Preparation and corrosion protective properties of nanostructured titania-containing hybrid sol–gel coatings on AA2024

Titania-containing organic–inorganic hybrid sol–gel films have been developed as an alternative to chromate-based coatings for surface pretreatment of aluminium alloys. Stable hybrid sols were prepared by hydrolysis of 3-glycidoxypropyltrimethoxysilane and different titanium organic compounds in 2-propanol solution in the presence of small amounts of acidified water. Different diketones were used as complexing agents in this synthesis for controllable hydrolysis of titanium organics. The properties of the obtained coatings were compared with those of zirconia-containing films. Electrochemical impedance spectroscopy (EIS) measurements and standard salt spray tests were performed to investigate the corrosion protection performance of the hybrid coatings. It was revealed that their protective properties depend significantly on the nature of metalorganic precursors and complexing agents used in the process of sol preparation. The best anticorrosive protection of AA2024 in chloride solutions is provided by the titania-containing sol–gel films prepared with titanium(IV) tetrapropoxide and acetylacetone as starting materials. In the case of zirconia-containing films, better protective properties were found when applying ethylacetoacetate as a complexing agent.