Sol-Gel Coating

4. Conclusion The influence of cerium concentration on behavior against corrosion of hybrid sol–gel coating on martensitic X13VD stain- less steel has been investigated. The increase of cerium content improved hydrophobic character of coating and its tightness toward water. Electrochemical impedance spectroscopy showed an optimal cerium concentration of 0.01 M in the hybrid layer. Besides, the increase of cerium concentration above 0.01 M decreased the barrier effect of sol–gel coating. The coatings were performed by scratch tests, and no significative cerium influence has been proved on the adhesion properties of coatings to the martensitic sub- strate; so a direct correlation between anticorrosion and adhesion performances of sol–gel coating is not possible. Liquid 29 Si NMR spectroscopy was then carried out to determinate the cerium influ- ence on sol condensation. It was shown that an excess of cerium (>0.01 M) lead to a chemical shift of T species and can change the chemical structure of sol–gel network. Therefore a reduction of corrosion resistance of coatings up was noted to the same cerium concentration (0.01 M).

4. Conclusion Crack free organic–inorganic coatings were deposited onto AA 2024-T3 aluminum alloy from sols prepared by mixing GPTMS and ASB with different cerium contents (0–0.1 M). The influence of the inhibitor concentration on anti-corrosion and mechanical properties of hybrid sol–gel coating has been investigated. An optimal cerium concentration of 0.01 M has been determined, while for higher cerium concentrations a significant collapse of coating electrochemical performances has been noted. The results of NMR evidenced the effect of cerium content on the hybrid structure: for a concentration higher than 0.01 M, there are important modifications of the organic and inorganic polymeri- zation rates and of the aluminum incorporation into the silicon network. These important structural modifications could explain the important collapses of coating performances when it is deposited on aluminum alloy.

This first comparison shows that GZA coating presents a better corrosion resistance than GZ and GA coatings. For one sample of GZA coating (Figure 3 C), resistance to NSS is even higher than the recommendation of 500 h. This performance can be attributed to the concomitant presence of aluminium and zirconium in the sol-gel matrix. Furthermore, coating adhesion properties are brought by covalent bonds such as Al-O-Si between the aluminium substrate and the inorganic network as already reported [ 12 , 21 ]. Aluminium element allows increasing adhesion of sol-gel coating on substrate with strong chemical bonds [ 14 ]. Furthermore, the zirconium contributes to the barrier effect and improves coating durability [ 15 – 19 ]. This positive aspect can be confirmed by GZ coating, which presents a better corrosion resistance than GA coating (respectively around 300 h against 200 h, (Figure 3 B and 3 A).

This first comparison shows that GZA coating presents a better corrosion resistance than GZ and GA coatings. For one sample of GZA coating (Figure 3 C), resistance to NSS is even higher than the recommendation of 500 h. This performance can be attributed to the concomitant presence of aluminium and zirconium in the sol-gel matrix. Furthermore, coating adhesion properties are brought by covalent bonds such as Al-O-Si between the aluminium substrate and the inorganic network as already reported [ 12 , 21 ]. Aluminium element allows increasing adhesion of sol-gel coating on substrate with strong chemical bonds [ 14 ]. Furthermore, the zirconium contributes to the barrier effect and improves coating durability [ 15 – 19 ]. This positive aspect can be confirmed by GZ coating, which presents a better corrosion resistance than GA coating (respectively around 300 h against 200 h, (Figure 3 B and 3 A).

The present work proposes an architectural approach to the production of an epoxy-based sol–gel coating, for the corrosion protection of a cast magnesium alloy (Mg–Nd–Gd–Zr–Zn). Usually, the metallic pieces obtained by low-pressure casting may contain a large amount of impurities at the surface, also presenting a very irregular and rough surface resulting from the contact with the mold. Hence, a chemical etching is generally applied for this kind of pieces in order to remove the contaminated and rough exter- nal layer before the application of protective coatings [26] , though this surface pretreatment may expose the microstructure of the metallic substrate. On the other hand, the high reactivity of the magnesium substrate in aqueous media manifests itself when the sol contains a large portion of water, complicating the preparation of homogeneous coatings without pores and defects. Therefore, a bilayer epoxy-based sol–gel system is presented, where the first created sol–gel layer of sol would undergo the chemical reactions with the magnesium alloy, but thereby providing an inert sur- face for a second sol–gel layer which contribute to the thickening of the sol–gel coating. The morphological aspects of the sol–gel systems are analyzed by scanning electron microscopy, and their electrochemical properties are studied by OCP (open circuit poten- tial) monitoring and electrochemical impedance spectroscopy in chloride media (0.05 M NaCl solution), in order to evaluate their corrosion resistance.

