Electrochimica Acta 52 (2007) 7486–7495 Electrochemical study of modified bis-[triethoxysilylpropyl] tetrasulfide silane films applied on the AZ31 Mg alloy M.F. Montemor a,∗ , M.G.S. Ferreira a,b a Instituto Superior T´ ecnico, ICEMS, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b University of Aveiro, Department of Ceramic and Glass Engineering, 3810-193 Aveiro, Portugal Received 5 July 2006; received in revised form 10 October 2006; accepted 17 December 2006 Available online 26 January 2007 Abstract This work investigates the protective behaviour of bis-[triethoxysilylpropyl] tetrasulfide silane pre-treatments on the AZ31 Mg alloy. The silane solution was modified by the addition of cerium nitrate or lanthanum nitrate in order to introduce corrosion inhibition properties in the silane film. The corrosion behaviour of the pre-treated AZ31 magnesium alloy was studied during immersion in 0.005 M NaCl solution, using electrochemical impedance spectroscopy and the scanning vibrating electrode technique (SVET). The electrochemical experiments showed that the presence of cerium ions or lanthanum ions improve the protective behaviour of the silane film. The SVET experiments evidenced that the presence cerium in the silane film led to an important reduction of the corrosion activity. The results demonstrate that either cerium ions or lanthanum ions can be used as additives to the silane solutions to improve the performance of the pre-treatments for the AZ31 magnesium alloy. © 2007 Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy AZ31; Silanes; SVET; Cerium; Corrosion 1. Introduction The use of magnesium alloys for different applications has been increasing during the last years. The need of lighter mate- rials for specific applications such as the electronic industry led to an increasing interest on the corrosion behaviour of Mg alloys. They are used in the production of cellular phones, note- books and other electronic devices. The alloys are also used in the automotive and aeronautical industry, mainly in structural components. The increasing interest on the use of Mg alloys is due to the fact that these alloys exhibit an attractive combination of low density, high strength, good damping capacity, castabil- ity and machinability. The AZ grades of Mg alloys contain aluminium and zinc. Zinc is effective to increase strength. Alu- minium increases strength and corrosion resistance, but reduces castability and weldability. ∗ Corresponding author. Tel.: +351 218417234; fax: +351 218419771. E-mail address: mfmontemor@ist.utl.pt (M.F. Montemor). Mg alloys show a surface film composed essentially of a mixture of MgO and Mg(OH) 2 , which provides reasonable cor- rosion protection in air, but becoming very unstable in aqueous or in high humidity environments. Thus, Mg alloys are generally used in environments with reduced aggressiveness because they are very sensitive to corrosion attack. To improve the lifetime of Mg alloys different chemical treatments and organic coatings are generally applied on their surface. In addition to improved durability, organic coatings also confer specific functionalities and aesthetic properties. For such purposes, adhesion of the coat- ing to the bare substrate is an important issue for the durability of the coated material under service. Thus, the performance of coated Mg alloys is strongly dependent on the corrosion resis- tance and on the adhesion properties of the coating to the metallic substrate. The corrosion resistance of Mg and its alloys can be enhanced by different procedures. Anodising [1–4] has proved to improve the corrosion resistance of the alloys under aggressive envi- ronments. Anodic oxidation produces an oxide film with good corrosion resistance and reasonable adhesion properties. The deposition of chemical conversion layers using modified per- manganate solutions led to the formation of a nearly protective 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.086