Electrochimica Acta 54 (2009) Contents lists available at ScienceDirect. Electrochimica Acta

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1 Electrochimica Acta 54 (2009) Contents lists available at ScienceDirect Electrochimica Acta journal homepage: Chemical composition and corrosion protection of silane films modified with CeO 2 nanoparticles M.F. Montemor a,, R. Pinto a, M.G.S. Ferreira a,b a Instituto Superior Técnico, Technical University of Lisbon, ICEMS, DEQB, Av. Rovisco Pais, Lisboa, Portugal b University of Aveiro, CICECO, Department of Ceramic and Glass Engineering, Aveiro, Portugal article info abstract Article history: Received 30 October 2008 Received in revised form 8 January 2009 Accepted 9 January 2009 Available online 24 January 2009 Keywords: Ceria Silanes Pre-treatments Self-healing The present work aims at understanding the role of CeO 2 nanoparticles (with and without activation in cerium(iii) solutions) used as fillers for hybrid silane coatings applied on galvanized steel substrates. The work reports the improved corrosion protection performance of the modified silane films and discusses the chemistry of the cerium-activated nanoparticles, the mechanisms involved in the formation of the surface coatings and its corrosion inhibition ability. The anti-corrosion performance was investigated using electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and d.c. potentiodynamic polarization. The chemical composition of silanised nanoparticles and the chemical changes of the silane solutions due to the presence of additives were studied using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance spectroscopy (NMR), respectively. The NMR and XPS data revealed that the modified silane solutions and respective coatings have enhanced cross-linking and that silane cerium bonds are likely to occur. Electrochemical impedance spectroscopy showed that the modified coatings have improved barrier properties and the SVET measurements highlight the corrosion inhibition effect of ceria nanoparticles activated with Ce(III) ions. Potentiodynamic polarization curves demonstrate an enhanced passive domain for zinc, in the presence of nanoparticles, in solutions simulating the cathodic environment Elsevier Ltd. All rights reserved. 1. Introduction Ceria nanoparticles are a versatile material, which found applications in many different fields, ranging from catalysis [1], ceramics [2 3], fuel cells [2 4] sensors [5 6], biomaterials [7 8], cosmetics [9] and coatings [10 11] among others. In the coatings domain, ceria nanoparticles have shown to improve wear and corrosion resistance in acidic media [11]. NiAl intermetallic coatings containing CeO 2 exhibit higher hardness, improved elastic modulus, fewer defects and decreased porosity [12]. Ce 2 O 3 CeO 2 layers show a pronounced stabilizing effect on the passive state of steel and their corrosion resistance in sulfuric acid medium, allows these coated steels to be used as construction materials for reactors neutralizing sulfuric acid-containing emissions [13]. Literature also reports that nano CeO 2 and nano SiO 2 particles can increase the thermal stability of Ni W P alloy coatings at high temperature, as well as micro-hardness [14]. The anti-corrosion properties of ceria and other cerium oxides/hydroxides coatings have been studied in different media Corresponding author. Tel.: ; fax: address: mfmontemor@ist.utl.pt (M.F. Montemor). and in different materials. Generally, the literature reports an improvement of the anti-corrosion properties in the presence of these oxides/hydroxides [15 21] and many explanations have been proposed to explain these improved corrosion resistance. The beneficial effects of cerium oxides/hydroxides have been attributed to improved barrier properties, cathodic and anodic polarization effects and enhancement of the passive behaviour among others. Nevertheless, the mechanisms are not fully understood as proved by the increasing number of papers being published in the topic during the most recent years. Several procedures for the preparation of effective cerium oxide-containing coatings have been proposed in literature. The most traditional ones were based on the formation of conversion coatings in solutions of cerium nitrate [16 17] or cerium chloride and the most recent ones have been based on the sol gel route [15,20 21]. This latter procedure has become very attractive since the use of functional molecules allows preparing tailored surfaces, with specific functionalities. Surface functionalisation is extremely important for materials that need to be painted. Therefore, metallic surface pre-treatments based on the use of functional molecules, like organosilanes, are nowadays a common route for surface functionalisation of materials that require paint. The formation of the silane layer is a simple procedure, being achieved by dipping the metal in diluted alcohol or /$ see front matter 2009 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 5180 M.F. Montemor et al. / Electrochimica Acta 54 (2009) water based solutions for a short period. The final result is a hybrid functional self assembled coating that shows high stability and very good coupling properties. This thin coating works like a molecular bridge, which promotes the adhesion between the substrate and other organic layers, like paints and adhesives. The interface silane/metal is characterised by the presence of chemically stable and high strength bonds that must present (i) good adhesion between the inorganic surface and organic polymers both in wet and dry environments; (ii) good barrier properties, preventing moisture uptake and (iii) improved surface properties like scratch, wear, thermal and oxidation resistance. Organosilane-based coatings have been successfully tested as anti-corrosion pre-treatments for aluminum alloys, steel, copper, magnesium and galvanized steel and literature generally reports improved corrosion resistance [22 51]. Among the organosilane molecules that can be used as pre-treatments for corrosion protection of metallic substrates, bisfunctional silanes have attracted special attention. Example of these silanes are the bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) and bis-1,2-[triethoxysilyl] ethane (BTSE). Electrochemical measurements and accelerated corrosion tests, like salt spray tests, show that these silanes provide enhanced corrosion protection of different metallic substrates [31 40]. However, the anti-corrosion properties are, in most cases, consequence of the good barrier effect created by the silane coating. Therefore, the corrosion performance of silane pre-treated substrate will depend upon the silane layer thickness, uniformity, hydrophobicity and chemical stability. This makes the silane coatings effective, but also inert, since they cannot play any active role when the corrosion processes start to