Progress in Organic Coatings 59 (2007) 224–229 Silane coating of metal substrates: Complementary use of electrochemical, optical and thermal analysis for the evaluation of film properties I. De Graeve ∗ , J. Vereecken, A. Franquet, T. Van Schaftinghen, H.Terryn Vrije Universiteit Brussel, Department of Metallurgy, Electrochemistry and Materials Science, Pleinlaan 2, 1050 Brussels, Belgium Abstract Silane coating is a metal surface pre-treatment introduced as a replacement for chromium treatments. It is supposed to be suited for various metals, including aluminium, steel and galvanised steel. Good corrosion performance has been assigned to these hybrid organic–inorganic thin film deposition systems. The silane molecules contain Si O C n H (2n+1) groups, which after hydrolysis into reactive silanol groups Si OH form a covalent bonded layer on the metal surface. Curing of these films is considered essential for corrosion protection; during heat treatment, condensation of unreacted silanol groups in the film result in the formation of a Si O Si network with enhanced barrier properties. This contribution gives a chemical and morphological characterisation, obtained with spectroscopic ellipsometry (SE), of BTSE silane coated aluminium as a function of curing. Further, with thermal gravimetric analysis combined with mass spectroscopy (TGA-MS), the mechanism and kinetics of the curing process are explained, and the resulting barrier properties are measured using electrochemical impedance spectroscopy (EIS). © 2006 Elsevier B.V. All rights reserved. Keywords: Silane; Surface characterisation; Curing; Barrier properties 1. Introduction Silanes were originally used as glass coupling agents, i.e. adhesion promoters for coatings on glass substrates. In the early nineties, the silane technology was introduced for adhesion pro- motion and corrosion protection on metals, as an alternative for the carcinogenic chromium VI containing conversion treat- ments. In the research group of the Vrije Universiteit Brussel many research efforts have been dedicated to the silane coating tech- nology [1–13], most work focussing on aluminium and some on steel. Silanes are considered for use on various metals: alu- minium and aluminium alloys [14–16], copper [17,18], iron and steel [19], zinc [20] and more recently for magnesium alloys [21]. Silanes are hybrid molecules containing functional organic groups, such as methoxy or ethoxy groups, on inorganic silicon atoms. Some silane types also contain other types of func- tional groups, such as chlorine, amine, sulphur or epoxy. The latter silane types are so-called functional silanes where the ∗ Corresponding author. Tel.: +32 2 629 3534; fax: +32 2 629 3200. E-mail address: idgraeve@vub.ac.be (I. De Graeve). additional functional groups promote adhesion with overlying organic films such as paint coatings. The ethoxy or methoxy groups are hydrolysed when adding water to the system, and the resulting silanol groups Si OH can react with metal hydroxide groups on the substrate surface, thus forming a Si O M cova- lent bonded metal/film interface [22]. Hence, unlike chemical conversion treatments such as chromating [23] or the Cr-free zirconium/titanium conversion [24], in which metal oxidation and species reduction processes govern the surface conversion, silanes do not require the metal to electrochemically participate in the film deposition mechanism. For corrosion protection purposes, curing of the silane layer is considered essential. Heating of the coated substrates results in the crosslinking between silane molecules in the bulk of the deposited film; silanol groups which have not reacted with the metal surface condense to form Si O Si siloxane chains. Crosslinking and branching results in the formation of a dense network limiting electrolyte access to the underlying metal and hence forming an effective barrier against corrosive attack. In the present paper, the curing of BTSE coated aluminium is investigated using a complementary analysis approach based on spectroscopic ellipsometry (SE) [1,7–9,11,12], electrochemical impedance spectroscopy (EIS) [7,9,11,12], differential scanning 0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.09.006