HYBRID Ce-CONTAINING SILICA-METHACRYLATE SOL-GEL COATINGS FOR CORROSION PROTECTION OF ALUMINIUM ALLOYS Mario Aparicio, Nathaly C. Rosero-Navarro, Sergio A. Pellice*, Yolanda Castro, Alicia Durán Instituto de Cerámica y Vidrio (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain * Instituto de Investigación en Ciencia y Tecnología de Materiales (CONICET). Av. J. B. Justo 4302, B7608FDQ Mar del Plata, Argentina Abstract The alternative we proposed for substituting chromates conversion coatings (CCC) for application on aluminium substrates treatable up to 150-200°C is based on the development of cerium doped silica-methacrylate hybrid polymer sol-gel coatings. These coatings have to combine barrier properties to delay the penetration of corrosion agents and inhibition properties to hinder the corrosion process because of the presence of a pore, crack or scratch in the coating. Cerium has the requirements for alternative corrosion inhibitors: the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors; they have a low toxicity and are relatively abundant in nature. A sol-gel coating based on silica-methacrylate using silicon alkoxides, monomers and modified silicon alkoxides offers the opportunity to design a structure with the adequate level of cross-linking to optimize the cerium diffusion in order to provide long-term corrosion protection. The coatings prepared using tetraethylorthosilicate (TEOS), 2-hydroxyethylmethacrylate (HEMA) and 3-methacryloxypropyltrimethoxysilane (MPS) provide a small barrier functionality because of the low degree of cross-linking in the structure and the high hydrophilic nature. The incorporation of cerium ions in the sols originates the increase of defect concentration due to the disruption of the structure, reducing the barrier functionality of the coatings, and a self-healing effect precipitating as yellowish oxide-hydroxide. Another signal of this inhibition mechanism is the increase of the impedance modulus at 0.01 Hz with immersion time, contrary to that observed in coatings without cerium. Several modifications, as removal of HEMA, and incorporation of commercial silica nanoparticles and ethylene glycol dimethacrylate (EGDMA), produces an increase of cross-linking and density with a significant improvement of the barrier functionality. However, the only way to combine barrier functionality and self-healing effect in this system is to develop a multilayer coating where each layer has a specific role. The results present a very good behaviour against corrosion as a barrier and signals of self-healing effect after a long immersion time in NaCl solutions. 1. INTRODUCTION 1.1. Protection of metal substrates with sol-gel coatings The sol–gel method is a process in which a precursor in the form of a solution undergoes gelation by evaporation of the solvent and is subsequently cured or sintered to produce a broad range of materials. Industrial applications of the sol–gel process currently focus on protective and functional coatings, utilising the ability of the process to modify substrate surface properties while preserving those of the bulk. This process is attractive because it requires relatively low processing temperatures, gives highly homogeneous products and can be applied to materials with a wide range of compositions and properties. Low viscosity, fluid sols are ideal for preparing films, as

