Optimization of hybrid sol–gel coatings by combination of layers with complementary properties for corrosion protection of AA2024

The coating system usually employed for corrosion protection of metal substrates consists of different layers which can be constituted of a chemical conversion coating applied on the metal surface followed by a number of organic layers. Hybrid films prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces, although it is necessary to combine the barrier functionality with an active protection mechanism to avoid corrosion when the coating is damaged. Previous works have shown that it is very difficult to reach this result in a mono-layer sol–gel because the amount of inhibitors incorporated tends to increase significantly the porosity of the coating and reduces drastically the barrier properties. This work presents the characterization of a multi-layer sol–gel hybrid inorganic–organic coating system with a structure composed of an intermediate cerium inhibited layer deposited between two un-doped layers on AA2024 alloy. The comparison between the inhibited system and a bi-layer non-inhibited one has allowed to assess the migration of the cerium ions into the hybrid coating towards the substrate corresponding to the improvement of the corrosion properties. The combination of the physical barrier and the active protection enables to obtain an effective protective system.