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Electrochimica Acta 55 (2010) 5100–5109

Contents lists available at ScienceDirect

Electrochimica Acta

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ffect of cerium (IV) ions on the anticorrosion properties ofiloxane-poly(methyl methacrylate) based film applied on tin coated steel

.H. Suegamaa, V.H.V. Sarmentob, M.F. Montemorc,1, A.V. Benedettib,1, H.G. de Meloa,1, I.V. Aokia,1,

.V. Santilli b,∗

Departamento de Engenharia Química, Escola Politécnica, Universidade de São Paulo, CP 61548, 05424-970 São Paulo, SP, BrazilDepartamento Fısico-Química, Instituto de Química, Universidade Estadual Paulista, UNESP, CP 355, 14801-970 Araraquara, SP, BrazilICEMS, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

r t i c l e i n f o

rticle history:eceived 15 September 2009eceived in revised form 1 April 2010ccepted 3 April 2010vailable online 10 April 2010

a b s t r a c t

This work investigates the influence of the addition of cerium (IV) ions on the anticorrosion propertiesof organic–inorganic hybrid coatings applied to passivated tin coated steel. In order to evaluate thespecific effect of cerium (IV) addition on nanostructural features of the organic and inorganic phases ofthe hybrid coating, the hydrolytic polycondensation of silicon alkoxide and the radical polymerization

eywords:ybrid organic–inorganic coatingsISerium (IV) ions

of the methyl methacrylate (MMA) function were induced separately. The corrosion resistance of thecoatings was evaluated by means of linear polarization, Tafel type curves and electrochemical impedancemeasurements. The impedance results obtained for the hybrid coatings were discussed based on anelectrical equivalent circuit used to fit the experimental data. The electrochemical results clearly showedthe improvement of the protective properties of the organic–inorganic hybrid coating mainly when the

the onsely

nticorrosion coatingsin coated steel

cerium (IV) was added toof a more uniform and de

. Introduction

Inorganic–organic hybrids have applications in many branchesf materials chemistry because they are simple to process andmenable to design on a molecular scale. Moreover, they cane easily processed to form coatings and thin films, and haveeen successfully used to improve the corrosion resistance of sev-ral metallic substrates [1–6]. The corrosion resistance providedy a coating applied onto a metal depends upon its barrier anddherence properties. Therefore, one possible way to enhance theorrosion protection afforded by the hybrid coating is by improvingts degree of polymerization and cross-linking nodes density.

In organic chemistry, polymerization initiated by a reactionetween an oxidizing and a reducing agent is termed redox poly-erization. In this context the Ce(IV) ions, in the form of cerium (IV)

mmonium nitrate (CAN), cerium (IV) ammonium sulfate, ceriumIV) sulfate or cerium perchlorate, have been used in the oxidation

tep for the production of different organic compounds [7] suchs acryl amide [8], acrylonitrile [9,10], or ketones [11], and partic-larly in vinyl polymerization [12–14]. In the presence of Ce(IV),ydroxyl-ended organic molecules form complexes, which then

∗ Corresponding author. Tel.: +55 16 3301 6645; fax: +55 16 3301 6692.E-mail address: santilli@iq.unesp.br (C.V. Santilli).

1 ISE members.

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.04.002

rganic phase solution precursor, which seemed to be due to the formationreticulated siloxane-PMMA film.

© 2010 Elsevier Ltd. All rights reserved.

decompose unimolecularly to produce a free radical, Ce(III) ionsand protons, with the free radical initiating polymerization [15].Ce(IV) ions have also already been successfully employed to pro-duce block copolymers [16–18]. For such polymerization it has beenreported that the addition of Ce(IV) ions is beneficial up to a certainamount, above which oxidative termination leads to a decrease inthe yield of the process [18].

Addition of Ce(IV) ions can improve the anticorrosion propertiesof silane coatings [19–23], and it has been reported that there is amaximum amount of cerium that can be added to the hydrolysissolution with positive results [24]. In recent work using 29Si NMR,Suegama et al. [20] demonstrated that Ce(IV) increases the retic-ulation of the siloxane chains. The authors suggested that in thepresence of Ce(IV) ions, alcohol molecules of the hydrolysis solu-tion can be incorporated in the siloxane chains, increasing theirlength. All the evidences indicate that the addition of Ce(IV) ions tothe initial solution leads to the formation of free radicals, improvingthe reticulation of the silane films.

The aim of this work was to study the influence of the addi-tion of cerium (IV) ions in the precursor solution, on the corrosionresistance of hybrid organic–inorganic coatings applied onto com-

mercial passivated tin coated steel. The anticorrosion propertiesof Ce(IV) doped and undoped hybrid coated samples were inves-tigated using classical electrochemical techniques, after differentperiods of immersion in 3.5 wt.% NaCl solution. The chemical statesof the species present in the hybrid film were explored using 13C

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nd 29Si NMR. To evaluate the efficiency of the modifier in the poly-erization of each individual phase, coatings were produced where

erium (IV) ions were added to the precursor solution of either therganic or the inorganic phase.

. Experimental

.1. Solutions

All chemicals used were commercially available. 3-ethacryloxypropyltrimethoxysilane, MPTS (Fluka, 98% purity),

etraethyl orthosilicate, TEOS (Aldrich, 98% purity), ethanolMallinckrodt, 99.8% purity), and ceric ammonium nitrate CANVetec, 99% purity), were used as received. Methyl methacrylate,

MA (Fluka, 99% purity), was distilled to remove the polymer-zation inhibitor (hydroquinone) and impurities, and stored in

freezer prior to use. Benzoyl peroxide, BPO (Reagen), wasecrystallized from ethyl alcohol.

