Investigation of the corrosion behaviour of a bilayer cerium-silane pre-treatment on Al 2024-T3 in 0.1 M NaCl

In the last few years great efforts have been made in order to find environmentally friendly substitutes for Cr6+ pre-treatments applied to aluminium alloys used in the aircraft industry. In this work we have investigated the electrochemical response of a bilayer pre-treatment consisting of a Ce conversion bottom layer and a non-functional silane (bis-1,2-(triethoxysilyl) ethane (BTSE)) top layer applied on Al 2024-T3, and compared its behaviour with monolayer coated samples. The investigation was carried out in 0.1 M NaCl solution, and the electrochemical techniques employed were anodic polarization curves and electrochemical impedance spectroscopy (EIS). EIS experiments performed with bilayer coated samples have shown a continuous increase of the impedance response during the whole test period, which was interpreted on the basis of a pore blocking mechanism supported by scanning electron microscopy (SEM) images and equivalent circuit fitting. Moreover, the impedance of the bilayer coated samples was approximately one order of magnitude higher than that presented by monolayer coated ones. On the other hand, mechanical tests have evidenced the good adhesion of the silane layer to the Ce conversion layer, which can be likely attributed to a better linking between the silane molecules and the cerium bottom layer.