Despite the good leveling effect of the hybrid sol –gel coating, often reported in the literature on several metallic alloys [33] , on El21 casted alloy, these defects are not covered by the hybrid coating. Moreover, the presence of bubbles in the coating suggests a reactivity of the substrate with the sol during the immersion. These defects seem to have its origin on the microstructure of the cast El21 alloy and especially where the intermetallic compounds of neodymium

sol–gel coating. Likewise, the appearance of a second time constant at the middle frequency range (126 Hz) is attributed to the presence of an intermediate mixed layer, consisting of the conversion layer and corrosion products, located between the hybrid sol–gel coating and the magnesium substrate. Both time constants are visible on the fol- lowing hours of immersion, but the high frequency constant gradual- ly decreases its phase angle. The apparition of the second time constant after 24 h of immersion is afforded to the liquid take of the sol–gel hybrid film which finally gets in contact with the metallic sub- strate. The frequency range and the value of its phase angle are very similar to those of the bare substrate. This suggests that this interme- diate layer has the same morphological nature of the passive layer de- veloped by bare substrate, both consisting of a porous-ceramic layer. The impedance modulus shows a substantial reduction between 1 h and 24 h of immersion, slightly decreasing to the end of the immer- sion. After 192 h of immersion, the impedance modulus of the duplex system conversion (10 g L − 1 )/hybrid film is around 40 kohm·cm 2 .

In our case, efficient protections against corrosion associated with good adhesion properties are researched directly on phosphated zinc for applications mainly in the building sector. Commonly, sol-gel processes involve alkoxysilane precursors as starting materials. In this study, two sol-gel formulations were first investigated based on previous work developed on aluminum alloys [ 17 ] or stainless steel [ 18 ], as well as literature [ 4 ]: (i) 3-glycidoxypropyltrimethoxysilane (GPTMS)/aluminum-tri-sec-butoxide (ASB), and (ii) 3-(trimethoxysilyl)propylmethacrylate (MAP)/tetraethylorthosilicate (TEOS), whose chemical structures are shown in Figure 1 . In both formulations, the precursors contributed to creating the organic and inorganic parts to provide organoaluminosilicated (i) and organosilicated (ii) hybrid coatings, respectively. These precursors are mixed in alcohol and water, and cerium (III) nitrate hexahydrate is systematically added as an eco-friendly corrosion inhibitor in the optimized concentration of 0.01 mol L −1 [ 16 – 18 ]. The behavior of the coated samples is evaluated, especially the corrosion resistance with accelerated corrosion testing (climatic chamber and salt spray) and the adhesive properties by mechanical tests (shock test and scratch test). This work proposes an optimization of the most efficient formulations by adding an ureido precursor 1-[3-(trimethoxysilyl)propyl]ureido (UPS) (Figure 1 iii). This ureido precursor has been chosen to increase the chemical compatibility with the polyurethane top-coat paint applied on zinc substrates. It should be noted that bis[(ureapropyl)triethosilyl groups are introduced in sol-gel coating to obtain a hydrophobic urea-polydimethylsiloxane sol-gel coating on aluminum alloys [ 19 , 20 ]. In these reported works, the bis-urea derivative is introduced in an amount of less than 4% wt in the sol-gel network. Introduction of such precursors constitutes an innovative sol-gel formulation on natural or phosphated zinc substrates, which could be developed on many other metal substrates.

The present work proposes an architectural approach to the production of an epoxy-based sol–gel coating, for the corrosion protection of a cast magnesium alloy (Mg–Nd–Gd–Zr–Zn). Usually, the metallic pieces obtained by low-pressure casting may contain a large amount of impurities at the surface, also presenting a very irregular and rough surface resulting from the contact with the mold. Hence, a chemical etching is generally applied for this kind of pieces in order to remove the contaminated and rough exter- nal layer before the application of protective coatings [26] , though this surface pretreatment may expose the microstructure of the metallic substrate. On the other hand, the high reactivity of the magnesium substrate in aqueous media manifests itself when the sol contains a large portion of water, complicating the preparation of homogeneous coatings without pores and defects. Therefore, a bilayer epoxy-based sol–gel system is presented, where the first created sol–gel layer of sol would undergo the chemical reactions with the magnesium alloy, but thereby providing an inert sur- face for a second sol–gel layer which contribute to the thickening of the sol–gel coating. The morphological aspects of the sol–gel systems are analyzed by scanning electron microscopy, and their electrochemical properties are studied by OCP (open circuit poten- tial) monitoring and electrochemical impedance spectroscopy in chloride media (0.05 M NaCl solution), in order to evaluate their corrosion resistance.

