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 noninhibited 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.Peer reviewe

Inhibition effect of cerium in hybrid sol–gel films on aluminium alloy AA2024

Aluminium alloys such as AA2024 are susceptible to severe corrosion attack in aggressive solutions (e.g. chlorides). Conversion coatings, like chromate, or rare earth conversion coatings are usually applied in order to improve corrosion behaviour of aluminium alloys. Methacrylate-based hybrid films deposited with sol–gel technique might be an alternative to conversion coatings.Barrier properties,paint adhesion andpossibly self-healing ability are important aspects for replacement of chromatebased pre-treatments. This work evaluates the behaviour of cerium as corrosion inhibitor in methacrylate silane-based hybrid films containing SiO2 nano-particles on AA2024. Hybrid films were deposited on aluminium alloy AA2024 by means of dipcoating technique. Two different types of coating were applied: a non-inhibited film consisting of two layers (non-inhibited system) and a similar film dopedwith cerium nitrate in an intermediate layer (inhibited system). The film thickness was 5 μmfor the non-inhibited system and 8 μm for the inhibited system. Film morphology and composition were investigated by means of GDOES (glow discharge optical emission spectroscopy). Moreover, GDOES qualitative composition profiles were recorded in order to investigate Ce content in the hybrid films as a function of immersion time in 0.05 M NaCl solution. The electrochemical behaviour of the hybrid films was studied in the same electrolyte by means of EIS technique (electrochemical impedance spectroscopy). Electrochemicalmeasurements provide evidence that the inhibited system containing cerium displays recovery of electrochemical properties. This behaviour is not observed for the non-inhibited coating. GDOES measurements provide evidence that the behaviour of inhibited system can be related to migration of Ce species to the substrate/coating interfacePeer reviewe

Study of the effect of cerium nitrate on AA2024-T3 by means of electrochemical micro-cell technique

This work evaluates the effect of cerium nitrate as corrosion inhibitor for AA2024-T3 in the view of its introduction in sol–gel coatings able to provide self-healing ability. Since it is well established that deposition of Ce species is activated by the local pH increase, the objective of this paper is to investigate the behavior of AA2024-T3 (open circuit potential and polarization curves) in the presence of Ce species in aggressive solutions by means of a local technique, the electrochemical micro-cell. This technique enables the investigation of small areas with resolution in the micrometer range by the use of glass capillaries to define the working electrode area. The micro-cell results clearly displayed that the entire AA2024-T3 area exposed to the ceriumcontaining electrolyte was involved in the cerium precipitation mechanism. The heterogeneous electrochemical behavior of the microstructure is minimized by the formation of a cerium-containing layer able to protect the metal substrate.Peer reviewe