EIS Characterization of UV-Cured Hybrid Sol-Gel Coatings for Corrosion Protection of Aluminum Alloy 2024-T3

Sébastien Chanfreau,a Nadia Moreau,b Lingli Ni,b Abraham Chemtob,b Céline Croutxe-

Barghorn,b Nadine Pébèrea aInstitut Carnot CIRIMAT, ENSIACET, 4, allée Emile Monso, BP 44362, 31030 TOULOUSE Cedex 4, France

bLaboratory of Photochemistry and Macromolecular Engineering, ENSCMu, University of Haute-Alsace, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France

E-mail:Sebastien.chanfreau@ensiacet.fr

Introduction Organic-inorganic hybrid materials attract great interest due to properties that arise from synergism between components.1-3 It is almost associated with the sol-gel route which is based on change in a colloidal system through formation of an inorganic or hybrid sol followed by its gelation to form a continuous polymer network (gel).4 As important stage after coating deposition, drying of the sol-gel film is commonly achieved either by high temperature annealing or low temperature drying. A relatively new approach uses UV radiation. So, hybrid sol-gel films were achieved via simultaneous organic-inorganic UV-curing process using a photoacid generator.5 Unique glass-like properties of these materials have led to emerging research and their use as substitutes for commonly used processes. Sol-gel technology also appears as good candidate for substitution of environmentally unfriendly chromate pretreatments that are widely used for corrosion protection of structural aluminum alloy 2024-T3 (AA2024-T3), especially in aeronautic industry.6 Sol-gel films treated by UV radiation at room temperature show high increase in corrosion protection that can be attributed to formation of dense sol-gel coating layers.4 The present work concerns corrosion protection of AA2024-T3 by UV-cured hybrid sol-gel layers. The coating performance was evaluated by using electrochemical impedance spectroscopy and the impedance diagrams of the samples were obtained for various exposure times to a 0.5 M NaCl solution.

Experimental Coatings were deposited onto AA2024-T3 currently used in the aerospace industry. The specimens consisted of 125 × 80 × 1.6 mm plates machined from rolled plate. Before coating, surface pretreatment free from hexavalent chromium was used. Samples were treated successively by an alkaline degreasing solution and an acid etching. Hybrid films were prepared by mixing an organic resin that can polymerized under UV light with an organoalkoxysilane.5,7 Before UV irradiation, hybrid photocurable mixtures were applied at different thicknesses onto different substrates by means of automatic film applicators equipped with wire wound bars. A classical three-electrode cell was used with a platinum auxiliary electrode, a saturated calomel reference electrode (SCE) and the sample as working electrode with exposed area of 24 cm2

. The test solution was a 0.5 M NaCl solution. Electrochemical impedance measurements were performed using a Biologic VSP device over a frequency range from 200 kHz to 3 mHz with ten points per decade.

Results and Discussion Several samples with different thicknesses were analyzed by EIS for one month duration. As an example, Fig. 1 gives the impedance spectra in Bode coordinates for different immersion times for AA2024-T3 coated with a UV-cured sol-gel film.

Fig. 1: Bode plots for AA2024-T3 coated with a UV-cured sol-gel film for various immersion times

in 0.5 M NaCl solution. Coating thickness= 18±5 µm. Three time constants were observed: at middle frequency (MF) e.g. 200 Hz, low frequency (LF) e.g. 40 mHz and very low frequency (VLF) lower than 10 mHz. At MF, the impedance modulus significantly decreased when the exposure time to the aggressive solution increased while low change was observed at LF and VLF. Thus, the presence of two layers was considered in the coating. A denser layer may have been achieved at the vicinity of the substrate while a porous layer may have been achieved as outer layer of the coating. Presence of the two layers could not be assessed by conventional characterization techniques. The VLF time constant was ascribed to the natural oxide film at the surface of the aluminum alloy.6

In order to estimate the barrier properties associated to the two layers during immersion as well as corrosion protection, impedance spectra were analyzed by an equivalent circuit composed of three hierarchical RC circuits (Fig. 2a). Constant phase elements (CPE) were used to take into account the non ideal behavior of the system (phase shift differing from -90°). As an example, given in Fig. 2b, the experimental diagram is perfectly fitted with the equivalent circuit. Fig. 3 presents the variations of Rporous, Rdense, Qporous and Qdense obtained from the fitting of the experimental data. It can be seen that values of Rporous decrease with coating ageing while no significant change in values of Rdense was considered. Assuming that Qporous and Qdense are pure capacitances, an outer to inner layer thickness ratio of 100 can be considered. This result underlines that the inner layer (at the metal/coating interface) is thin (about 200 nm), compact and poorly modified during immersion. This layer would be responsible of the high corrosion protection afforded by the UV-coating. The outer layer (at the coating/electrolyte interface) represents most of the coating thickness and is rather porous (water uptake mainly occurs in the outer layer).

Fig. 2: (a) Equivalent circuit used to analyse the impedance spectra

and (b) Fitted and experimental Bode plots for AA2024-T3 coated with a UV-cured sol-gel film at 168 h.

Fig. 3: (a) Rporous and Rdense and (b) D) Qporous and Qdense with immersion time in the 0.5 M NaCl solution.

AA2024-T3 coated with a UV-cured sol-gel film. Coating thickness = 18±5 µm. Several coating thicknesses were considered to characterize the two layers in the UV-coating and to determine both their influence on the corrosion protection and an optimum coating thickness. Analytical techniques may be also used to confirm the presence of the two layers which are assumed to be chemically different.

Acknowledgements Agence Nationale de la Recherche (ANR-08-MAPR-0003, research program MHYRCEA) is gratefully acknowledged for kindly financial support.

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