Benzoxazine resin coated on anodized aluminium alloys: electrochemical properties of a Cr-free corrosion protection system

Aluminum alloys are widely used in aircraft applications, especially series 2xxx (Cu) and 7xxx (Ti). Those alloys offer improved mechanical properties compared to pure aluminum, but are strongly sensitive to corrosion. Chromic anodizing combined with chromate containing epoxy primer was used as an efficient system to prevent corrosion. Nevertheless, due to health and environmental issues, this treatment is going to be forbidden. Alternative solutions have to be developed to reach aircraft standards considering health and environmental concerns.
This work aims at proposing an alternative protective system, consisting in an oxide layer obtained from sulfotartaric anodizing (TSA) and coated with a benzoxazine organic layer. Benzoxazine resins offer high chemical resistance and low shrinkage and so are very good candidates for coating applications. Investigated layers have been prepared on both bare and clad Al-2024 T3 substrates.
The morphology and the thickness of coating systems were determined using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Electrochemical Impedance Spectroscopy (EIS) was used to characterize the electrochemical behaviour of obtained systems and separated layers in order to emphasize synergetic effects. Existing chrome-containing coating was also characterized and compared to the investigated system. Equivalent electrical circuits were used to model the electrochemical properties of different layers and the evolution of their properties over immersion time in an aggressive saline solution.
The results confirmed the strong potential of such systems for future corrosion protection applications.