Layer-by-Layer Assembled Nanocontainers for Self-Healing Corrosion Protection**

Transcription

1 DOI: /adma Layer-by-Layer Assembled Nanocontainers for Self-Healing Corrosion Protection** By Dmitry G. Shchukin,* Mikhail Zheludkevich, Kiryl Yasakau, Sviatlana Lamaka, Mario G. S. Ferreira, and Helmuth Möhwald The corrosion of metals is one of the main destructive processes that leads to huge economic losses. Polymer coating systems are normally applied on a metal surface to provide a dense barrier against the corrosive species in order to protect metal structures from corrosive attack. When the barrier is damaged and the corrosive agents penetrate to the metal surface the coating system can not stop the corrosion process. The most effective solution so far for designing anticorrosion coatings for active protection of metals is to employ chromate-containing conversion coatings. [1] However, hexavalent chromium species are responsible for several diseases, including DNA damage and cancer, [2] which is the main reason for banning Cr 6+ -containing anticorrosion coatings in Europe from The deposition of thin inorganic or hybrid films on metallic surfaces has been suggested as a pretreatment to provide an additional barrier against the corrosion species and mainly to improve adhesion between the metal and polymer coating system. [3] The films are usually deposited by the plasma polymerization technique or the sol gel route. Sol gel-derived thin films that contain either inorganic (phosphates, vanadates, borates, and cerium and molybdenum compounds) or organic (phenylphosphonic acid, mercaptobenzothiazole, mercaptobenzoimidazole, triazole) inhibitors are investigated as substitutes for chromates. [3a e] Among them, the highest activity is shown for sol gel coatings with a cerium dopant of a critical concentration in the wt % range. However, the negative effect of the free inhibitor occluded in the sol gel matrix on the stability of the protective film is observed for all types [*] Dr. D. G. Shchukin, Prof. H. Möhwald Max Planck Institute of Colloids and Interfaces Potsdam (Germany) dmitry.shchukin@mpikg.mpg.de Dr. M. Zheludkevich, K. Yasakau, Dr. S. Lamaka, Prof. M. G. S. Ferreira Department of Ceramics and Glass Engineering CICECO, University of Aveiro Aveiro (Portugal) [**] This work was supported by EU FP6 project Nanocapsule (contract #MIF1-CT ), FCT project POCI/CTM/59234/2004. D.S. acknowledges the NATO Collaboration Linkage Programme (grant #CLG ). The authors thank R. Pitschke for TEM analysis, H. Zastrow and A. Praast for electrophoretic mobility measurements, F. Montemor and P. Cecilio for SVET experiments. Supporting Information is available online from Wiley InterScience or from the author. of inhibitors (for instance, a higher concentration of Ce leads to the formation of microholes in the sol gel film [3f] ). This shortcoming calls for the development of nanometer-scale reservoirs to isolate an inhibitor inside and prevent its direct interaction with the sol gel matrix. Nanoreservoirs should be homogeneously distributed in the film matrix and should possess controlled and corrosion-stimulated inhibitor release to cure corrosion defects. Mixed-oxide nanoparticles (e.g. ZrO 2 /CeO 2 ), [4] b-cyclodextrin-inhibitor complexes, [3c] hollow polypropylene fibers, [5] and conducting polyaniline [6] have been explored as prospective reservoirs for corrosion inhibitors to be incorporated in the protective film. The common mechanism of the nanoreservoir activity is based on the slow release of inhibitor triggered by corrosion processes. Ion exchangers have also been investigated as smart reservoirs for corrosion inhibitors. Chemically synthesized hydrocalmite behaves as an anion exchanger: adsorbing corrosive chloride ions and releasing corrosion-inhibiting nitrite anions. [7] Despite considerable efforts devoted to the development of new, complex anticorrosion systems, practically no single solution is able to fulfill the requirements of sufficient corrosion protection while avoiding chromates in the coating, especially in the case of aluminum alloys used for aerospace applications. The recently developed technology of layer-by-layer (LbL) deposition [8] of oppositely charged species (polyelectrolytes, nanoparticles, enzymes, dendrimers) from their solutions on the substrate surface represents an interesting approach to prepare reservoirs with regulated storage/release properties assembled with nanometer-thickness precision. LbL coatings are of practical interest in photonics (optical filters, luminescent coatings), [9] electrocatalysis (electrodes for DNA transfer, enzyme-catalyzed oxidation), [10] as membranes, [11] and chemical reactors. [12] LbL-assembled polyelectrolyte multilayers reveal controlled permeability properties. Depending on the nature of the assembled monolayers, the permeability of multilayer films can be controlled by changing ph, ionic strength, and temperature, or by applying magnetic or electromagnetic fields. [13,14] Polyelectrolyte assemblies have never been used in corrosion-protection coatings, although storage of corrosion inhibitors in polyelectrolyte multilayers can confer several advantages: they can prevent a negative effect of the corrosion inhibitor on the stability of the coating, decrease the influence of the coating polymerization on the inhibitor, and provide intelligent release of the corrosion inhibitor, as the permeability WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18,

