Development and industrial scale-up of ZrO2 coatings and hybrid organic–inorganic coatings used as pre-treatments before painting aluminium alloys

Development of Cr-free coatings is an important issue for replacement of chromate conversion coatings in aerospace and automotive industry. This aspect was targeted in the framework of MULTIPROTECT IP project funded by the European Community. This paper is an overview of research activities carried out by our groups in the framework of the MULTIPROTECT IP project.Our research approach in the project considered the development of thin ZrO2 coatings (100-200nm) by means of sol-gel technology for AA2024 aluminium alloy. ZrO2 films were deposited employing solutions of Zr precursors. Coating application was carried out by dipping and spraying technology at a laboratory scale. In addition, substrates pre-treated with ZrO2 films were painted with an organic primer and top coat in order to evaluate paint adhesion and electrochemical behaviour of painted systems. Corrosion inhibitors were introduced in the ZrO2 coatings in order to improve the corrosion behaviour of the sol-gel films. In parallel, hybrid organic-inorganic methacrylate silane-based films containing SiO2 nano-particles doped with cerium nitrate were developed in the framework of the project. At a later stage of the project, coating deposition was carried out with a robot-controlled air pressure gun in spraying cabins in order to evaluate industrial production of the ZrO2 films and hybrid organic-inorganic films. The results obtained in the development phase clearly indicate that it is possible to deposit ZrO2 coatings and hybrid organic-inorganic films on the substrate by dipping technique. These thin pre-treatments provide good barrier properties to the substrate and promote adhesion of an organic primer and top-coat. Cerium nitrate introduced in hybrid organic-inorganic films evidences an inhibition effect on corrosion processes, which can be attributed to cerium compounds. The up-scaling phase proves that it is possible to produce water-based ZrO2 pre-treatments and hybrid organic-inorganic films by spraying technology using industrial equipment.