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Progress in Organic Coatings 58 (2007) 127–135

Nanoporous titania interlayer as reservoir of corrosion inhibitorsfor coatings with self-healing ability

S.V. Lamaka a,∗, M.L. Zheludkevich a, K.A. Yasakau a, R. Serra a,S.K. Poznyak a,b, M.G.S. Ferreira a,c

a University of Aveiro, CICECO, Department of Ceramics and Glass Engineering, 3810-193 Aveiro, Portugalb Research Institute for Physical Chemical Problems, Belarusian State University, 220050 Minsk, Belarus

c Instituto Superior Tecnico, ICEMS, Department of Chemical Engineering, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Received 15 June 2006; accepted 30 August 2006

Abstract

Active corrosion protection of AA2024-T3 alloy has been provided by an environmental-friendly, well adhering pre-treatment system consistingof an inhibitor-loaded titanium oxide porous layer and a sol–gel based thin hybrid film. A novel approach aimed at developing a nanoporousreservoir for storing of corrosion inhibitors on the metal/coating interface has been proposed. The nanostructured porous TiO2 interlayer wasprepared on the aluminium alloy surface by controllable hydrolysis of titanium alkoxide in the presence of template agent. The morphology andthe structure of the TiO2 film were characterized with TEM, EDS, SEM, and AFM techniques. Different ways of loading of the inhibitor in thepre-treatment coating were discussed. In contrast to direct embedding of the inhibitors into the sol–gel matrix, the use of the porous reservoireliminates the negative effect of the inhibitor on the stability of the hybrid sol–gel matrix. TiO2/inhibitor/sol–gel systems show enhanced corrosionprotection and self-healing ability confirmed by EIS and SVET measurements.© 2006 Published by Elsevier B.V.

Keywords: Inhibitor; Titanium oxide; Pre-treatment; Self-healing; Nanoreservoir

1. Introduction

In view of the forthcoming date of the prohibition of useof chromate-containing anticorrosion coatings, the number ofreports dealing with the exploration and development of effec-tive substitutes for chromates has considerably increased in thelast 5–8 years. Among them the sol–gel or ceramic like thinfilms takes a significant part. The recent works in the field ofcorrosion protection of the metallic substrates by sol–gel filmsare reviewed in [1]. It has been reported in [2,3] that the cover-ing of the surface of stainless steel with TiO2 nanoparticles byusing a dip-coating technique leads to sufficient improvement ofcorrosion resistance properties. However, the protection mech-anism of such a coating implies that they can work only untiltheir surface is intact. As soon as even small defects appear inthe sol–gel or TiO2 layer, these coatings are not longer ableto protect the exposed zone. The development of new coating

∗ Corresponding author. Tel.: +351 234 378 146; fax: +351 234 378 146.E-mail address: [email protected] (S.V. Lamaka).

systems with active corrosion protection instead of the carcino-genic chromates is an issue of prime importance for the widerange of industrial applications where effective corrosion protec-tion is required. Active protection implies not only mechanicalcovering of the protected surface with a dense barrier coatingbut also provides self-healing properties which allow contin-ued protection even after partial damage of the coating. Theseproperties can be achieved by impregnation of specific corrosioninhibitors into the coating system. Indeed, inhibitors can providelocal anticorrosion protection in places where the main coatingis damaged.

Usually the coating system consists of several layers includ-ing a pre-treatment layer applied directly to the metal surfaceand one or several organic coating layers. Although the mainfunction of pre-treatment is to provide a good adhesion of paintto the metal, enhancement of its protective properties is verydesirable as well. Embedding of inhibitors exactly in this layerhas benefit since this layer is the closest to the protected metal.The main problem which can arise when adding the extraneoussubstances to the coating is the disturbance of its structure whatleads to serious deterioration of protective properties of the

0300-9440/$ – see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.porgcoat.2006.08.029

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128 S.V. Lamaka et al. / Progress in Organic Coatings 58 (2007) 127–135

pre-treatment layer. Several attempts have been made to enrichthe pre-treatment coating with the inhibitors [4–9].

