroxyl groups on the etched copper surface was ascertained by XPS analysis(Supporting Information, Fig.S1).A vinyl-terminated trimethoxysilane coupling agent was immobilised on the etched substrate surfaces via self-assembly to provide the active functional groups for the subsequent surface functionalisation.3.1.Silanisation of copper surfaces and surface-graft polymerisation of HFBACoupling of the copper surface with3-(trimethoxysilyl)propyl methacrylate(TMSPMA)results in the formation of silane mono-layers containing active carbon–carbon double bonds.The thick-ness of the silane monolayer is about1.7±0.4nm.Parts a and b of Fig.2shows the respective wide scan,C1s and Si2p core-level spectra of the Cu–TMSPMA surface.In comparison with the wide scan spectrum of the etched copper surface(Supporting Informa-tion,Fig.S1a),in addition to the C1s and O1s spectral lines with substantially enhanced intensities,two photoelectron lines at binding energies(BEs)of about99and151eV,attributable to Si 2p and Si2s species[42],respectively,are also observed in the wide scan spectrum of the Cu–TMSPMA surface(Fig.2a),indicative of the successful immobilisation of TMSPMA on the copper surface. The[Si]:[C]ratio,as determined from the Si2p and C1s core-level spectral area ratio,is approximately0.13,which is close to the the-oretical value of0.142for TMSPMA.The curve-fitted C1s core-le-vel spectrum consists of four peak components with BEs at283.9, 284.6,286.2and288.4eV,attributable to the C A Si,C A H,C A O and O@C A O species[42],respectively(Fig.2b).The area ratio of the C A Si,C A H,C A O and O@C A O peak components of the Cu–TMSPMA surface(about1.0:3.9:1.1:0.8)is comparable to the theoretical va-lue of1:4:1:1for the molecular structure of TMSPMA.The single Si 2p peak component at the BE of about102.5eV is associated with Si A O species[42](inset of Fig.2b),indicating that the TMSPMA monolayer is immobilised on the copper surface via the robust Si A O bonds.Graft polymerisation of2,2,3,4,4,4-hexafluorobutyl acrylate was conducted by surface-initiated free radical polymerisation. The resulting surfaces from2and6h of polymerisation reaction are referred to respectively as the Cu-g-P(HFBA)1and Cu-g-P(HFBA)2surfaces.Parts c and d in Fig.2shows the respective wide scan,C1s and F1s core-level spectra for the Cu-g-P(HFBA)1surface.The appearance of two additional photoelec-tron lines,F1s(at a BE of685eV)and F KL23L23(at a BE of 832eV),as well as the decrease in intensity of Si and Cu signals, is discernible in the wide scan spectrum of the Cu-g-P(HFBA)1 surface(Fig.2c),indicative of successful grafting of P(HFBA) brushes from the Cu–TMSPMA surface.The presence of sur-face-grafted HFBA polymer can also be deduced from the appearance of C1s peak components with BEs at about285.6, 289.4,291.4and293.8eV,attributable to C A CF x,CF A CF x,C A F2 and C A F3species[20,42],respectively(Fig.2d),as well as the appearance of the F1s core-level signal at the BE of about 689eV[42](inset of Fig.2d).Thesefluorine peak components are characteristics of the P(HFBA)brushes.On the other hand, the persistence of Si and Cu signals in the wide scan spectrum indicates that the thickness of the P(HFBA)brushes is less than the probing depth of the XPS technique(about8nm in an organ-ic matrix[42])after2h of reaction.The thickness of the P(HFBA)films grafted on the copper substrate is about7±2nm after2h of polymerisation.This point is further confirmed by the devia-tion of the peak area ratios of C A O,O@C A O,CF A CF x,C A F2and C A F3(about 2.1:1.7:2.1:1.0:1.1)from the theoretical value of 1:1:2:1:1.The increase in the relative amount of the C A O and O@C A O species probably has resulted from the contribution of the underlying TMSPMA monolayer,as well as from(at least in part)the adsorption of adventitious carbon and hydrocarbons during sample handling.Upon prolonging the reaction time to6h,the wide scan spec-trum of the Cu-g-P(HFBA)2surface shows only four photoelectron lines at the BEs of about285,530,685and832eV,associated with the C1s,O1s,F1s and F KL23L23species,respectively(Fig.2e).The disappearance of the Si and Cu signals on the Cu-g-P(HFBA)2sur-face indicates that the thickness of the P(HFBA)brushes is larger than the probing depth of the XPS technique