long as some limitations are respected. The structure and characteristics of coatings are determined both by the physical properties of the sol (viscosity, surface tension, evaporation rate of the solvent), and by the characteristics of the ‘clusters’ formed from polymerisation of hydrolysed precursors (size, extent of branching or aggregation and fractal dimension of the network). Also, a wide range of coating compositions and properties can be achieved by varying the method for sol preparation (colloidal or hydrolysis–polycondensation), the synthesis parameters (pH, precursors, H2O/precursor ratio, temperature of reaction, sol concentration) and the thermal treatment for consolidation and/or sintering. The sol characteristics and thermal treatments determine a number of the films properties, including hardness, critical thickness, wear resistance, chemical behaviour, hydrophobicity and adherence (1-4). First silica sol-gel coatings were prepared using tetraethylorthosilicate (TEOS) after thermal treatment at 500°C. These pure silica coatings have a critical thickness of 400 nm (measured by profilometry on glass substrates), and they are hard, brittle and dense. The hybrid character is conferred through silicon alkylalkoxides with one or more non-hydrolysable Si–C bonds, or more complex organic groups allowing some polymerisable functionality. The non-hydrolysable Si–C bond allows the structural incorporation of the organic groups. Depending on the composition of the organic component, it is possible to distinguish between hybrid and hybrid polymer coatings. ‘Hybrid’ coatings, which contain low organic concentrations of highly stable organic groups, are sintered at 350–500°C. ‘Hybrid polymer’ coatings, cured below 200°C, contain higher concentrations of organic groups and usually involve organic and inorganic polymerisations. The number of pathways for sol synthesis can be increased by combining alkoxides and alkylalkoxides, with and/or without polymerisable groups, to obtain the desired properties. Activation of organic polymerisation through epoxy groups or double bonds can be used to produce hybrid networks giving a nanostructured final material consisting of two interpenetrating networks (organic and inorganic) connected by chemical bonds (5). In the context of corrosion protection, silica layers have the potential to provide a significant improvement because SiO2 has very low oxygen diffusivity and are resistant to corrosion over a wide pH range (6,7). Furthermore, the incorporation of other oxides (ZrO2, TiO2, Al2O3, B2O3) has enlarged the range of corrosion applications to alkaline and neutral media for different metal substrates. A complete review of protective sol-gel coatings can be found in the literature (8,9). Most of the initial shortcomings experienced with pure SiO2 coatings have been overcome by the development of hybrid coatings using methyltriethoxysilane (MTES) in combination with TEOS. It is possible to obtain crack free coatings up to 2 µm in thickness in one step with a 40TEOS/60MTES molar ratio. The improvement in protection of these hybrid coatings has been attributed to the higher thickness, a lower concentration of defects because of the higher ductility, and the presence of methyl groups that confers higher hydrophobicity, delaying the access of the electrolyte to the substrate surface. While pure and hybrid SiO2 layers are effective as a barrier against oxygen diffusion when the coating is treated at temperatures higher than 350°C, other types of coatings that can be cured or sintered below 200°C are required for metals susceptible to deterioration at higher temperatures. Hybrid polymer coatings meet these requirements (8). 1.2. Lanthanides as environmentally friendly corrosion inhibitors Chromate conversion coatings (CCCs) provide an excellent protection mechanism against the localized corrosion, being the most common option used up to now by industry. CCCs presents self-healing behaviour after superficial damage; a scratch or

defect appearing in the film can be protected by migration of soluble Cr (VI) species from the coating that precipitate on the surface healing the defect (10). However CCCs are extremely dangerous for human health and generate serious problems of environmental contamination. Therefore, it is necessary to find a replacement that supplies a self-healing protection against corrosion. A recent trend is the development of sol–gel coatings doped with environmentally friendly inhibitors (11,12). The aim is to incorporate corrosion inhibitors having the ability to play an active protective role in the case of coating damage. These species would diffuse in the presence of an electrolyte and precipitate on bare metal areas, blocking the corrosion reaction. However, this is not easy to achieve, because the ability of the inhibitor to protect and passivate an active corrosion site on the metal substrate will depend on a balanced combination of properties such as inhibitor efficiency, solubility and diffusivity. Over and above their barrier properties, coatings should behave as reservoirs, slowly releasing the inhibitor compound to ensure long lasting protection. This behaviour is intimately related to the precursor form (oxides, ions, nanoparticles) in which the inhibitor has been included in the coating matrix, conditioning the effectiveness of the protective system. Special attention should also be paid to the compatibility between the coating matrix and the inhibitor, because inappropriate entrapment of the inhibitor may lead to performance problems such as blistering caused by osmotic pressure when water migrates within the coating to dissolve the inhibitor, or formation of clusters, which could create new defects (13). Several routes to introduce a wide variety of inhibitors into sol–gel coatings have been reported in the literature (8). Lanthanides, especially cerium, fulfil the basic requirements for alternative corrosion inhibitors: the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors; they have a low toxicity and are relatively abundant in nature. Cerium has a high affinity for oxygen and the bond between cerium and oxygen is unlikely to be broken under the potentials applied. For some aluminium alloys, cerium precipitation from aqueous solutions of cerium salts was observed on cathodic intermetallic compounds and in some instances, the oxide covered the entire specimen surface (14-19). Hybrid silica coatings prepared with TEOS and MTES doped with Ce(NO3)3 and (NH4)2Ce(NO3)6 on stainless steel and aluminium alloys produce an improvement in corrosion protection with increased immersion time in NaCl solutions. This effect can be explained by the deposition of mixed cerium (III and IV) and chromium hydroxides/oxides on cathodic areas, triggered by an increase of pH. However, thermal treatments at temperatures higher than 350°C are applied to sinter these coatings (20,21). In consequence, these can not be used to protect metal substrates susceptible to deterioration at lower temperatures. For this reason, most of the silica sol–gel coatings doped with cerium tested on aluminium substrates are epoxy based hybrid polymer coatings. This composition guarantees good compatibility and bonding with the epoxy based primer or topcoat typically used with aluminium substrates. Moreover, the availability of silica sol–gel precursors with an epoxy functional group promotes chemical bonding between the sol–gel coating and the substrate. Some studies have incorporated cerium nitrate into the epoxy based sol–gel coatings prepared by combining glycidoxypropyl trimethoxysilane (GPTMS) and different silicon alkoxides or zirconium (IV) tetrapropoxide (TPOZ) (22-26). To evaluate the self-healing properties of cerium doped coatings on aluminium, an artificial defect was produced on the film with a diamond tipped scribe. Results obtained with epoxy based sol–gel coatings doped with cerium acetate indicated that the inhibitor interacts with the sol structure, creating particles with cracks that can act as