In our previous paper [9], for the first time it was shownthe effect of active corrosion protection for an aluminium alloycoated with an inhibitor-loaded titanium oxide layer and sol–gelfilm. The influence of the surface structure of the substrate onthe assembling of titanium oxide nanoparticles on the alloy aredemonstrated as well.

Herein, this approach is further developed, the analysis ofthe chemical structure of the titanium oxide is presented andthe results of different ways of loading the sol–gel thin filmswith inhibitors are compared. The use of a well adhering thinnanoporous layer of TiO2 as a reservoir for storage and releaseof organic and inorganic corrosion inhibitors is proposed.

2. Experimental section

2.1. Samples preparation and sols syntheses

The AA2024-T3 aluminium alloy was cleaned with analkaline cleaner TURCOTM, nitric acid and distilled water asdescribed elsewhere [9]. The alloy surface for AFM, Ramanand FTIR measurements was polished sequentially with differ-ent grate of CafroTM polishing diamond pastes down to 500 nm.All syntheses and measurements were carried out at ambienttemperature if other is not specified.

TiO2 as an interlayer and sol–gel films as a pre-treatment weresynthesized and applied on aluminium substrates as describedbelow.

The titanium oxide based sol was prepared by hydrolysis oftitanium(IV) iso-propoxide. Ti(OPri)4 was added to the ethanolsolution of the complexing agent Pluronic F 127 in 1:30 weightratio. The final concentration of the Ti(OPri)4 was 4 × 10−2 M.After stirring for 1 h, the solution was hydrolyzed by addition ofacidified water (pH ∼ 1, HNO3) in 1:25 precursor to water molarratio. Hydrolysis was lasted for 30 min. Then aluminium plateswere dip-coated in the acquired sol. The time of deposition inthe sol was 3 min followed by withdrawal with speed 18 cm/min.

The hybrid sol–gel films were synthesized using a con-trollable sol–gel route mixing two different sols. The first solwas prepared by hydrolysis of 70% zirconium(IV) propoxide(TPOZ) solution in iso-propanol mixed with ethylacetoac-etate in 1:1 volume ratio. After 20 min the synchronousultrasonic agitation was started and water with pH 0.5 wasadded in stoichiometric quantity needed for TPOZ hydrol-ysis and condensation, which continued for 90 min. Thesecond organosiloxane sol was prepared hydrolyzing (3-glycidoxypropyl)-trimethoxysilane (GPTMS) in iso-propanolby addition of acidified water (pH 0.5, HNO3) in 8:8:1 volumeratio for 1 h under the stirring. The hybrid solution was obtainedby mixing of the zirconium-based sol with the organosiloxaneone in 1:2 volume ratio. The final sol–gel solution was stirredunder ultrasonic agitation for 1 h and then aged for 1 h. Thesol–gel films were produced by dip-coating of the aluminiumsubstrates at a dipping speed of 18 cm/min and exposition time100 s. Then all samples were cured at 130 ◦C for 1 h in air forcross-linking and gelation of sol–gel and solvent evaporation.

To ascertain the contribution of each component on theprotective properties of sol–gel with inhibitors-doped titaniumoxide interlayer, five types of samples were prepared:

(I) aluminium alloy coated with sol–gel film only;(II) alloy immersed for 1 h successively in Ce(NO3)3 (5 g/l

of water) and benzotriazole (20 g/l of ethanol) solutions,dried for 30 min at 80 ◦C and coated with sol–gel film;

(III) alloy treated with TiO2 sol, dried at 120 ◦C for 1 h andcoated with sol–gel film;

(IV) alloy treated with TiO2 sol, dried at 120 ◦C for 1 h, suc-cessively immersed for 1 h in Ce(NO3)3 (5 g/l aqueous)and benzotriazole (20 g/l ethyl alcohol) solutions, driedfor 30 min at 80 ◦C and coated with sol–gel film;

(V) alloy coated with a sol–gel film using a sol to which0.13 wt.% of benzotriazole was directly added.

