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Electrochimica Acta 52 (2006) 538–545

Improving the formation and protective properties of silane films by thecombined use of electrodeposition and nanoparticles incorporation

Liang Liu, Ji-Ming Hu ∗,1, Jian-Qing Zhang, Chu-Nan CaoDepartment of Chemistry, Zhejiang University, Hangzhou 310027, PR China

Received 14 February 2006; received in revised form 24 May 2006; accepted 24 May 2006Available online 5 July 2006

bstract

The electrodeposition and nano-silica incorporation were used together to prepare the novel composite dodecyltrimethoxysilane/SiO2 thin filmsrom silane sol–gel system on aluminum substrate. The results showed that both the two techniques can improve the films formation and theirrotectiveness. The influences of the deposition potential and the silica content in silane solution were investigated. A “critical deposition potential”nd a “critical silica content” were both observed, under which the obtained silane films had the highest protective properties. The enhancementn film thickness has been detected by these two techniques from the elemental depth profiles of silane films as measured by secondary-ion mass

pectroscopy (SIMS). Current–time curves were on-line recorded on aluminum electrodes in silane solutions. As compared with that in blankolution, the current response was found to be larger in silica-contained precursor, probably suggesting that the silica particles participate in thelm deposition.2006 Elsevier Ltd. All rights reserved.

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eywords: Silane films; Electrodeposition; Nanoparticles; Corrosion protection

. Introduction

Silane coupling agents have long been used as adhesionromoters between metal surfaces and organic resins [1,2].ecent developments suggest that silane pre-treatments relied onydrolysis and condensation reactions occurring on the substrateuring film formation can help protect metals against corro-ion, and investigations are being directed at developing thispproach as an alternative to the currently used carcinogenichromating and pollutive phosphating processes [3–6]. So farvariety of silane agents have been investigated for steel and

luminum alloys treatments. It was found that non-functionalilane films such as bis-1,2-[triethoxysilyl] ethane (BTSE) per-orm better corrosion resistance than the organofunctional silanelms, such as �-aminopropyltriethoxysilane (�-APS) and �-

lycidoxypropyltrimethoxy silane (�-GPS) [7].

Silane films were usually prepared by conventional dip-oating method, that is, simply dipping metal substrates into

∗ Corresponding author. Tel.: +86 571 87952318; fax: +86 571 87951895.E-mail address: kejmhu@zju.edu.cn (J.-M. Hu).

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re-hydrolyzed silane/water/alcohol solutions then drying anduring at a certain high temperature. Some corrosion inhibitorsere employed as either the pre-treated layers (e.g. rare earth

alts [8] and alkanethiols [9] films) or as the active additivese.g. cerium salts [10,11], tolyltriazole [11], benzotriazole [11]nd phenylphosphonic acid [12]) in order to further improve theorrosion resistance of silane films. Recently van Ooij’s group13] applied non-active silica nanoparticles as the additive ofilane films, indicating that the obtained composite films areore anti-corrosive.On the other hand, it should be noted that a novel tech-

ique, namely electrodeposition, was also developed in theilane sol–gel chemistry and was proved to be effective tomprove the corrosion performance of silane films. Literature14] might be the first work reporting the electrodeposition ofilane films, but its purpose was to improve the adhesive perfor-ance. Lately, Mandler’s group [15,16] and van Ooij’s group

17] applied this technique in the anti-corrosion treatment ofetals and the results showed that silane films prepared at a

ertain cathodic potential present higher corrosion resistancehan those obtained by conventional dip-coating method. In ourrevious works, several different silane films (such as BTSE18], dodecyltrimethoxysilane (DTMS) [19,20]) have been

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L. Liu et al. / Electrochim

lectrodeposited on aluminum alloys, and the results showed thathe physical structure and corrosion performance of the obtainedlms are sensitively dependent of deposition potential. Criticalathodic potential (CCP) was found for each above-mentionedilane system. Films deposited at CCP exhibit the highest com-actness and uniformity thereby the best barrier properties.