after6h of reaction. The thickness of the P(HFBA)films grafted on the copper substrate is approximately19±3nm after6h of polymerisation.The[F]/[C] ratio,as determined from the sensitivity factor corrected F1s and C 1s core-level spectral area ratio,is approximately0.83,which is close to the theoretical[F]/[C]ratio of0.86for the HFBA unit of the P(HFBA)polymers.The curve-fitted C1s core-level spectrum of the Cu-g-P(HFBA)2surface consists of seven peak components with BEs at284.6eV for the C A H species,at 285.6eV for the C A CF x species,at286.2eV for the C A O species, at288.0eV for the O@C A O species,at289.4eV for the CF A CF x spe-cies,at291.4eV for the C A CF2species,and at293.8eV for the2740S.Yuan et al./Corrosion Science53(2011)2738–2747C A CF3species[42].The area ratio of these peak components (about3.4:2.1:1.1:1.2:1.9:1.0:1.0)is consistent with the chemical structure of P(HFBA)(theoretical component ratio of 3:2:1:1:2:1:1).Thus,the P(HFBA)films have been successfully immobilised on the copper surface.3.2.Surface wettability and morphologyThe surface wettability and surface microstructure of various substrates were characterised respectively by water contact angle measurements and SEM.After etched in the nitric acid and hydro-gen peroxide oxidising mixtures,the copper substrate has a rough surface with nubble-protrusion and island structures(Fig.3a and b),and is hydrophilic with a water contact angle of about61°(Fig.3a0).The photo image of water setting on the etched copper surface is shown in Fig.3a00.The hydrophilic property exists over the entire copper surface.After coupling of the TMSPMA mono-layer,the copper surface becomes relatively smooth and hydro-phobic with a water contact angle of about119°without island structures(Supporting Information,Fig.S2a,S2a0and2b).The sub-stantial increase in hydrophobicity further confirms the successful immobilisation of TMSPMA on the copper surface.Parts c and d of Fig.3shows the representative SEM images at different magnifications and water contact angle images of the Cu-g-P(HFBA)1surface.In comparison with those of the etched and silanised copper surfaces,the roughness of the substrate surface changes dramatically after grafting of the P(HFBA)brushes.The SEM image in Fig.3c shows a micro-porous P(HFBA)film covering the entire substrate surface.The high-resolution SEM image (Fig.3d)reveals the foliage-like structure of P(HFBA)film.The foli-age-like microstructures of the P(HFBA)film are essential for superhydrophobicity.It is well-known thatfluorine is the most effective element for lowering the surface free energy because it has a small atomic radius and the highest electronegativity among all atoms.It forms a stable covalent bond with carbon,resulting in a surface with low surface energy[43].However,even aflat surface with the lowest surface free energy,that of the closest hexagonal packed A CF3groups,shows a water contact angle of only119°[44].The combination of low surface free energy and surface roughness,arising from the foliage-like microstructures in Cu-g-P(HFBA)1,gives rise to a superhydrophobic surface with a water contact angle of around154°(Fig.3c0).The optical images of water droplets at different locations of the Cu-g-P(HFBA)1substrate sug-gest uniform surface superhydrophobicity.Upon increasing the reaction time to6h,the high-resolution SEM image(Fig.3f)shows the randomly packed nanoslices or petal-likefluoropolymer structures on the Cu-g-P(HFBA)2surfaces,leading to the further increase in the water contact angle to about159°(Fig.3e0).Thus, the superhydrophobicity of thefluoropolymer-grafted substrate surface wasascertained.S.Yuan et al./Corrosion Science53(2011)2738–274727413.3.Determination of the anticorrosion behaviour of the surface-functionalised copper couponsThe anticorrosion performance of superhydrophobic fluoropoly-mer films was determined by the measurements of Tafel polarisa-tion curves and electrochemical impedance spectra (EIS).Fig.4shows the Tafel polarisation curves of bare and surface-functional-ised coupons after 1day and 21days of exposure in a 3.5wt.