In our case, efficient protections against corrosion associated with good adhesion properties are researched directly on phosphated zinc for applications mainly in the building sector. Commonly, sol-gel processes involve alkoxysilane precursors as starting materials. In this study, two sol-gel formulations were first investigated based on previous work developed on aluminum alloys [ 17 ] or stainless steel [ 18 ], as well as literature [ 4 ]: (i) 3-glycidoxypropyltrimethoxysilane (GPTMS)/aluminum-tri-sec-butoxide (ASB), and (ii) 3-(trimethoxysilyl)propylmethacrylate (MAP)/tetraethylorthosilicate (TEOS), whose chemical structures are shown in Figure 1 . In both formulations, the precursors contributed to creating the organic and inorganic parts to provide organoaluminosilicated (i) and organosilicated (ii) hybrid coatings, respectively. These precursors are mixed in alcohol and water, and cerium (III) nitrate hexahydrate is systematically added as an eco-friendly corrosion inhibitor in the optimized concentration of 0.01 mol L −1 [ 16 – 18 ]. The behavior of the coated samples is evaluated, especially the corrosion resistance with accelerated corrosion testing (climatic chamber and salt spray) and the adhesive properties by mechanical tests (shock test and scratch test). This work proposes an optimization of the most efficient formulations by adding an ureido precursor 1-[3-(trimethoxysilyl)propyl]ureido (UPS) (Figure 1 iii). This ureido precursor has been chosen to increase the chemical compatibility with the polyurethane top-coat paint applied on zinc substrates. It should be noted that bis[(ureapropyl)triethosilyl groups are introduced in sol-gel coating to obtain a hydrophobic urea-polydimethylsiloxane sol-gel coating on aluminum alloys [ 19 , 20 ]. In these reported works, the bis-urea derivative is introduced in an amount of less than 4% wt in the sol-gel network. Introduction of such precursors constitutes an innovative sol-gel formulation on natural or phosphated zinc substrates, which could be developed on many other metal substrates.

sol–gel processes involve silane precursors as starting materials. Silane based coatings usually exhibit good barrier properties due to the development of a dense –Si–O–Si– network [11–15] , which inhibits the penetration of aggressive species towards the metallic substrate. Thus, the efficiency of the metal surface pre-treatments based on silane coatings is strongly dependent on the barrier properties of the film [16,17] . However, when a defect is formed in the barrier layer, the coating is not able to stop the localized corrosion process. Thus, the presence of inhibitor species is essential to decrease or to avoid corrosion activity in these cases [18] . Recently, different inhibitors have been studied to prevent corrosion of metal surfaces. Cerium compounds appeared, through lot of works, as a promising corrosion inhibitor to replace chromate compounds for the corrosion protection of various metallic compounds, especially aluminum alloys [19–24] . The aim of this paper consists, first, in the characterization of both microstructure and barrier effect of an alumino-silane hybrid sol–gel coating. Then cerium nitrate has been introduced into the hybrid sol–gel layer in order to improve the corrosion protection. The cerium nitrate was added at different ratios in the sol to understand the possible interactions of the inhibitor with components of the sol–gel system. Anti-corrosion protection and mechanical properties of alumino-silane hybrid coating containing different cerium contents were evaluated, depending on cerium content, by electrochemical analyses (EIS) and nano- indentation. Finally, 29 Si, 13 C and 27 Al RMN analyzes were

∗ Corresponding author. E-mail address: cambon@chimie.ups-tlse.fr (J.-B. Cambon). Sol–gel coatings are one of the most promising alternative pre- treatments, completely chrome-free with other advantages such as cost-effectiveness, low life-cycle environmental impact and simple application procedures. The potential application of sol–gel coat- ings as a corrosion protection system for metal substrates was highlighted 15 years ago by Guglielmi [4]. Then several works have been undertaken to make various sol–gel based protective coat- ings, as described in numerous recent reviews. Submicron metal oxide films inhibit corrosion by providing a chemically inert barrier to the diffusion of corrosive species, and the protective character- istics of sol–gel films containing SiO 2 [5], TiO 2 [6], Al 2 O 3 [7], and

Sol–gel Coating Spectral selectivity SiC-based materials are good candidates for the application as solar receivers except concerning their optical properties. Indeed, considering the use at high temperature, materials used as solar receivers have to efficiently absorb the visible-near infrared waves (corresponding to solar spectral range) and simultaneously reflect the mid and far-infrared rays but SiC is absorbent in all the whole visible-infrared spectral domain. In this challenging work, a suitable YBa 2 Cu 3 O 7-δ oxide which can present appropriate optical properties is studied. It was synthe- sized following a sol–gel route and it was obtained with a high level of purity. YBa 2 Cu 3 O 7-δ pellets were realized and heat treated at different temperatures revealing that the higher the heat treatment is, the better the oxygen stoichiometry (7- δ) is and the smoothest the surface is. This directly acts on the YBa 2 Cu 3 O 7-δ optical properties. Considering these results, an YBa 2 Cu 3 O 7-δ coating was realized on SiC pellets by dip-coating. A homogenous and covering layer of about 10 μm was obtained presenting very promising optical properties which were predomi- nant in the FIR-MIR range whereas absorptance was increased in NIR-visible range.

