Corrosion protection of high strength al alloys by mono or bi-layers hybrid coatings

In the last years organic-inorganic hybrid coatings applied by the sol-gel route have emerged as a promising surface treatment for different metals. These films are versatile, as they can be applied to a variety of substrates, and, due to functionalization, they can also act as adhesion promoters between the base metal and primers and/or organic coatings used in the overall anticorrosion protection system. Furthermore, they provide additional barrier protection to the substrate, hindering the access of aggressive species to the surface. However this passive protection mechanism is not enough to replace the efficient chromate-based pre-treatments. Due to this fact in the last few years they have been tailored with the addition of modifiers in order to afford some active corrosion protection properties [1,2]. Among these modifiers Ce ions seem to be one of the most promising [3,4].
The aim of the present work is to investigate the anticorrosion performance of mono and bi-layer TEOS-GPTMS (tetraethyl orthosilicate/3-glycidoxypropyltrimetoxysilane) hybrid coatings applied on AA 2024-T3. The hydrolysis solution is aqueous-based with the addition of only 10% of ethanol in volumetric percentage. The hydrolysis time, the cerium content and curing conditions were optimized with the help of electrochemical techniques and FT-IR determinations. The electrochemical investigation was carried out in 0.1 M NaCl solution by means of electrochemical impedance spectroscopy (EIS) and polarization curves, while microstructural characterization of uncorroded and corroded samples was performed by SEM-FEG observations. The coatings were modified either by the addition of Ce3+ or Ce4+ ions to the hydrolysis solution. The experimental results show that Ce modified coatings perform better than the non-modified ones, being more stable with immersion time. However, addition of modifier excess hinders the anticorrosion behavior of the protective coating, likely due to introduction of excess defect in the film structure. The TEOS-GPTMS amount in the hybrid solution influences the rheological behavior of the solution and the layer performance.