Environmental friendly and efficient coatings on Ultra-high strength steels for the aviation sector

The H2Free project will develop a degassing model, which aims to be a key tool to substitute the toxic metal cadmium by zinc-nickel

Metals remain an important structural material to meet the rigorous requirements of industries including aerospace, military, mining and automotive. In recent years, ultra-high-strength steels (UHSSs) have been gaining traction. These materials are used due to their extraordinary resistance mainly for landing gears in the aeronautic industry. To resist corrosion, these materials require a coating that has usually been made by cadmium (Cd) electroplating. Cadmium is a toxic metal whose use is advisable to avoid (by European REACH regulation). Aiming to make industrial processes more sustainable, the European aviation sector is working on substituting Cd with innovative, environmentally friendly coatings. ZnNi coatings are considered to be a viable and REACH-compliant substitute. Electroplating tends to liberate hydrogen from the cathode, which can lead to hydrogen embrittlement. The evolved hydrogen may diffuse outward and become trapped in the substrate/coating interface or migrate inward into the steel lattice causing delayed embrittlement when the component is subjected to stress.

The aim of the H2Free project is to develop a practical guide for the degasification of hydrogen from UHSS steel coated with Low Hydrogen Embrittlement (LHE) ZnNi and Cd, in order to save production costs and minimise environmental impact. The challenging part is the lack of knowledge about degassing kinetics as it is till now unknown in advance if a standard degasification process is always necessary or effective in lowering the hydrogen content below the critical level, leading in some cases to scrap preventively entire components.

The H2Free project is now in its second year of project. The project is coordinated by CIDETEC Surface Engineering. CIDETEC is electrodepositing both LHE-ZnNi and LHE-Cd on different UHSSs, both at lab and pilot plant scale, under different conditions in order to produce and characterize a broad spectrum of morphologies with different H contents and supports the other partners of the project with coated specimens to be characterized regarding H permeability and hydrogen embrittlement. The consortium is made up of four outstanding research centres and two SMEs with complementary profiles and great experience. The Institute of Surface Science of HELMHOLTZ-ZENTRUM HEREON has a lot of experience in developing predictive modelling and simulation of service-life aspects. ELSYCA provides its experience in modelling processes of functional electroplating applications, while AZTERLAN is evaluating the H content in coated UHSS prior and after different degassing treatments and studies hydrogen embrittlement. In addition, the consortium is complemented with the technical knowledge to carry out surface treatments for the aerospace industry from ELHCO and MAX-PLANCK-INSTITUT FÜR EISENFORSCHUNG, a leading materials research institute focusing on advanced materials research with capabilities to perform hydrogen permeation measurements with very high precision.

This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007713. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union. This communication reflects only the author’s view and that the JU is not responsible for any use that may be made of the information it contains.

Figure: Graphic representation of Hydrogen Induced Embrittlement.. Hydrogen atoms are absorbed by the UHS-steel and diffuse to the metal grain boundaries forming bubbles. These bubbles exert pressure on the metal grains resulting in a reduction of metal ductility and strength.