Green and light high-performance alloys
Airplanes routinely lift their own tremendous weight and that of their passengers and cargo high into the air. They then fly hundreds or thousands of miles against friction and drag to deliver the load to its destination. Understandably, air transport makes a significant contribution to carbon dioxide emissions due to fuel combustion. Lightweight aluminium (Al) or magnesium (Mg)-based alloys could efficiently replace iron-based materials for many aircraft structures or the engine. Unfortunately, they are particularly sensitive to corrosion and require processing to coat and seal them, often using harsh chemicals now forbidden or strongly restricted. An electrochemical oxidation process could provide the solution and was investigated with EU funding of the project 'Corrosion protective coating on light alloys by micro-arc oxidation' (CO-PROCLAM). Plasma electrolytic oxidation (PEO), also called micro-arc oxidation, is a very promising eco-friendly process to induce reliable coatings and harden metal components. PEO uses high voltages that produce plasma discharges that modify the component's surface structure, producing multifunctional, ceramic coatings. Researchers varied processing parameters and assessed the physics of the micro-discharges, and characterised the resulting coatings using a variety of advanced techniques. Scientists employed PEO on Al and Mg alloys using the patented Ceratronic process. Electrolyte composition, electrical parameters, treatment time, electrode configuration and electrolyte temperature were all investigated. Effects on the ceramic coating layer were determined with techniques including scanning electron microscopy and X-ray diffraction. The samples were also subjected to standardised aeronautical neutral salt spray to investigate resistance to corrosion. The micro-arc discharge was studied via optical emission spectroscopy and fast video imaging. CO-PROCLAM researchers gained important insight into the PEO process. Optimisation will enable production of a cost-effective, eco-friendly corrosion-resistant and durable coating on lightweight Al and Mg alloys for the aerospace industry. The process eliminates the use of most of the hazardous chemicals associated with anodisation and use of the alloys will significantly decrease fuel consumption and emissions. That is a win-win situation for the aerospace industry and the environment.
Keywords
Aircraft, lightweight materials, corrosion, light alloys, micro-arc oxidation