%NaCl solution.The values of corrosion current densities (i corr ),corrosion potentials (E corr ,vs.Ag/AgCl),and Tafel slopes (b a and b c )were ob-tained by extrapolating the linear portions of the anodic and catho-dic branches to their intersection using procedures described previously [41].Parameters associated with Tafel polarisationcurves are summarised in Table 1.Inhibition efficiency (g )is calcu-lated by Eq.(1)below [41],g ¼i o Ài corri oÂ100%ð1Þwhere i o and i corr correspond to current densities of the bare copper and the surface-functionalised coupons,respectively.As shown in Table 1,the slight shift in the corrosion potential,E corr (vs.Ag/AgCl),to positive direction and the decrease in corrosion current density,i corr ,with exposure time are observed on the bare copper coupons,attributable to the formation of protective oxide films (consisting mainly of Cu 2O)on the substrate surfaces.As compared to the bare copper,the Cu–TMSPMA coupons display more noble values oftheSEM images at low (1000Â)and high (5000Â)magnifications of the (a and b)the etched Cu,(c and d)Cu-g -P(HFBA)1and (e 0)are contact angle profiles of water droplet on the etched Cu,Cu-g -P(HFBA)1and Cu-g -P(HFBA)2surfaces,respectively.optical images of water droplets at different locations on the etched Cu,Cu-g -P(HFBA)1and Cu-g -P(HFBA)2surfaces,corrosion potential and smaller i corr values throughout the exposure periods(Table1),indicative of the protective nature of the self-assembled silane monolayers[45].However,the i corr value of the Cu–TMSPMA coupon increases approximately by50%relative to the initial value after21days of exposure,indicating that the pro-tective capability of the silane monolayer is compromised by the prolonged attack of aggressive ClÀanions.As for the superhydrophobicfluoropolymer-coated copper cou-pons,the corrosion potential,E corr(vs.Ag/AgCl),undergoes a posi-tive shift to more noble values as compared to those of the bare copper and the Cu–TMSPMA coupons,and remain relatively con-stant throughout the exposure periods(Table1),attributable to the presence of protective superhydrophobicfluoropolymerfilms on the substrate surface.The ennoblement in corrosion potential is a common phenomenon for the polymer-coated coupons with respect to the bare metal coupon[46].The magnitude of i corr for the Cu-g-P(HFBA)1and Cu-g-P(HFBA)2coupons is lower by about 8-and12-fold,respectively,as compared to that of the bare copper coupon after21days of exposure in a3.5wt.%aqueous NaCl solu-tion,indicating that the superhydrophobicfluoropolymerfilms render the desired protection capability against corrosion.The g values of the Cu-g-P(HFBA)2coupons,calculated from Eq.(1),re-main higher than90%throughout the exposure periods,and are al-ways larger than those of the Cu-g-P(HFBA)1coupons(Table1), suggesting that the anticorrosion property is associated with wet-tability,thickness and the structure of the grafted-fluoropolymer films.The protective capability of the superhydrophobicfluoro-polymerfilms probably originates from both the water repelling property and the physical diffusion barriers of thefluoropolymer coatings.The barrier behaviour of the coatings has been known to be dependant mainly on their thickness and structures,as well as the chemical properties of inhibitor molecules[47].EIS can provide important information on the reaction mecha-nisms and kinetics of an electrochemical system under investigation [46].Fig.5shows the EIS data of the bare copper and surface-func-tionalised copper coupons after1day and21days of exposure in a 3.5wt.%aqueous NaCl solution.These EIS data are analysed by using the Boukamp’s EQUIVCRT program with an appropriate equivalent electrical circuit(EEC).Insets of Fig.5d(i.e.,Fig.5d0and d00)shows two EECs proposed to model the respective EIS data of the bare cop-per and the surface-functionalised copper coupons.EEC(d0)is used to model a single charge transfer reaction on the bare metal sub-strate,while ECC(d00)has been widely used to estimate the barrier, protection and degradation properties of polymer coatings[46].In ECC(d00),the pore resistance(R po)represents the extent of ionic con-duction through a polymer in an electrolytic environment,and is commonly used as a criterion for assessing the extent of corrosion protection derived from the organic coatings,while the constant phase element(CPE)of coating,Q c,is used to substitute coating capacitance,C c,by taking into account of the phenomenarelatedTable1Analysis results of Tafel polarisation curves of the bare copper and the surface-functionalised copper coupons after immersion in a3.5wt.