damage the surface. During corrosion attack the cathodic reactions release hydroxyl ions that increase the ph, inducing the decomposition of the silica network. As consequence, there is acceleration of the degradation and delamination processes of the silane coating and the barrier effect is lost. In order to overcome these limitations a new challenge is imposed, consisting on the modification of the bulk properties of the silane coatings in order to make them more efficient face to the corrosion processes. This will increase the corrosion resistance of the metallic substrate and therefore the lifetime of the painted systems. Modification of silane coatings with species able to provide corrosion inhibition properties constitutes a procedure, which introduces a new functionality in the organic coating: active corrosion protection ability, making it more efficient face to the corrosion processes. The first approaches proposed in literature [48 49] consisted in the addition of alumina or silica particles in order to improve the mechanical properties of the silane coating. The addition of active ions, i.e., ions with well-known corrosion inhibition ability like cerium, zirconium or lanthanum to the silane formulations was proposed in previous works [50 51]. The active species become trapped in the siloxane network, and then are released to the active corrosion sites, where they show its anticorrosion ability. Furthermore, it was observed an improvement of the barrier properties of the coating due to reduced porosity, increased thickness and decreased conductivity. In previous works [22 24] a new approach, consisting on the modification of silane coatings with nanoparticles, like CeO 2, SiO 2 or CeO 2 ZrO 2 was reported. In some cases these nanoparticles were previously activated with cerium ions to obtain a synergistic effect and improved corrosion resistance ability [22,23]. Results demonstrated that ceria nanoparticles are very effective fillers, leading to improved barrier properties of the silane coatings and improved corrosion resistance. In this work further investigation on the modifications occurring on the surface of the ceria nanoparticles (with and without previous activation with cerium ions) before and after silanization is investigated and discussed. The corrosion inhibition ability was investigated by electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and d.c. polarization. The chemical changes were assessed by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) spectroscopy. 2. Experimental 2.1. Silane-modified solutions Ceria nanoparticles from Sigma Aldrich with an average diameter of nm were dispersed in water in a 250 ppm concentration. This aqueous suspension was used to prepare one the silane solutions tested. Another set of ceria nanoparticles was ultrasonically dispersed in an aqueous solution of cerium nitrate Ce(NO 3 ) 3 6H 2 O (Sigma Aldrich), in order to obtain a concentration of 250 ppm of nanoparticles and 250 ppm of cerium nitrate. This aqueous dispersion was then used for the preparation of a second silane solution. The bis-[triethoxysilylpropyl] tetrasulfide silane (Sigma/Aldrich product) solutions were prepared by dissolving 5% (v/v) of silane in a mixture of methanol (90% v/v) and the aqueous dispersions of nanoparticles. The silane solution was stirred for one hour and stored for few days before used for the pre-treatment of the galvanized steel coupons. For comparative purposes, identical silane solutions were prepared with silica nanoparticles instead of ceria nanoparticles in order to obtain a silane coating modified with silica nanoparticles activated with cerium ions Silane pre-treatments The silane solutions were used to treat galvanized steel coupons. These coupons, having a zinc coating weight of approximately 140 g/m 2 and a thickness of approximately 10 m, were degreased using an alkaline cleaner (Novomax )for4 5minat50 C, washed twice with distilled water and dried in air. The cleaned coupons were immersed in the silane solutions for 10 s and then cured in an oven at 120 C for 40 min Electrochemical experiments Silane-coated samples The EIS measurements were carried out using a Gamry FAS1 Femtostat with a PC4 Controller Board. The experiments were performed at room temperature, in a Faraday cage, at the open circuit potential, using a three-electrode electrochemical cell, consisting of working electrode ( 3.15 cm 2 of exposed area), saturated calomel electrode (SCE) as reference and platinum as counter electrode. The measuring frequency ranged from 100 khz down to 5 mhz. The rms voltage was 10 mv. Spectra were treated using the Z-view Software using the adequate equivalent electric circuits. Measurements were taken periodically in samples deliberately damaged during immersion in M NaCl. The SVET measurements were performed using Applicable Electronics equipment, controlled by the ASET program (Sciencewares). The vibrating electrode was made of platinum iridium covered with polymer, leaving only an uncovered platinum black tip with a diameter of m. The average distance of the tip to the surface waskeptat200 m. The coated specimens were immersed for one day in 0.05 M NaCl solution (10 times more aggressive than the one used in the EIS measurements) to evaluate the effectiveness of the corrosion inhibition ability. After this period, a defect was created on the surface using a needle. The diameter of the circular defect is around 0.2 m and only the vertical component of the current was used for the current maps.

3 M.F. Montemor et al. / Electrochimica Acta 54 (2009) Zinc exposed to CeO 2 -containing solutions Zinc coupons (purity 99.95%) from Goodfellow were polished with SiC paper down to 2400 grit and used as working electrodes for potentiodynamic experiments. These experiments were performed in solution of 0.05 M NaCl (as the one used in the SVET measurements) containing 250 ppm of CeO 2 nanoparticles, with or without cerium activation. Solutions of different ph, ranging from 6 to 11 were prepared by addition of NaOH. These experiments aim at understanding the behaviour of the zinc surface under ph conditions identical to those generated by the anodic and cathodic reactions when the galvanized steel substrate coated with silane is corroding. The potentiodynamic polarization curves were performed using a RADIOMETER-VOLTALAB PGZ 100 apparatus. The scan rate was 10 mv/s in the anodic and in the cathodic directions, starting at the open circuit potential. The electrochemical cell used in the d.c. experiments was similar to the one used for the EIS measurements X-ray photoelectron spectroscopy (XPS) The XPS experiments were carried using a Microlab 310 (Thermo Electron former VG Scientific) equipped with a Mg (nonmonochromated) anode and a concentric hemispherical analyzer. The XPS spectra were taken in CAE mode (20 ev), using an Al (nonmonochromate) anode. The