initiation sites for localised corrosion, as a result of which the performance worsens with immersion time. Thus, it looks necessary to adapt the sol–gel chemistry to include inhibitors for an improved corrosion protection system. Cerium incorporated in ionic form could diffuse too quickly, reducing the protection with time. On the other hand, stabilising the inhibitor within the coating matrix through chemical bonds could limit its ability to diffuse to active reaction sites. In situ formation of ZrO2 nanoparticles within hybrid coatings using TPOZ was found to enhance the corrosion protection (27). The presence of these amorphous nanoparticles is believed to have a pore blocking effect that improves the barrier properties of the coating. 2. DESIGN OF SILICA-METHACRYLATE HYBRID POLYMER SOL-GEL MATERIALS The alternative we proposed for substituting chromates conversion coatings (CCC) for application on metal substrates treatable up to 150-200°C is based on the development of thick hybrid polymer sol-gel coatings. These coatings have to combine barrier properties to delay the penetration of corrosion agents and inhibition properties to hinder the corrosion process because of the presence of a pore, crack or scratch in the coating. The inhibition action through a self-healing or self-sealing process has several steps: diffusion of the inhibitors (cerium ions in this case) across the coating towards the corrosion sites, precipitation of insoluble products and deposition covering the corrosion area. The efficiency of the cerium ions diffusion depends on the hybrid structure porosity: percentage, size and connectivity. A highly cross-linked and dense structure could provide an excellent barrier against corrosion reactives, as oxygen, water, Cl-, etc., but a very slow cerium ions diffusion making ineffective the inhibition action. On the other hand, an open hybrid structure would facilitate the Ce diffusion, but the barrier function would surely diminish permitting corrosion to proceed quickly being the inhibitor insufficient to stop the corrosion process. The design of the hybrid polymer structure of the coating for a specific metal substrate and corrosive conditions is crucial to combine adequately both properties, barrier and inhibition, to obtain an efficient corrosion protection system. As it was pointed out above, most of the hybrid polymer sol-gel coatings in the literature are based on silica-epoxy structures because of their highly cross-linked structure and the chemical compatibility with the epoxy based primer and topcoat used in most applications of Al alloys. However, a sol-gel coating based on silica-methacrylate offers the opportunity to design a structure with the adequate level of cross-linking to optimize the cerium diffusion in order to provide long-term corrosion protection. The control of the organic polymerization, through double bonds, together with inorganic condensation makes possible the design of the concentration of chemical bonds in the structure. The types of precursors used to prepare these hybrid structures are: 1) silicon alkoxides to build the inorganic structure through the hydrolysis and condensation of alkoxide groups; 2) monomers of methacrylate groups to control the organic/inorganic ratio; 3) modified silicon alkoxides with methacrylate groups to bond both components, organic and inorganic, in order to build a nano-structured materials with two interpenetrating networks joined by chemical bonds. These materials belong to the Hybrid class II following the classification of P. Gómez-Romero et al.(5). The compensation of the polymerization (organic and inorganic) kinetics is very important to avoid the formation of a composite material with big organic and inorganic domains and a low connectivity between them. The control of the processing parameters: temperature, pH, concentration and water molar rate is the key to produce