2.2. Characterization and measurements

A Bruker Equinox IFS 55 infrared spectrophotometer wasused to collect the Fourier transform infrared (FTIR) absorbancespectra of solutions simulating the synthesis of titanium sol,while BIO-RAD FTS 6000 was employed for measurements ofFTIR spectra of TiO2 film deposited on the aluminium alloy. Forthe preparation of the samples, a polished aluminium substratewas coated with the above described titanium oxide sol for sixtimes to increase the thickness of the layer and consequentlyamplify the signal. After each dipping in titanium oxide sol, thealuminium substrate was dried at 120 ◦C for 30 min. Samplesfor Raman and XRD measurements were prepared in the sameway.

Raman spectra were recorded at room temperature in air usinga SPEX 1401 spectrometer with an Ar-ion laser (λ = 514.5 nm).The diameter of the laser beam was 10 �m; the beam powerat the sample surface was 100 mW. Spectra of pure TiO2 wererecorded using commercial ultra pure rutile and anatase phases.

X-ray diffraction (XRD) was performed using a PhilipsX’Pert instrument (Cu K� radiation, 2θ = 10–70◦, step 0.05◦,50 s/step).

The morphology and structure of the obtained TiO2 coat-ings and nanoparticles were specified by scanning electronmicroscopy (SEM) and transmission electron microscopy(TEM). SEM images were taken with a Hitachi S-4100 system,beam energy is 25.0 keV. To prepare TEM samples one drop oftitanium oxide sol was dripped on the tiny (0.1 mm × 1.0 mm)strips of aluminium alloy on glass base, dried at 120 or 450 ◦C,included in Agar Resin and ultramicrotomed by a diamond knife“Diatome”. The thickness of the sample prepared in this way wasaround 30 nm.

Electrochemical impedance spectroscopy (EIS) was used toestimate the corrosion protection performance of the preparedcoatings during immersion in 0.05 M NaCl solution. EIS mea-surements were carried out in a Faraday cage with a GamryFAS2 Femtostat coupled with a PCI4 Controller at open cir-cuit potential with applied 10 mV sinusoidal perturbations in the100 kHz to 10 mHz frequency range with 10 steps per decade. Athree-electrode cell was used, consisting of a saturated calomel

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reference electrode, a platinum foil counter electrode and thecoated AA2024 substrate as a working electrode with a surfacearea of 3.4 cm2.

Scanning vibrating electrode technique (SVET) measure-ments were performed with Applicable Electronics Apparatusto observe cathodic and anodic currents based on local flows ofions. The scanned area was 2 mm × 2 mm.

3. Results and discussion

3.1. Chemical composition and structure of TiO2 layer

Chemical reactions occurring during the hydrolysis oftitanium alkoxides in the presence of various complexagents such as alkoxy- and aminoalcohols, �-diketones, �-ketoesters, phosphonic acids, carboxylic, including fatty- andamino-acids, non-ionic surfactants (alkylamines, oligomericalkyl-poly(ethylene oxide)), were discussed in detail in manypapers [10–13]. Formation of complexes with non-ionic block-copolymer Pluronic F127 decreases the speed of Ti(OPri)4hydrolysis, which proceeds concurrently with water and alcoholcondensation and polycondensation of titanium propoxide lead-ing to generation of semi-condensed titanium oxo-propoxide.Chemical conversions which can take place in the system con-taining Ti(OPri)4 and Pluronic F 127 are briefly discussed in[9].

FTIR spectra of initial substances (Ti(OPri)4, Pluronic F 127and CHCl3 used as a solvent) are shown in Fig. 1. They are in agood agreement with the spectra published in the monographs[14–16] and Sigma–Aldrich online catalog-database [17]. Well-defined absorbance peaks are observed at 1120, 1001, 948, 852,and 621 cm−1 before the hydrolysis of Ti(OPri)4 and can beassigned to ν (R–O)–Ti vibration. The vibrations in infraredregion of the reactive mixture were assigned by comparison withIR spectra of individual substance and literature sources [18–21].After interaction of Ti(OPri)4 with Pluronic F 127 and hydroly-sis, most of the changes occurred in the 550–1200 cm−1 region

Fig. 1. FTIR spectra of individual components and model reactive mixture ofsemihydrolyzed Ti(OPri)4 with Pluronic F 127 as well as transmittance spectraof the aluminium alloy substrate covered with titania sol and heat treated at120 ◦C.