Formation of the sol–gel coating was carried out in two mainteps. The polymerization of organic and inorganic phases wasnitially begun separately, and then the mixtures were com-ined. The hydrolysis/polycondensation reaction involving theilicon alkoxide was performed by mixing water, acidified withCl (pH = 1), to the solution of TEOS and MPTS in ethanol main-

ained at 60 ◦C. This inorganic phase precursor solution wastirred for 1 h. The radical polymerization of the organic phaseas performed in a separate receptacle, where MMA and BPOere mixed and stirred together at room temperature until totalomogenization of BPO. The contents of the two receptacles werehen mixed and stirred for 5 min, at 45 ◦C, forming a transpar-nt sol in which the tin coated steel samples were immersed.he hybrid sol used in the coatings was prepared using theollowing molar ratios: TEOS:MPTS:MMA = 2:1:6; H2O:Si = 3.5:1;thanol:H2O = 1:2; BPO:MMA = 1:100.

In order to determine the influence of Ce(IV) on the structurend anticorrosion properties of the film, hybrid sols were preparedy dissolving 500 ppm of CAN in either the inorganic or the organicrecursor solution. The corresponding films will be denoted IP andP coating, respectively. The amount of Ce(IV) was chose basedn the fact that hybrid coating containing 500 ppm revealed theighest corrosion protection performance, comparatively to coat-

ng containing 100 or 1000 ppm of such ion. Blank films (no Ce(IV)ddition) were also prepared and defined as undoped films – UF,or reference.

.2. Substrate treatment

The commercially available (L-type tinplate from CSN, Brazil)in coated steel samples were degreased ultrasonically in acetonend thoroughly rinsed with distilled water. The substrates coupons2 cm × 2 cm) were then dipped in the hybrid sol, and withdrawnrom the solution at a constant rate of 14 cm min−1. The samplesere then air-dried for approximately 10 min. This process was

arried out 3 times. Afterwards, the coated substrates were heatedt 55 ◦C for 24 h, and cured at 160 ◦C for 3 h. This procedure favorshe breakdown of BPO and the production of free radicals, leadingo the polymerization of MMA monomers and densification of theel network [25]. Homogeneous, transparent and crack free filmsere obtained using all the formulations, having a thickness around

.5 �m as determined by profilometry (Talystep, Taylor & Hobson).

.3. Electrochemical techniques

The corrosion resistance of hybrid coated and uncoated tinoated steel was evaluated by means of electrochemical measure-

Acta 55 (2010) 5100–5109 5101

ments carried out in 400 mL of naturally aerated and unstirred3.5 wt.% NaCl solution. An Ag/AgCl/KClsat electrode, connected tothe working electrolyte through a Luggin capillary, was used asreference, and a Pt network as counter electrode. The workingelectrode was mounted in an EG&G electrochemical flat cell, expos-ing an area of 1 cm2 to the electrolyte. Open circuit potential(EOC versus time) and electrochemical impedance spectroscopy(EIS) measurements were carried out using an EG&G Model 283potentiostat/galvanostat coupled to a Solartron-SI1255 frequencyresponse analyzer. The EIS diagrams were acquired for immer-sion times up to 240 h, and were performed applying a 10 mV(rms) sinusoidal perturbation signal to the EOC, from 5 × 104

down to 5 × 10−3 Hz with 7 points per frequency decade. The firstimpedance measurement was obtained after 24 h of immersionin the test solution. The impedance data were fitted using the Z-View software, version 3.1c. Linear polarization curves from −20to +20 mV versus EOC/Ag/AgCl/KClsat, at a scan rate of 0.167 mV s−1,were recorded in the electrolyte solution for immersion times iden-tical to those of the EIS measurements. Polarization curves (Tafeltype plots) were recorded for all samples in a potential rangefrom −150 to +350 mV versus EOC/Ag/AgCl/KClsat, at a scan rate of0.5 mV s−1, after 3 h of immersion.

2.4. Analytical techniques

For NMR analysis, unsupported hybrid materials were obtainedby drying the sols in a Petri dish at 65 ◦C for 24 h. After the curingprocess, at 160 ◦C for 3 h, the films were detached from the dish.Solid-state 29Si and 13C magic-angle spinning nuclear magneticresonance (MAS-NMR) spectra were recorded using a VARIAN spec-trometer operating at 300 MHz and 7.05 T. The Lamor frequenciesfor 29Si and 13C were 59.59 and 75.42 Hz, respectively. The spectrawere obtained from the Fourier transformation of the free inductiondecays (FID), following a single �/2 excitation pulse and a dead timeof 2 s. Chemical shifts were referenced to tetramethylsilane (TMS),used as external standard. Proton decoupling was always used dur-ing spectra acquisition. Because of the high sensitivity of the 29Siand 13C NMR measurements, the uncertainty in the chemical shiftvalues was less than 0.2 ppm.

3. Results

3.1. Electrochemical characterization

XPS examination of the commercial passivated tin coated steelrevealed a stratified structure. From top to bottom, it was com-posed by a thin SnO2 passivating layer (≈0.02 �m) followed by atin layer (0.8–1.5 �m) a mixed tin/iron alloy (0.07–0.15 �m) andfinally the underlying steel (150–250 �m). For the sake of simplic-ity, hereafter, this whole structure will be denominated tin coatedsteel.

The open circuit potentials (EOC) measured as a function ofimmersion time for the blank tin coated steel and for couponscoated with hybrid films produced according to the different pro-tocols are presented in Fig. 1. After 3 h of immersion in the 3.5 wt.%NaCl solution, the EOC values for hybrid coated samples weremore positive than those for the commercial substrate. Additionof cerium (IV) ions further increased the EOC indicating a noblercharacter of the IP and OP coated samples, compared with the UFcoated sample.