In this study, we investigated the electrodeposition and char-cterization of silica nanoparticle-filled DTMS films on alu-inum substrates. The advantages of the combination of two

ovel techniques (i.e. electrodeposition technique and nanopar-icle incorporation process) were realized and the results showedhat both the two techniques could facilitate the film formationnd enhance the film protectiveness. Choice of DTMS lies in theact that this formed silane film performs high hydrophobicitynd a potentiality of protection usage due to the existence of aong dodecyl chain in bone structure as reported in our previousork [19,20].

. Experimental

Pure aluminum rods (99.999%, 6.35 mm diameter,≈ 0.4 cm2, Aldrich) embedded with poly-tetrafluoroethylene

PTFE) were used as the metal substrates. The rods wereechanically polished to mirror-like smooth and highly

eflecting surfaces prior to film deposition by emery paper (800rits) followed by alumina paste (2.5 �m). After polishing, theamples were thoroughly rinsed with home-made surfactant-ased low alkaline cleaner, and finally washed with deionizedDI) water and then blow-dried with warm air. All samplesere kept in a desiccator for at least 24 h before use.Silane agent (DTMS: CH3(CH2)11Si(OCH3)3) was pur-

hased from DaDi Chemical (Hangzhou, China) and useds received without further purification. Silica nanoparticles20 ± 5 nm) were purchased from Chemat Chemical (Xia’men,hina). The deposition solutions used here consist of 5 vol.%

ilane monomer dissolved in 75/25 (v/v) ethanol/water mixedolvent. The pH of the solutions was adjusted to 4.5 usingcetic acid. The obtained solutions were pre-hydrolyzed at 35 ◦Cor 48 h forming sol–gel precursors, then appropriate amountf silica nanoparticles were added (silica content: 0, 20, 40,0, 100, 150 �g/L, respectively) and the mixed solutions wereurther stirred for about 2 h to disperse the nanoparticles. Thelectrodeposition was performed by using three-electrode com-artment. A saturated calomel electrode (SCE) was used as theeference electrode, and a platinum plate as the counter elec-rode (2.0 cm × 2.0 cm). Apart from the open-circuit potentialOCP), ca. −0.3 V/SCE for pure Al in DTMS solution, var-ous cathodic potentials (i.e. −0.6, −0.8, −1.0, −1.2 V/SCE,espectively) were selected to deposit the silane films. The depo-ition was conducted for 200 s, after which samples were takenut and blow-dried with nitrogen to remove any excess liq-id, finally cured at 100 ◦C for 30 min at air atmosphere in anven.

The surface morphology of silane films was observed on aIRION field emission scanning electron microscopy (SEM)roduced by FEI Co. Ltd. In situ EDX was also done to deter-ine the element compositions of the film. The depth profiles

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cta 52 (2006) 538–545 539

f the relevant atoms were conducted on a Cameca IMS-6Fecondary-ion mass spectroscopy (SIMS). A 12.49 kV O2+ ioneam with a beam current of ∼600 nA was used to raster over250 �m × 250 �m area. Electrochemical impedance spec-

roscopy (EIS) was employed to evaluate the corrosion perfor-ance of the silane-treated specimens. The testing electrolyteas a 3.5 wt.% NaCl aqueous solution prepared with DI water. A

imilar three-electrode compartment was used as stated above.he EIS measurement of silane-covered electrodes was carriedut at the OCP in NaCl aqueous solution, ca. −0.7 V/SCE, andhe frequency range was from 105 Hz to 10 mHz, with an ACxcitation amplitude of 10 mV. All the electrochemical testsere carried out at 25 ◦C on an M273 model potentiostat com-ined with an M5210 model lock-in amplifier.

. Results and discussion

EIS has been widely applied for evaluating corrosion inhi-ition of coated metals in corrosive media [21,22]. Conditionsequired by stability, causality and linearity should be satisfied inreliable EIS measurement [23]. The OCP of the film-covered

lectrode in NaCl solution shows high oscillation, which may bettributed to the high hydrophobicity of DTMS films [20], caus-ng the difficulty for electrolyte access onto the metal substratehrough the films. Such oscillation makes the EIS data acquisitedn the early immersion not reliable (usually, the obtained EIS datahow highly scattering in low-frequency domain). Immersedor longer times, e.g. 12 h (see Fig. 1), however, EIS data

ig. 1. Nyquist diagram of DTMS/150 �g/L SiO2 filmed-Al electrode preparedy dip-coating method measured after 12 h immersion in NaCl solution.

