Montemor 2008

10
Electrochimica Acta 53 (2008) 5913–5922 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta The synergistic combination of bis-silane and CeO 2 ·ZrO 2 nanoparticles on the electrochemical behaviour of galvanised steel in NaCl solutions M.F. Montemor a,, W. Trabelsi a,b , S.V. Lamaka c , K.A. Yasakau c , M.L. Zheludkevich c , A.C. Bastos c , M.G.S. Ferreira a,c a ICEMS, Instituto Superior T´ ecnico, Technical University of Lisbon, Av. Rovisco Pais Lisboa 1049 001, Portugal b ENIT, Unit´ e de Recherche de Corrosion, Tunes, Tunisia c CICECO, University of Aveiro, Department of Ceramic and Glass Engineering, Aveiro 3810 193 Portugal article info Article history: Received 7 March 2008 Accepted 26 March 2008 Available online 10 April 2008 Keywords: Silanes Ceria Zirconia Nanoparticles SVET SIET abstract Bis-1,2-[triethoxysilylpropyl]tetrasulfide silane films containing CeO 2 ·ZrO 2 nanoparticles were deposited by dip-coating on galvanised steel substrates. The morphological features of the coated substrates were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The anti-corrosion performance of the modified silane film applied on galvanised steel substrates was studied by electro- chemical impedance spectroscopy (EIS). The ability of nanoparticles to mitigate localized corrosion activity at artificially induced defects was investigated via the scanning vibrating electrode technique (SVET) and by the scanning ion-selective electrode technique (SIET). The results showed that the addition of nanopar- ticles provides good corrosion protection of the galvanised steel substrates pre-treated with the modified silane solutions. The corrosion activity was reduced by more than one order of magnitude. Complementary d.c. experiments, using zinc electrodes exposed to NaCl solutions containing the nanoparticles were also performed in order to better understand the role of the nanoparticles. An ennoblement of the corrosion potential and polarisation of the anodic reactions could be detected. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction The need of alternatives to replace the very effective but environmentally harmful chromate-based surface treatments pro- moted the investigation of several classes of new pre-treatment systems. Among these, pre-treatments based on organosilanes attracted considerable interest as they provide the formation of a thin organic coating that confers surface functionalisation. Therefore, silane pre-treatments work as adhesion promoters for improved paintability. The corrosion resistance of the painted sys- tem is mainly controlled by the anti-corrosion properties of the paint scheme, which includes the pre-treatment layer. Thus, this layer plays an important role, since it is the one in direct contact with the metal. The pre-treatment layer must provide adhesion, good barrier properties [1–4], and if possible, corrosion inhibition ability. The major drawback of the silane pre-treatments is their inert character regarding the corrosion processes. The silane coat- ing, by itself, does not provide any “active” protection when the aggressive species reach the metallic surface and initiate the cor- Corresponding author. E-mail address: [email protected] (M.F. Montemor). rosion activity. Recent research efforts have been focused on the modification of the bulk properties of silane coatings by adding active” anti-corrosion species in order to further improve the cor- rosion resistance, or to introduce self-healing ability in the silane coating. Some corrosion inhibitors were employed as active addi- tives like tolyltriazole [4], benzotriazole [4], phenylphosphonic acid [5] and cerium [6–9], zirconium [8] and lanthanum [9] salts. The organic inhibitors added to the silane coatings improved the corro- sion resistance but did not impart self-healing activity. Conversely, cerium salts present potential as additives since they seem to pro- vide self-healing ability [4,7–9]. New pre-treatments for galvanised steel substrates based on bis-[triethoxysilylpropyl] tetrasulfide doped with small amounts of cerium nitrate or zirconium nitrate were studied in a previous work [8] and it was found that the corrosion resistance of the treated substrates before painting was significantly improved by addition of those dopants. This effect could be associated to reduced poros- ity and increased thickness of the coatings [8,9]. It was shown that the presence of zirconium ions provided very good barrier prop- erties, whereas the presence of cerium provided better corrosion inhibition ability. Nanoparticles of several oxides are very promising fillers for coating materials. Generally, these small oxide particles provide 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.03.069

Transcript of Montemor 2008

Electrochimica Acta 53 (2008) 5913–5922

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

The synergistic combination of bis-silane and CeO2·ZrO2 nanoparticles on the

electrochemical behaviour of galvanised steel in NaCl solutions

M.F. Montemora,∗, W. Trabelsi a,b, S.V. Lamakac, K.A. Yasakauc, M.L. Zheludkevichc,A.C. Bastosc, M.G.S. Ferreiraa,c

a ICEMS, Instituto Superior Tecnico, Technical University of Lisbon, Av. Rovisco Pais Lisboa 1049 001, Portugal

10 193

yl]tesed stctronfied sroscocts wive elsion psion ac ele

ter unof th

b ENIT, Unite de Recherche de Corrosion, Tunes, Tunisiac CICECO, University of Aveiro, Department of Ceramic and Glass Engineering, Aveiro 38

a r t i c l e i n f o

Article history:Received 7 March 2008Accepted 26 March 2008Available online 10 April 2008