Linear polarization curves were recorded for the hybrid coatedsubstrates after different immersion times. For 3 h of immersion,the data confirmed the trend previously observed for EOC evolution,and showed a decrease in polarization resistance (Rp) following theorder: OP (7.7 M� cm2) > IP (2.4 M� cm2) > UF (1.8 M� cm2) > tin

5102 P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109

Table 1Polarization resistance, Rp, and |Z| values at 5 mHz obtained after different immersion times in 3.5 wt.% NaCl solution.

Time (h) IP OP UF

|Z| (M� cm2) Rp (M� cm2) |Z| (M� cm2) Rp (M� cm2) |Z| (M� cm2) Rp (M� cm2)

24 1.1 1.7 2.4 4.4 0.59 0.7648 0.85 1.2 2.3 4.2 0.26 0.1172 0.44 0.66 1.3 2.8 0.15 0.08596 0.53 0.66 1.3 1.4 – –

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144 0.32 0.34 0.98168 0.20 0.23 0.90192 0.19 0.19 0.79240 0.13 0.14 0.74

oated steel (0.013 M� cm2). The values of the Rp, estimated fromhe experimental data for immersion times equal to and higherhan 24 h are presented in Table 1. Samples UF (72 h) and IP (240 h)howed Rp values one order of magnitude higher than the tin coatedteel (0.013 M� cm2 after 3 h immersion), and for 72 h OP showedp value one order of magnitude higher than the UF sample. Theeneficial effect of Ce(IV) ions addition was reflected in the higherp for the doped films, with the highest value being obtained for theample coated with the OP film, which showed a Rp value fivefoldigher than IP sample after 240 h immersion. On the other hand theF sample presented an elevated degree of degradation after 72 hf test and its electrochemical behavior was no longer followed foronger immersion periods.

Electrochemical impedance spectroscopy (EIS) diagrams within coated steel and hybrid coated samples were obtained aftertabilization of the EOC, and for different immersion times. Thexperimental and fitted diagrams and the electrical equivalentircuits (EEC) used to fit the experimental data are presented inigs. 2–5. The impedance diagrams are represented as Nyquist andode (−� and log |Z| versus log(f)) plots and show that for all theamples impedance decreases as immersion time increases. For thein coated steel, a low impedance modulus (10 k� cm2) and onlyne time constant was observed after 3 h of immersion (Fig. 2And B), which was attributed to the SnO2 passivating top layeresponse. After 24 h, two time constants were observed, the onen the high frequency (HF) region being related to the SnO2 topayer, and the one in the middle frequency domain being related

o the charge transfer reaction, involving the underlying metallicubstrate (tin + tin/iron + steel), since the SnO2 passivating layer isorous. After 48 h, both time constants overlapped, indicating highegradation of the SnO2 passivating layer. When the experimentas finished, red corrosion products were observed on the elec-

ig. 1. Open circuit potential (EOC) versus time for commercial tin coated steel andamples coated with UF, IP and OP hybrids. Data acquired in aerated and unstirred.5 wt.% NaCl solution, at 25 ◦C.

1.3 – –0.79 – –0.77 – –0.78 – –

trode surface, indicating that at certain time during the experimentiron is dissolved and its diffusion to the surface occurred to formiron oxides. When bare steel substrate (without the tin coating) wasused (Fig. 2A and B) only one time constant with the phase anglemaximum located at approximately 10 Hz was detected, and theimpedance modulus at 5 mHz was 2.2 k� cm2 after 3 h of immer-sion in the electrolyte. This impedance value is lower than thatmeasured for the passivated tin coated steel, at the same immer-sion time, and the phase angle maximum is similar to that observedfor commercial tin coated steel in the middle frequency range afterthe SnO2 top layer degradation, confirming that the second timeconstant observed for passivated tin coated steel is related to theunderlying metallic substrate.

In agreement with the Rp experiments (Table 1), samples coatedwith hybrid films containing Ce(IV) ions (Figs. 4 and 5) exhib-ited higher impedance values (higher impedance modulus andslower degradation rate) than those coated with the UF (Fig. 2Cand D), indicating improved corrosion resistance performance. Forthe Ce(IV) doped hybrid coatings, the HF time constant is shiftedto higher frequencies, whereas the LF phenomena are displaced tolower frequencies, comparatively to the UF coated samples. Thesefeatures indicate that electrolyte uptake and further attack of themetallic substrate are delayed by the enhanced barrier propertiesof the Ce(IV) doped hybrid coating. On the other hand, the samplecoated with the OP film presented the highest overall impedancevalues. The ability of this sample to maintain higher phase anglesvalues at higher frequencies (Fig. 4B) and the increased totalimpedance (Fig. 4A and C), even after long immersion periods,indicates that this hybrid film possesses the best anticorrosionproperties.

Table 1 shows that Rp and the impedance modulus |Z| calcu-lated at 5 mHz for hybrid coated samples immersed in 3.5 wt.% NaClsolution presented the same trend.

For more accurate interpretation of the EIS data, the exper-imental plots measured for hybrid coated samples at differentimmersion times were fitted using electrical equivalent circuits(EEC). The fitting procedure revealed that two different EECs,showed in Fig. 3, were necessary to take into account the evolu-tion of the EIS response of the hybrid coated samples: one EEC withtwo time constants (Fig. 3A), and another with three time constants(Fig. 3B). In the circuit depicted in Fig. 3A, R1//CPE1 and R2//CPE2 arecascade sub-circuits corresponding to the response of the HF andLF time constants, attributed to the coating system (hybrid + SnO2passivating layer) and to the underlying metallic substrate (tinlayer + tin/iron alloy + steel), respectively. In this circuit CPE1 andCPE2 correspond to constant phase elements constituted by theadmittance CPE-T and the exponent CPE-P. A value of CPE-P of1 corresponds to the response of an ideal capacitor C; CPE-P val-

ues of 0.5 suggests a diffusion response or a porous material, and0.5 < CPE-P < 1 values are associated to heterogeneous, rough ornon-homogeneous current distribution [26,27]. On the other hand,R1 corresponds to the resistance of conductive pathways throughthe hybrid and SnO2 coating system and R2 is associated to the

P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109 5103

F d D)l ne).

