One of the most favorable reasons for aluminium worldwide usage is its well-known capability to be easily covered with a protective oxide layer when exposed to aerated environment and, thus, kinetically inhibiting the electrical contact needed for corrosion to take place. As aluminum oxide is an insulating material, the thicker and the denser the oxide layer is, the better its anticorrosive behavior. As all barrier-type layers, its efficiency against corrosion is directly correlated with the presence of preferential paths into the layer, where electrolyte can penetrate thus creating the electrical contact between metal and depolarizing species. Accordingly, the key-point for corrosion protection of aluminum is to allow the growth of a protective layer virtually with no defects. Thickness of this layer is then a matter of the final use. Among all the possibilities, anodizing in strongly acidic media is nowadays the most relevant procedure for aluminum oxide growth. Aim of the present study is the preferential and controlled growth of an oxide layer on AA1050 aluminium alloy. When moving from the "synthesis" of the oxide layer to its capability to act as a barrier to corrosion, the presence of the above-described pores may bring to protection failure. Addition of a different material, capable of occluding pores can in principle improve corrosion protection. Zirconia is an easily synthesizable oxide, whose resistance to corrosion is well established. Many synthetic methods are available in the literature, the newest being microwave assisted synthesis in non-aqueous media. Application of non-aqueous systems to microwave assisted precipitation gives the possibility to obtain nanoparticles with extremely narrow size distribution by extremely short reaction times. Some our previous studies demonstrated the effective accessibility to new transition metal oxides of nano dimensions, with highly homogeneous shapes. Application of this, and other methods on zirconium will be discussed in this paper. Combination of the above two compounds - namely aluminium oxide layer and Zr02 nanoparticles - and their joined anti-corrosion properties have been disclosed by polarization experiments. Aluminum oxide was prepared by room-temperature selective oxidation/dissolution of AA1050 foils, according to Montero [J.M. Montero-Moreno et al. / Microporous and Mesoporous Materials 136 (2010) 68-74]. Procedure was modified adding a step of mechanical polishing in order to obtain the most reproducible and ordered structure of pores. ZrO_2 nanoparticles were produced by different methodologies from commercial zirconium salts. Aluminium oxide layers thus prepared were examined by SEM and evaluated by quasi-stationary polarization curves for corrosion resistance. Zirconia nanoparticles were prepared with 4-7 nm diameters, while crystallites composition was a mix of tetrahedral and monoclinic phases. Evaluation of their practical use for the closure of the oxide layer pores was tested taking into account the different nature of the synthetic methods. In this respect, microwave assisted preparation seems to be promising. In particular, microwave energy source permits reaction times of a few seconds. Polarization tests on as-prepared aluminium oxide layers show an effective corrosion protection, with passivity potential window widening as the aluminium oxide layer grows more and more ordered. Corrosion tests on anodized AA1050, when pores were occluded by zirconia, together with a study of pores thickness, were also performed and the results were discussed as a function of the zirconia synthesis.
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