Keywords:SilanesCeriaZirconiaNanoparticlesSVETSIET

a b s t r a c t

Bis-1,2-[triethoxysilylpropby dip-coating on galvanievaluated by scanning eleperformance of the modichemical impedance spectat artificially induced defeby the scanning ion-selectticles provides good corrosilane solutions. The corrod.c. experiments, using zinperformed in order to betpotential and polarisation

1. Introduction

The need of alternatives to replace the very effective butenvironmentally harmful chromate-based surface treatments pro-moted the investigation of several classes of new pre-treatmentsystems. Among these, pre-treatments based on organosilanesattracted considerable interest as they provide the formationof a thin organic coating that confers surface functionalisation.Therefore, silane pre-treatments work as adhesion promoters forimproved paintability. The corrosion resistance of the painted sys-tem is mainly controlled by the anti-corrosion properties of thepaint scheme, which includes the pre-treatment layer. Thus, thislayer plays an important role, since it is the one in direct contactwith the metal. The pre-treatment layer must provide adhesion,good barrier properties [1–4], and if possible, corrosion inhibitionability. The major drawback of the silane pre-treatments is theirinert character regarding the corrosion processes. The silane coat-ing, by itself, does not provide any “active” protection when theaggressive species reach the metallic surface and initiate the cor-

∗ Corresponding author.E-mail address: [email protected] (M.F. Montemor).

0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2008.03.069

Portugal

trasulfide silane films containing CeO2·ZrO2 nanoparticles were depositedeel substrates. The morphological features of the coated substrates weremicroscopy (SEM) and atomic force microscopy (AFM). The anti-corrosionilane film applied on galvanised steel substrates was studied by electro-py (EIS). The ability of nanoparticles to mitigate localized corrosion activityas investigated via the scanning vibrating electrode technique (SVET) andectrode technique (SIET). The results showed that the addition of nanopar-rotection of the galvanised steel substrates pre-treated with the modifiedctivity was reduced by more than one order of magnitude. Complementaryctrodes exposed to NaCl solutions containing the nanoparticles were alsoderstand the role of the nanoparticles. An ennoblement of the corrosion

e anodic reactions could be detected.© 2008 Elsevier Ltd. All rights reserved.

rosion activity. Recent research efforts have been focused on themodification of the bulk properties of silane coatings by adding

“active” anti-corrosion species in order to further improve the cor-rosion resistance, or to introduce self-healing ability in the silanecoating. Some corrosion inhibitors were employed as active addi-tives like tolyltriazole [4], benzotriazole [4], phenylphosphonic acid[5] and cerium [6–9], zirconium [8] and lanthanum [9] salts. Theorganic inhibitors added to the silane coatings improved the corro-sion resistance but did not impart self-healing activity. Conversely,cerium salts present potential as additives since they seem to pro-vide self-healing ability [4,7–9].

New pre-treatments for galvanised steel substrates based onbis-[triethoxysilylpropyl] tetrasulfide doped with small amounts ofcerium nitrate or zirconium nitrate were studied in a previous work[8] and it was found that the corrosion resistance of the treatedsubstrates before painting was significantly improved by additionof those dopants. This effect could be associated to reduced poros-ity and increased thickness of the coatings [8,9]. It was shown thatthe presence of zirconium ions provided very good barrier prop-erties, whereas the presence of cerium provided better corrosioninhibition ability.

Nanoparticles of several oxides are very promising fillers forcoating materials. Generally, these small oxide particles provide

chimic

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improved resistance to oxidation, corrosion, erosion and wear.Many efforts have been made to enhance the corrosion resistance ofmetallic substrates by using ZrO2 [10,11], CeO2 [12,13], SiO2 [14,15],Al2O3 [16], and other mixed oxides. Among these, CeO2 and ZrO2,particularly, are very interesting due to their resistance to hightemperature oxidation, corrosion, mechanical abrasion and wear[12,13,17]. Cerium oxides and cerium hydroxides are also reportedas cathodic inhibitors [18–20] and have been proposed as effectivespecies for the protection of metals from corrosion.

Zirconium oxides also possess very attracting properties, such asimproved resistance to wear and corrosion, biocompatibility, heatresistance and good adhesion to metallic surfaces [21–32]. ZrO2thin films have been applied for corrosion protection of metals.Chemical vapour deposition [25], eletrophoretic deposition [26]and sol–gel deposition by dip coating procedure [24,27,33] are com-mon routes to prepare ZrO2 coatings for anti-corrosion purposesand for the improvement of mechanical properties [28–32].

Non-active SiO2 [34–38] or CeO2 [38] nanoparticles havebeen applied as additives in silane coatings, leading to compos-ite systems with improved anti-corrosive resistance. ZrO2–CeO2films were deposited on metals by sol–gel processing [39]. Theirimproved corrosion behaviour was attributed to a decreased flow ofaggressive species through the coating. The extent of the improve-ment depended upon the thermal treatment of the sample andcoating thickness [40]. New hybrid sol–gel coatings containing ZrO2nanoparticles and doped with cerium ions, provide longer cor-rosion protection, since the ZrO2 particles could play the role ofnanoreservoirs for storage and controllable release of the inhibitor[41]. Despite the large number of works published in literaturethat explored the unique properties of CeO2 and ZrO2 very littlehas been published using a combination of both in the field of thepre-treatments.