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ig. 2. EIS diagrams: (A and B) for commercial tin coated steel and bare steel, (C anog |Z| versus log(f) Bode plots (right). Experimental (symbol) and EEC fitted (solid li

nterfacial response of the underlying metallic substrate, probablyhe charge transfer resistance. With this circuit a good fitting (withow errors associated to all passive elements estimations) was pos-ible for the UF at all immersion times, while for the IP and OPlms good quality fitting were obtained only up to 72 and 168 hf immersion, respectively. For these two latter films, for test peri-ds above those previously mentioned, the CPE exponents stronglyeviated from 1 and the fitting quality greatly decreased, indicating

he need to introduce a new time constant.

As shown in the inserts of Figs. 4 and 5, after a certain immersionime, different for both coatings, the HF region of the Nyquist dia-rams for the IP and OP samples presents a 45◦ angle corresponding

ig. 3. Electrical equivalent circuits used for adjusting the impedance data for shortA) and long (B) immersion times.

for UF samples, obtained in 3.5 wt.% NaCl solution. Nyquist plots (left) and −� and

to a Warburg type response; this coincided with the large devia-tions in the fitting quality previously mentioned, indicating thata new time constant should be introduced in the EEC. In this newconfiguration, presented in Fig. 3B, C1 stands for the coating (hybridand SnO2 passivating layer) capacity and R2//CPE2 are ascribed tothe same processes as in the EEC of Fig. 3A. In addition a parallel CPE(CPEpo) resistance (Rpo) sub-circuit was introduced in series withR2//CPE2 to take into account the HF Warburg-like response sincebetter fitting results were obtained using this arrangement whencompared with a single Warburg element. As stated by Campestriniet al. [28], parallel R//CPE can adequately simulate Warburg-typeresponse in active pores, when the length of the diffusion path isfinite. However, due to the extremely high frequency of this newtime constant no diffusion controlled process can be associatedwith it; therefore the couple Rpo//CPEpo (pore resistance and porecapacitance) was introduced to fit a porous electrode response,when the pore length is not semi-infinite, as justified in the fol-lowing. During the years 1960s, de Levie [29–31] showed thatfor porous electrodes with semi-infinite pore length the Nyquistdiagrams are characterized by a Warburg-type response with a45◦ angle of the diagram with the real axis, which means thatimpedance of porous electrodes is proportional to the square rootof flat ones. Even though the developed theory relied in a series of

simplifying assumptions, since than it has been used to explain theEIS response of different systems [32,33].

In the diagrams presented in Figs. 4 and 5 the 45◦ responsetypical of porous electrode is detectable only in their HF regions,indicating that this behavior is effective only when the ac per-

5104 P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109

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ig. 4. EIS diagrams: (A) Nyquist plots, (B and C) −� and log |Z| versus log(f) Bode pEC fitted (solid line).

urbation is fast. In recent work Song et al. [34] have shownathematically that porous electrode responses depends on the

ore size distribution, and that the shallower the pore the higherhe frequency at which the EIS response deviates from thatxpected from a porous electrode approaching to that exhibitedy a flat surface. In a recent work Frateur et al. [35] modelinghe cast iron/drinking water system verified experimentally that

ixed porous-flat electrode EIS response can be obtained in theame experimental diagram with the transition frequency beingetermined by the pore length (the shallower the pore the higherhe transition frequency). This transition frequency is related to theenetration depth (�) of the ac perturbation sign inside the pore.

n the de Levie [36] model the impedance of a cylindrical pore isiven by the following relation:

P =√

RC · Z0 · cothlC�

(1)

here RC is the resistance of the electrolyte per unit length withinhe pore (� cm), Z0 is the impedance per unit length of the flatlectrode developed in the cylindrical pore (� cm), lC is the poreength (cm) and � corresponds to the penetration of the ac sig-al in the pore (cm). According to de Levie [29], � corresponds to

he fraction of the pore that effectively participates in the chargingrocess. One pore behaves as semi-infinite when � is inferior to theore length [33], when lC � �, coth (lC/�) ∼= 1, and the impedance ofhe pore is proportional to the square root of the impedance of theat electrode characterizing the 45◦ response. On the other hand

r OP coated sample, obtained in 3.5 wt.% NaCl solution. Experimental (symbol) and

when lC/� ≤ 0.3, the coth term tends to (lC/�)−1 with an accuracybetter than 97% [37] and, in this case, the impedance response isthat presented by a flat electrode.

Based on this literature survey we propose that, for the IP and OPsamples, a porous electrode behavior with shallow pores developswith exposure time to the NaCl solution, and that, for such elec-trode, the corrosion activity would take place at the walls of thepores developed in the conductive phase formed by the underlyingmetallic substrate. In the EEC of Fig. 3B this response is fitted by thecouple Rpo//CPEpo, which is meant to represent the double layercharging in parallel with the charge transfer resistance in the HFregion, when the porous electrode behavior dominates the elec-trochemical response of the underlying metallic substrate. Thesepores would be formed when flaws of the hybrid coatings matchthe porosities of the SnO2 passivating layer, forming preferentialconductive pathways, allowing the electrolyte to reach the under-lying metallic substrate. It is worth to emphasize that this effectwas not observed for the UF coated sample probably due to earlydisbondement of the hybrid coating from the sample caused by alarge number of conductive pathways through the coating defects.

