New coatings for aircraft wings
To develop an aluminium oxide coating on an Al2024 alloy to replace the current one, based on conversion treatments with chromate and primers.
The aluminium oxide coating developed, using plasma electrolytic oxidation (PEO), successfully passed the tests required in the appropriate aeronautical protocol.
Currently, the Al2024 aluminium aircraft wings carry a primer between the aluminium substrate and the external paint layer so as to improve the adhesion and the corrosion resistance of the material. These types of primer contain chromates, the treatment of which generates highly toxic hexavalent chromium. The PEO coating of aluminium oxide applied directly on the Al2024 substrate would cover the functions of ensuring adherence of the subsequent external paint layer and ensuring high corrosion resistance for the whole system.
The specific objective in this case was to develop an oxide layer of just a few microns of thickness which would be relatively dense near the substrate and porous in the outermost part, to protect against corrosion and promote adhesion with the outer paint layer of the component.
To this end, various plasma electrolytic oxidation procedures were developed to generate Al2O2 coatings of different thicknesses and porosity and the behaviour of the coatings obtained was evaluated by the various standardised tests required by aeronautical protocols.
Lastly, the aluminium oxide PEO coating successfully passed the following tests and was applied on a consistent demonstrator on a part of an actual aircraft wing:
- Resistance to the pretreatment salt spray
- Cutting by mesh
- Adherence to sticky tape
- Flexibility test
- Resistance to the pretreatment salt spray system and priming
- Resistance to immersion in distilled water
- Filiform corrosion resistance
- Resistance to hydraulic fluids and lubricating oils
- Adherence strength (PULL OFF TEST)
- Moisture resistance
- Forming
- Bend flexibility
- Scratch resistance