The present work reports the anti-corrosion properties of anew pre-treatment for galvanised steel obtained by dipping themetallic substrate in a silane solution modified with 250 ppmof CeO2·ZrO2 nanoparticles. Two approaches are reported: thefirst one investigates the morphological features and the electro-chemical behaviour of silane coatings containing the CeO2·ZrO2nanoparticles, whereas the second approach examines the cor-rosion activity of zinc electrodes immersed in NaCl solutionscontaining the nanoparticles. Furthermore, we present an elec-trochemical study in which advanced localised electrochemicaltechniques (SVET and SIET) are combined with conventional d.c.measurements in order to better understand the corrosion mecha-

nisms.

The results show that silane coatings modified with nano-particles enhanced the corrosion protection of galvanised steelsubstrates. The role of the nanoparticles is analyzed and discussed.

2. Experimental procedure

2.1. Materials

2.1.1. Silane coatingsBis-[triethoxysilylpropyl]tetrasulfide silane (BTESPT) was cho-

sen because it forms films with very high hydrophobicity andtherefore good potentialities for corrosion protection, as reportedin previous works [8,9]. The silane solution was prepared bydissolving 5% (v/v) of BTESPT (Sigma–Aldrich) in a mixture ofmethanol (90%, v/v) and aqueous dispersion of nanoparticles (5%,v/v). The aqueous dispersion was produced by dispersing ultrasoni-cally, in water, CeO2·ZrO2 (CAS number 53169–24–7) nanoparticles(Sigma–Aldrich) with purity 99% and an average diameter around25 nm (measured by XRD). The final concentration of nanoparticles

a Acta 53 (2008) 5913–5922

in the silane solution was 250 ppm. The solution was stirred for onehour and then stored for a few days before use.

Galvanised steel coupons, having a zinc coating weight ofapproximately 140 g/m2 were used. No post annealing was per-formed on this material. Pure zinc coupons (Goodfellow; 99.9%)were also prepared. The coupons (either galvanised steel or zinc)were degreased using an alkaline cleaner (Novomax®) for 4–5 minat 50 ◦C, washed with distilled water, dried in air and immersedin the modified silane solution for 10 s. The treated coupons werecured in the oven at 120 ◦C for 40 min.

2.1.2. Nanoparticles added to aqueous solutionsAqueous solutions of 0.005 M NaCl containing 250 ppm of

nanoparticles were prepared and continuously stirred. d.c. elec-trochemical experiments were performed under stirring. Two testsolutions were prepared: 0.005 M NaCl – pH 6 and 0.005 M NaClplus NaOH – pH 10. The nanoparticles were CeO2·ZrO2 or, forcomparative purposes, CeO2 (30 < ∅ < 40 nm), both obtained fromSigma–Aldrich.

2.2. Electrochemical techniques

2.2.1. EISThe EIS measurements were carried out using a Gamry FAS1

Femtostat with a PC4 Controller Board. The experiments were per-formed at room temperature, in a Faraday cage, at the open circuitpotential, using a three-electrode electrochemical cell, consistingof the working electrode (≈3.15 cm2 of exposed area), saturatedcalomel electrode (SCE) as reference and platinum as counter elec-trode. The measuring frequency ranged from 100 kHz down to5 mHz. The rms voltage was 10 mV. The EIS experiments wereperformed during immersion of the pre-treated galvanised steelsubstrates in solutions of 0.005 M NaCl for one week. Spectra weretreated using the Z-view Software using the adequate equivalentelectric circuits.

2.2.2. SVETThe SVET measurements were performed using Applicable Elec-

tronics equipment, controlled by the ASET program (Sciencewares).The vibrating electrode was made of platinum–iridium coveredwith polymer, leaving only an uncovered platinum black tip with adiameter of 20–30 �m. The average distance of the tip to the sur-face was kept at 200 �m and the scanned area was 2 mm × 2.5 mm,

with maps of 50 × 50 points. To evaluate the corrosion inhibitionperformance of the modified silane film, a scratch was made onthe surface using an edge knife. The dimensions of the scratchwere approximately 1 mm (length) × 0.1 mm (wide). The zinc sur-face was exposed in the scratch. Silane treated galvanised steel orsilane treated zinc coupons were immersed in the aggressive solu-tion (0.005 M NaCl) and measurements were taken periodically.

2.2.3. SIETThe SIET measurements were performed with Applicable Elec-

tronics equipment, controlled by the ASET program (Sciencewares).The pH micro-potentiometric electrodes were made in the lab-oratory as described in reference [42] using hydrogen I cocktailB ionophore (Fluka, Ref. 95293). These electrodes have a linearpotential–pH response from pH 5.5–12. The microelectrodes werecalibrated before and after the measurements with commercial pHbuffers. Maps with 30 × 30 points were obtained in a plane 50 �mabove the sample’s surface. SIET measurements were performed onthe silane treated zinc coupons immersed in 0.005 M NaCl. Prior toimmersion a scratch of approximately 1 mm length by 0.1 mm widewas performed on the silane coating, exposing the zinc substrate.