these nano-structured materials. Solutions were synthesized from TEOS, 2-hydroxyethylmethacrylate (HEMA) and 3-methacryloxypropyltrimethoxysilane (MPS), the last one as the coupling agent between the organic and inorganic networks thanks to a double bond and three methoxy groups, respectively (Figure 1). The free-radical co-polymerization of the C=C groups of MPS and those of the co-monomer was carried out by using a suitable initiator (2,2´-azobis(isobutyronitrile), AIBN) (28). The organic polymerization and inorganic condensation lead to a sol constituted by microgels of sizes between 100 and 300 nm in an alcoholic medium. The concentration of the sol is the key parameter to control the degree of C=C polymerization in order to design a more or less permeable structure, and also to define the stability of the solutions. As an example, a schematic view of the chemical structure of a sol with a concentration of Si of 19 g L-1, an inorganic condensation of 2.86 and a complete organic polymerization is shown in Figure 2. The initial viscosity of this sol is 2 mPa.s and the gelation time of around 180 hours at 25°C. The sol has excellent properties to produce coatings: good wettability, homogeneity, adequate viscosity and high stability (more than 6 months) at 5°C. The structure also allows the incorporation of corrosion inhibitors, as cerium ions, due to the presence of hydrophilic groups as OH groups from HEMA and silanols. 3. SILICA-METHACRYLATE HYBRID POLYMER SOL-GEL COATINGS WITH SELF-HEALING PROPERTIES FOR THE CORROSION PROTECTION OF ALUMINIUM ALLOYS 3.1. TEOS-MPS-HEMA coatings As it has been pointed out, TEOS-MPS-HEMA sols have the adequate properties to prepare coatings on aluminium alloys. Figure 3 shows a SEM photograph of an AA2024 alloy protected with a three-layer 60TEOS-10MPS-30HEMA (molar composition) coating prepared by dip-coating. The coating is homogeneous, crack-free and well adhered to the substrate, with a thickness around 6.5 µm. These coatings provide a small barrier functionality at initial immersion times in NaCl solutions. The low degree of cross-linking in the structure and the high hydrophilic nature because of the presence of hydroxyl groups from HEMA leads to quick electrolyte permeation and, consequently, to the deterioration of the coating (29). The incorporation of cerium ions in the sols, through the dissolution of cerium salts, as cerium chloride (CeCl3·7H2O) and cerium nitrate (Ce(NO3)3·6H2O), originates important modifications in the coating response. On one hand, the increase of defect concentration due to the disruption of the structure reduces the barrier functionality of the coatings. On the other, although cerium ions provide a self-healing effect precipitating as yellowish oxide-hydroxide, the amount of cerium is not enough for the numerous corrosion points. The mechanism proposed by the self-healing or self-sealing ability provided by cerium ions is based on the tendency for the oxides formation, as the values of free energy (ΔGf) is -1024.6 kJ mol-1 for CeO2 and -1706.2 kJ mol-1 for Ce2O3. The oxidation state of cerium ions in the coatings is mainly the same in the salt used in the synthesis, although a mix of Ce3+ and Ce4+ is always present (20,21). Both cerium ions contained in the coating present a high reactivity with oxygen, being the driving force for the cerium migration through the coating to react with OH- groups produced in the cathodes and to form cerium oxides/hydroxides that act blocking the subsequent entrance of oxygen to the reactive site:

33 )(3 OHCeOHCe →

←−+ +

OHOCeOHCe 2323 3)(2 +→←

+→

←−+ + 2

24 )(2 OHCeOHCe

OHCeOOHOHCe 22

22 22)( ++ →

←−+

3.2. TEOS-MPS-SiO2 coatings Based on the results of the previous chapter, a new design of the hybrid structure is required to improve the cross-linking and density of the coatings in order to prevent quick electrolyte permeation. The new design is supported on two modifications (30):

1. Removal of HEMA in order to eliminate its C-OH groups that provide high hydrophilicity but very low bonding capacity.

2. Incorporation of commercial silica nanoparticles (Levasil 200A, Bayer, particle size 20 nm) with surface Si-OH groups to increase the density of the coatings.