2 of the polyelectrolyte assemblies can be regulated by ph, humidity, and light. [14,15] A change of ph is a more preferable trigger for corrosion protection systems since, as is well known, corrosion activity leads to local changes of ph in the cathodic and anodic areas. [3b] A smart coating that contains polyelectrolyte reservoirs may use the corrosion reaction to release the corrosion inhibitor. Polyelectrolyte reservoirs in sol gel anticorrosion coatings need to fulfill two significant conditions: they should be compatible with the sol gel matrix material to prevent matrix distortion and have a nanometer-scale size to uniformly distribute the loaded inhibiting species in the matrix. These requirements challenge the search for nanometer-scale structures built by the LbL approach. In the present study, the selfhealing effect of LbL-assembled nanoreservoirs embedded in hybrid epoxy-functionalized ZrO 2 /SiO 2 sol gel coatings deposited onto the aluminum alloy AA2024 is investigated. As nanoreservoirs, 70 nm SiO 2 particles coated with poly(ethylene imine)/poly(styrene sulfonate) (PEI/PSS) polyelectrolyte layers are employed. The inhibitor, benzotriazole, is entrapped within the polyelectrolyte multilayers during the LbL-assembly step; its release is initiated by ph changes during corrosion of the aluminum alloy. SiO 2 nanoparticles are chosen as supporting hosts for the benzotriazole because of their ability to be incorporated inside the hybrid silica-based sol gel matrix and to preserve its structure. [4] To produce an inhibitor-loaded polyelectrolyte shell, the LbL-deposition procedure is followed for both large polyelectrolyte and small benzotriazole molecules. Initial SiO 2 nanoparticles are negatively charged, so the adsorption of positive PEI (Fig. 1) is performed during the first stage (mixing 20 ml of 15 wt % SiO 2 colloidal solution with 3 ml of 2 mg ml 1 PEI solution, 15 min of incubation). The resultant composite nanoparticles are washed after each adsorption step with distilled water. The adsorption of the second negative layer is then carried out using a PSS solution (2 mg ml 1 ) in 0.5 M NaCl. Benzotriazole is only slightly soluble in water at neutral ph, so that the adsorption of the third inhibitor layer is accomplished from an acidic solution (10 mg ml 1, ph 3). PSS/benzotriazole adsorption is repeated to increase the inhibitor loading in the LbL structure. The final nanoreservoirs have a SiO 2 /PEI/PSS/benzotriazole/PSS/ benzotriazole layer structure (Fig. 1). Taking into account the amount of inhibitor that remains in the supernatant solutions after two adsorption steps, the benzotriazole content in the final SiO 2 -based nanoreservoirs is estimated at about 95 mg per gram of SiO 2 nanoparticles. The zeta (f)-potential of the initial SiO 2 nanoparticles is negative (Fig. 2a). Electrophoretic measurements indicate the charging of the nanoparticles coated with the adsorbed polyelectrolyte or inhibitor layers upon the addition of each layer. Figure 2a shows a drastic increase of the surface charge after deposition of the first PEI layer followed by a similar decrease after PSS adsorption during the next stage. Benzotriazole deposition leads to a more positive f-potential without complete recharging of the surface. The difference between the f-potentials of nanoparticles with outermost layers of PSS or benzo- a) b) Figure 2. a) Electrophoretic mobility measurements of nanoreservoirs in water during LbL assembly. Layer number 0: initial SiO 2 ; 1: SiO 2 /PEI; 2: SiO 2 /PEI/PSS; 3: SiO 2 /PEI/PSS/benzotriazole; 4: SiO 2 /PEI/PSS/benzotriazole/PSS; and 5: SiO 2 /PEI/PSS/benzotriazole/PSS/benzotriazole. b) Growth of the particle size during nanoreservoir assembly. COMMUNICATIONS Figure 1. Left: Schematic representation of the fabrication of a composite ZrO 2 /SiO 2 coating loaded with benzotriazole nanoreservoirs. Right: The scanned topography of the resulting sol gel coating containing nanoreservoirs, using atomic force microscopy (AFM). Adv. Mater. 