Progress in Organic Coatings 74 (2012) 281–287 Contents lists available at SciVerse ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat Influence of deposition conditions on the protective behavior of tetraethyl orthosilicate sol–gel films on AA5754 aluminum alloy A. Altube a , E. García-Lecina a,∗ , N. Imaz a , J.A. Díez a , P. Ferrón b , J.M. Aizpurua b a Surface Finishing Department, CIDETEC-IK4, Paseo Miramón, 196, 20009 San Sebastián, Spain b Departamento de Química Orgánica-I, Universidad del País Vasco UPV/EHU, Joxe Mari Korta R&D Center, Avda. Tolosa-72, 20018 San Sebastián, Spain article info Article history: Received 31 May 2011 Received in revised form 12 October 2011 Accepted 29 October 2011 Available online 22 November 2011 Keywords: Tetraethyl orthosilicate Aluminum Sol–gel Corrosion abstract The aim of this work is to compare the effect of two deposition methods, dip-coating and electrophoretic deposition, on the characteristics of tetraethyl orthosilicate (TEOS) sol–gel films on AA5754 aluminum alloy, especially in what concerns to their resistance against corrosion. The influence of pH bath on the hydrolysis of the system was analyzed. Moreover, the effect of some experimental parameters, such as deposition time and deposition voltage, was also evaluated. The results showed that pre-hydrolysis rate of the orthosilicate bath depends on the solution pH, and that at pH 2 complete hydrolysis of the solution was obtained after only 20 min. Moreover, it was observed that electrodeposited TEOS films provided better corrosion resistance than films obtained by dip-coating. The improved corrosion resistance was ascribed to a higher uniformity and density of the polysiloxane films and to an in situ modification of the aluminum–polysiloxane interface by the cathodic voltage. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Aluminum is a remarkable material, whose outstanding prop- erties make it the material of choice for many applications in the automotive, aircraft, aerospace, architectural and packaging indus- try, among others. However, aluminum is not the perfect material, as its exposure to the natural environment leads to corrosion of the metal which may cause damage and limit its durability. One of the most convenient methods to protect the surface of alu- minum is by applying a protective coating. In this sense, chromate conversion coatings have been largely used to retard and prevent corrosion, and improve the appearance of a range of metals and alloys, including aluminum alloys [1,2]. However, the use of chro- mates is discouraged due to the carcinogenicity and environmental hazards of Cr(VI), which is a main component in chromate con- version baths. For this reason, it is necessary to investigate and implement more ecological alternatives able to replace conven- tional toxic chromates. Among some of the “green” alternatives explored based on Cr(III), zirconate, silicate or vanadate com- pounds [3–6], silanes and orthosilicates, a group of silicon-based organic–inorganic chemicals, have emerged as a very promising alternative for toxic chromate-based treatments in metal-finishing industries. Orthosilicates are well known cross-linking agents that can provide bond between inorganic and organic materials. Their ∗ Corresponding author. Tel.: +34 943 309 022; fax: +34 943 309 136. E-mail address: egarcia@cidetec.es (E. García-Lecina). general formula may be written as Si(OR) 4 where R is a hydrolysable alkoxy group such as methoxy (–OCH 3 ), ethoxy (–OC 2 H 5 ) or ace- toxy (–OCOCH 3 ). The formation of orthosilicate based coatings comprises several steps: first of all, the alkoxy (–SiOR) groups must be hydrolyzed in water or water/alcohol mixtures in order to form active silanol (–SiOH) groups that can be adsorbed when con- tacted with a clean metal surface forming hydrogen bonds between silanol groups and surface hydroxyls. After adsorption, such bonds slowly convert to metallo-siloxane bonds. Moreover, inside the adsorbed film, the silanols react with themselves giving place to –Si–O–Si– (siloxane) bonds. The solid formation of the polysiloxane film can be speeded up by heating the film to moderate tempera- tures. Thin polysiloxane films may be obtained by a wide range of experimental techniques such as dip coating [7], spraying [8], plasma assisted chemical vapor deposition (PACVD) [9–11] or elec- trophoretic deposition (EPD) [12,13]. Among them, dip-coating is the most widely used technique for its simplicity and low price. It consists of immersing the substrate into a partially hydrolyzed orthosilicate ester solution for a given period of time, followed by drying in air or in an oven. However, several authors [14] have proposed the use of electrophoretic deposition [15] to grow the protective coatings, accelerating the condensation step by altering the pH on the surface. Thus, the application of a negative potential to the substrate to be coated consumes protons at the metal surface, causing a local increase of the pH, and allowing the deposition of the sol–gel film. According to the literature [16] electrophoretic depo- sition technique allows the obtaining of denser and more uniform films than dip-coatings, leading to a higher protection ability. But to date, the results are not conclusive, probably due to the absence of a 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.10.018