Figure 4 presents the Bode plots for AA2024 protected with a 42TEOS-23MPS-35SiO2 (molar composition) coating (3.7 µm) after different immersion times in 3.5 wt % NaCl solution compared with the bare alloy. The phase angle curve of the bare alloy presents two time constants at 30 and 0.015 Hz assigned to the intermediate aluminium oxide layer and the electron charge transfer process from corrosion, respectively (27). The incorporation of the sol-gel coating originates the presence of a new time constant at higher frequencies (above 105 Hz). The sol-gel coating leads to an increase of the impedance modulus at 0.01 Hz in two orders of magnitude as a consequence of the additional barrier functionality. However, Bode plots of coated samples after only one hour of immersion showed signals of corrosion activity by the presence of a time constant at 0.02 Hz. Although the modification of the structure produces an improvement of the barrier functionality compared with TEOS-MPS-HEMA coatings, a porous structure remains in this new sol-gel coatings explaining the presence of corrosion after one hour of immersion in NaCl solution. On the other hand, a relatively open structure like this can be adequate for the objective of combination of barrier properties and self-healing effect when an inhibitor as cerium is incorporated in the coating. The increase of the immersion time produced a slow deterioration of the corrosion protection system. The total impedance at 0.01 Hz decreases slightly with time as a first signal of degradation. As well, the plateau observed in the impedance plot between 1 and 100 Hz, associated with the contribution of the resistances assigned to the NaCl solution, sol-gel coating and intermediate layer, decreases with immersion time, also indicating coating degradation. The reduction of phase angle of the two higher frequency time constants with immersion time indicates a less capacitive response due to the permeation of the solution through the pores of the sol-gel coating and intermediate layer. The incorporation of cerium ions from a salt, Ce(NO3)3·6H2O, reduces the barrier functionality, but generates an inhibition process. Figure 5 presents the Bode plots for AA2024 protected with a 40TEOS-22MPS-33SiO2-5Ce (molar composition) coating (4.3 µm) after different immersion times in 3.5 wt % NaCl solution. The values of impedance at 0.01 Hz are almost two orders of magnitude lower than those obtained with sol-gel coatings without cerium and very close to the bare substrate. This change is an indication of a coating with a higher content of defects (31). Although it is not as evident as in the case of coatings without cerium, the phase angle curves maintain the three time constants already observed and assigned to sol-gel coating, intermediate aluminium oxide layer and electron charge transfer process. The presence of diffusion

processes through pores is exhibited in a phase angle shift of 45° in the phase plot and a decreasing slope in the impedance modulus plot at lower frequencies. The reduction of the impedance at around 103 Hz is also a signal of the deterioration of the barrier functionality provided by the sol-gel coating and intermediate layer. Although the incorporation of cerium ions has induced a partial degradation of the coating as barrier, an inhibition action has been also provided. A first signal of this inhibition mechanism is the increase of the impedance modulus at 0.01 Hz with immersion time, contrary to that observed in coatings without cerium. As resistances assigned to the sol-gel coating and intermediate layer decrease with immersion time, the rise of the impedance at 0.01 Hz can only be associated with a significant increasing of the resistance describing the corrosion of the metal substrate (Rct). This resistance is inversely proportional to the corrosion rate, and should be normalized respecting to the electrode area. Rct can be defined as:

Rct = Rct0 / Acorr

Rct0 is the Rct of the metal substrate without coating, and Acorr is the substrate area

affected by corrosion (32). On the other hand, the increase of the phase angle from 30° for 1 hour to almost 80° after 215 hours of immersion indicates a more capacitive behaviour. Both factors (increase of Rct and phase angle) indicate the presence of deposits or precipitates well adhered and with enough density to reduce the area affected by corrosion. Figure 6 shows photographs of the tested areas of coated samples after the EIS tests. The alloy protected with the 42TEOS-23MPS-35SiO2 coating (Figure 6a) presents a transparent and almost intact film with only scarce isolate pitting. This coating provides a good barrier, although the initiation of the pitting can not be stopped due to the absence of a corrosion inhibitor. Figure 6b shows a photograph of the sample protected with the 40TEOS-22MPS-33SiO2-5Ce coating after the EIS-time test. Well adhered yellowish precipitates all over the test area can be observed. Although the initiation of a wide corrosion process is evident, macroscopic pitting, as those observed on samples protected with coatings without cerium, have not been detected. This behaviour can be attributed to the self-healing effect provided the cerium. The improvement of the barrier functionality, compared with TEOS-MPS-HEMA coatings, and the confirmation of the inhibition properties of cerium ions have been showed. However, the increase of defect concentration based on the disruption in the hybrid structure makes necessary the re-design of the coating network and the model of a single layer. 3.3. TEOS-MPS-EGDMA-SiO2 multilayer coatings The model of a protective coating with a single layer that combines barrier functionality with inhibition action of cerium ions has demonstrated to be an extremely difficult objective. For this reason, a new design was based on the synthesis of a multilayer coating where some layers do not contain cerium ions and have to perform only as a barrier against diffusion of corrosive agents, while other layers doped with cerium ions should provide a self-healing effect when the barrier is damaged. A new monomer, ethylene glycol dimethacrylate (EGDMA), is incorporated to the structure in order to increase the network cross-linking through the polymerization of its two double bonds (Figure 7). Figure 8 compares the polarization curves after one hour of immersion in 0.05 M NaCl of the bare AA2024, the alloy protected with a 42TEOS-23MPS-35SiO2 monolayer coating, and the same alloy protected with a tri-layer coating formed by an inner and outer 42.5TEOS-17MPS-8.5EGDMA-32SiO2 coating and an intermediate 40.5TEOS-