due to the partial opening of (R–O)–Ti bonds and the formationof new Ti–O–Ti bridges. Thus, the absorbance of the most inten-sive peak in this group at 1120 cm−1 was essentially decreasedand a wide peak from ν C–O–C vibration of Pluronic locatedat 1160–1060 cm−1 became more pronounced again. Weakerpeaks located at 1001, 852 and 621 cm−1 almost disappearedand the peak at 948 cm−1 was doubled. The latter can be relatedto Ti–O stretching vibration [19]. The appearance of a new peakat 817 cm−1 can be assigned to the formation of Ti–O–Ti bridgesdue to reaction of Ti(OPri)4 condensation [18]. Moreover, a riseof the absorbance at 817 and 948 cm−1 can be explained by lib-eration of free iso-propanol in the reaction solution. Peaks at820 and 948 cm−1 are the most intensive in its spectrum. Bothprocesses Ti–O–Ti bonds formation and iso-propanol evolutionare likely to contribute in the appearance of these peaks, sincethey run in the direction of Ti(OPri)4 polycondensation.

Analysis of the position alteration of the diagnostic peaksin 1200–550 cm−1 region allows to conclude that the solutionof the titanium sol, deposited on aluminium substrates, con-tains the products of the titanium tetraisopropoxide hydrolysisand polycondensation and likely some amount of unhydrolyzedTi(OPri)4 [18]. The formation of complexes of PEO–PPO–PEOwith semi-hydrolyzed Ti(OPri)4 is accompanied by appearanceof hydrogen bonds between OH groups of hydrolyzed Ti(OPri)4and oxygen of PEO or PPO [22]. Basing on the presented spectra,which reflect the reactive mixture where O–H stretching can cor-respond to TiOH, PrOH groups as well as H2O, discussion aboutformation of hydrogen bonds seems not appropriate. Neverthe-less, the strong interaction under similar conditions between thetitania sub-units and the polymer has been reported in manypapers [11,20–23] thus, the presence of titania complexes withthe templating agent is out of question.

Although characteristic peaks of pure TiO2 (wide intensivepeaks for anatase at 650 and rutile at 660 cm−1) were not exhib-ited in the spectra of model solutions, its presence becameevident on the aluminium substrate after heating the samples at120 ◦C (Fig. 1). A wide peak at 600–900 cm−1 can be explainedby overlapping of stretching mode of vibration frequencies ofAl–O at 750–850 cm−1 with those of Ti–O at 750–650 cm−1

[24]. Template signals of ν CH at 2923 and 2870 cm−1 and ofδ CH at 1470–1230 cm−1 and a wide intensive ν C–O–C peakat 1096 cm−1 are also presented on the surface of aluminiumsubstrate after heat treatment [25].

Raman spectrum of the same sample prepared and heated at120 ◦C as for FTIR analysis and the spectra of pure rutile andanatase are presented in Fig. 2. Both the anatase and rutile phasesare evident in the spectrum, although the formation of the rutilephase is not typical at such temperature [26,27]. All the peaks inthe spectra are characteristic of vibrations of Ti–O bonds. Themost intensive band at 151 cm−1 can be unambiguously assignedto the anatase phase of TiO2, while Raman peaks centered at 440and 614 cm−1 contain convincing signs of both rutile and anatasestructure. A peak at 511 cm−1 confirms the presence of anatase-like structure, and at 254 cm−1 rutile-like one [16,23,28–30].Thus, the Raman spectroscopy data indicate the presence ofanatase and rutile nanocrystals in the template-prepared TiO2nanoporous film.

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Fig. 2. Raman spectra (a) of the pure TiO2 modification anatase and rutile (thespectra are in a good agreement with the data [26–28]) and (b) of the TiO2 layerdeposited by dip-coating on the polished aluminium alloy substrate.