In Fig. 6 is presented the physical models, together with theassociated EEC, for the IP and OP systems for short (Fig. 6A) and

long (Fig. 6B) immersion times in the test electrolyte. On the otherhand, Fig. 6C shows the representation of the pore electrochemicalresponse, according to the de Levie [30] theory, which, in our fit-ting procedures, was substituted for the Rpo//CPEpo couple, whichsimulates the 45◦ response. Before going on with the discussion

P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109 5105

F lots foE

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ig. 5. EIS diagrams: (A) Nyquist plots, (B and C) −� and log |Z| versus log(f) Bode pEC fitted (solid line).

f the results it is worth to emphasize that, for the system repre-ented in Fig. 6B the electrochemical activity takes place mainly athe pore walls as the surface area is much bigger than the surfacef the pores bottom.

Tables 2–4 show the evolution of the passive elements of theECs for hybrid coated samples at different immersion times. Theum of square deviations, �2 around 10−3, the low errors (%) asso-

iated with all the parameters estimation, and the good matchingetween the fitted and the experimental diagrams (see continu-us lines in Figs. 4 and 5) indicate the adequacy of the proposedECs.

ig. 6. Physical model with associated EEC for IP and OP samples for short (A) and long (evie [36] theory. In the representation the general term coating stands for the ensemblein layer + tin/iron alloy + steel.

r IP coated sample, obtained in 3.5 wt.% NaCl solution. Experimental (symbol) and

The results of the fitting procedure show that for the samplecoated with the UF (Table 2) both R1 and R2, which were associ-ated with the pathways of the coating system (hybrid film + SnO2passivating layer) and the charge transfer resistance of the under-lying metallic substrate (tin + tin/iron alloy + steel), respectively,decreased as immersion time increased. This suggests that theelectrolyte easily reaches the substrate, activating the corrosion

process. On the other hand, CPE1-T increases, indicating electrolyteuptake by the hybrid coating, while CPE2-T remains almost con-stant, which is indicative that the active area does not increasewith immersion time, pointing towards a good adhesion between

B) immersion times. (C) Equivalent circuit model inside a pore according to the dehybrid coating + SnO2 passivating layer and the metallic substrate represents the

5106 P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109

Table 2Values of the EEC parameters obtained from the fitting of the experimental data for the UF coated sample. The error % associated with each estimate is given in parenthesis.

Time (h) Rs (� cm2) CPE1-T (�F cm−2 s(˛−1)) CPE1-P or ˛ R1 (k� cm2) R2 (M� cm2) CPE2-T (�F cm−2 s(˛−1)) CPE2-P or ˛ �2 (10−3)

24 49 (1.2) 0.32 (5.4) 0.76 (0.72) 20 (2.0) 0.77 (3.3) 7.4 (1.2) 0.72 (1.0) 1.8348 45 (2.4) 1.1 (5.4) 0.70 (0.78) 7.4 (2.3) 0.27 (1.4) 7.8 (1.4) 0.73 (0.75) 0.8972 42 (2.7) 2.9 (6.1) 0.66 (0.92) 4.7 (3.8) 0.16 (1.1) 6.9 (3.0) 0.76 (0.97) 0.75

Table 3Values of the EEC parameters obtained from the fitting of the experimental data for the OP coated sample. The error % associated with each estimate is given in parenthesis.

Time (h) 24 48 72 96 144 168 192 240R1 (k� cm2) 39 (1.2) 25 (1.4) 18 (1.5) 11 (2.3) 8.3 (3.1) – – –CPE1-T (�F cm−2 s(˛−1)) 0.048 (3.8) 0.075 (4.9) 0.11 (5.0) 0.15 (8.4) 0.37 (9.8) – – –CPE1-P or ˛ 0.86 (0.42) 0.83 (0.57) 0.80 (0.59) 0.80 (0.99) 0.74 (1.2) – – –R2 (M� cm2) 10(11) 6.0 (7.3) 1.8 (3.2) 2.0 (5.8) 1.3 (4.4) 0.99 (1.2) 0.88 (1.1) 0.77 (0.94)CPE2-T (�F cm−2 s(˛−1)) 4.0 (0.82) 4.2 (0.94) 4.8 (0.97) 5.3 (1.4) 5.3 (1.8) 5.6 (0.61) 5.7 (0.61) 5.8 (0.58)CPE2-P or ˛ 0.69 (0.66) 0.73 (0.66) 0.71 (0.68) 0.75 (0.89) 0.76 (0.97) 0.84 (0.47) 0.85 (0.47) 0.85 (0.45)C1 (�F cm−2) – – – – – 9.9 10−3 (2.0) 9.9 10−3 (2.2) 9.8 10−3 (2.4)

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Rpo (k� cm ) – – –CPEpo-T (�F cm−2 s(˛−1)) – – –CPEpo-P or ˛ – – –�2 (10−3) 3.1 4.4 3.6

he hybrid coating and the substrate at non-defective sites, evenhough, as stressed earlier, a large number of conductive pathwayss present in this coating leading to early disbondement.