16.2MPS-8.1EGDMA-30.6SiO2-4.6Ce coating. The bare alloy presents active dissolution of the metal substrate in the anodic branch, without signs of passivation. The coatings provide a good barrier against corrosion, both showing a icorr decreasing of almost three orders of magnitude compared to the bare alloy. The difference in the coating thickness, 3.7 µm for the 42TEOS-23MPS-35SiO2 coating and 11 µm for the tri-layer coating, can be distinguished in the passive region showed in the anodic branch. The former presents a passive zone of 500 mV against the 1200 mV of the tri-layer coating. The EIS analysis clarify the role and performance of the different coatings. The impedance study (EIS) as a function of the immersion time in 0.05 M NaCl of a bi-layer coating (8 µm) prepared with 42.5TEOS-17MPS-8.5EGDMA-32SiO2 sol is displayed in Figure 9. The curve for one hour of immersion also reflects the barrier properties of these coatings, because only two time constants at around 104 and below 10-2 Hz, assigned to the sol-gel coating and the alumina layer, respectively, can be observed. The first signals of corrosion activity can be seen after 17 hours of immersion where three time constants appear. The increase of immersion time originates a continuous slow deterioration of the protection system with a reduction of the impedance in all the frequency range, mainly the impedance associated with the sol-gel coating. In spite of the good barrier properties of these coatings, the absence of an inhibitor avoid to stop the corrosion process initiated in any defect of the coating. Figure 10 shows the Bode plots after different immersion times in 0.05 M NaCl solution of AA2024 protected with a tri-layer coating (11 µm) formed by an inner and outer layer prepared using a 42.5TEOS-17MPS-8.5EGDMA-32SiO2 sol, and an intermediate layer prepared using a 40.5TEOS-16.2MPS-8.1EGDMA-30.6SiO2-4.6Ce sol. The results show a very good behaviour against corrosion. First signals of corrosion activity only appear after 1870 hours of immersion with a time constant centred around 0.3 Hz. The stability of the impedance value at 0.01 Hz after 400 hours of immersion pointed out a significant difference compared with the previous coating without cerium. This behaviour could be related with the healing of the first corrosion sites produced. The presence of a new time constant at 80 Hz after 3000 hours of immersion could be related with the formation of cerium based precipitates. This subject is under study. Figure 11 displays several photographs of the test area of protected AA2024 alloy with both types of coatings after different immersion times in 0.05 M NaCl solution. The images show clearly the influence of the incorporation of cerium ions in the coatings. Figures 11 A and B present the photographs of the alloy protected with the bi-layer 42.5TEOS-17MPS-8.5EGDMA-32SiO2 coating, showing the continuous degradation of the coating with the immersion time. Although this type of coating has good barrier properties, the slow penetration of water through the defects can not be stopped because of the absence of inhibitors. On the other hand, the coatings doped with cerium present a very different behaviour. Figure 11 C and D show the photographs of the alloy protected with the tri-layer coating formed by an inner and outer 42.5TEOS-17MPS-8.5EGDMA-32SiO2 layer, and a 40.5TEOS-16.2MPS-8.1EGDMA-30.6SiO2-4.6Ce intermediate layer. In this case, there is not evidence of aggressive corrosion, and the aspect of coating surface is unchanged with immersion time. Only a big pit appears in the surface, and it seems to be stabilized by the effect of the cerium ions. The SEM study of this sample after 3130 immersion hours (Figure 12) shows a cracked coating with the presence of precipitates. The EDS analysis shows a very significant difference of cerium content between the precipitate and the coating, taking into account that the nominal Si/Ce molar rate is 95/5. This is another evidence of the cerium diffusion and precipitation, probably in cathode regions delaying the corrosion process.