M.F. Montemor et al. / Electrochimic

2.2.4. d.c. polarizationCurrent vs. time experiments were performed on galvanised

steel substrates. The applied potential was 50 mV above the opencircuit potential.

Potentiodynamic polarization experiments were performed insolutions of 0.005 M NaCl and NaOH (pH 10) containing 250 ppmof CeO2·ZrO2 or 250 ppm of CeO2. These experiments aim at under-standing the behaviour of the zinc surface under pH conditionsidentical to those generated by the cathodic activity when the gal-vanised steel substrate coated with silane is corroding. Thus, theexperiment intends to simulate a cathodic delamination processand to investigate the role of the nanoparticles in such process.

The potentiodynamic polarization curves were performedusing a RADIOMETER-VOLTALAB PGZ 100 apparatus. The scan ratewas 1 mV/s in the anodic or in the cathodic direction, start-ing from the open circuit potential after different stabilizationtimes. The working electrode was pure zinc with dimensions10 mm × 10 mm × 1 mm. The samples were mounted in an epoxy

Fig. 1. (a) SEM image of the Zn coupon coated with the silane coating containing CeOcross-section image and (d) EDX mapping of the same coating deposited on zinc substrainto silane coating.

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resin mould, polished with SiC papers (Buehler) until 1200 grit,washed with deionised water and ethanol and finally dried withcompressed air.

The electrochemical cell used in the d.c. experiments was similarto the one described for the EIS measurements.

2.3. Microscopic techniques

2.3.1. SEM/EDXThe microstructure, qualitative chemical composition and thick-

ness of the bis-1,2-[triethoxysilylpropyl]tetrasulfide-based filmscontaining CeO2·ZrO2 nanoparticles before immersion in NaCl solu-tion were studied by scanning electron microscopy (SEM) coupledwith energy dispersive X-ray spectroscopy. Semi-in-lens Hitachi SU-70 UHR Schottky (Analytical) FE-SEM microscope with an electronbeam energy of 15 keV or 5 keV was used. The sample for cross-section analysis was prepared by embedding a coated coupon intoepoxy resin. The sample was polished sequentially with 400, 800,

2·ZrO2 nanoparticles, (b) overall EDX spectrum of the image presented in (a), (c)tes, (e) SEM image and (f) EDX mapping of CeO2·ZrO2 nanoparticles incorporated

chimic

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1200, 2400 and 4000 grade sand paper in ethanol. The sample wasthen polished with CafroTM diamond paste down to 0.5 �m andcleaned ultrasonically in ethanol.

2.3.2. AFM

The morphology of the pre treated Zn surface was assessed using

a PicoLETM Atomic Force Microscope from Molecular Imaging. Mea-surements were done in AC Mode using Si probes highly doped todissipate surface charge with tip radius <10 nm.

3. Results

3.1. Morphological Investigation

The SEM/EDX images depicted in Fig. 1 show the plane view(a), the overall EDX spectrum (b) of the area presented in (a),the cross-section representation (c) and the EDX mapping (d) ofa nanoparticles-doped silane coating applied on zinc coupons. Thefilm is uniform, without relevant defects. All elements of the coat-ing and nanoparticles were identified by EDX analysis (b). No cracksinduced by addition of CeO2·ZrO2 were visualized on the surface ofthe coating. Some agglomerates of nanoparticles could be observedat low magnification (Fig. 1a) and the EDX analysis of this regionclearly shows the presence of cerium and zirconium peaks. A closerview (Fig. 1e) and the respective EDX maps taken at low accelerat-ing voltage (5 keV) to avoid the cracking of the coating revealed

Fig. 2. AFM topography (a) and phase (b) maps obtained at the silane films containing Cimmersion in 0.005 M NaCl. Z height of images (a) and (c) is 23 nm and 144 nm, respectiv

a Acta 53 (2008) 5913–5922

the presence of cerium and zirconium, confirming the presenceof CeO2·ZrO2 nano-sized particles agglomerates—Fig. 1(e and f)and a relatively uniform distribution of nanoparticles in the coat-ing. The thickness of the bis-1,2-[triethoxysilylpropyl]tetrasulfidesilane film deposited on the Zn substrate was around 0.8–1.1

micrometers. The EDX analysis of the cross-section zones revealedthat the main components of the coating were Si and S togetherwith Zn from the substrate – Fig. 1(b and d).

Fig. 2 presents atomic force microscopy (AFM) topography andthe phase images taken on the zinc coupons coated with thenanoparticles-modified silane coating and on the coating withoutaddition of nanoparticles, which was used as a reference. The sin-gle nanoparticles cannot be clearly seen in the topography image(Fig. 2a), however, the phase image presented in Fig. 2b showsthe presence of agglomerates and very small nanoparticles. Thesenanoparticles appear to be 40–60 nm in diameter. The size dif-ference between the initially added nanoparticles and the valuesmeasured by AFM can be explained by the presence of a silane layeron the top of the nanoparticles. Thus, AFM measures the convexityof the surface originated by the underlying nanoparticles.