The analysis of the fitting results for the IP and OP samplesemains more complex as two different EECs were used to fit thexperimental data. To understand the transition between the twoECs it is important to stress that, as corrosion takes place throughhe conductive pathways formed at the coating system (hybridlm + SnO2 layer) flaws, the length of the pores increases (theransition frequency between porous and flat electrode behaviorecreases [34]). This makes possible the co-existence of porous andat electrode responses that can be detected in the EIS measure-ent [35]. So the two series sub-circuits Rpo//CPEpo and R2//CPE2

epresent the response of the same phenomenon, the former beingepresentative of the porous electrode behavior (when the penetra-ion depth of the ac signal is inferior to pore depth – HF region) andhe latter being representative of the flat electrode response (whenhe penetration depth of the ac signal is superior to pore depth andhe signal sees the electrode as a flat one – lower frequency region).

oreover in the Rpo term a contribution of the resistance withinhe pore is also comprised, as indicated in Fig. 6C.

For the IP sample up to 72 h, when the experimental data weretted with the EEC presented in Fig. 3A, like in the UF sample, there

s a decrease in R1 and an increase in CPE1-T, which is indicativef coating deterioration. For immersion periods above 72 h, dueo the onset of porous electrode response, R1 was substituted for

po//CPEpo and CPE1 by a pure capacitor (C1), Fig. 3B. For this latteronfiguration Rpo decreases and CPEpo increases with immersionime, which is indicative of increasing pore depth (increasing sur-ace exposed to the electrolyte) and active corrosion. C1 continueso increase, indicating further enrichment of the coating system

able 4alues of the EEC parameters obtained from the fitting of the experimental data for the IP

Times (h) 24 48 72R1 (k� cm2) 12 (1.3) 5.8 (1.7) 3.6 (3.0)CPE1-T (�F cm−2 s(˛−1)) 0.070 (6.1) 0.20 (7.8) 0.95 (11)CPE1-P or ˛ 0.87 (0.66) 0.80 (0.90) 0.69 (1.4)R2 (M� cm2) 2.47 (6.9) 1.5 (5.4) 0.59 (3.8)CPE2-T (�F cm−2 s(˛−1)) 9.7 (1.0) 11 (1.1) 11 (1.8)CPE2-P or ˛ 0.76 (0.68) 0.77 (0.68) 0.77 (0.92)C1 (�F cm−2) – – –Rpo (k� cm2) – – –CPEpo-T (�F cm−2 s(˛−1)) – – –CPEpo-P or ˛ – – –�2 (10−3) 5.6 6.7 8.5

– – 15 (4.3) 15 (4.7) 14 (4.6)– – 8.7 (5.9) 10 (4.9) 11 (4.2)– – 0.41 (1.5) 0.42 (1.2) 0.43 (1.1)8.5 9.7 1.0 0.88 0.77

(hybrid film + SnO2 layer) with water molecules. The evolution ofthe fitting parameters (irrespectively to the EEC employed) showsthat R2 decreases, while CPE2-T remains almost constant, indicat-ing that, although corrosion increases there is no augmentation ofthe exposed surface (no disbondment is progressing). For the OPsample the tendency verified for the evolution of the EEC param-eters was the same, with the difference that the porous electroderesponse developed later due to the better anticorrosion propertiesof this particular hybrid coating.

The two EECs presented in Fig. 3 to fit the experimental EIS dataof IP and OP samples may raise two questions: how the resistanceR1 was transformed in a parallel Rpo//CPEpo and why, when passingfrom the EEC of Fig. 3A to the EEC of Fig. 3B, a CPE is transformedto a pure capacitor (C1). The data presented in Tables 3 and 4 showthat in the transition from the EEC of Fig. 3A to the EEC of Fig. 3B,the value of Rpo is superior to that previously exhibited by R1, so it islikely that some contribution of the pathways of the coating (hybridfilm + SnO2 passivating layer) is included in the estimations of theRpo parameter, which represents the resistance of the electrolytewithin the pores. On the other hand the answer to the second ques-tion remains unclear, even though the quality of the fitting wasclearer superior when a pure capacity was employed instead of aCPE.

Polarization curves for all samples were recorded after 3 h ofimmersion in naturally aerated NaCl solution. The comparison ofresults shown in Fig. 7 evidences that the OP sample presents a

slightly more positive Ecorr and lower anodic current densities,indicating higher corrosion resistance, as already demonstrated bythe Rp determination and EIS experiments. The UF sample showedbehavior intermediate between that of IP and the commercial passi-vated tin coated steel samples. It is interesting to note that only the

coated sample. The error % associated with each estimate is given in parenthesis.

96 144 168 192 240– – – – –– – – – –– – – – –0.62 (0.85) 0.33 (0.87) 0.20 (0.76) 0.19 (0.70) 14 (0.72)

12 (0.36) 13 (0.44) 13 (0.52) 13 (0.48) 14 (0.72)0.82 (0.28) 0.82 (0.35) 0.85 (0.41) 0.86 (0.37) 0.84 (0.48)0.011 (2.9) 0.016 (11) 0.046 (6.6) 0.052 (7.0) 0.10 (4.7)5.2 (3.2) 2.4 (4.3) 2.4 (5.4) 2.1 (5.2) 2.5 (6.6)

25 (3.5) 28 (4.2) 35 (4.6) 38 (4.2) 38 (4.4)0.42 (0.84) 0.51 (0.96) 0.52 (1.1) 0.53 (1.0) 0.53 (1.1)0.41 0.63 0.70 0.61 0.63

P.H. Suegama et al. / Electrochimica Acta 55 (2010) 5100–5109 5107

FU

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3

omec(1(tn

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ig. 7. Polarization curves for commercial tin coated steel and samples coated withF, IP and OP hybrids, obtained after 3 h of immersion in 3.5 wt.% NaCl solution.