2006, 18, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 triazole further decreases with the deposition of a second PSS/benzotriazole bilayer. This is caused by the different molecular weight and size of the layer components. Large multicharged chains of PEI or PSS have stronger electrostatic forces and can be adsorbed in quantities sufficient to recharge the surface, while small molecules of monocharged benzotriazole only compensate for the excess of negative-charge-forming PSS/benzotriazole complexes that are insoluble in slightly acidic media. As seen in Figure 2b, the average diameter of the nanoreservoirs obtained from the light-scattering measurements increases with the layer number. For the first PEI and PSS monolayers, the increment is about 8 nm per layer. Benzotriazole layers increase the size of the nanoreservoirs by a smaller ca. 4 nm step, which confirms the electrophoretic-mobility data for the lower adsorption efficiency of benzotriazole as compared with the polyelectrolytes. Growth of the average diameter of the nanoreservoir unambiguously proves LbL assembly of the polyelectrolytes and the inhibitor on the surface of the SiO 2 nanoparticles. A transmission electron microscopy (TEM) image of the resulting SiO 2 /PEI/PSS/benzotriazole/ PSS/benzotriazole nanoreservoirs is shown in Figure 3. Nanoreservoirs are separate individual particles of ca. 100 nm diameter. The optimal number of the PSS/benzotriazole bilayers deposited onto the silica nanoparticles is two. One bilayer is not sufficient to demonstrate the self-healing effect of 400 nm Figure 3. TEM image of the final benzotriazole nanoreservoirs in a water suspension. the protective coating, while three or more bilayers drastically increase aggregation of the nanoreservoirs, which negatively affects the integrity of the protective coating and inhibitor distribution. In the final step, a suspension of benzotriazole-loaded nanoreservoirs is mixed with ZrO 2 and organosiloxane sols, following the sol gel protocol, and is deposited onto aluminum alloy AA2024 by a dip-coating procedure. Figure 1 presents the surface topography of the hybrid sol gel film with inhibitor nanoreservoirs. The uniformly distributed nanoparticles are impregnated into the sol gel film deposited onto the aluminum substrate. These particles in the sol gel matrix have a diameter of about 100 nm. Atomic force microscopy (AFM) does not show any sign of nanoreservoir agglomeration, which confirms the high stability of the reservoir suspension used to dope the hybrid sol gel film. The thickness of the film measured by scanning electron microscopy (SEM) is about lm. The electrochemical impedance measurements, which can provide a numerical evaluation of the physicochemical processes on the coated substrate during corrosion tests, [16] are taken to estimate the corrosion-protection performance of the hybrid sol gel films. A low concentration of chloride ions is used in order to decrease the rate of the corrosion processes, since the sol gel film is not a complete coating and is being used only as a pretreatment. The decreased rate of corrosion allows a more accurate estimation of the processes during the early stages. The behavior of developed composite films in 0.5 and M electrolytes is quite similar (results not shown). However, as shown below, such chloride-ion concentrations are sufficient to have a drastic corrosion impact on the untreated AA2024 substrate. Figure 4 demonstrates typical Bode plots of an aluminum alloy coated with different hybrid films (a ZrO 2 /SiO 2 film doped with nanoreservoirs, an undoped ZrO 2 /SiO 2 film, and ZrO 2 /SiO 2 films containing free inhibitor in the film matrix). The impedance spectra are obtained after 48 h of immersion in sodium chloride solution. The assignment of the components of the impedance spectra to certain processes is a very complicated issue and can not be performed without support by other localized techniques, especially when such complex systems are used. Therefore, a model proven elsewhere [17] is used for impedance spectra interpretation. The first high-frequency maximum observed at Hz is characteristic for the capacitance of the sol gel film (see equivalent circuit scheme used for fitting impedance spectra in Supporting Information, Fig. 1s). Another time-dependent process appears at medium frequencies between 0.1 and 10 Hz, depending on the hybrid film. This time constant can be clearly ascribed to the capacitance of the intermediate oxide film formed by both the native Al 2 O 3 layer and chemical Al O Si bonds. [17] The first well-defined signs of a third relaxation process appear on the impedance spectra at low frequencies of about 0.01 Hz for sol gel films with benzotriazole directly introduced into the hybrid matrix (benzotriazole concentrations of 0.13 and 0.63 wt %). This low-frequency time constant appears to be a result of the corrosion processes started on the surface of the coated substrate (see also Supporting Information, Fig. 2s). [3a] No signs of corrosion are found in the impedance spectra for the undoped hybrid film and the film loaded with nanoreservoirs, which indicates an effective barrier against corrosive species. The active corrosion processes on an alloy surface protected by sol gel films with directly introduced benzotriazole indicate the absence of any inhibition effect, despite the fact that free benzotriazole added to an aqueous solution has proven to be a very effective inhibitor of the corrosion processes on AA2024. [18] WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18,

