Capacitance and photoelectrochemical studies for the assessment of anodic oxide films on aluminium J.C.S. Fernandes a, * , R. Picciochi a , M. Da Cunha Belo a , T. Moura e Silva b , M.G.S. Ferreira a,c,1 , I.T.E. Fonseca d,1 a Department of Chemical Engineering, Instituto Superior Te ´cnico, 1049-001 Lisboa, Portugal b Instituto Superior de Eng. de Lisboa, Department of Mechanical Engineering, 1950-062 Lisboa, Portugal c Department of Ceramics and Glass Engineering, University of Aveiro, 3810-193 Aveiro, Portugal d CECUL, Faculty of Sciences, University of Lisbon, 1749-016 Lisboa, Portugal Received 5 February 2004; received in revised form 18 May 2004; accepted 19 May 2004 Available online 25 June 2004 Abstract Photoelectrochemical spectroscopy and capacitance measurements were used in this work to assess the electronic properties of the oxide films formed on 99.5% aluminium and 2024-T3 aluminium alloy by anodising in a sulphuric-boric bath. The morphology of these films was also studied by transmission electron microscopy cross-section observations. The results obtained indicate that the oxide films formed on aluminium show a n-type semiconductive behaviour, with bandgap energies that are identical for the oxides studied, despite their different characteristics. It was found out that capacitance measurements may be used as a valuable technique for the assessment of the quality of anodised layers, allowing the distinction between an efficient and an inefficient sealing. Therefore, they may be used to predict the corrosion resistance of these materials. ß 2004 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Anodising; Semiconductivity; Capacitance; Photoelectrochemistry 1. Introduction Corrosion protection of metals and alloys is often achieved by the formation of passive oxide films, which usually exhibit semiconductive properties. The resistance of the metal to corrosive attack has been found to depend upon the solid-state characteristics of the oxide film [1,2]. In this frame, photo- electrochemical spectroscopy and capacitance measurements (Mott–Schottky approach) have been successfully used in the past as in situ techniques for the characterization of passive films formed on different metals [3–5]. Although anhydrous Al 2 O 3 has been usually reported in the literature as an insulator, with a bandgap ranging from 8 to 9 eV [6], recent studies of the oxide films formed on aluminium seem to indicate that they show n-type semi- conducting properties, with optical transitions ranging between 2.8 and 4.5 eV [7-9]. It should be pointed out that the distinction between a semiconductor and an insulator is however purely arbitrary. In both cases the electronic struc- ture can be described by the band model, but the bandgap of insulators is generally higher than that of semiconductors and the absence of suitable dopants prevents conductivity. The relationship between the solid-state properties of Al oxide films and its corrosion resistance has been studied through different approaches. McCafferty [7] reported a dependence of the pitting potential on the flatband potential (E fb ) and the isoelectric point of the oxide. Similar results were presented by Menezes et al. [8], who found out that E fb increases with the tendency of Al to undergo pitting. On the other hand, Di Quarto and coworkers [9,10] have related the www.elsevier.com/locate/electacta Electrochimica Acta 49 (2004) 4701–4707 * Corresponding author. Tel.: +351 218417964; fax: +351 218404589. E-mail address: joao.salvador@ist.utl.pt (J.C.S. Fernandes). 1 ISE member. 0013-4686/$ – see front matter ß 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.05.025