Benzoxazine resin coated on anodized aluminium alloys for corrosion protection: impact of the oxide morphology

Aluminium alloys are widely used in aircraft applications, especially series 2xxx (Cu) and 7xxx (Ti). Those alloys offer improved mechanical properties compared to pure aluminium, but are strongly sensitive to corrosion. Chromic anodizing was used as an efficient corrosion protection. But because of 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.
In this work, an alternative corrosion-protection system has been developed. It consists in a cross-linked benzoxazine resin [1] coated on an oxide film obtained by sulfotartaric anodizing (TSA). The role of the anodic film is to improve compatibility between coating and substrate by providing a great interaction surface to the resin thanks to the porous structure of the oxide layer.
We investigated the influence of the nanoporosity of the oxide (pore size, surface porosity) on expected properties by playing with anodizing parameters (voltage, time, bath temperature). The influence of the substrate surface composition has also been studied by comparing anodized bare and cladded AA2024-T3 substrates. Oxide layers were observed and characterized using Field Emission Gun Scanning Electron Microscopy (FEG-SEM).
An organic coating based on benzoxazines, a new class of thermoset resins offering low shrinkage and high chemical resistance was used. Two different thermal crosslinking cycles have been performed and compared using DiElectric Analysis (DEA).
Coatings were applied by spin coating on anodized aluminium substrates and characterized using Electrochemical Impedance Spectroscopy (EIS) to emphasize anti-corrosion behaviour. All systems exhibited very good barrier properties.

[1] L. Dumas, L. Bonnaud, M-G. Olivier, M. Poorteman and Ph. Dubois - Facile preparation of a novel high performance benzoxazine-CNT based nano-hybrid network exhibiting outstanding thermo-mechanical properties - Chem. Commun. 49 (2013)