The previous examples of Ce-containing silica-methacrylate sol-gel coatings on Al2024 alloy provide different evidences of the appearing of self-healing or self-sealing effect related to the presence of Ce ions. A major objective of further researches now in progress is to increase the Ce content in the film with the aim of obtaining coatings acting as reservoirs of inhibitors for a more prolonged corrosion protection effect. 4. CONCLUSIONS The manuscript describes the evolution of the structure design of cerium doped silica-methacrylate hybrid polymer sol-gel coatings to protect aluminium alloys combining barrier functionality and inhibition properties. The coatings prepared using tetraethylorthosilicate (TEOS), 2-hydroxyethylmethacrylate (HEMA) and 3-methacryloxypropyltrimethoxysilane (MPS) provide a small barrier functionality, although the incorporation of cerium ions originates a self-healing effect precipitating as yellowish oxide-hydroxide. Another signal of this inhibition mechanism is the increase of the impedance modulus at 0.01 Hz with immersion time, contrary to that observed in coatings without cerium. Several modifications as removal of HEMA, and incorporation of commercial silica nanoparticles and ethylene glycol dimethacrylate (EGDMA), produces an increase of cross-linking and density with a significant improvement in the barrier functionality. However, the only way to combine barrier functionality and self-healing effect in this system is to develop a multilayer coating where each layer has a specific role. The results present a very good behaviour against corrosion as a barrier showing the first signals of corrosion activity after 1870 hours. On the other hand, the stability of the impedance value at 0.01 Hz after 400 hours of immersion pointed out a significant difference compared with the previous coating without cerium. This behaviour could be related with the healing of the first corrosion sites produced. After the EIS-time test, the alloy protected with the coating without cerium presents a general degradation, while the coatings doped with cerium do not show evidence of aggressive corrosion. In this case, only a big pit appears in the surface, and it seems to be stabilized by the effect of the cerium ions. The EDS analysis shows a very significant difference of cerium content between the precipitate and the coating, evidencing the cerium diffusion and precipitation, probably in cathode regions delaying the corrosion process. 5. REFERENCES 1. J. D. Mackenzie, J. Non-Cryst. Solids, 1982, 48, 1–10. 2. C. J. Brinker and G. W. Scherer, ‘Sol–gel science: the physics and chemistry of sol–

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Ferreira, Surf. Coat. Technol., 2006, 200, 3084–3094. 28. S. A. Pellice, R. J. J. Williams, I. Sobrados, J. Sanz, Y. Castro, M. Aparicio and A.

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Surf. Coat. Technol. (in press). 30. N. C. Rosero-Navarro, S. A. Pellice, A. Durán and M. Aparicio, Corr. Sci. (in

press). 31. M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Möhwald and M.G.S.

Ferreira, Chem. Mat., 2007, 19, 402-411. 32. A. Amirudin and D. Thierry, Prog. Org. Coat., 1995, 26, 1-28.

TEOS

HEMA MPS

TEOS

HEMA MPS

Figure 1. Chemical structures of TEOS, HEMA and MPMS.

SiliconOxygenCarbon

SiliconOxygenCarbon

Figure 2. Schematic view of the chemical structure of a 60TEOS-10MPS-30HEMA sol with a concentration of Si of 19 g L-1, an inorganic condensation of 2.86 and a complete organic polymerization.

Figure 3. SEM photograph of an AA2024 alloy coated with a three-layer 60TEOS-10MPS-30HEMA coating.

10-3 10-2 10-1 100 101 102 103 104 105100

101

102

103

104

105

106

107

Zmod

(ohm

cm2 )

Frequency (Hz)

1 hour 42 hours 164 hours Bare, 1 hour

10-3 10-2 10-1 100 101 102 103 104 1050

-10

-20

-30

-40

-50

-60

-70

-80

-90

Zphz

(º)

Frequency (Hz)

Figure 4. Bode plots for AA2024 protected with 42TEOS-23MPS-35SiO2 coatings after different immersion times in 3.5 wt % NaCl solution compared with the bare alloy.