Apart from single nanoparticles, some agglomerates (rangingfrom 100 to 600 nm) are visible on the topography map. The phaseimage does not show significant differences between the matrixand agglomerates, which can be explained by the presence of thesilane film at the top of CeO2·ZrO2 agglomerates. The roughness(RMS) of the topographic images presented in Fig. 2 are 85 and

eO2·ZrO2 nanoparticles and on silane films without nanoparticles (c and d), beforeely.

M.F. Montemor et al. / Electrochimic

Fig. 3. Impedance Bode plots obtained on the galvanised steel sample pre-treatedwith the silane film filled with CeO2·ZrO2 nanoparticles. Spectra for the blank silanefilm and for untreated galvanised steel (HDG) obtained after one day of immersionare also presented for comparative purposes.

Fig. 4. Equivalent circuit used for numerical fitting of the EIS spectra and fitted spectra forin 0.005 M NaCl.

a Acta 53 (2008) 5913–5922 5917

31 nm for samples with addition of nanoparticles and without,respectively. The differences in roughness are caused by the addi-tion of the nanoparticulated mixed oxides to the silane solution inthe course of synthesis.

3.2. Electrochemical investigation

3.2.1. Barrier propertiesThe role of the nanoparticles in the barrier properties of the

silane coating applied on galvanised steel substrates was evalu-ated via EIS in 0.005 M NaCl solutions. The concentration of theNaCl solution is low because the main goal of this work is tounderstand how the nanoparticles affect the corrosion behaviour.Therefore, it is necessary to delay the corrosion processes to getas much information as possible. Fig. 3 depicts the electrochemi-cal impedance spectra obtained on the galvanised steel substratespre-treated with the silane film filled with CeO2·ZrO2 nanoparti-cles. Spectra for a silane film without nanoparticles (blank silane)

and for untreated galvanised steel were also included in Fig. 3 forcomparative purposes. One broad time constant could be observed.The impedance spectra show a resistive behaviour at very highfrequencies followed by a capacitive response and, in the low fre-quency domain, a resistive plateau with impedance values around108 � cm2. With further increase of the immersion time, a smalldrop of the low frequency impedance values was observed. At theend of the tests no signs of corrosion activity could be detected onthe pre-treated surface. In previous works [8] we proposed threedifferent equivalent circuits, for the fitting of the EIS data obtainedon doped silane films. The number of time constants dependedupon the dopants and immersion time as previously reported [8].Since in this work EIS was used to evaluate the barrier properties ofthe nanoparticles filled coating the impedance spectra were fittedusing the equivalent circuit shown in Fig. 4. Only one time constantwas considered due to the very high impedance values. This optiongave the smallest error in the fitting routines. The equivalent cir-cuit includes the electrolyte resistance and a R/CPE contribution tosimulate the capacitive and low frequency resistive behaviour. Thefitting of the capacitive part of the spectra was made using a con-stant phase element (CPE). Generally the CPE is associated with

the silane coatings containing CeO2·ZrO2 nanoparticles after one day of immersion

5918 M.F. Montemor et al. / Electrochimic

or as current plots across lines. The SVET maps give quantitativeinformation about anodic and cathodic currents densities and showhow the anodic vs. cathodic activity changes during immersion.The SIET maps allow monitoring the changes of the pH, both in theanodic and cathodic areas when the corrosion activity proceeds.The combination of both techniques is a relatively new approach incorrosion studies and constitutes a strong tool to understand corro-sion and corrosion inhibition mechanisms at the microscale level.

Fig. 7 shows the SVET maps obtained on the galvanised steelsubstrates pre-treated with blank silane and with a silane coatingfilled with CeO2·ZrO2 nanoparticles. Prior to immersion a defect

Fig. 5. Evolution of the CPE parameters during immersion of the treated galvanisedsteel coupons in 0.005 M NaCl. Values obtained for a blank silane coating are alsopresented.

a distribution of the capacitance over the surface or to changesof the capacitance with frequency [43]. One example of the fit-ting procedure adopted is presented in Fig. 4. The evolution ofthe fitting parameters obtained from the numerical simulation of

experimental spectra are depicted in Figs. 5 and 6. Generally then exponent of the CPE element was in the range 0.95–0.9. Fig. 5shows a very small increase of the coating capacitance during thefirst day of immersion. This increase was due to electrolyte uptakethrough conductive pathways that developed inside the film. How-ever, changes were limited to a few nF/cm2 sn−1 suggesting a veryhigh stability of the silane coating during the test period. The silanecoating also presented very high resistances, that decreased dur-ing immersion from 100 M � cm2, down to 10 M � cm2 (Fig. 6).These values are about three orders of magnitude higher than thoseobserved in a blank silane film. The thickness of modified silanefilms deposited on galvanised steel substrates is generally higherthan those of the blank films (0.5–1 �m). These differences couldcontribute to the improved barrier properties, but other factors likelower porosity and lower conductivity of the filled films can alsocontribute to the improved behaviour. This result shows that thepresence of CeO2·ZrO2 nanoparticles enhanced the barrier proper-ties of the silane coating, delaying corrosion onset.