accelerating voltage was 15 kv. The quantitative XPS spectra were fitted using a Gaussian Lorentzian product function and an algorithm based on the Simplex optimisation as used in the Avantage software. The binding energies assigned to the different species were always determined after peak fitting. For chemical identification of the same compounds, the deviations assumed in the fitting procedures were of ±0.1 ev. The background subtraction was performed using the Shirley algorithm, which gives an S-shape curve and assumes that the intensity of the background is proportional to the peak area on the higher kinetic energy side of the spectra. XPS experiments were carried in powder samples of the nanoparticles treated and untreated with the cerium nitrate, before and after silanization Nuclear magnetic resonance ( 29 Si) Nuclear magnetic resonance (NMR) was used to investigate in more detail the changes in the chemical composition of the silane solution due to the presence of CeO 2 nanoparticles and cerium (III) ions. The 29 Si spectra were acquired at MHz in a Bruker Avance III 500. A solution of 80% Methanol-d4 (fully deuterated methanol) was used for lock and 20% Hexamethyldisiloxane (HDMS) was used as reference in a 3 mm capillary. HDMS has a 29 Si chemical shift of 7.22 ppm from TMS (tetramethylsilane) which is the zero for the 29 Si scale. A BBO 10 mm probe was used. Spectra were acquired with 8.5 s (60 ) and a delay of 10 s. For the blank solution and for the CeO 2 -containing solution the signal was the result of 2000 scans; for the solution containing CeO 2 nanoparticles activated with cerium the signal was the result of 5000 scans. 3. Results and discussion 3.1. Analytical characterisation of the nanoparticles The surface of the CeO 2 nanoparticles prior and after activation in the cerium nitrate solution, as well as after silanization was investigated by X-ray photoelectron spectroscopy. Fig. 1a shows the Ce3d ionisation spectra for the nanoparticles, before and after treatment in cerium nitrate. The Ce3d spectra for Fig. 1. XPS spectra for the Ce3d ionisation. Spectra were obtained on the CeO 2 nanoparticles and on the CeO 2 nanoparticles after cerium nitrate pre-treatment (a) and on SiO 2 nanoparticles after cerium nitrate pre-treatment (b). cerium compounds exhibit complex features related to hybridization with ligand orbitals and partial occupancy of the valence orbital 4f. An important feature for the identification of the cerium species [52 53] is the presence of satellite peaks. Ce(IV) is clearly recognised by the satellite peak at approximately ev, which arises from a transition of the 4f 0 initial state to the 4f 0 final state and is exclusive of the presence of Ce 4+. The spectra obtained on the CeO 2 nanoparticles show predominantly the presence of Ce 4+ species, as expected for pure CeO 2. The spectra obtained for the ceriumtreated nanoparticles show a different shape with predominance of Ce(III) species. The signal arising from the core nanoparticles is still observed as confirmed by the Ce(IV) characteristic satellite. For comparative purposes, a spectrum obtained on the surface of SiO 2 nanoparticles treated in cerium nitrate solution is also presented in Fig. 1b. This spectrum is completely different from that obtained over the CeO 2 nanoparticles; the Ce 4+ satellite is absent, meaning that the surface of the SiO 2 nanoparticles is covered only with Ce 3+ ions, due to its treatment with cerium nitrate. The O1s ionisation spectra also reveal a number of interesting features (Fig. 2). For the pure ceria nanoparticles, a main peak is identified at ev, corresponding to the oxygen in the ceria structure. However, a shoulder is also observed at higher binding energies, at ev. This shoulder is due to the formation of

4 5182 M.F. Montemor et al. / Electrochimica Acta 54 (2009) Fig. 2. XPS spectra for the O1s ionisation. Spectra were obtained on the CeO 2 nanoparticles and on the CeO 2 nanoparticles after cerium nitrate pre-treatment and on SiO 2 nanoparticles after cerium nitrate pre-treatment. hydroxyl groups on the surface that could result of particles interaction with humidity of air. This shoulder becomes the most intense peak in the cerium-treated nanoparticles due to the presence of hydrolysed cerium ions, like Ce(OH) 3, formed during the treatment of the particles. The silica treated nanoparticles reveal essentially one main O1s peak, at (±0.1) ev, which can be assigned to the oxygen in the SiO 2 structure, and hydroxyl groups formed due to hydrolysis of the nanoparticles. After silanization of the nanoparticles, the surface composition suffers some changes: the cerium signal decreases markedly, revealing an effective coverage of the nanoparticles Fig. 3. The same trend was observed for the SiO 2 nanoparticles after silanization, in which the Ce3d was completely hindered (not shown). The Si2p ionisation from the silane layer in the silanised CeO 2 nanoparticles suffers some changes. For the reference silane film Fig. 4a, the Si2p ionisation is composed of only one main peak, at (±0.1) ev, corresponding to the SiO 2 component in the silane film. However, the Si2p spectra obtained on the surface of the Cetreated CeO 2 nanoparticles reveals an important broadening and Fig. 4. XPS spectra for the Si2p ionisation. Spectra were obtained on the reference silane film (a) and on the cerium-treated CeO 2 nanoparticles after silanization (b). Peak fitting envelopes are also enclosed in (b). the formation of a new peak at ev, which may correspond to interactions between the silicon atoms and the ceria groups on the surface of the nanoparticles. This interaction has been reported in literature [54], for CeO 2 nanoparticles coated with SiO 2 layers. The interaction was attributed to the formation of cerium silicate and was proved by a shift in the Ce3d ionisation spectra. In the present work, this shift could not be observed due to the low signal to background ratio of the Ce3d ionisation after silanization Fig. 3. But, in addition, it was observed in the broadening of the Si2p ionisation for the ceria and silica nanoparticles treated with cerium ions Fig. 5. The trends observed in the Si2p ionisation of the silane film formed on the CeO 2 and SiO 2 nanoparticles activated with cerium, can only be explained by interaction of silicon (from the silane film) with the cerium ions present on the nanoparticles surface. The broadening of the Si2p ionisation was not observed on the untreated SiO 2 nanoparticles after silanization Fig. 5b. In fact, for Fig. 3. XPS spectra for the Ce3d ionisation. Spectra were obtained on the Ce-treated CeO 2 nanoparticles after silanization. Fig. 5. XPS spectra for the Si2p ionisation. Spectra were obtained on the reference silane film (a); on the SiO 2 nanoparticles after silanization (b) and on the Ce-treated SiO 2 nanoparticles after silanization (c).

