Inhibition effect of cerium in hybrid sol–gel films on aluminium alloy AA2024†

Aluminium alloys such as AA2024 are susceptible to severe corrosion attack in aggressive solutions (e.g. chlorides). Conversion coatings, like chromate, or rare earth conversion coatings are usually applied in order to improve corrosion behaviour of aluminium alloys. Methacrylate-based hybrid films deposited with sol–gel technique might be an alternative to conversion coatings. Barrier properties, paint adhesion and possibly self-healing ability are important aspects for replacement of chromatebased pre-treatments. This work evaluates the behaviour of cerium as corrosion inhibitor in methacrylate silane-based hybrid films containing SiO2 nano-particles on AA2024. Hybrid films were deposited on aluminium alloy AA2024 by means of dipcoating technique. Two different types of coating were applied: a non-inhibited film consisting of two layers (non-inhibited system) and a similar film doped with cerium nitrate in an intermediate layer (inhibited system). The film thickness was 5 μm for the non-inhibited system and 8 μm for the inhibited system. Film morphology and composition were investigated by means of GDOES (glow discharge optical emission spectroscopy). Moreover, GDOES qualitative composition profiles were recorded in order to investigate Ce content in the hybrid films as a function of immersion time in 0.05 M NaCl solution. The electrochemical behaviour of the hybrid films was studied in the same electrolyte by means of EIS technique (electrochemical impedance spectroscopy). Electrochemical measurements provide evidence that the inhibited system containing cerium displays recovery of electrochemical properties. This behaviour is not observed for the non-inhibited coating. GDOES measurements provide evidence that the behaviour of inhibited system can be related to migration of Ce species to the substrate/coating interface. Copyright c © 2010 John Wiley & Sons, Ltd.