X-ray diffraction pattern (Fig. 3) of TiO2 layer depositedon aluminium substrate and heated at 120 ◦C showed diffrac-tion peaks corresponding to aluminium, native aluminium oxide,template agent Pluronic F 127, and both TiO2 phases rutile aswell as anatase, in a good agreement with the data of the Ramanspectrum. The presence of organic shells on the TiO2 parti-cles provides them good adhesion to the sol–gel film and thealuminium alloy.

Fig. 3. X-ray diffraction pattern of TiO2 layer deposited on the aluminium alloysubstrate.

Fig. 4. SEM micrographs of alloy AA2024 coated with TiO2 film.

The morphology and porosity of the titanium oxide inter-layer is also an issue of prime importance from the standpointof utilization of this layer as a reservoir for active anticorrosionspecies, since increased porosity means an enhanced capacityas a reservoir. SEM micrograph (Fig. 4) demonstrates that thestructure of the porous TiO2 layer which covers the alloy surfaceconsists of nanoparticles with a diameter 80–120 nm. A higherresolution cross-section image of titanium oxide layer was madeby TEM for a sample heated at 120 ◦C (Fig. 5a). Elemental com-position was proven with EDS analysis (Fig. 5a, inset). Fig. 5ashows that the titanium oxide layer has its own nanostructure,which becomes more pronounced after removing of the organictemplate agent at 450 ◦C (Fig. 5b).

Thus, the TiO2 layer has a porous structure at the micro-and nano-level. The formation of a nanoporous TiO2 structurewith an enlarged surface on the metallic substrate offers greatopportunity for loading of this layer with active substances.

Although the increase of the TiO2 porosity is very desir-able from the point of view of loading capacity of this TiO2intermediate layer, a high temperature such as 450 ◦C drasti-cally changes the structure and composition of the substratematrix, intermetallic inclusions and thickness of native alu-minium oxide. Heat treatment in this temperature region isnot employed on manufacturing and processing of aluminiumalloys. Therefore, the samples consecutively coated with TiO2layer and loaded with inhibitors were thermally treated at 120 ◦Conly.

3.2. Active corrosion protection

All the tested samples were coated with equal sol–gel filmsat the same time. The thickness of sol–gel films is about 2 �m.It has been determined by cross-section analysis with SEMobservation and described in [6]. The viscosity of the sol–gelsolution after preparation and before heating was determinedby a rotational CSL Rheomiter at a shear rate of 600 s−1 andamounts 9.5 cP. The evolution of the viscosity of this hybrid solis described elsewhere [6].

Benzotriazole and its derivates as well as salts of Ce3+ are wellknown and investigated inhibitors for AA2024-T3 [31–33].

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Fig. 5. TEM micrographs of alloy AA2024 coated with TiO2 film (a) cross-section of the sample heated at 120 ◦C for 1 h; inset demonstrates EDS spectrumof titanium oxide layer; (b) the sample was calcinated at 450 ◦C for 3 h.

The electrochemical impedance spectroscopy is one of themost intensively used and powerful technique for investigationand prediction of the anticorrosion protection [34,35]. EIS wasused in this work to estimate the evolution of barrier and activeanticorrosion properties of the systems under study. The fre-quency dependence of the complex impedance of the coatedsubstrate permits effective evaluation of the different compo-nents of the system such as capacitance and resistance of theprotective layers, polarization resistance and double layer capac-itance. Representation of these parameters as a function of timeallows assessment of the corrosion protection performance ofthe coating system.

The low frequency impedance is one of parameters which canbe easily used to compare corrosion protection performance ofdifferent systems. Higher impedance demonstrates better pro-tection. Fig. 6 shows the Bode plots for the different coatingsafter 14 days of immersion in an aqueous NaCl solution. Thealloy sample coated with TiO2 nanoporous layer loaded withbenzotriazole and Ce3+ (IV) shows the highest corrosion protec-tion with the impedance module value of about 4 × 106 � cm2 at0.01 Hz. The low frequency impedance in the case of the sol–gelfilm deposited directly on the alloy surface (I) is almost an order

Fig. 6. Bode plots of aluminium alloy substrates coated with different pre-treatments after 14 days of immersion in 0.05 M NaCl. Sample V was immersedin 0.005 M NaCl solution.

of magnitude lower in comparison with the sample without theTiO2 layer. Sample II, where inhibitors were applied directlyon the alloy surface and sample III in which sol–gel film wasdeposited on undoped TiO2 layer take middle order. The weak-est corrosion protection demonstrates aluminium alloy coatedwith the hybrid sol–gel film directly doped with benzotriazole(sample V), showing the impedance values of only 105 � cm2

in spite of the one order lower electrolyte concentration used forimmersion in this case.