%aqueous NaCl solution for1day and 21days.Time(days)Specimen b c e(mV/dec)b a f(mV/dec)E corr.g(V,vs.Ag/AgCl)i corr.(l A cmÀ2)m h(mm yÀ1)g i(%) 1Bare Cu aÀ19588À0.1934.00.40–Cu–TMSPMA bÀ231259À0.1210.50.1269.1Cu-g-P(HFBA)1cÀ2562720.04 2.250.0393.4Cu-g-P(HFBA)2dÀ2592890.08 1.610.0295.321Bare CuÀ217194À0.1622.20.26–Cu–TMSPMAÀ244259À0.1315.90.1928.6Cu-g-P(HFBA)1À2462770.01 2.790.0487.5Cu-g-P(HFBA)2À2552740.08 1.930.0291.3a Bare Cu refers to a newly polished copper coupon.b Cu–TMSPMA refers to the etched Cu coupon with immobilised TMSPMA via self-assembly reaction.c,d Reaction conditions:1mmol of HFBA in a5mL of DMF solution with0.02mmol of AIBN as initiator at65°C for2and6h,respectively.e bcrefers to the Tafel slope of the cathodic polarisation curve.f barefers to the Tafel slope of the anodic polarisation curve.g Ecorrrefers to the potential where the current reaches zero under polarisation.h m denotes corrosion rate.i g denotes inhibition efficiency of the grafted polymeric coatings,calculated from equation g¼ði corrÀi origÞ=i origÂ100%.S.Yuan et al./Corrosion Science53(2011)2738–27472743to the heterogeneous surface and diffusion process[46].Thefitted EIS parameters are shown in Table2.The diameters of the Nyquist loop of thefluoropolymer-grafted coupons are significantly larger than those of the bare copper and Cu–TMSPMA coupons,indicative of the enhancement of polarisa-tion resistance by the superhydrophobicfluoropolymerfilms (Fig.5a and c).The magnitude of total impedance plot of theflu-oropolymer-grafted coupon is larger by more than one order of magnitude than that of the bare copper coupon(Fig.5b and d). For the bare copper coupon,the charge transfer resistance,R ct, increases gradually from0.85to1.6k X cm2after21days of expo-sure in a3.5wt.%aqueous NaCl solution(Table2),indicating that the corrosion rate of copper decreases owing to the formation of protective oxidefilms in an aerated solution.The R ct value of the Cu–TMSPMA coupon undergoes a gradual decrease with exposure time,and becomes slightly larger than that of the bare copper cou-pons after21days of exposure,indicative of the gradual loss of passivity of the self-assembled silane monolayer.This result is in good agreement with the previousfindings that the durability and the corrosion resistance of SAMs are limited under harsh aque-ous environments[47].In the case offluoropolymer-grafted coupons,the R ct values are significantly larger than those of the bare copper and the Cu–TMSPMA coupons(Table2),indicative of the substantial decrease in the corrosion rate of the copper substrates under the protection of superhydrophobicfluoropolymerfilms.Generally,thecorrosionTable2Fitted parameters of EIS spectra of the bare copper and the surface-functionalised copper coupons after immersion in a3.5wt.%aqueous NaCl solution for1day and21days.Time(days)ParametersSpecimen R s c(X cm2)R f/R po d(k X cm2)Q f/Q c e R ct f(k X cm2)Q dl g g h(%)Y oÂ10À3(XÀ1cmÀ2s n)n0Y1Â10À3(XÀ1cmÀ2s n)n11Bare Cu a12.9–––0.850.610.78–Cu–TMSPMA b19.00.550.200.69 3.440.180.8075.2Cu-g-P(HFBA)1b21.4 4.520.160.4711.80.210.6792.8Cu-g-P(HFBA)2b24.6 5.310.060.5012.90.190.6193.4 21Bare Cu13.60.210.260.85 1.62 1.030.48–Cu–TMSPMA24.60.320.410.64 2.280.510.8128.8Cu-g-P(HFBA)118.3 3.310.170.5310.80.1484.9Cu-g-P(HFBA)225.1 4.640.110.5111.80.130.6686.2a The EIS spectrum of the bare Cu coupons after1day of immersion in a3.5wt.%NaCl solution isfitted with the equivalent circuit(d0)of Fig.4.b The EIS spectra of the bare Cu coupons after21days of immersion and the surface-functionalised coupons arefitted with the equivalent circuit(d00)of Fig.4.c Rsresistance of the electrolyte solution.d Rpothe pore resistance of the grafted polymeric coatings and R f resistance of the surface oxidefilm.e Qcconstant phase element(CPE)of the grafted polymeric coatings and Q f CPE of the surface oxidefilm.f Rctcharge transfer resistance of the electrical double layer(EDL).g QdlCPE of EDL.h g refers to inhibition efficiency of the grafted polymeric coatings,calculated from equation g¼ðR ctÀR oct Þ=R ctÂ100%.2744S.Yuan et al./Corrosion Science53(2011)2738–2747。