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ystem after the immersion of this time. For this reason, if unlessentioned otherwise, in this paper all EIS data are recorded

fter the immersion of treated Al samples in NaCl solutionsor 12 h.

.1. Protective properties of dip-coated silane films

Fig. 2 compares the EIS behaviors of conventional dip-coatedTMS films filled with different amount of silica nanopar-

icles on Al substrates. Similar change tendencies of phasengle as a function of testing frequency have been found at

oth the pure (curve 2) and composite DTMS-covered (curves–7) samples. Three relaxations are all observed: one in high-requency domain, which is associated with the existence ofrotective silane films; another at intermediate frequencies

ig. 2. Bode plots of bare aluminum (1) and dip-coated DTMS filmed-Al elec-rodes loaded with 0 �g/L (2), 20 �g/L (3), 40 �g/L (4), 70 �g/L (5), 100 �g/L6) and 150 �g/L (7) silica nanoparticles in NaCl solution.

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cta 52 (2006) 538–545

around 102 Hz), being attributed to the chemical formation ofn “interfacial inorganic layer” (e.g. Al–O–Si bonds) betweenluminum base and up-coated silane films [24–27]; the thirdne at the lowest frequency range associated with the corro-ion onset of alloy base. The high-frequency relaxation wasbsent at uncoated samples (curve 1). Unlike those in theases of metallic samples coated with many other convention-lly used silanes (e.g. BTSE [18]), the DTMS-covered systemsresent extremely higher phase angles in the high-frequencyomain, indicating the higher hydrophobicity of DTMS filmsnd potentially better barrier performance. An equivalent elec-ric circuit containing three relaxations has been discussedor DTMS-treated aluminum alloys previously [19,20], and ithould be also proper for DTMS/SiO2 composite films. Cor-osion resistance of silane-covered electrodes can be quantita-ively evaluated by the low-frequency impedance values (Zlf)13]. It is shown from Fig. 2(a) that the corrosion resistancef DTMS films can be improved by loading a proper amountf silica nanoparticles. As the silica content loaded increases,lf values for DTMS films firstly increase and then decrease.

critical silica content, namely 70 �g/L, is observed, underhich the composite silane films perform the highest pro-

ectiveness. It is interesting that as the silica content loadedncreases, the phase angle values of low-frequency domainor composite films also initially increase and then decrease,eaching highest when the silica content is 70 �g/L. This phe-omenon might indicate that the corrosion processes of Alubstrates would be inhibited by silica nanoparticles. Van Ooijnd co-workers suggested that silane films are thickened andtrengthened by loading silica nanoparticles and the enhance-ent of films’ protectiveness was explained by the suppression

f cathodic reactions in corrosive electrolyte, which was realizedia a small amount of silica reacting with cathodically gen-rated OH− ions then forming passive Al-silicate compounds13].

Fig. 3 displays the SEM images of DTMS films loaded withifferent amount of silica nanoparticles. The figure shows thathe unloaded silane film is un-uniform and incomplete cov-red (Fig. 3(a)). Some other silane agents, e.g. BTSE [18]nd phenyltrimethoxysilane (PTMS) [16], were reported alsoot able to form integrate films on the aluminum substratesy dip-coating method. The non-uniformity as high as 30%as been reported in dip-coated BTSE films covered on alu-inum as quantitatively measured by IR spectroscopic ellip-

ometry (IRSE) [28]. When the loaded silica content increaseso 70 �g/L, the silane film performs much more compact with

any tiny clusters on the surface (Fig. 3(b)). Higher Si peaks detected on these tiny clusters than on the other areas of thelm by in situ EDX, showing that these clusters may be related

o silica nanoparticles. However, large amount of clusters withize up to several microns are observed in the 150 �g/L silica-oaded silane film, causing the film porous and deterioratinghe film protectiveness (Fig. 3(c)). The critical silica content

70 �g/L) is higher than the value in literature [13] (15 �g/L),robably because the size of silica nanoparticles used in ourork (20 ± 5 nm) is much smaller than that used there (∼1 �m)

nd they could be dispersed better.

L. Liu et al. / Electrochimica A

Fig. 3. SEM images of dip-coated DTMS films loaded with 0 �g/L (a), 70 �g/L(b) and 150 �g/L (c) silica nanoparticles.