For quantitative estimation of the corrosion protection, exper-imental impedance spectra were fitted using equivalent circuits.Fig. 7 represents different stages of the formation of corrosiondefects and respective equivalent circuits which can be used formodelling of these processes. Constant phase elements (CPE)instead of capacitances were used in all fittings presented in thiswork. Such modification is obligatory in the case the phase angleof capacitor is different from −90◦. The capacitance values forcoating, oxide and double layer elements can be calculated usingthe following equation:

C = Q(ωmax)n−1

where ωmax is the frequency at which the imaginary impedancereaches a maximum for the respective time constant, Q and nare parameters for the constant phase element that characterizesthe capacitance of the inhibitor film [36].

During the initial 1–5 h of immersion, when the resistance ofsol–gel coatings is the highest, the capacitance of sol–gel filmhas the lowest numerical values and the intermediate oxide layeris intact, impedance spectra were modelled using the schemeshown at Fig. 7A. Rsolut is the resistance of solution, Rcoat andCcoat (which consists of Qcoat and n-coat in the electrical circuit)are the resistance and capacitance of the sol–gel film. Wateruptake into the sol–gel film and penetration of active chlorideions in the oxide layer lead to its partial destruction. At thisstage the impedance spectra were fitted with an equivalent circuitshown in Fig. 7B, which contains additionally the parameters ofthe oxide layer. At the first signs of the corrosion process which

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Fig. 7. Schematic representation of the different stages of defect formation and respective equivalent circuits.

are accompanied by appearance of a new time constant in thelow frequency region, the spectra were fitted with the circuitdepicted in Fig. 7C, where the polarization resistance Rpolar anddouble layer capacitance (Qdl and n-dl) are added. The circuitdepicted in Fig. 7D with a Warburg element was used for datafitting with extensive corrosion, which, in fact, was observedonly in the case of the sample V.

The resistance and capacitance of the dielectric sol–gel filmdepend on the porosity of the film, its crack ability and amount ofabsorbed water [37–43]. The evolution of resistance and capac-itance of the sol–gel film are presented in Fig. 8. As it canbe concluded from Figs. 6 and 8, for all samples, except V,the resistance of the sol–gel film, Rcoat, after 14 days (336 h)of immersion is similar (about 103 � cm2). The low corrosion

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Fig. 8. Evolution of the sol–gel films parameters: capacitance Ccoat and resis-tance Rcoat during 14 days of immersion in 0.05 M NaCl. Sample V wasimmersed in 0.005 M NaCl solution.

protection of sol–gel coatings doped with benzotriazole can beexplained by the interfering effect of the inhibitor with the cross-linking process during sol–gel synthesis. The evolution of thecapacitance of the sol–gel film during the immersion (Fig. 8)confirms this conclusion. Continuous increase of the capaci-tance for inhibitor doped sol–gel film (sample V) is the evidenceof continuous deterioration of the sol–gel film by increasingthe amount of absorbed water, while for others samples afterthe initial water uptake, the capacitance changes insignificantly.Comparison of Rcoat and Ccoat for samples I–IV allows to con-clude that the barrier properties of sol–gel films of these samplesare rather close.

Fig. 9. Resistance evolution of the oxide intermediate layer for all types of testedsamples during 14 days of immersion in 0.05 M NaCl. Sample V was immersedin 0.005 M NaCl solution.

Fig. 10. General scheme of the pre-treatment system with inhibitors doped TiO2

layer and mechanism of self-healing after sol–gel and oxide film damage.

