Alternative for CR(VI) in aircraft coatings

Despite the increasing use of CFRP in aircraft manufacturing, the worldwide best sellers single aisle aircrafts (A320, B737), contain around 65% of aluminium in their structures. Corrosion protection systems for this material contain somehow Cr(VI), either in anodization processes or anticorrosive primer coatings.

Up to now, there is no simple and robust Cr(VI)-free treatment that meets all the requirements of component manufacturers for the aeronautical sector. This sector presents a very demanding requisites and the alternatives available to date present room for improvement, especially in terms of corrosion, since they lose the repairing capacity of formulations with Cr(VI).

The alternative under study in the ACTIVAL project is the replacement of Cr(IV) based primers by Cr-Free anaphoretic treatment (e-coat), which consists of the immersion of the components to be treated in a colloidal suspension of organic compounds. The immersion system ensures a correct protection of complex geometries, as for example hidden areas of bended parts, as well as a significant reduction of VOCs emissions and coating efficiencies around 95%. The e-coating being tested in this project is provided by LVH Coatings.

The consortium, supported by the Basque technological centre CIDETEC and the University of Cadiz, is developing this technology as a prove of concept for aeronautic future certification and use. In doing this, each partner is covering different aspects of anaphoretic technology implementation

Cr-free pilot line implemented

Titania, in its role of technology validator, has implemented an anaphoretic treatment pilot line in its facilities. Coupons and demonstrators are being e-coated at titania, and validation tests are being conducted, including thickness control, adhesion performance, damage tolerance, and environmental, chemicals, and corrosion resistances.

Moreover, Titania is also developing an early corrosion detection system based on an ultrasonic inspection technique called acoustic emission (AE). Some aluminium specific corrosion mechanisms are being studied in order to identify corresponding AE signal transmitted in e-coated coupons.

At the moment, first coat adhesion and filiform corrosion resistance tests have been carried out with good results. Regarding Salt Spray Chamber corrosion tests, a better performance has been identified when using CHEMPLATE´s pre-treatment chemicals.

Testing with aircraft aluminium alloys

Mecanizados y Montajes Aeronaticos, in its role of potential e-coating end-user, has proposed most interesting aluminium alloys for the project, series 2000, 6000 and 7000, as well as final demonstrators of the project. They are based on real helicopter parts and one demonstrator specifically designed to push e-coating technology to the limit, including demanding geometries and drills, commonly present in aircraft structures. All required demonstrators have been already manufactured, and first e-coating trials have been performed with excellent results.

In addition to this, M&M is developing in the project an e-coat repair system based on brush plating technique, setting up a scenario in which this e-coating will have to be repaired directly on the aircraft surfaces.

Chemplate Materials, as a chemicals developer, is improving the pre-treatments required for good e-coating results, trying to offer better corrosion resistance by selective removal of metallic inclusions on aluminium substrate.

Automated pre-treatment replenisher

Additionally, Chemplate is also working in an innovative system for bath quality monitoring and automatic replenishment, based on a battery of sensors developed for each bath stage. By now, a first automated bath replenisher is being tested at lab scale, using a web interface. The system will be validated in the next scale-up stage.

Innerspec Technologies as a specialist in NDT inspection, is developing a non-destructive real time monitoring system for the thickness of the e-coat. This system is based on dual technology sensors (EMAT-EC), capable to survive to aggressive bath conditions, while obtaining good measurements in the electric filed promoted by e-coat electrodes. Good results have been reached measuring coat thickness (around 1,5 µm accuracy), despite the interferences of anodic the current being applied.

Innerspec will also intent to develop this system to work in real environments, to cope with the requirements of brush-plating repairing activities.

The ACTIVAL project will finish by the end of 2023 after 28 months of development, but two of the partners, Chemplate and Titania, are already working together in follow up-project. Also the project will be presented by speakers from Tiatnia and Cidetec at the next IntAIRCOAT conference which will take place at 7-8 June in Sevilla, Spain. Registration for the conference is already open.