4 Figure 4. Bode plots of the AA2024 aluminum alloy coated with different sol gel films after 48 h of immersion in M NaCl solution. Z mod is an impedance module. The resistive plateau at Hz represents the pore resistance of the hybrid sol gel coating (Fig. 4). The resistance of the undoped sol gel coating and that of the film containing nanoreservoirs show high values of about 10 4 ohm cm 2.On the contrary, the coatings with benzotriazole directly introduced into the sol gel matrix exhibit significantly lower resistances (one and two orders of magnitude in the case of 0.13 and 0.63 % of benzotriazole, respectively). This decrease of sol gel film resistance with an increase of free-benzotriazole concentration is evidence of a strong adverse effect of the inhibitor on the weathering stability of the sol gel matrix (see also Supporting Information, Fig. 3s). Another resistive part is observed at low frequencies ( Hz) in all spectra except the one doped with nanoreservoirs (Fig. 4). This part of the impedance spectrum characterizes the pore resistance of the Al 2 O 3 oxide layer. After two days of immersion, the aluminum sample coated by the sol gel film with nanoreservoirs still shows pure capacitative behavior, which confirms the intactness of the protective film. However, samples with other coatings reveal mixed capacitative/resistive behavior at low frequencies because of corrosion defects appearing in the Al 2 O 3 oxide layer. Time-dependent evolution of the Al 2 O 3 oxide-layer resistance, which is derived from impedance measurements, for the sol gel film doped with nanoreservoirs demonstrates the highest value when compared with the other systems under study (Fig. 5a). This resistance slightly decreases at the beginning of the corrosion tests and then maintains a constant value, which indicates very high corrosion protection. The sample coated with an undoped sol gel film has a low-frequency resistance one order of magnitude lower than that of the sample containing nanoreservoirs. The hybrid films with benzotriazole directly impregnated into the sol gel matrix confer significantly lower corrosion protection and show fast degradation of the intermediate-oxide layer, especially with higher contents of the inhibitor in the sol gel matrix. The fast degradation of barrier properties observed for the free-benzotriazole-impregnated hybrid film can be explained in terms of the strong influence of benzotriazole on the hydrolysis/polymerization processes during coating preparation. The capacitance of the undoped sol gel film increases by about one order of magnitude over 20 h and then shows stable behavior. In contrast, the sol gel film that contains 0.13 % of benzotriazole shows a capacitance increase of four orders of magnitude. Such a high change of the capacitance cannot be explained by water uptake alone and evidently indicates hydrolytic destruction of the sol gel matrix, which leads to the change of dielectric properties of the hybrid film. This fact confirms, once again, the strong negative effect of free benzotriazole on the barrier properties of the sol gel coating. An additional experiment was performed in order to reveal evidence of the self-healing effect of nanoreservoirs added to the coating. Artificial defects were formed in the sol gel film after immersion in M NaCl by using a microneedle (5 defects, each 50 lm in size, per sample) to provide direct ingress of the corrosive medium to the alloy surface. The impedance spectra obtained immediately after beginning the immersion of the defected samples show a decrease of impedance and a scattering of data at low frequencies that originates from active processes in the damaged zone. In the COMMUNICATIONS Adv. Mater. 