UV-cured hybrid sol-gel coatings for aeronautical and Direct-To-Metal (DTM) applications

Corrosion protection in the aeronautic industry is a key issue. Nowadays, 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 previously prepared surface. However, hexavalent chromium is an extremely toxic, carcinogenic, mutagenic and environmentally hazardous compound. From 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, EADS 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 so far and reported. To date, none of these approaches has been found to be as efficient as chromate-based coatings.

In the framework of MHYRCEA project belonging to ANR research program, we have combined a UV-curing technology and hybrid sol-gel chemistry to develop a single-step route toward chromate-free coatings. Our 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 initiators2-4. This single-step process combines a photoinduced sol-gel process and a cationic (or free-radical) organic photopolymerization5,6. Through the catalysis of a phototoacid generator, the organic groups carried by the organoalkoxysilanes can photopolymerize with an organic resin while an inorganic network is formed simultaneously by inorganic polymerization of the alkoxysilyl groups7,8. The organic resin affords more flexible and thicker coatings than pure inorganic sol-gel layers.

The present study focuses on the characterization of these novel UV-cured hybrid sol-gel coatings, and the understanding of the protective mechanism. The first part presents a range of characterizations of hybrid coatings prepared with an epoxy resin and n-alkyltrimethoxysilane precursors exhibiting different chain lengths. The second part discusses the performances of UV-cured hybrid sol-gel coatings in terms of corrosion protection on various substrates including aeronautical substrates and composites, solvent resistance, UV stability and adhesion.