Fig. 6. Evolution of the coating resistance (Rc) during immersion of the treatedgalvanised steel coupons in 0.005 M NaCl. Values obtained for a blank silane coatingare also presented.

a Acta 53 (2008) 5913–5922

3.2.2. Corrosion inhibition at localized defectsElectrochemical impedance spectroscopy provides data about

the average response over the entire sample surface. However, thescanning vibrating electrode technique (SVET) and the scanningion-selective electrode technique (SIET) allow local electrochemicalinformation to be obtained in the form of local current density mapsor pH maps, respectively, over the exposed metallic surface. SIETmeasures potentiometrically the concentration of specific ions insolution (pH in this work) at selected distances from the surface[42,44,45].

These local measurements are very useful to understand theability of the nanoparticles to inhibit corrosion activity in localizeddefects, such as scratches. The measured distributions of currentor potential can be represented as two or three dimensional maps

was made on the specimen surface in order to expose the sub-strate. This procedure ensures that corrosion activity starts at thesame time in both substrates. Therefore, it allows a better com-parison of the results and a better understanding of the role ofthe nanoparticles. After one hour of immersion, the SVET mapsshowed significant differences. The substrate coated with the blankfilm (Fig. 7A) developed strong anodic activity over the scratch;the remaining surface behaving essentially as cathodic. Since oxy-gen reduction is the main cathodic reaction expected, the largecathodic areas revealed that oxygen could easily access the inter-

Fig. 7. SVET maps obtained in the scratched BTESPT silane films (A) and CeO2·ZrO2

filled BTESPT silane films (B) after one hour of immersion in NaCl 0.005 M. Scan size:2 mm × 2.5 mm. Current units: �A/cm2.

M.F. Montemor et al. / Electrochimica Acta 53 (2008) 5913–5922 5919

e scratched BTESPT silane films (images A and B), CeO2·ZrO2 filled BTESPT silane filmssize: 2 mm × 2.5 mm. Current units: �A/cm2.

galvanised steel substrates. They provide better barrier propertiesand delay corrosion activity at localised defects.

In order to better understand the role of the nanoparticles anumber of SVET, SIET and potentiodynamic polarisation tests werecarried in coated and uncoated zinc samples immersed in NaClsolutions.

3.2.3. Role of the nanoparticlesThe mechanisms associated with the corrosion inhibition in

the presence of nanoparticles were studied in pure zinc electrodescoated with the silane film modified with the nanoparticles, usingthe SVET and SIET measurements. The study was complemented

Fig. 8. SVET maps (ionic currents – left and surface images – right) obtained in th(images C and D) after 24 and 48 h of immersion in NaCl 0.005 M, respectively. Scan

face. The surface of the system pre-treated with the silane coatingfilled with CeO2·ZrO2 nanoparticles (Fig. 7B) showed much loweranodic/cathodic current densities, revealing much weaker corro-sion activity on the exposed surface. The anodic current densitieswere about one order of magnitude below those measured in theblank film. This trend was observed during the whole experimentthat lasted for 48 h of immersion.

For the blank silane film, with time, there was a decrease ofthe current density on the initial active area due to precipitationof zinc corrosion products, but several new anodes developed and,after 24 h, significant delamination could be observed around theinitial scratch (Fig. 8A and B). After 48 h of immersion the blank

coating was strongly deteriorated and generalised corrosion activ-ity was observed. For the coating modified with the nanoparticles,the anodic current densities were always below 10 �A/cm2 andafter 48 h (Fig. 8C and D) the first signs of delamination couldbe observed around the scratch. The SVET results indicated thatthe galvanised steel substrates pre-treated with the silane coatingscontaining CeO2·ZrO2 nanoparticles provide a significant delay ofthe corrosion activity at localized scratches.

Fig. 9 shows the current vs. time curves obtained on the gal-vanised steel substrates treated with the blank silane coating andwith the silane coating modified with CeO2·ZrO2 nanoparticles.Prior to measurement a scratch was made on the surface. Forthe blank coating the current increased and attained stable val-ues around 100 �A/cm2. However, in the presence of nanoparticlesthe initial shift was followed by a slow decrease and stabilisation atvalues around 1 �A/cm2, two orders of magnitude below those ofthe blank film. Although this measurement gives an average valueof the electrode behaviour, it showed a very good agreement withthe localised SVET measurements.

The previous electrochemical results clearly showed that thenanoparticles delay the development of the corrosion activity of

Fig. 9. Current vs. time curves recorded for the scratched blank silane film andCeO2·ZrO2 filled silane film during immersion in NaCl 0.005 M.

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BTESP

Fig. 10. SVET maps (A and C) and SIET pH maps (B and D) obtained on scratcheddifferent immersion times in NaCl 0.005 M. Scan size: 2 mm × 2.5 mm.

with potentiodynamic polarisation curves carried using untreated

zinc electrodes in solutions containing the nanoparticles.