5 M.F. Montemor et al. / Electrochimica Acta 54 (2009) The 29 Si NMR spectra obtained for the blank silane solution, for the solution modified with CeO 2 and solutions modified with CeO 2 previously treated with cerium ions are depicted in Fig. 6. For the reference silane solution one resonance composed of three peaks was detected at shifts of ppm, ppm and ppm. This resonance seems to correspond to the presence of hydrolysed silane molecules containing two hydrolysed (Si OH) components and probably one Si O Si component. The addition of CeO 2 has little effect on these spectra, inducing only a small shift to the right side of the spectra. The new peak positions were ppm, ppm and ppm. Based on literature [55 56], shifts towards more negative resonances in the 29 Si spectra of silane molecules are related with a higher degree of hydrolysis and crosslinking. Nevertheless, the changes observed here are too small to be assigned to a higher degree of hydrolysis. The major differences were detected for the solution containing ceria nanoparticles activated with cerium ions. Three resonances were detected. The one around ppm can be assigned to the fully hydrolised silane groups Si (OH) 3. The central resonance in the range 49 to 51 ppm can be assigned to silane molecules containing one or two silanol groups Si(OH) 2 and to a higher contribution of Si O Si. The resonances at ppm, may be attributed to the presence of condensed species of higher molecular weight, probably containing a high content of cross-linked Si O Si components. This trend was also reported in literature for another silane molecule [55]. The NMR results indicate two important features; (i) the CeO 2 nanoparticles have little ability to influence the formation of silanol reactive groups s; (ii) the activation of the CeO 2 nanoparticles with Ce(III) ions strongly influences the chemistry of the silane solution, leading by one hand, to a higher degree of reactive Si(OH) 3 silanol groups and, on the other hand, to the formation of more condensed species. These ones may contain the ceria-activated nanoparticles, since the cerium groups in their surface seem to have the ability to establish bonds with silicon. These results are, at our knowledge by the first time reported for silane solutions containing ceria nanoparticles activated with cerium(iii) ions. These results are in agreement with a previous work [57], in which, by XPS analysis, it was demonstrated that the direct addition of cerium(iii) ions to the silane solutions improved the degree of cross-linking in silane coatings applied on magnesium alloys (AZ31) Corrosion behaviour of silane coatings doped with CeO 2 nanoparticles Fig Si NMR spectra the for the blank silane, for the silane-modified with CeO 2 and for the silane-modified with CeO 2 previously treated with cerium ions this case, the Si2p spectrum matches with that of the blank silane film Fig. 5a. Therefore, the Si2p core level clearly supports the formation of new bonds after the silanization of the cerium-activated nanoparticles Analytical characterisation of the silane solutions In previous work [22 23] it was reported that silane coatings containing ceria and silica nanoparticles modified with cerium ions provided very good barrier properties and improved corrosion protection comparatively to non-modified silane coatings, when applied on galvanized steel substrates. It was shown that systems modified with ceria nanoparticles were more effective than those modified with silica nanoparticles. Previous results also suggested that the cerium-activated ceria nanoparticles also provided self-healing ability when defects are formed in the silane coating. Self-healing can be defined as the partial recovery of the protective properties of the coated system when damaged. The most suitable way to evaluate self-healing ability is via formation of an artificial defect on the surface and monitoring of the electrochemical behaviour using for example electrochemical impedance spectroscopy and the SVET technique [58]. In the present work, the coated samples were immersed for one day and after this period a defect was created on the surface as described in the experimental section. Fig. 7 shows the impedance spectra obtained during one-week immersion period. During the first hours of immersion the spectra Fig. 7. EIS Bode plots obtained for the silane coating containing CeO 2 nanoparticles modified with cerium ions. Spectra were obtained during immersion in M NaCl, before and after defect formation.

6 5184 M.F. Montemor et al. / Electrochimica Acta 54 (2009) Fig. 8. Fitting of the EIS Bode plots obtained for the silane coating containing CeO 2 nanoparticles modified with cerium ions. (a) Spectra obtained one day after the formation of the defect and during immersion in M NaCl. Spectra were fitted using a two-time constants equivalent circuit (b). R1 corresponds to the solution resistance; CPE1 and R2 were used for the fitting of the high frequency region and correspond to the silane coating capacitance and resistance, respectively; CPE2 and R3 were used for the fitting of the low frequency region. R1 = 1560 cm 2 ;R2=898720cm 2 ; CPE1 = 3.45E 8 1 cm 2 s n ; n 1 = 0.93; CPE2 = 3.3E 6 1 cm 2 s n ; R3 = cm 2 ; n 2 = =2E 4. were characterised by a resistive response in the high frequency range related with the solution resistance, followed by a capacitive slope, corresponding to the coating capacitance. Despite a small decay of the phase angle, no clear resistive plateau could be observed, meaning that the coating behaves as a very effective barrier layer. After one day of immersion, the resistive answer in the low frequency region was well-defined, revealing that the coating developed some conductive pathways. The low frequency impedance was around 100 M cm 2. After this time (one day of immersion), a defect was made on the coating surface in order to create a corroding area. One day after the formation of this defect, the impedance showed an important drop and a new time constant, was visible in the low frequency region of the phase angle plot. Two days after the defect formation the impedance values showed a small increase, and, after one week the low frequency impedance increased by about eight times. The impedance results were fitted using different equivalent circuits as proposed elsewhere [59]. These equivalent circuits make use of constant phase elements, which correspond to a capacitor when the CPE exponent (n) is one. For the first day of immersion, only one time constant was considered, corresponding to the coating capacitance (CPE 1 ) and resistance (R 2 ). The Y 0 values of the CPE were nearly constant, around cm 2 s n and the n values were all above After defect formation the spectra were simulated with an equivalent circuit consisting of two time constants. The fitting results Figs. 8 and 9 reveal that after defect formation there is a decrease of the coating resistance (high frequency time constant) by more than one order of magnitude, as expected, since the coating was damaged, but the values remained nearly Fig. 9. Evolution of the EIS fitting parameters for the silane coating containing CeO 2 nanoparticles modified with cerium ions. constant, around 1 M cm 2 until the end of the experiment. The Y 0 value increased about five times. The low frequency behaviour was characterised by resistance values that increased from 0.5 M cm 2 one day of defect formation up to 5 M cm 2 four days after defect formation. The Y 0 value decreased from 3 1 cm 2 s n down to cm 2 s n. The evolution of the fitting parameters shows that the silane coating partially recovers its protective properties. Since the low frequency behaviour can be assigned to the corrosion process occurring at the interface, it is possible to conclude that this process was slowed down. The same trend could not be observed for the films modified with CeO 2 only (Fig. 10). For these films the impedance values slightly decrease after defect formation, but no recovery could be detected. Despite this fact, the values did not decrease below 1 M cm 2, suggesting that the coating was still providing some protection. For the films modified with silica nanoparticles (not shown) an important drop of the impedance values was observed after defect formation and severe deterioration was observed on the surface at the end of the test period. The behaviour of these systems was also investigated using the SVET technique, which allows assessing local electrochemical information in the form of local current density maps over the exposed coated surface. The SVET maps give quantitative information about anodic and cathodic currents densities and show how the anodic vs.

