Corrosion resistance of 2524 Al alloy anodized in tartaric-sulphuric acid at different voltages and protected with a TEOS-GPTMS hybrid sol-gel coating H. Costenaro a,b, ⁎, A. Lanzutti c , Y. Paint d , L. Fedrizzi c , M. Terada a , H.G. de Melo a , M.-G. Olivier b,d a Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, n. 2463, CEP 05508-030 São Paulo, SP, Brazil b Service de Science des Matériaux, Université de Mons, Place du Parc 23, 7000 Mons, Belgium c Polytechnic Department of Engineering and Architecture, University of Udine, Via del Cotonificio 108, 33100 Udine, Italy d Materia Nova asbl, Avenue Copernic 1, 7000 Mons, Belgium abstract article info Article history: Received 3 February 2017 Revised 3 April 2017 Accepted in revised form 31 May 2017 Available online 1 June 2017 Corrosion is a major problem for high strength aluminium alloys. Thickening of the naturally formed oxide layer through anodizing is one of the main approaches to improve the corrosion resistance of these materials. Chromate anodizing is extremely efficient to produce anodized layers with good corrosion resistance and painting adhesion, however chromate based surface treatments must be banished from industrial use. The corrosion resistance of al- uminium alloy 2524 (AA2524) anodized in tartaric/sulphuric acid (TSA) bath and protected with a hybrid sol-gel coating was evaluated by means of electrochemical impedance spectroscopy (EIS) and salt-spray tests. The mor- phologies of the obtained layers were characterized by SEM-FEG while the chemical in depth distribution of the hybrid layers was evaluated by means of Rf-GDOES. The effect of anodizing voltage on the sol-gel impregnation and the protection afforded by the layers was evaluated. Electrical equivalent circuit fitting of the EIS data has shown that the anodized layer thickness plays an important role in the protection mechanism of the sol-gel layer. Salt-spray tests highlighted the significant contribution of the sol-gel distribution in the anodized layer. © 2017 Elsevier B.V. All rights reserved. Keywords: Anodizing voltage TSA bath EIS Sol-gel Salt-spray 1. Introduction Aluminium alloys from 2xxx series are very favourable for aircraft applications due to their physical characteristics, such as lightweight, high specific strength and durability [1]. The most employed brand is AA2024. It is used in parts such as fuselages and shear webs where toughness, fatigue and mechanical strength are major requirements [2]. However, this particular alloy presents a high amount of intermetal- lics (IMs) in its microstructure, achieving a few hundred thousand par- ticles per cm 2 [3] having inhomogeneous sizes [4], compositions [3] and small scale spatial surface distributions [5]. As a consequence, the onset of localized corrosion processes is likely to occur, due to the develop- ment of domains with different electrochemical activities either within the alloy microstructure or within the IM particles themselves. More- over, it has been reported in the literature that IM particles have a ten- dency to cluster [5], favouring the development of stable pits, eventually leading to intergranular corrosion [6]. IM particles are formed during alloy solidification and their number and size depend on several factors such as alloy composition and impurity content, as the solid solution solubilities of the alloying ele- ments are interdependent [7]. Therefore, in recent years, new genera- tions of 2xxx aluminium alloys have been developed in order to improve mechanical and anticorrosion properties through optimization of the microstructure, with stricter limits on Fe and Si impurity levels and by adjusting the content of the Cu and Mg alloying elements [2,8]. Aluminium alloy 2524 (AA2524) is a new kind of aerospace alloy with high damage-tolerance and excellent fatigue properties [9] and is con- sidered as a viable substitute for commercial AA2024 [10]. Chen et al. [9] reported that by diminishing impurity levels and controlling process parameters, AA2524-T3 could exhibit about a 15–20% increase in frac- ture toughness, attain twice the fatigue crack growth resistance and a 30–40% longer lifetime before failure when compared to AA2024-T3, without loss of strength or corrosion resistance. Moreover, DeBartolo and Hillberry [11] reported that the number of S–type (Al 2 CuMg) parti- cles as well as the β-type particles (containing iron) in AA2524-T3 is sig- nificantly reduced in comparison to AA2024 T3, though less reduction is obtained in the latter case. Aggressive environments can affect the fatigue strength of aircraft components by favouring the formation of stable corrosion pits, acting as stress concentrators where crack nucleation can take place [12]. Re- gardless of the brand, due to their complex microstructure, 2xxx Al alloys are particularly sensitive to localized corrosion in chloride environments. Surface & Coatings Technology 324 (2017) 438–450 ⁎ Corresponding author at: Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, n. 2463, CEP 05508-030 São Paulo, SP, Brazil. E-mail address: hellencostenaro@gmail.com (H. Costenaro). http://dx.doi.org/10.1016/j.surfcoat.2017.05.090 0257-8972/© 2017 Elsevier B.V. All rights reserved. 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