2006, 18, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Figure 5. a) Evolution of Al 2 O 3 oxide-layer resistance (R ox ) with immersion time in M NaCl for a sol gel-coated alloy. b) Evolution of coating resistance in 0.05 M NaCl. The defects were formed after immersion of samples in M NaCl for 14 days, in the case of the film impregnated with reservoirs, and for 1 day, in the case of the undoped hybrid film. case of an undoped sol gel film, the formation of defects led to scattering of data over a long period; therefore, the defects were formed in this film after only 1 day in M electrolyte, whereas the composite film was artificially defected after 14 days of immersion. Figure 5b demonstrates evolution of the oxide-film resistance in 0.05 M NaCl after defect formation. The resistance of the intermediate-oxide layer of the defected coatings is calculated from the low-frequency part of the impedance spectra. The initial resistance of the undoped coating is high as a result of the shorter immersion period before defect formation. A rapid decrease of oxide resistance occurs immediately upon introduction of defects. This progressive drop of resistance demonstrates degradation of the corrosion-protection performance. The resistance of the protective coating also decreases immediately after the formation of defects in a hybrid film impregnated a) b) with nanoreservoirs. However, after the initial drop of impedance, a very important recovery of the oxide-film resistance occurs over a further 60 h of immersion. The increase of resistance clearly indicates the self-healing of defects in the nanoreservoir-doped sol gel coating. Such a self-healing action is not observed in the case of AA2024 coated with an undoped sol gel film. Hence, the self-healing effect originates from the release of benzotriazole from nanocontainers in the damaged area. The scanning vibrating electrode technique (SVET) is employed to prove the self-healing ability of nanocomposite pretreatments. This method can reveal localized corrosion activity by mapping the distribution of cathodic and anodic currents along the surface. Defects of about 200 lm in diameter are formed on the sol gel pretreated AA2024 surface, as shown in Figure 6a and b. A high cathodic current density appears immediately in the origin of the defect when the undoped coating is immersed in 0.05 M NaCl, revealing well-defined corrosion activity. The defects remain active during all tests (Fig. 6c, e, and g). The sample coated with a hybrid film doped with nanocontainers behaves completely differently. During the first 10 h, there are no remarkable currents in the defect zone (Fig. 6d). Only after about 24 h does a cathodic current appear. However, 2 h after the activity has started, effective suppression of corrosion takes place to decrease the local current density (Fig. 6h). Cathodic activity in the location of the defects becomes almost undetectable again after 48 h of continuous immersion. This effective suppression of the corrosion activity at a relatively large artificial defect formed in the coating system clearly proves the self-healing ability of the hybrid pretreatment films doped with nanocontainers. The aluminum alloy AA2024, used as a model substrate, contains Al 2 CuMg intermetallics, which are the first target for corrosive attack. The corrosive medium contacts the intermetallic surface after penetrating the Al 2 O 3 barrier layer to c) e) g) d) f) h) Figure 6. SVET maps of the ionic currents measured above the surface of the artificially defected (a,b) AA2024, coated either with an undoped sol gel pretreatment film (c,e,g) or with that impregnated by nanoreservoirs (d,f,h). The maps were obtained 5 (c,d), 24 (e,f), and 26 h (g,h) after defect formation. Scale units: lacm 2. Scanned area: 2 mm 2 mm WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18,

6 cause the chemical reaction of aluminum and magnesium with water: 2Al+6H 2 O 2Al 3+ +6OH +3H 2 (1) Mg + 2 H 2 O Mg 2+ +2OH +H 2 (2) In addition, the electrochemical evolution of hydrogen is possible on these intermetallic particles because of their cathodic potential in respect to the surrounding alloy matrix: 2H 2 O+2e 2OH +H 2 (3) Water oxidation of the aluminum alloy is accompanied by oxygen reduction, which occurs according to the following equation: O 2 +2H 2 O+4e 4OH (4) Simultaneously, the oxidation of magnesium and aluminum occurs at the anodic zones of the corrosion defect: Al Al 3+ +3e (5) Mg Mg 2+ +2e (6) As one can see, both the hydrogen evolution and the oxygen reduction processes lead to a local increase of ph at the micrometer-scale defect. [17] Nanoreservoirs can also be found in this micrometer-scale area because they have a homogeneous and densely packed distribution in the hybrid sol gel film (approximately nanoreservoirs per 1 lm 2 of the film, as estimated from the AFM image in Fig. 1). The increase of ph in the surrounding media of the nanoreservoirs leads to distortion of the polyelectrolyte layer structure and decomposition of the PSS/benzotriazole complex, [13a,19] which provokes the release of benzotriazole from the nanocontainers around the formed defect. The released benzotriazole forms a thin adsorption layer on the damaged metallic surface to sufficiently hinder the anodic and cathodic corrosion processes and passivate the alloy by replacing the damaged Al 2 O 3 film. Thus, the LbL-assembled nanoreservoirs incorporated into the hybrid matrix release the inhibitor on demand to heal the defects in the coating and provide active