Fig. 9 presents the comparison of evolution of intermedi-ate oxide layer resistance Rox, formed between sol–gel andaluminium. This layer is the ultimate protection, since afterits disruption the corrosion process is started on the metal.Breakdown of this layer provides access of aggressive speciesprovoking metal corrosion. After 14 days of immersion, sampleIV (aluminium substrate coated with inhibitor-doped TiO2 layerand pure sol–gel film) demonstrates the most stable behaviour

Fig. 11. SVET maps of the ionic currents measured 200 �m above the surface of the AA2024 coated with (c and e) sol–gel film (I) and (d and f) sol–gel filmpre-treated with inhibitors loaded TiO2 layer (IV). (a) and (b) present microphotos of samples I and IV with artificial defects. Scanned area: 2 mm × 2 mm.

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with a final Rox value of around 8 × 106 � cm2. It is at least oneorder of magnitude higher than for sample I (Al coated with pureundoped sol–gel film) and eight times higher than for sample II(sol–gel film with intermediate layer consisting of inhibitor only)and III (sol–gel film with intermediate layer consisting of TiO2only). The recovery of the damaged oxide layer was observedafter 4 days of immersion of the sample IV. The oxide layer wassuccessfully repaired due to self-healing effect.

A schematic of the constructed protective system is shownin Fig. 10. Self-healing is the consequence of the releaseof the inhibitors from the TiO2 layer, promoted by wateraccess followed by fast precipitation of interaction productsof inhibitors with first products of corrosion process in thedefect.

To confirm the self-healing effect which was revealed by EIStesting, the same coating systems but with an artificial defectwere investigated by means of SVET method. The SVET mea-surements proved superior corrosion protection of the coatingwith the TiO2 nanoreservoir layer. The distribution of local cur-rents along the alloy surface was measured for the samples Iand IV. After 24 h of immersion in 0.05 M NaCl solution twodefects (around 100 �m in diameter) were made on both samples(Fig. 11a and b). Well-defined cathodic activity due to active cor-rosion processes exhibited in the defects formed on the surfaceof the AA2024 treated with sol–gel film only, while no cor-rosion activity appeared on the alloy pre-treated with inhibitordoped TiO2 and covered with sol–gel film (Fig. 11c and d).Even after 24 h of immersion this situation lasted—no activityappeared on the sample IV, while the cathodic current densityincreased in the case of sample I (Fig. 11e and f). The self-healing effect is exhibited due to the release of inhibitors fromthe TiO2 layer, which plays the role of inhibitor container, withvery good compatibility to both metal substrate and sol–gelfilm.

4. Conclusions

The negative role of benzotriazole on the stability and barrierproperties of the sol–gel film confirms the unsuitability of directinhibitor incorporation in the sol–gel hybrid matrix as a wayto impart the active corrosion protection to the pre-treatmentsystem.

Using TiO2 nanoporous layer, which contains rutile andanatase phases, for storage and release of corrosion inhibitoreliminates the negative effect of the inhibitor on the stability ofthe hybrid sol–gel matrix.

Different ways of loading of sol–gel coating and intermediatetitanium oxide film with corrosion inhibitors are considered.

The most effective way, which provides the active corrosionprotection of the aluminium alloy, is the deposition of a titaniumoxide layer loaded with inhibitors and coated with sol–gel film.

The TiO2 nanostructured reservoir layer for corrosioninhibitors imposes the self-healing properties to the hybridsol–gel films deposited on 2024 aluminium alloy and can bea promising substitute for the environmentally unfriendly chro-mate pre-treatments.

Acknowledgements

The authors acknowledge the financial support from thePortuguese “Fundacao para a Ciencia e Tecnologia (FCT)”(contract POCTI/CTM/59234/2004). S.V. Lamaka, M.L. Zhe-ludkevich, K.A. Yasakau and R. Serra also gratefullyacknowledge their personal FCT Grants SFRH/BPD/12538/2003, SFRH/BPD/20537/2004, SFRH/BD/25469/2005 andSFRH/BD/21541/2005. We thank Dr. M.F. Montemor and P.Cecılio for the SVET measurements; Dr. D. Shchukin for theFTIR investigation in transmittance mode of the surface of alu-minium alloy coated with titanium based sol; Dr. M.P. Samtsovfor Raman measurements.

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