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cta 52 (2006) 538–545 541

.2. Protectiveness of electrodeposited silane films

Though the merit of silica nanoparticle incorporation is real-zed in the silanization, the improvement in corrosion protectionf the conventionally dip-coated composite silane films might betill limited. Recently, a novel electrodeposition technique haseen successfully applied in silane film preparation. Our pre-ious works have shown that the protectiveness of silane filmss intensively affected by deposition potential [18,19]. In thisection, silica nanoparticle-filled DTMS films are prepared bylectrodeposition for the first time and the influences of deposi-ion potential as well as silica content are investigated.

.2.1. The influence of deposition potentialAs indicated in Section 3.1, DTMS films loaded with 70 �g/L

ilica nanoparticles perform the best corrosion resistance whenrepared by dip-coating method. Then, in this section the deposi-ion potential dependence of corrosion resistance is investigatedn DTMS/70 �g/L SiO2 films. Fig. 4 shows the Bode plots foromposite films prepared at various potentials in NaCl solution.ere, preparation process with E = −0.3 V/SCE corresponds to

he dip-coating method. It is shown that when the depositionotential shifts negatively from the OCP, the protectiveness asell as the low-frequency phase angle of the composite filmsrepared initially increases and then decreases, with the maxi-um at E = −0.8 V/SCE. Deposition potentials higher or lower

oth deteriorate the protectiveness of films, which is in agree-ent with our previous work in the electrodeposition of pure

ilane films (e.g. BTSE [18] and DTMS [19,20]).SEM image (Fig. 5(a)) shows that the composite film pre-

ared at −0.8 V/SCE performs higher compactness than thatrepared by dip-coating method (Fig. 3(b)). The facilitation inlm formation at cathodic potentials was believed being causedy the alkaline-aided condensation of silanols (Si–OH) by oxy-en reduction (reaction (1)) or water electrolysis reaction (reac-ion (2)) near the electrode surface [15,16,18]:

1/2)O2 + H2O + 2e− → 2OH− (1)

2O + e− → OH− + (1/2)H2 (2)

n Fig. 5(a), many tiny clusters which may be related to loadedilica are also observed, indicating that silica nanoparticles par-icipate in the silane film formation and improve the corrosionesistance of silane films.

However, when the deposition potential shifts too negative,he hydrogen evolution reaction (HER) tends to be enhancedimultaneously accompanying with the generation of OH− ionn water decomposition reaction (reaction (2)). The HER waslso believed to happen by the so-called “cathodic corrosion” ofl, as described as follows [29]:

l + 3H2O → Al(OH)3 + (3/2)H2 (3)

he intensive evolution of H2 gas, which attacks the electrode

urface, leading to the non-uniformity and high porosity of silanelms, thereby to the deterioration of film protectiveness, haseen evidenced by the sharp increase in cathodic currents asbserved from the voltammetric curve of aluminum electrode in

542 L. Liu et al. / Electrochimica Acta 52 (2006) 538–545

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ig. 4. Bode plots of bare aluminum (1) and DTMS/70 �g/L SiO2 filmed-Allectrodes in NaCl solution prepared by dip-coating method (2), and electrode-osited at −0.6 V (3), −0.8 V (4), −1.2 V (5) and −1.5 V (6).

ilane solution [18]. However, for silica-contained silane films,lthough porous morphology caused by hydrogen evolution istill observed when the deposition potential is negative enoughe.g. E = −1.5 V/SCE, see Fig. 5(b)), some deep holes are foundo be filled with large clusters which may be attributed to the clus-ered silica nanoparticles. This indicates that silica nanoparticlestill benefit the film protectiveness even though the depositionotential is very negative.

.2.2. The influence of silica contentAs stated above, composite silane film prepared at

0.8 V/SCE performs the best corrosion protectiveness. Inhis section, we investigate the influence of silica content

n corrosion performance of silane films electrodeposited at0.8 V/SCE. EIS results shown in Fig. 6 indicate that the cor-

osion resistance as well as the low-frequency phase angle ofTMS films first increases then decreases as a function of loaded

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ig. 5. SEM images of DTMS/70 �g/L SiO2 films electrodeposited at −0.8 Va) and −1.5 V (b).

ilica content, which is similar to the case of dip-coating. The0 �g/L silica-loaded film also performs the best corrosion resis-ance.

