Influence of the composition of a hybrid sol-gel layer as a pre-treatment for cataphoretic paint

 
Aluminium is naturally protected by a very thin alumina (Al2O3) layer which endows the material with a remarkable corrosion resistance. In addition, over the alumina layer, an outer film of variable thickness consisting of mainly bayerite (Al(OH)3) is commonly present. Triggered by the environmental restrictions related to the traditional surface pretreatments, silicon alkoxides sol-gel coatings has been widely studied as an effective solution to protect aluminium and aluminium alloys components. Hybrid sol-gel films were proved to be suitable both as a chemical conversion treatment alone or as a coupling agent between the underling substrate and an organic coating [1]. In a previous study carried out on AA6061 it was proved that a sol-gel coatings are suitable as pre-treatment to promote the adhesion and improve the durability of a cataphoretic paint [2]. In this work the effect of the composition of a sol-gel layer employed as pre-treatment for a cataphoretic paint application was investigated. In particular, hybrid films with different GLYMO:MTES molar ratios were investigated in order to evaluate the best compromise to improve the corrosion protection properties of the complete system (substrate+hybrid film+cataphoretic paint) Prior sol-gel deposition the aluminium alloy plates were alkaline etched in a NaOH solution. Films of different GLYMO:MTES molar ratios (1:0 - 0,8:0,2 - 0,6:0,4 - 0,4:0,6 - 0,2:0,8 - 0:1) were afterward applied on the substrates from a 5wt% solution in acidified water. The substrates were immersed in the solution containing the silicon alkoxide precursors for two minutes. The withdraw speed was set to 20 cm/min. A curing treatment in oven at about 160°C in 30 minutes was carried out to promote the condensation reactions of the sol-gel film. The cataphoretic paint applied on the sol-gel coated samples consisted in a commercial epoxy-polyester clearcoat. About 20 ?m (dry thicnkness) of paint were applied. The effect of the different GLYMO:MTES molar ratios was investigated both in terms of morphology by means of scanning electron microscope (SEM), in terms of surface chemistry by means of FT-IR exploiting ATR geometry and in terms of electrochemical activity of the surface using electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy. The corrosion resistance of the complete protection system (substrate+hybrid film+cataphoretic paint) was evaluated carried out EIS measurements of the intact coatings, filiform corrosion test according to EN 3665:1997 standard and exposure in AASS (acetic acid salt spray) cabinet according to UNI EN ISO 9227 standard. Dry adhesion test were also carried out following ASTM D4541 standard.