10-3 10-2 10-1 100 101 102 103 104 105101

102

103

104

105

1 hour 41 hours 114 hours 215 hours

Frequency (Hz)

Zmod

(ohm

cm2 )

10-3 10-2 10-1 100 101 102 103 104 1050

-10

-20

-30

-40

-50

-60

-70

-80

-90

Zphz

(º)

Frequency (Hz)

Figure 5. Bode plots for AA2024 protected with 40TEOS-22MPS-33SiO2-5Ce coatings after different immersion times in 3.5 wt % NaCl solution.

10 mm

a)

10 mm10 mm10 mm

a)

10 mm

b)

10 mm10 mm10 mm

b) Figure 6. Photographs of the tested areas of samples with coatings after the EIS - immersion time analysis: a) 42TEOS-23MPS-35SiO2 coating and, b) 40TEOS-22MPS-33SiO2-5Ce coating. EGDMA

O

O

O

O

O

O

O

OEGDMA

O

O

O

O

O

O

O

O

Figure 7. Chemical structure of ethylene glycol dimethacrylate (EGDMA).

-1,2

-0,4

0,4

1,2

1,0E-12 1,0E-10 1,0E-08 1,0E-06 1,0E-04 1,0E-02

j (A/cm2)

V v

s. R

ef (V

)

B

C

A

Figure 8. Polarization curves after one hour of immersion in 0.05 N NaCl of AA2024 alloys: A) bare, B) alloy protected with a 42TEOS-23MPS-35SiO2 monolayer coating and, C) alloy protected with a tri-layer coating formed by an inner and outer 42.5TEOS-17MPS-8.5EGDMA-32SiO2 coating, and an intermediate 40.5TEOS-16.2MPS-8.1EGDMA-30.6SiO2-4.6Ce coating.

10-2 10-1 100 101 102 103 104 105101

102

103

104

105

106

107

1 hour 17 hours 170 hours 1870 hours 2613 hours

Zmod

(ohm

cm2 )

Frequency (Hz)

10-3 10-2 10-1 100 101 102 103 104 1050

-20

-40

-60

-80

-100

Zphz

(º)

Frequency (Hz)

Figure 9. Bode plots after different immersion times in 0.05 N NaCl solution of AA2024 protected with a bi-layer coating prepared with the 42.5TEOS-17MPS-8.5EGDMA-32SiO2 sol.

10-3 10-2 10-1 100 101 102 103 104 105102

103

104

105

106

107

1 hour 305 hours 430 hours 1870 hours 3130 hours

Zmod

(ohm

cm2 )

Frequency (Hz)

10-3 10-2 10-1 100 101 102 103 104 1050

-20

-40

-60

-80

-100

Zphz

(º)

Frequency (Hz)

Figure 10. Bode plots after different immersion times in 0.05 N NaCl solution of AA2024 protected with a tri-layer coating formed by an inner and outer layer prepared using a 42.5TEOS-17MPS-8.5EGDMA-32SiO2 sol, and an intermediate layer prepared using a 40.5TEOS-16.2MPS-8.1EGDMA-30.6SiO2-4.6Ce sol.

B

0.3 cm

BB

0.3 cm0.3 cm

A

0.3 cm

AA

0.3 cm0.3 cm

C

0.3 cm

CC

0.3 cm0.3 cm

D

0.3 cm

DD

0.3 cm0.3 cm

Figure 11. Photographs of the test area of protected AA2024 alloy with both types of coatings after different immersion times in 0.05 NaCl solution: A) bi-layer coating without cerium after 1870 hours; B) bi-layer coating without cerium after 2613 hours; C) tri-layer coating with cerium after 1870 hours; D) tri-layer coating with cerium after 3130 hours.

⊗

⊗

Si/Ce = 99/1

Si/Ce = 54/46

⊗

⊗

⊗

⊗

Si/Ce = 99/1

Si/Ce = 54/46

Figure 12. SEM micrograph of the tri-layer coating with cerium after 3130 hours showing some cracks and a precipitate. The EDS results of the Si/Ce molar rate is included for two locations.