In order to quantitatively compare the corrosion resistance ofilane films prepared at different conditions, the Zlf values mea-ured at 10 mHz of DTMS films prepared at −0.8 V/SCE andCP, as a function of silica content, respectively, are displayed

n Fig. 7. Systematically higher values of Zlf are observed forlectrodeposited samples than those of dip-coated ones, indicat-ng the improvement in protectiveness by electrodeposition. Theiggest extent of such improvement is reached as 70 �g/L silicaanoparticles are incorporated into the DTMS films.

.3. Mechanistic aspects of the electrodeposition of

omposite silane films

To investigate the electrodepositing process and the forma-ion of the silica-loaded DTMS films, the in situ current–time

L. Liu et al. / Electrochimica Acta 52 (2006) 538–545 543

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ccesmopfigfeeto shift towards higher pH values [32]. Actually, the chargingstate of particles may be changeable from the bulk solution tonear surface electrolyte in electrode/solution interface during

ig. 6. Bode plots of bare aluminum (1) and −0.8 V electrodeposited DTMSlmed-Al electrodes loaded with 0 �g/L (2), 20 �g/L (3), 40 �g/L (4), 70 �g/L5), 100 �g/L (6) and 150 �g/L (7) silica nanoparticles in NaCl solution.

urves of aluminum electrodes in silane solutions during thelectro-formation and ex situ depth profiles of element concen-rations of as-prepared silane films are recorded. Fig. 8 showshe current response immediately measured after the appliedotential shifts to −0.8 V/SCE from the OCP in DTMS solutionontaining 70 �g/L silica nanoparticles (denoted “compositeolution”) and in the blank solution (i.e. DTMS solution with-ut silica nanoparticle), respectively. A typical “potential-step”ehavior is observed in both solutions, that is, the current sharplyecreases during the initial seconds of recording, after whichradually reaches a quasi-steady value. The relaxation is associ-ted with the charging process on double-layer structure at elec-

rode/electrolyte interface [30]. Larger current response is foundn the “composite solution” than that in the blank solution duringhe first 50 s, afterward it shows almost no difference for the twoolutions. This phenomenon demonstrates that silica nanoparti-

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ig. 7. The Zlf values of DTMS/SiO2 filmed-Al electrodes in NaCl solution asfunction of silica nanoparticle content in silane solution.

les might participate in the film formation but they would nothange the kinetics of electrochemical reactions occurring in thelectrodeposition system. The larger currents in the “compositeolution” during the first 50 s deposition indicate that DTMS-odified silica nanoparticles may promote the charging process

f the electric double-layer on the electrode/solution interfacerobably via absorbing the charged particles on the cathode sur-ace. The charging state of SiO2 particles has been extensivelynvestigated in aqueous solutions in literatures [31–33]. In inor-anic solutions, for instance, the particle surface was usuallyound being negative-charged over a wide pH range [31]. How-ver, the recent studies showed that after surface modification,.g. by amino-silane, the isoelectric point (i.e.p.) of silica tends

ig. 8. Current response of Al recorded at −0.8 V in DTMS solutions in thebsence (1) and presence (2) of 70 �g/L silica nanoparticles.

5 ica Acta 52 (2006) 538–545

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he process of electrodeposition due to the significant alter ofH there as stated from Eqs. (1) and (2). For this reason, its still unclear for us whether the silica is exactly positive- oregative-charged in the double-layer of cathodic interface. Buthe enhanced current response as shown in Fig. 8 may suggesthat the silica particles are more possibly being positive-charged.

In addition, the figure shows non-linear oscillations in cur-ent response in both silane solutions as the deposition timencreases, which may be caused by the oscillatory adsorptionf silane species on aluminum alloy surfaces. The oscillatorydsorption has been previously reported on dip-coated PTMS34] and �-APS [35] on variety of metal substrates. The simi-ar oscillatory current response was also observed on Al alloysuring the electrodeposition of BTSE films [18].

