Hybrid sol–gel coatings doped with transition metal ions for the protection of AA 2024-T3

The protective capabilities of sol–gel coatings are determined by their physical barrier properties. For an effective protection, a homogenous crack-free material is required, which prevents from attacks of corrosive species. When the coating is damaged, active corrosion protection is usually achieved by the use of inhibitors. Among the different inhibitors rare earth ions and especially cerium have shown effective inhibiting properties. Due to the complexity of the corrosion processes, a combination of inhibitors is expected to be superior to a monocomponent inhibiting. The aim of this study was to prove which other ions, used in combination with cerium, can improve the corrosion protection abilities of hybrid silica based inorganic–organic sol–gel coatings applied on aluminium alloy 2024 substrates. Mixtures of cerium nitrate with two other potential inhibitor substances were incorporated into a sol–gel matrix and their behaviour in neutral salt spray test and during EIS measurements was investigated. The Ce–P–Pr inhibitor combination (Ce3+, PO43−, Pr3+) has shown the best long-term corrosion protection properties at low doping levels.