7 M.F. Montemor et al. / Electrochimica Acta 54 (2009) shows that this system provides very effective corrosion protection in the defects present on the surface. The electrochemical tests show that the activation of the ceria nanoparticles with cerium ions results in the formation of a protective coating, either in terms of barrier properties or self-inhibiting corrosion activity. The improved barrier properties can be consequence of a higher degree of reactive silanol groups Si(OH) 3 and more condensed species that may contribute to increase the silane solution viscosity and coating thickness as previously reported [22]. In fact, silane films filled with CeO 2 showed thicknesses around m that increase to values around 5.5 m when the nanoparticles were activated with cerium ions. This increase of thickness can be consequence of the higher degree of polymerisation of the hydrolysed silane molecules in the silane solution as demonstrated by NMR. The ceria-modified films were also thicker than the blank silane film, which showed thicknesses below 1 m [22] The electrochemistry of the nanoparticles in solution Fig. 10. EIS Bode plots obtained for the silane coating containing CeO 2 nanoparticles. Spectra were obtained during immersion in M NaCl, before and after defect formation. cathodic activity behaves during immersion. These local measurements are very useful to understand the ability of the nanoparticles to inhibit corrosion activity in localized defects. Fig. 11 shows the SVET maps obtained on the blank coating, one day after the formation of the artificial defect. The samples showed very intense anodic activity, centred on the round-shaped defect and covering more than one third of the exposed area. A sample coated with the silane film containing ceria nanoparticles (Fig. 12) revealed current intensities that are more than five times lower than those measured in the reference coating, showing that this system provides some degree of corrosion inhibition. However, some new active areas developed in the exposed surface, revealing that the protective properties of the coating tend to decrease. Nevertheless, an important reduction of the corrosion activity was observed, comparatively to the blank coating. The SVET maps obtained on the cerium-treated CeO 2 nanoparticles (Fig. 13) reveal negligible corrosion activity for a period up to four days after the formation of the round-shaped defect. This trend In order to understand the improved corrosion inhibition ability observed for the silane coatings modified with ceria nanoparticles a set of electrochemical tests were performed using pure zinc electrodes immersed in 0.05 M NaCl solution. CeO 2 nanoparticles were added to this solution in order to obtain a concentration of 250 ppm of nanoparticles. In some cases, 0.05 M NaCl solution containing nanoparticles previously treated in the cerium nitrate solution (250 ppm of cerium nitrate) was also used, but only in the near neutral ph range, since in more alkaline solutions there was a severe precipitation and agglomeration of the nanoparticles treated with the cerium ions. One important point related with the use of nanoparticles suspensions is their stability and tendency to form aggregates and flocculates in solution. The Zeta potential is an indicator of the electronic charge on the surface of the nanoparticles, in a specific solvent, and can be used to predict and control the stability of nanoparticles suspensions or emulsions. The Zeta potential is dependent on the ph and by controlling it is possible to tailor the nanoparticles surface for different applications [60]. Generally, the larger the modulus of the Zeta potential, the more likely the suspension is to be stable because the charged particles repel one another, overcoming their natural tendency to aggregate. The Zeta potential of the aqueous suspension of CeO 2 nanoparticles (ph 6) was deter- Fig. 11. SVET maps obtained on the reference silane coating immersed in NaCl 0.05 M, one day after defect formation. Scan size was 2 mm 1.9 mm and the current units are Acm 2.

8 5186 M.F. Montemor et al. / Electrochimica Acta 54 (2009) Fig. 12. SVET maps obtained on the silane coating modified with CeO 2 nanoparticles immersed in NaCl 0.05 M, one day after defect formation. Scan size was 2 mm 1.5 mm and the current units are Acm 2. mined, being around 13.3 ± 2.1 mv. However for suspensions with the same ph, containing the CeO 2 nanoparticles previously activated with cerium ions, there was an increase of the Zeta potential for values of 25.5 ± 2.7 mv. The results indicate that the surface of the CeO 2 nanoparticles is positively charged and that the suspension stability increases after addition of cerium ions. Potentiodynamic polarization curves using pure zinc electrodes were obtained in CeO 2 -containing solutions of ph values ranging from 6 to 11. These ph values intend to simulate the distribution of ph that develops in the anodic areas (ph 5 6) and in the cathodic areas (above 9) as demonstrated elsewhere [24] using the scanning ionic electrode technique (SIET). In near neutral solutions (Fig. 14) it is possible to observe an important shift of the corrosion potential to more negative values in the presence of cerium-activated nanoparticles. Another important feature is that both the anodic and cathodic current densities are affected by the presence of the cerium ions. The most important effect was observed in the cathodic branch for which a marked Fig. 13. SVET maps obtained on the silane coating modified with cerium-treated CeO 2 nanoparticles immersed in NaCl 0.05 M, four days after defect formation. Scan size was mm and the current units are Acm 2.

