Corrosion protection in the aeronautic industry is a key issue, and multilayer coatings are used to protect aeronautical metallic substrates. A typical system contains hexavalent chromium, or chromate, to provide high, active corrosion protection, and is made up of three layers applied on a prepared surface. However, hexavalent chromium is an extremely toxic, carcinogenic, mutagenic and environmentally hazardous compound. Beginning in 2017, the use of chromate will be subjected to authorization under the REACH directive. Hence, the replacement of chromate compounds is an urgent and major issue for the aeronautic industry. Up to now, Airbus Group Businesses Units, including Airbus, have succeeded in replacing chromate in anodizing and etching (chemical acid treatment to deoxidize and prepare metallic surfaces) steps. In addition, different alternatives based on inorganic and organic inhibitors, low-temperature plasma deposition or sol-gel coatings1 have been investigated. To date, none of these approaches are as efficient as chromate-based coatings.
In the framework of the UV-Curable Hybrid Sol-Gel Coatings for Aeronautical Substrates (MHYRCEA) project belonging to The French National Research Agency (ANR) research program, we combined a UV-curing technology and hybrid sol-gel chemistry to develop a single-step route toward chromate-free coatings. The UV-driven procedure is compliant with novel environmental regulations and the reduction of manufacturing cycle time in the aeronautic industry. In addition, the hybrid sol-gel materials have demonstrated good passive corrosion resistance for metal substrates because of their good adhesion on metals and their ability to form dense barriers to the penetration of corrosion initiators.2-4 This single-step process combines a photoinduced sol-gel process and a cationic (or free-radical) organic photopolymerization.5,6 Through the catalysis of a photoacid generator, the organic groups carried by the organo-alkoxysilanes can photopolymerize with an organic resin, while an inorganic network is formed simultaneously by inorganic polymerization of the alkoxysilyl groups.7,8 The organic resin affords more flexible and thicker coatings than pure inorganic sol-gel layers.