P sample showed a well-defined breakdown potential at around0.33 V, and if a potential of −0.35 V is considered, the current den-

ities (i/A cm−2), based on geometric area, for the different samplesre 2.1 × 10−8 (OP), 6.5 × 10−8 (IP), 3.5 × 10−7 (UF) and 1.3 × 10−6

commercial tin coated steel), indicating that current densities forybrid coated samples were almost two orders of magnitude lowerhan that for the commercial tin coated steel. The electrochem-cal results, therefore, showed that the surface degradation ratencreased as follows: OP < IP < UF � commercial tin coated steel.

.2. 13C and 29Si NMR

13C NMR spectra for unsupported IP, OP and UFrganic–inorganic hybrid materials are shown in Fig. 8. The poly-erization of methacrylate groups belonging to MPTS and MMA is

videnced by the resonance signals corresponding to quaternaryarbon atoms (b′), C O groups bonded to aliphatic carbon groups′ ′

c ), and aliphatic –CH2– groups (a ) [38]. The absence of signals at

25 ppm (a) and 137 ppm (b) associated with vinylic carbon atomsC C), and with C O groups bonded to these atoms (c), evidenceshe absence of unpolymerized acrylate structures. It is worthoting that the relative intensity of peak (a′) is stronger for the OP

ig. 8. 13C NMR spectra of the unsupported UF, IP and OP organic–inorganic hybridaterial.

Fig. 9. 29Si NMR spectra of the unsupported UF, IP and OP organic–inorganic hybridmaterial. The percentages of each trifunctional, Ti, and tetrafunctional, Qj, speciesare displayed near the corresponding resonance.

sample, indicating that the organic phase is more fully polymerizedwhen Ce(IV) ions are added to the organic precursor solution.

The 29Si NMR spectra of the unsupported UF, IP and OP hybridmaterials (Fig. 9) were used to evaluate the effect of CAN additionon the polycondensation of alkoxysilane precursors. The samplespresented five resonances at approximately −59, −65, −92, −102and −109 ppm, corresponding to T2, T3, Q2, Q3 and Q4 species,respectively [39–41]. The conventional notation corresponding totrifunctional “Ti” and tetrafunctional “Qj” species is adopted asshown below, where R is H or an alkyl group, and R′ is the methacry-loxypropyl group:

T1 = (RO)2Si(OSi)R′, T2 = (RO)Si(OSi)2R′, T3 = Si(OSi)3R′

Q 2 = (RO)2Si(OSi)2, Q 3 = (RO)Si(OSi)3, Q 4 = Si(OSi)4

The absence of the T1 (−49 ppm) and Q1 (−86 to −80 ppm) res-onance corresponding to terminal units indicates the predominantpresence of polyhedral cyclic structures in the UF and IP hybrids.In fact, the resonances observed at −66.3 and −58.7 ppm can beassigned to T3 and T2 units, belonging to a mixture of fully con-densed octahedron (R′

8Si8O12) and incomplete condensed silanolcage, respectively. In the UF hybrid, the resonances correspond-ing to tetrafunctional silicon are dominated by the broad bandcentered between −100 and −103 ppm, characterizing the highlyvaried environment of Q3 species, and a broad shoulder around−110 ppm, evidencing the secondary presence of the fully con-densed Q4 species. It is noteworthy that the intensities of bandscorresponding to the fully condensed T3 and Q4 species increasedwhen cerium ions were added to the inorganic phase (IP), indicat-ing an improvement of the condensation degree (CD). The averageCD calculated from the proportion of each species, determined from

29

the areas of the Si resonance peaks, shows similar values for theUF (CD = 79%) and OP (CD = 78%) samples, and a maximum for theIP hybrid (CD = 88%). For the OP sample, it is interesting to notethe curious predominant coexistence of fully condensed Q4 specieswith terminal or dimeric T1 species.

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108 P.H. Suegama et al. / Electroc

. Discussion

Electrochemical characterization of commercial passivated tinoated steel samples coated with the different hybrid films hashown that anticorrosion performance was improved when Ce(IV)ons were added to either of the initial precursor solutions, withest results achieved when addition was made to the so calledrganic phase. It is recognized that Ce(IV) ions are versatile reagentsor the oxidation of numerous functional groups in organic synthe-is, as well as in transition metal chemistry [42–44] and in redoxystems for initiation of alcohol polymerization [18,45]. From aractical point of view, the most striking feature of a redox pro-ess is that it enables polymerization to occur at rates substantiallyaster than conventional methods. Ce(IV) forms a complex withlcohol, and the decomposition of the coordination complex leadso the formation of a free radical that can react faster, leading toolymerization [46].

Improvement of coatings systems performances due to the addi-ion of Ce ions to the initial precursor solution has been reportedn investigations of corrosion protection of different metals usingilane coatings [19,47–49]. This has frequently been ascribed to bet-er reticulation of the silane film [20,47]. Indeed, in a recent work,uegama et al. [20], using different techniques, showed that theddition of Ce(IV) ions to the sol–gel precursor solution enhanceshe formation of siloxane bonds and increases the organic chainengths. According to the authors [20], the interaction of these acti-ated complexes with alcohol complexes from the solution wouldead to stronger reticulation of the film and to better anticorrosionroperties. Even though Suegama et al. recognize that during thisrocess Ce(III) ions are formed from Ce(IV) reduction, the amountf this former ion within the hybrid coating is negligible to beonsidered as having a classical inhibiting effect, which has beenetected when Ce(III) ions are added to the silane hydrolysis solu-ion [48,50,51].