The depth profiles of the concentrations of the relevant atomsre measured by SIMS on electrodeposited pure DTMS filmprepared at −0.8 V/SCE), dip-coated DTMS/70 �g/L SiO2omposite film and electrodeposited DTMS/70 �g/L SiO2 com-osite film (also prepared at −0.8 V/SCE) covered aluminumubstrates, respectively. Fig. 9 displays the secondary-ion inten-ities versus sputtering time from surfaces for these three speci-ens. In the electrodeposited pure DTMS film (Fig. 9(a)), the Si

ignal first decreases then increases during the first 300 s sputter-ng, afterward it sharply drops to nearly zero while the Al signalapidly increases and then remains constant. This phenomenonndicates the existence of a clear Al/silane film interface. Theime required for the complete removal of pure silane film is00 s. The complete sputtering times required for SiO2-filledomposite films (∼1100 s for dip-coated and ∼1400 s for elec-rodeposited one as shown in Fig. 9(b) and (c)), however, are faronger than that for pure DTMS film, indicating the promotionf film formation by silica presence or (and) the existence of thearticles themselves in silane films. This may be the main reasonor the improvement of protectiveness of silica-filled films. Moreffective promotion is observed in electrodeposition process asndicated by bigger thickness and richer Si secondary-ion inten-ity. In contrast to the rapid drop of Si and sharp rise of Al in purelm, both the Si and Al signals change slightly and gradually

n composite films. The “continuous” distributions of elementsndicate the ambiguous boundaries between substrate and com-osite silane films, as proposed in our previous paper [36]. Theailure to form a clear Al/composite film interface is undoubtedlyssociated with the presence of silica particles in the compositelms. Moreover, the “continuous” declining of Si element over

he wide thickness may suggest the continuous distribution ofilica particles in the whole depth of composite films.

If deeply analyze the depth profiles, one can observe fourregions I–IV) and three (regions II–IV) distinct regions forlectrodeposited and dip-coated films, respectively. Region IVorresponds to the bulk of Al substrate, and region III is char-cterized by the inner dense layer of silane films. In region II,owever, Si element is found to be lowered outwards the surface.imilar phenomena were found in the depth profiles of silane

lms measured by Auger electron spectroscopy (AES) [27,37]nd scanning Auger microscopy [38]. One of the possibilities ishat the formed external layer is less compact as compared withhe inner one. The “loose to dense” structure across the whole

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ig. 9. SIMS depth profiles obtained on aluminum covered with electrode-osited pure DTMS film prepared at −0.8 V (a), dip-coated DTMS/70 �g/L SiO2

lm (b), and electrodeposited DTMS/70 �g/L SiO2 film prepared at −0.8 V (c).

ayer is observed in all three samples (Fig. 9(a)–(c)). Unfortu-

ately, it is still unclear in the current stage for the appearancef region I, where the obviously higher content of Si elements presented on the top surface of electrodeposited films. Moreeep investigation is in progress in our laboratory.

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. Conclusions

Silica nanoparticle-filled DTMS films were successfullyrepared by electrodeposition technique. Experimental resultshowed that even tiny amounts of silica nanoparticles≤70 �g/L) could enhance the protectiveness of DTMS-treatedluminum, while higher amounts of silica nanoparticles wouldake the film porous, leading to the facilitation of electrolyte

enetration and the deterioration of film protectiveness. DTMSlms were observed to be greatly thickened by loading silicaanoparticles which were found continuously distributing in thehole depth of composite films. Electrodeposition at cathodicotentials facilitates the silane films formation by producingH− ions and thereby enhances their protective properties, but

pplying extra negative cathodic potentials causes intensive evo-ution of H2 gas which attacks the electrode surface, leadingo the non-uniformity, high porosity and low protectiveness ofilane films. Silica nanoparticles were supposed to participate inhe cathodic electrodeposition of DTMS films, which may fur-her enhance the films’ thickness and protectiveness. “Criticalathodic potential” (−0.8 V/SCE) and “critical silica content”70 �g/L) were both observed, at which the film prepared per-orms the highest protectiveness.

cknowledgements

This work was supported by National Natural Science Foun-ation of China (nos. 50571090 and 50499336) and Zhejiangrovincial Natural Science Foundation (no. Y404295). Theuthors also gratefully acknowledge the financial support fromhe Chinese State Key Laboratory for Corrosion and Protection.

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