9 M.F. Montemor et al. / Electrochimica Acta 54 (2009) Fig. 14. Potentiodynamic polarization curves obtained in 0.05 M NaCl solutions at ph 6 in the presence of CeO 2 nanoparticles and CeO 2 nanoparticles previously activated with cerium ions. For comparative purposes a plot is also inserted, in which the potential is depicted as the difference between the imposed potential and the corrosion potential. This approach allows a better separation of the anodic and cathodic polarization effects. diffusion control effect could be observed for the oxygen reduction (Fig. 14). Therefore, it seems that in neutral ph the presence of cerium ions induce an important polarization of the cathodic processes. This trend has been reported earlier in literature for corrosion inhibition of aluminium alloys [61] and stainless steel [62] in the presence of cerium ions. When the ph of the NaCl solution containing the CeO 2 nanoparticles was shifted to more alkaline values important features could be observed Fig. 15. For zinc electrodes exposed to 0.05 M NaCl solution of ph 10 and 11 without nanoparticles, there is a shift of the corrosion potential by about 45 mv towards more cathodic values, with a unit increase of the ph. Simultaneously, the current density decreases and the kinetics of the anodic process change, showing a more marked diffusion controlled processes. This is consequence of the formation of a layer of zinc corrosion products, which becomes more protective, decreasing the area available for the anodic processes. In the presence of CeO 2 nanoparticles there is an opposite trend, concerning the corrosion potential evolution. The corrosion potential increases by about 70 mv towards more positive values as the ph shifts from 9 to 11 and, simultaneously, passive plateaux could be detected in all cases. At the end of these plateaux it was possible to observe a breakdown potential and a strong increase of the anodic current density. The current density just before the breakdown potential was about 2 Acm 2 in the presence of CeO 2 (ph 9, 10 and 11). After the breakdown the current increases by more than two orders of magnitude for all the samples. The breakdown potential, increased with ph. Moreover, the potential ranges characteristic of these plateaux widen with ph. For ph 9, the passive plateau ranged for about 144 mv; for ph 10 for about 190 mv and for ph 11 for about 200 mv. The current densities slightly decrease Fig. 15. Potentiodynamic polarization curves obtained in 0.05 M NaCl solutions with different ph in the presence of CeO 2 nanoparticles and in 0.05 M NaCl solutions without nanoparticles. from ph 9 to 11, but the values remained below 2 Acm 2, therefore being characteristic of a passive film. The presence of ceria nanoparticles affects the kinetics of the anodic processes decreasing the current density generated by the anodic reactions occurring on the zinc surface. The beneficial effect of ceria has been reported in literature and attributed to different effects. For example, Alves et al. [63] consider that CeO 2 is an oxidant, and this is the reason for the potential shift for more positive values. Nikolova et al. [13] attributes the beneficial effect of cerium oxides in the corrosion behaviour of stainless steel to a more protective and Cr-rich surface layer as well as a possible decrease of the oxygen content due to the high affinity of ceria towards oxygen. In the present work, the polarization curves under alkaline media, suggest that CeO 2 polarises the anodic reaction, stabilizing the passive layer formed on the zinc surface. The XPS analysis of the Zn electrodes polarised in a solution of 0.05 M NaCl containing 250 ppm of ceria, at ph 10, from the open circuit potential up to 0.7 V, thus in the passive domain, revealed the features depicted in Fig. 16. The Ce 3d 5 ionisation could be detected, revealing that cerium species are present on the surface. However, the spectrum has a very low signal to background ratio, revealing that cerium is vestigial. Consequently, accurate binding energies could not be obtained. Nevertheless, the shape of the spectra indicates the presence of Ce(IV) species of the ceria nanoparticles. On the other hand, the Zn LMM ionisation reveals the presence of different peaks, which can be assigned to the presence of zinc hydroxides (predominant species) and Zn oxides in agreement with literature [64]. However, other species [64] like ZnCl +, Zn(OH) +,Zn 5 (OH) 8 Cl 2 and zinc carbonates can also co-exist in the corrosion products layer. The most interesting feature could be observed at approximately 494 ev. There is a peak, more intense for the electrode polarised in CeO 2 -containing solutions. This peak can be attributed to the

10 5188 M.F. Montemor et al. / Electrochimica Acta 54 (2009) was a decrease of the current density in the passive domain, too. The results reported in the present work, show that ceria nanoparticles, can be used for storage of cerium ions, but, it also demonstrates that the ceria nanoparticles, by themselves also play an important role and are able to improve the barrier properties of the silane coating and to decrease the corrosion activity both in zinc electrodes exposed to their suspensions and in silane coatings filled with the nanoparticles. The loading with cerium ions further improves the anti-corrosion performance, leading to very effective corrosion inhibition in artificially induced defects. One of the innovative results presented in this work is that the cerium ions, loaded in the nanoparticles (either ceria or silica) establish bonds with the siloxane network. A new peak, detected by XPS, at binding energies lower than those characteristic of SiO 2 was observed in the silane coating, revealing