For the IP films, the results of the 29Si NMR characterizationFig. 9) showed that both T3 and Q4 resonances associated with fullyondensed silicon species were more intense, indicating a higheregree of polycondensation. As the only difference between therocessing of the UF and the IP hybrids was the addition of Ce(IV)

ons to the so called inorganic phase precursor solution, we proposehe following explanation. The polycondensation reaction would benhanced by the formation of a complex between Ce(IV) and alco-ol, which consumes the ethanol produced in the hydrolysis (Eq.2)) and polycondensation (Eq. (3)) reactions [52]. It may displacehe equilibrium in Eqs. (2) and (3) to form more silanols and silox-nes groups, respectively. This effect on both reactions favors anncrease in the degree of polycondensation of hybrid materials.

Si–OR + H2O �≡ Si–OH + R–OH (2)

Si–OR + OH–Si ≡�≡ Si–O–Si ≡ + R–OH (3)

On the other hand, as demonstrated by the electrochemicalesults, the OP film presented the best anticorrosion properties.his can be explained by the formation of more highly reticulatedrganic domains in such films. BPO molecules contain an oxygentom free radical, which can likely initiate the radical polymer-zation of MMA monomers, as shown for the UF hybrid [53,54].herefore, the addition of Ce(IV) ions to the organic precursorolution would accelerate the opening of the carbon double bond

epresented by the a′ peak, suggesting a higher polymerizationield, as confirmed in the 13C NMR experiment (Fig. 8).

Three qualitative tests were performed, in the absence of BPO, torovide further evidence that the Ce(IV) ions could open the carbonouble bond and promote polymerization:

Acta 55 (2010) 5100–5109

(1) The MMA monomer was mixed with the Ce(IV) salt and it didnot dissolve, being the colorless liquid an indication that nocomplex had been formed with the Ce(IV) ion.

(2) The Ce(IV) salt was dissolved in ethanol and a red solutionwas formed, indicating complex formation [55]. This com-plex was stable for at least 2 days, and after this period thesolution became colorless, due to the conversion of Ce(IV) toCe(III) [18,56], with no observed changes in solution viscosity.However, when MMA monomer was added to the red solu-tion, the color became orange almost instantaneously and thesystem became colorless after 24 h. The MMA polymerizationwas evidenced by the continuous increase in viscosity of thesolution and the formation of a transparent gel after around24 h.

(3) The Ce(IV) salt was dissolved in water, and the addition of MMAmonomer formed two liquid phases. This system became ahomogeneous solution when ethanol was added. A white solidwas formed after around 12 h, indicating that in the presenceof alcohol Ce(IV) ions interacted with the MMA monomer, andno red color was observed even at the moment of ethanol addi-tion. Under these conditions, a greater amount of radicals wasprobably formed, than in test (2), leading to the formation ofshorter PMMA chains.

The results clearly show that Ce(IV) ions play an active role inthe polycondensation of silicon species and on the polymerizationof MMA. The formation of a more compact structure appears to befavored by the addition of these ions, leading to an increase in thebarrier effect to electrolyte penetration through the hybrid coatingresulting in a better protection of the substrate against corrosion,as demonstrated in all the electrochemical tests.

Therefore, in the present study, the effect of Ce(IV) ions can besummarized as following:

(1) Ce(IV) ions added to the organic precursor solution increasedthe organic phase polymerization, as evidenced by the absenceof unpolymerized acrylate structures in the OP sample (Fig. 8).These ions form a complex with alcohol, which decomposesleading to the formation of a free radical that can react fasterto increase the MMA polymerization. In this step, Ce(IV) ionsare reduced to Ce(III) ions. Probably, the addition of Ce(IV)ions to the organic precursor solution accelerates the openingof the carbon double bond, resulting in higher polymerizationyield.

(2) The addition of Ce(IV) ions also increased the polycondensationof alkoxysilane precursors as demonstrated for the OP sampleby the predominant coexistence of fully condensed Q4 specieswith terminal or dimeric T1 species (Fig. 9).

(3) Both effects favored the formation of a more compact struc-ture which delays electrolyte penetration through the coating,and consequently increases the corrosion resistance of the tincoated steel against chloride attack.

(4) The classical corrosion inhibitor effect like Ce(III)/Ce(IV) oxideor hydroxide passive layer was not observed here, probably dueto the low amount of cerium used and the compact structure ofthe hybrid coatings.

At the present we do not have enough information about the

location of Ce(III) ions coming from Ce(IV) reduction, mainly due tothe low amount of cerium salt used. Ce(III) and even some Ce(IV)ions may have been incorporated into the coating, precipitated asoxide or hydroxide inside the coating or on the substrate throughdefects of the film.

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P.H. Suegama et al. / Electroc

. Conclusions

Hybrid films prepared with the addition of Ce(IV) ions in thenitial step of sol–gel synthesis presented better corrosion resis-ance and can be used as a pre-treatment for corrosion protectionf commercial tin coated steels.

The addition of Ce(IV) ions into the organic phase precursorolution improved MMA polymerization, while addition to thenorganic phase precursor increased the degree of polycondensa-ion of the siloxane phase. They therefore act in the structures ofoth organic and inorganic phases to produce a more condensedlm, improving the barrier effect against corrosion. However, allhe experimental evidences indicated that more protective filmsere formed when the Ce(IV) ions were added to the organic phase,hich was confirmed with better EIS, Rp and anodic polarization

ehaviors.Ethanol contributes with OH groups in the complexation of

e(IV) ions, and the complex is essential to generate radicals whichnitiate the polymerization of methylmethacrylate. It is also impor-ant to note that the role of Ce(IV) ions, observed here, can bexploited in a more general way in the elaboration of hybrid mate-ials which are produced near low temperatures using radicalolymerization reactions.

cknowledgements

The authors are grateful to FAPESP (Proc. nrs. 05/51851-4 and7/53073-4), CNPq (Proc. nrs. 310860/2005-9, 300728/2007-7 and05073/2008-7) and CAPES (BEX-3167-08-9) for scholarships.

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