The SVET and SIET maps obtained on pure zinc electrodes coatedwith the CeO2·ZrO2 nanoparticles modified coating are depicted inFig. 10. The SVET map (Fig. 10A) obtained six hours after immersionrevealed some anodic activity over the scratched area, as expectedand in good agreement with the results obtained for the galvanisedsteel substrates. For the early stages of immersion the anodic cur-rents densities were very low and the pH at the anodic sites wasaround 6. The cathodic sites were characterised by higher pH values,which attained 8.5. After three days of immersion the behaviourwas significantly different. The anodic currents increased to valuesthat are one order of magnitude above the initial ones and newanodic sites appeared on the surface, extending the corroded area.The pH around these areas was nearly neutral since zinc corro-sion does not lead to significant acidification. The cathodic activitybecame more localised as it can be seen in the left bottom. ThepH starts to increase in the neighbours of the cathodic zones andattained 10.5, at the centre of the cathodic area. It should be notedthat the pH measurements were made 50 �m above the surfacewhich means that the pH of the solution in direct contact withthe metal is even higher in the cathodic regions. The SIET maps

T silane films filled with CeO2·ZrO2 nanoparticles applied on zinc substrates after

reveal that the increased anodic activity at the scratched area is

accompanied by a stronger alkalinisation at the cathodic sites.

In a complementary experiment, the zinc electrodes whereimmersed in a solution of NaCl 0.005 M plus NaOH at pH 10, sim-ulating the conditions that were observed at the cathodic areas(see Fig. 10D), where precipitation of zinc hydroxides takes place.Fig. 11 shows the potentiodynamic polarisation curves obtainedunder these conditions. In this experiments tests were performedusing the CeO2·ZrO2 nanoparticles and, for comparative purposes,CeO2 nanoparticles, too. For the zinc electrode immersed in thesolution containing the CeO2 nanoparticles a clear passivationrange, extending from the corrosion potential up to −0.7 V couldbe observed. Above −0.7 V the current increased due to zincdissolution. An identical trend could be observed for the solu-tion modified with the CeO2·ZrO2 nanoparticles. In this case, thecorrosion potential was slightly more negative, but the currentdensities were identical. For the blank solution (without nanopar-ticles) the corrosion potential was −1.0 V. A clear passivationrange could not be observed; however, the current changes werevery small for the potential range between −0.9 and −0.8 V, sug-gesting an incipient passivation range. In this case the anodiccurrent densities were more than one order of magnitude higher

M.F. Montemor et al. / Electrochimic

Fig. 11. Anodic potentiodynamic polarisation curves obtained in solutions of0.005 M NaCl—pH 10, with and without addition of 250 ppm of nanoparticles usingpure zinc electrodes.

comparatively to the electrodes polarised in the solutions contain-ing the nanoparticles.

The potentiodynamic polarisation results revealed that thepresence of nanoparticles improves the self-passivation of zincelectrodes in alkaline environments, decreasing the anodic cur-rent densities and shifting the breakdown potential towards morepositive values. The polarisation measurements showed that thebehaviour of zinc electrodes exposed to solutions containing theCeO2·ZrO2 or CeO2 nanoparticles revealed an identical behaviour.Therefore, it seems that the zirconia component of the nanoparti-cles probably do not provide significant corrosion inhibition effects.Conversely, the cerium oxide plays the role of active element.

The beneficial effects of CeO2 in improving the passivation rangeof stainless steels electrodes immersed in aggressive conditionshave been previously reported [46].

4. Discussion

The modification of functional coatings, like silanes or sol–gelcoatings with species able to inhibit corrosion or to impart selfhealing ability as well as improved barrier and mechanical prop-erties is nowadays a big challenge in the corrosion protection field.An attracting route to incorporate this active species in the coatingis through doping with nanoparticles or with organic or inorganicinhibitors or through encapsulation of reservoirs containing theinhibitor [5,47,49–52]. This last approach consists on the use of sil-ica nanoparticles covered layer-by-layer with polyelectrolyte layersand layers of inhibitor (benzotriazole) that is slowly released to theelectrolyte solution due to pH changes [51]. The use of nanoparticlesdirectly covered with ions able to provide corrosion inhibition prop-erties, like cerium ions [38,41,48,52], is also another approach thatmakes silane and sol–gel coatings “smarter” face to the corrosionprocesses.

These procedures raise some important questions: Whichnanoparticles shall be chosen? How stable are these nanoparti-cles and do they have an active role? In previous works [38,52]we demonstrated that CeO2 nanoparticles were more effective interms of corrosion protection comparatively to SiO2 nanoparticles.

a Acta 53 (2008) 5913–5922 5921

These nanoparticles enhanced the barrier properties of the silanecoatings comparatively to a blank film and, furthermore, it wasdemonstrated that the CeO2 nanoparticles, by themselves provideda delay of the corrosion activity, starting at defects. Additionally, ina previous work [8] we reported that Zr-modified silane films pre-sented very good barrier properties, but no significant corrosioninhibition ability.