the presence of a cerium silicate like bond. Therefore, the nanoparticles will be trapped in the silane coating via Si O Ce bonding. This may help to decrease the porosity and conductivity of the siloxane network and to improve the barrier properties. Another new result is related with the effect of the cerium(iii) ions in the chemistry of the silane solution, since more reactive silanol groups are formed and, simultaneously, new condensed species, which may contain groups bonded to the cerium ions deposited the ceria surface. The polarization curves obtained in zinc coupons exposed to ceria suspensions of different phs also evidenced a new result: in the absence of nanoparticles, the corrosion potential decreases with ph, as expected; however, in ceria suspensions there is an important polarization of the anodic reactions and the formation of a passivation plateau with low current density, which can be attributed to more stable layers of passive zinc corrosion products. This was supported by the analytical results that revealed the presence of cerium species on the surface of the zinc electrodes at the end of the polarization experiments. The beneficial effect of ceria nanoparticles modified with cerium ions in the silane coatings is therefore twofold: - increase of barrier properties. - inhibition of the corrosion activity due to the formation of a more stable and protective layer of corrosion products. The mechanism of improved corrosion protection can be described as follows: Fig. 16. Ce3d5 ionisation and Zn LMM ionisation obtained on zinc electrodes polarised in the CeO 2-containing solution of ph 11. The polarization experiment was stopped at 0.7 V (SCE). Zn Zn bonds or eventually to the Zn Ce bond. It was not possible to find any reference in literature for the assignment of such binding energies, but the increased intensity may be related with the precipitation of mixed Zn Ce species. The literature [65,66] states that a mixed Zn Ce layer of oxides/hydroxides may form over zinc hydroxides The mechanism of corrosion protection Ceria nanoparticles can be used as effective fillers in many coatings and have potential to be used as nanoreservoirs for the encapsulation of corrosion inhibitors either organic or inorganic. For example 8-hydroxiquinoline was successfully loaded inside CeO 2 nanoparticles with improved corrosion protection of aluminium alloys [67]. Furthermore, in that work the authors showed that in the presence of CeO 2 nanoparticles only, there (i) In the presence of a defect, the zinc substrate dissolves (Eq. (1)) and, simultaneously, oxygen is reduced releasing hydroxyl ions (Eq. (2)), which locally increase the ph to values up to as demonstrated in previous works [24]. Zn(s) Zn e (1) O 2 + 2H 2 O + 4e 4OH (2) (ii) Due to the local alkalinisation, the Si O Si network of the silane coating starts to decompose because silica becomes unstable at ph above 9. There is swelling of the silica network and in a first step the formation of silicates (Eq. (3)), which may provide some protection in small defects or pores. In fact the literature states that a passive film composed of Zn(OH) 2, ZnSi 2 O 5 may form on the active areas and that preferential precipitation of zinc silicate occurred on the defects of the passive film, delaying the corrosion activity [65]. SiO 2 + 2OH SiO H 2 O (3)

11 M.F. Montemor et al. / Electrochimica Acta 54 (2009) However, as the ph becomes more alkaline, the deterioration of the silica network starts to release the CeO 2 nanoparticles, which are, in contrast, very stable under alkaline ph. (iii) The CeO 2 nanoparticles, which have a very high affinity for oxygen and for charged ions, like Zn 2+, in order to compensate defects in the oxygen sub-lattice [68,69], can precipitate together with the zinc corrosion products, leading to a more stable and protective surface layer, which polarises the anodic reactions and inhibits the corrosion activity as demonstrated by the d.c. polarization experiments. (iv) Additionally, cerium ions released from the nanoparticles may precipitate in the cathodic sites, hindering the cathodic reactions. As consequence of the above-mentioned steps, in the presence of ceria nanoparticles a more stable surface layer is formed, the anodic reactions are polarised, the area available for the anodic and cathodic reactions decreases and the overall corrosion activity is reduced. 4. Conclusions Pre-treatments based on the use of bis-sulfur silane solutions modified with CeO 2 nanoparticles are effective for corrosion protection of galvanized steel substrates. The positive impact, both in the barrier properties and corrosion inhibition, is significantly improved with the pre-treatment of the nanoparticles in cerium nitrate solution. In this way, CeO 2 nanoparticles are used as effective nano hosts for the storage of cerium ions. The cerium ions loaded in the ceria nanoparticles have the ability to change the silane solution chemistry, promoting the formation of reactive silanol groups and also of more condensed species. In addition, the cerium ions may form bonds with the silane layer. This allows the storage of the nanoparticles inside the silane network and avoids run-off of the ceria-loaded nanoparticles from the silane coatings. The corrosion inhibition mechanism is linked with the formation of a more stable and protective layer of zinc oxides and hydroxides formed under the alkaline environment that characterises the cathodic processes in coated zinc substrates. The combination of ceria and cerium ions creates a synergistic effect that significantly improves the corrosion inhibition capability of thin hybrid silane films. Acknowledgments The authors acknowledge Prof. José Ascenso (IST) for the NMR measurements. The financial support from the projects PTDC/CTM/66041/2006 and PTDC/CTM/65632/2006 is also acknowledged. References [1] M. Aresta, A. Dibenedetto, C. Pastore, C. Cuocci, B. Aresta, S. Cometa, E. De Giglio, Catal. Today 137 (2008) 125. [2] E.V. Tsipis, V.V. Kharton, J. Sol. State Electrochem. 