Sol–gel Coating Spectral selectivity SiC-based materials are good candidates for the application as solar receivers except concerning their optical properties. Indeed, considering the use at high temperature, materials used as solar receivers have to efficiently absorb the visible-near infrared waves (corresponding to solar spectral range) and simultaneously reflect the mid and far-infrared rays but SiC is absorbent in all the whole visible-infrared spectral domain. In this challenging work, a suitable YBa 2 Cu 3 O 7-δ oxide which can present appropriate optical properties is studied. It was synthe-

During the last 30 years, sol-gel processes have been widely used for the preparation of glasses and ceramics. Usu- ally, starting with molecular precursors like alkoxides or ac- etates, the sol-gel process takes place in solution. This pro- vides definitive homogeneity for multicomponent systems. In the particular case of doping, the sol-gel process provides an ideal way to control the level and the homogeneity of doping. For luminescent materials, this is crucial since the light emission is usually due to doping ions like rare-earth or transition metals ions. Quenching concentrations are usually found higher for sol-gel-derived materials because of bet- ter dispersion of doping ions and thus higher average dis- tance between emitting centers. Several authors have also developed heterometallic precursors associating di fferent el- ements through chemical bonding and thus providing the highest homogeneity [ 18 , 19 ]. As far as alkoxides chemistry is concerned, it is useful to distinguish the case of silicon alkox- ides (Si being a semimetal) and the case of metal alkoxides. 2.1. Silicon alkoxides

chaleur ou la pose d'isolants. On peut enfin conjuguer l'application de ces deux méthodes. Il faut de plus ne négliger aucun effort pour assurer le drainage maximal. On peut classer les pistes et les entrepôts frigorifiques en deux catégories d'après leur genre d'utilisation: saisonnière ou continue. Sous les bâtiments en utilisation continue la profondeur de pénétration du gel peut atteindre des dizaines de pieds. Elle dépend de la température prévue par les constructeurs à l'intérieur des bâtiments, de la température moyenne de l'air extérieur, et de la surface d'assise du bâtiment. Dans le cas du service saisonnier, le niveau du gel s'abaisse normalement en hiver de plusieurs pieds à l'intérieur du sol, pour remonter ensuite pendant l'été. Pour simplifier l'exposé, on traitera ci-après les deux cas séparément. Utilisation continue d'un bâtiment à des températures inférieures au point de congélation

L’oxyde de cuivre (pratiquée dans ce mémoire) est chimiquement stable, dur, non toxique, abondant dans la nature et donc le bon marché. Il est préparé généralement sous forme de couches minces par différentes voies parmi lesquelles la voie Sol-Gel offre plusieurs avantages et pour laquelle nous avons opté pour la fabrication de notre oxyde. Outre le domaine photovoltaïque, il est utilisé dans d’autres domaines tels que la microélectronique (le capteur de gaz par exemple), la photo catalyse, l’électro catalyse …etc.

Robert Tomala, Anna Lukowiak, Beata Borak, Andrea Chiappini, Wilfried Blanc, Maurizio Ferrari, Adel Bouajaj, Davor Ristić, Stefano Taccheo,. Wieslaw Strek[r]

Figure 9. Relation théorique entre (a) la teneur en eau volumétrique et la constante diélectrique du sol (Ulaby et al. 1986) et (b) le coefficient de rétrodiffusion La figure 10 présente les relations entre les teneurs en eau mesurées au TDR (-10 cm) (tableau 1) et les coefficients de rétrodiffusion. Les données ont été groupées en fonction de l’utilisation du sol au point de mesures de la même façon que pour les modèles établis en fonction de la température. La classification liée aux classes de drainage du sol a été abandonnée puisque les 2 groupes se comportaient de manière identique et que le nombre de points de contrôle était limitant. Ces relations permettent de constater la forte variabilité des coefficients de rétrodiffusion (dB) lorsque les sols sont partiellement gelés. Ces figures représentent bien la très grande variabilité des teneurs en eau lorsque la température se situe aux alentours de 0 o C. La figure 11 présente la différence entre les teneurs en eau mesurées et la température du sol mesurée aux thermocouples (-5 cm). Même si la tendance est similaire celle présentée à la figure

5.5.Conclusions 129 Nous avons d’autre part montré que les résultats des tests photocatalytiques peuvent être prédits par les mesures précédentes : sous une irradiation dans [r]