corrosion protection with direct feedback. In conclusion, a new approach for the formation of smart self-healing anticorrosion coatings is demonstrated based on silica nanoparticles LbL-coated with polyelectrolyte molecules, which act as nanoreservoirs for a corrosion inhibitor, incorporated in the hybrid sol gel protective coating. The nanoreservoirs increase the long-term corrosion protection of the coated aluminum substrate and provide effective storage of the inhibitor and prolonged release on demand to damaged zones, thus conferring active corrosion protection with a selfhealing ability. The use of LbL-deposited polyelectrolytes in anticorrosion coatings opens a fresh opportunity to create easily made, cost-effective, intelligent corrosion-protection systems with active feedback to the corrosion processes, which impart effective self-repairing of the corrosion defects. Experimental Materials: PSS (molecular weight, MW ), PEI (MW 2000), benzotriazole, HCl, NaCl, zirconium n-propoxide (TPOZ), 3-glycidoxypropyltrimethoxysilane (GPTMS), propanol, ethylacetoacetate, and HNO 3 were obtained from Aldrich. LUDOX HS colloidal silica (40 % suspension in water; DuPont, France) was used as a source of silica nanoparticles. The aluminum alloy AA2024 was used as a model metal substrate. Before corrosion experiments, the surface of the aluminum alloy was pretreated with the alkaline cleaner TURCO 4215 (from TURCO S.A., Spain), which contained sodium tetraborate and sodium tripolyphosphate mixed with a combination of surfactants. The water used in all experiments was prepared using a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MX cm. Preparation of the Sol Gel Film: Hybrid films doped with benzotriazole-loaded nanoreservoirs were prepared using the controllable sol gel route by mixing two different sols. The first sol was synthesized by hydrolyzing a 70 wt % TPOZ precursor in propan-1-ol mixed with ethylacetoacetate (1:1 volume ratio). The mixture was stirred under ultrasonic agitation at room temperature for 20 min to cause complexation of the precursor. The water-based suspension of benzotriazole-loaded nanoreservoirs or acidified water containing Zr/H 2 O (1:3 molar ratio) was then added to the mixture drop by drop and agitated for 1 h. The second organosiloxane sol was prepared by hydrolyzing GPTMS in propan-2-ol by the addition of acidified water in a molar ratio (GPTMS/propan-2-ol/water) of 1:3:2. The zirconia-based sol was mixed with the organosiloxane sol in a 1:2 volume ratio. The final sol gel mixture was stirred under ultrasonic agitation for 60 min and then aged for 1 h at room temperature. The sol gel system was homogenous and transparent with a light-yellow color, and was stable with time as shown by viscosity measurements (see Supporting Information, Fig. 4s). The aluminum alloy AA2024 was pretreated in an alkaline aqueous solution containing 60 g L 1 of TURCO 4215 for 15 min at 60 C followed by immersion for 15 min in 20 % nitric acid. Such treatment is used industrially for AA2024 and leads to partial dissolution of intermetallic particles. The sol gel films were produced by a dip-coating procedure by soaking the pretreated substrate in the final sol gel mixture for 100 s followed by controlled withdrawal at a speed of 18 cm min 1. After coating, the samples were cured at 130 C for 1 h. Three reference coatings were prepared in order to obtain a comparative estimation of corrosion-protection performance of the hybrid film doped with nanoreservoirs. One coating was prepared as described above without the introduction of nanoreservoirs to the TPOZ solution. Two others were synthesized by adding two different quantities of free, nonentrapped benzotriazole directly to the TPOZ solution (final concentration of benzotriazole was 0.13 and 0.63 wt %). Characterization: TEM (Zeiss EM 912 Omega) was used for the visualization of the SiO 2 -templated nanoreservoirs. Coated copper grids were employed to support the samples. The size and electrophoretic mobility measurements were performed using a Malvern Zetasizer 4 instrument. The morphology of the sol gel films that contain benzotriazole-loaded nanoreservoirs was assessed by AFM (Nanoscope Digital Instruments) equipped with a NanoScope III controller. Electrochemical impedance spectroscopy was utilized to study the corrosion-protection performance of different hybrid sol gel coatings on AA2024 immersed in NaCl solution. A three-electrode arrangement in a Faraday cage was used that consisted of a saturated calomel reference electrode, a platinum foil as counter electrode, and the exposed sample (3.4 cm 2 ) as a working electrode. The impedance measurements were performed on a Gamry FAS2 Femtostat with a PCI4 COMMUNICATIONS Adv. Mater. 2006, 18, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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