12 (2008) [3] M. Dudek, J. Eur. Ceram. Soc. 28 (5) (2008) 965. [4] T. Mori, R. Buchanan, D.R. Ou, F. Ye, T. Kobayashi, J.D. Kim, J. Zou, J. Drennan, J. Sol. State Electrochem. 12 (2008) 841. [5] A. Trinchi, Y.X. Li, W. Wlodarski, S. Kaciulis, L. Pandolfi, S. Viticoli, E. Comini, G. Sberveglieri, Sens. Actuators B: Chem. 95 (2003) 145. [6] N. Izu, T. Itoh, W. Shin, I. Matsubara, N. Murayama, Sens. Actuators B: Chem. 123 (2007) 407. [7] Y. Yang, J.L. Ong, J. Tian, Biomaterials 24 (2003) 619. [8] M. Nawa, K. Yamada, G. Pezzotti, Key Eng. Mater. 361 (2008) 813. [9] T. Masui, M. Yamamoto, T. Sakata, H. Mori, G.-Y. Adachi, J. Mater. Chem. 10 (2000) 353. [10] S.M.A. Shibli, F. Chacko, Surf. Coat. Technol. 202 (2008) [11] S. Zhang, M. Li, J.H. Yoon, T.Y. Cho, C.G. Lee, Y. He, Appl. Surf. Sci. 254 (2008) [12] Y. Wang, Z. Wang, Y. Yang, W. Chen, Intermetallics 16 (2008) 682. [13] D. Nickolova, E. Stoyanova, D. Stoychev, I. Avramova, P. Stefanov, Surf. Coat. Technol. 202 (2008) [14] R. Xu, J. Wang, L. He, Z. Guo, Surf. Coat. Technol. 202 (2008) [15] A.S. Hamdy, Mater. Lett. 60 (2006) [16] M.F. Montemor, A.M. Simões, M.G.S. Ferreira, M.J. Carmezim, Appl. Surf. Sci. 254 (2008) [17] M.F. Montemor, A.M. Simões, M.G.S. Ferreira, Prog. Org. Coat. 43 (2001) 274. [18] X. Zhong, Q. Li, J. Hu, Y. Lu, Corros. Sci. 50 (2008) [19] H. Hasannejad, T. Shahrabi, A.S. Rouhaghdam, M. Aliofkhazraei, E. Saebnoori, Appl. Surf. Sci. 254 (2008) [20] A.S. Hamdy, D.P. Butt, A.A. Ismail, Electrochim. Acta 52 (2007) [21] A. Nazeri, P.P. Trzaskoma-Paulette, D. Bauer, J. Sol-Gel Sci. Technol. 10 (1997) 317. [22] M.F. Montemor, M.G.S. Ferreira, Prog. Org. Coat. 63 (2008) 330. [23] M.F. Montemor, M.G.S. Ferreira, Electrochim. Acta 52 (2007) [24] M.F. Montemor, W. Trabelsi, S.V. Lamaka, K.A. Yasakau, M.L. Zheludkevich, A.C. Bastos, M.G.S. Ferreira, Electrochim. Acta 53 (2008) [25] A. Sabata, W.J. Van Ooij, R.J. Koch, J. Adhesion Sci. Technol. 7 (1993) [26] F. Deflorian, S. Rossi, L. Fedrizzi, Electrochim. Acta 51 (2006) [27] A. Frignani, F. Zucchi, G. Trabanelli, V. Grassi, Corros. Sci. 48 (2006) [28] G. Pan, D.W. Schaefer, Thin Solid Films 503 (2006) 259. [29] J. Flis, M. Kanoza, Electrochim. Acta 51 (2006) [30] T. Van Schaftinghen, C. Le Pen, H. Terryn, F. Hörzenberger, Electrochim. Acta 49 (2004) [31] A. Franquet, C. Le Pen, H. Terryn, J. Vereckeen, Electrochim. Acta 48 (2003) [32] A. Franquet, H. Terryn, P. Bertrand, J. Vereckeen, Surf. Interface. Anal. 34 (2002) 25. [33] W.J. van Ooij, D. Zhu, M. Stacy, A. Seth, T. Mugada, J. Gandhi, P. Puomi, Tsinghua Sci. Technol. 10 (2005) 639. [34] V. Subramanian, W.J. van Ooij, Corrosion 54 (1998) 204. [35] W.J. van Ooij, G.P. Sundararajan, J. Corros. Sci. Surf. Eng. 2 (2001), paper 14. [36] G.P. Sundararajan, W.J. van Ooij, Surf. Eng. 16 (2000) 315. [37] T. van Schaftingher, C. Le Pen, H. Terryn, F. Horzenberger, Electrochim. Acta 49 (2004) [38] M.F. Montemor, M.G.S. Ferreira, R.G. Duarte, A.M.P. Simões, Silanes and rare earth salts as chromate replacers for pre-treatments on galvanised steel, Electrochim. Acta 49 (2004) [39] M.F. Montemor, M.G.S. Ferreira, Surf. Interface Anal. 36 (2004) 773. [40].A. Cabral, R.G. Duarte, M.F. Montemor, M.L. Zheludkevich, M.G.S. Ferreira, Corros. Sci. 47 (2005) 869. [41] W. Trabelsi, L. Dhouibi, E. Triki, M.G.S. Ferreira, M.F. Montemor, Surf. Coat. Technol. 192 (2005) 284. [42] D. Zhu, W.J. van Ooij, Corros. Sci 45 (2003) [43] W.J. van Ooij, D. Zhu, Corrosion 57 (2001) 413. [44] A.M. Beccaria, L. Chiaruttini, Corros. Sci. 41 (1999) 885. [45] A. Beccaria, G. Padeletti, G. Montesperlli, L. Chiaruttini, Surf. Coat. Technol. 111 (1999) 240. [46] D.Q. Zhu, W.J. van Ooij, Electrochim. Acta 49 (2004) [47] D.Q. Zhu, W.J. van Ooij, Prog. Org. Coat 49 (2004) 42. [48] V. Palanivel, D.Q. Zhu, W.J. van Ooij, Prog. Org. Coat 47 (2003) 384. [49] L. Liu, J.-M. Hu, J.-Q. Zhang, C.-N. Cao, Electrochim. Acta 52 (2006) 538. [50] M.F. Montemor, W. Trabelsi, M. Zheludevich, M.G.S. Ferreira, P. Cecílio, Prog. Org. Coat. 57 (2006) 67. [51] W. Trabelsi, P. Cecilio, M.G.S. Ferreira, M.F. Montemor, Prog. Org. Coat. 54 (2005) 276. [52] X. Yu, G. Li, J. Alloys Compd. 364 (2004) 193. [53] Y. Kobayashi, Y. Fujiwara, J. Alloys Compd (2006) [54] H. Cui, G. Hong, X. Wu, Y. Hong, Mater. Res. Bull. 37 (2002) [55] De Graeve, E. Tourwé, M. Biesemans, R. Willem, H. Terryn, Prog. Org. Coat. 63 (2008) 38. [56] D. Hoebbel, M. Nacken, H. Schmidt, J. Sol-Gel Sci. Technol. 19 (2000) 305. [57] M.F. Montemor, M.G.S. Ferreira, Prog. Org. Coat. 60 (2007) 228. [58] M.L. Zheludkevich, K.A. Yasakau, A.C. Bastos, O.V. Karavai, M.G.S. Ferreira, Electrochem. Commun. 9 (2007) [59] W. Trabelsi, E. Triki, L. Dhouibi, M.G.S. Ferreira, M.L. Zheludkevich, M.F. Montemor, Surf. Coat. Technol. 200 (2006) [60] S. Patil, A. Sandberg, E. Heckert, W. Self, S. Seal, Biomaterials 28 (2007) [61] A.J. Davenport, H.S. Isaacs, M.W. Kendig, Corros. Sci. 32 (1991) 653. [62] S. Virtanen, M. Ives, G. Sproule, P. Schmuki, M. Graham, Corros. Sci. 39 (1997) [63] V.A. Alves, L.A. da Silva, J.F.C. Boodts, Electrochim. Acta 44 (1998) [64] A. Amirudin, D. Thierry, Prog. Org. Coat. 28 (1996) 59. [65] K. Aramaki, Corros. Sci. 44 (2002) [66] K. Aramaki, Corros. Sci. 44 (2002) [67] I. Kartsonakis, I. Daniilidis, G. Kordas, J. Sol-gel Sci. Technol. 48 (2008). [68] B. Cabot, A. Foissy, J. Mater. Sci. 33 (1998) [69] R. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Sol. State Ionics 151 (2002) 235.