Therefore, in this work, we proposed another approach, whichaims at creating a synergistic effect, by the combination of zirconiaand ceria nanoparticles. The results demonstrated that the use ofCeO2 and ZrO2 nanoparticles as fillers in silane coatings resulted ina very effective procedure to improve both the barrier properties ofthe coating and to reduce corrosion activity. The EIS measurementsrevealed that the coating modified with the nanoparticles couldattain impedance values near 100 M � cm2, much higher than thevalues measured for a blank film and also higher than the valuesmeasured for films modified with CeO2 only [38]. The enhance-ment of the barrier properties can be attributed to the presenceof the inorganic component and probably to lower porosity andconductivity as proposed in previous works [52].

In this work the SVET and d.c. measurements revealed a sig-nificant decrease of the corrosion activity in the presence ofnanoparticles. This means that the nanoparticles behave not onlyas fillers for enhanced barrier properties as observed by EIS, butalso as active fillers, which reduce the corrosion activity. The d.c.polarization tests carried in solution, revealed two important fea-tures: the open circuit potential was shifted towards nobler valuesand there was a clear decrease of the anodic current density in thepresence of nanoparticles. Furthermore, by comparing the poten-tiodynamic polarisation results obtained in the presence of CeO2nanoparticles and in the presence of CeO2·ZrO2 nanoparticles, theinhibition effect could be assigned to the CeO2 component in theCeO2·ZrO2 nanoparticles rather than the ZrO2 component. How-ever, the ZrO2 component also plays an important role, contributingfor the enhancement of the barrier properties. We demonstrated inprevious works [8] that zirconium ions led to very good barrierproperties, whereas the addition of cerium led to more effectivecorrosion inhibition ability.

To interpret the improved corrosion resistance of the silane coat-ings modified with the nanoparticles the following explanation canbe drawn: the CeO2·ZrO2 nanoparticles are homogeneously dis-persed in the film as demonstrated by SEM/EDX and AFM. Whena scratch is made on the surface, there is development of anodic(exposed sites) and cathodic activity (around the scratch) where

oxygen is available. In the conditions tested in the present work,the main cathodic reaction is oxygen reduction with production ofhydroxyl ions. The Zn2+ ions produced at the anodic areas migrateto the cathodic places where they combine with OH− to form zinccorrosion products, which in the presence of chloride ions alsolead to the formation of simonkolleite and/or other zinc chloridecharged ions as reported in literature [53,54]. Simultaneously tothese processes, and under the increased pH that attained val-ues higher than 10 as demonstrated by SIET, the silica componentof the silane coating starts to decompose into an hydrated andexpansive gel, because silica is not stable at increased pH val-ues. Conversely, the CeO2·ZrO2 nanoparticles are very stable underthose alkaline conditions. The CeO2 nanoparticles are characterisedby a fluorite structure that easily develop oxygen vacancies, makingthese nanoparticles highly reactive. Furthermore, literature reportsthat the maximum of defects concentration develops at pH valuesbetween 10 and 11 [55]. The high surface reactivity of ceria has beenexplored for several applications like for example to remove ionicspecies in water treatment routines or for the fixation of metal-lic cations, like Zn2+ in the formulation of sunscreens. Generally,the species formed are very stable ceria-based oxides. Therefore,

chimic

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5922 M.F. Montemor et al. / Electro

the CeO2 nanoparticles may complex with the zinc charged speciesformed during anodic polarisation, stabilising the layer of protec-tive zinc corrosion products.

When the nanoparticles are embedded in the silane coating, themechanism can be assumed as similar. Thus, at the cathodic areas,where the pH reaches values over 10, the silica network decom-poses, releasing the nanoparticles that precipitate on the electrodesurface, forming complexes with the zinc charged species and rein-forcing their protective role. These more stable corrosion productsdecrease the active area available for the half-cell reactions, there-fore slowing down the corrosion activity. The results obtained inthis work evidence that CeO2·ZrO2 nanoparticles play an activerole in the corrosion protection performance when they are addedas fillers to hybrid silane coatings. Therefore, the reinforcement ofsilane coatings with such nanoparticles, leads to the formation ofmore protective surface layers, which improve the lifetime of themetallic substrate.

5. Conclusions

Bis-1,2-[triethoxysilylpropyl] tetrasulfide silane coatings modi-fied with CeO2·ZrO2 nanoparticles applied by dipping procedurescan be used as fillers to improve the corrosion resistance of silanepre-treatments for galvanised steel. The addition of nanoparticlesto the silane solution enhanced the barrier properties of the modi-fied silane coatings.

In the presence of defects the corrosion activity was delayed forthe coatings containing the CeO2·ZrO2 nanoparticles. The polariza-tion measurements suggest that the inhibition effects are mainlydue to the presence of the CeO2 component of the nanoparticles.The nanoparticles in solution induce an important polarization ofthe anodic reactions and a decrease of more than one order of

magnitude on the corrosion current density.

The increased anodic activity observed around the scratchedareas was accompanied by an increased pH over the cathodicregions.

The presence of nanoparticles improves the passivity range ofzinc in alkaline medium, therefore delaying the breakdown of thefilms under chloride attack.

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