CO-PROCLAM

COrrosion PROtective Coating on Light Alloys by Micro-arc oxidation

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 Obiettivo del progetto (Objective)

'Since it does not use any CMR or pollutant compounds, micro-arc oxidation (MAO) also known as plasma electrolytic oxidation (PEO) is a promising technique to replace conventional anodising processes such as chromic acid anodising or hard anodising to improve the mechanical and corrosion properties of valve metals such as aluminium or magnesium and their alloys. Though it has been known for many years, this technique is however still not well adapted to comply with the aeronautical requirements. In fact, one of the major challenge is to grow thin oxide layers (< 10 µm), which exhibit low roughness (Ra < 1 µm) and good thickness homogeneity, the latter being of prime importance when processing work pieces with complex shape. The present program aims at developing a particular technology of micro-arc processing of Al and Mg alloys in order to achieve MAO corrosion protective thin layers that comply with the aeronautical requirements while minimizing energy consumption. A detailed investigation of the process parameters and their influence on the grown ceramic layer will be coupled with the characterization of the layers regarding their microstructure and properties focusing on corrosion protection efficiency. Moreover using fast video imaging and time-resolved optical emission spectroscopy coupled with electrical measurements, a detailed study of the physics of the micro-discharges resulting from the dielectric breakdown will be carried out to shed light on the physico-chemical mechanisms that sustain the layer growth. It is thus expected to determine the response of the micro-discharges, and consequently of the ceramic layer characteristics, to a modification in the process parameters. The perfect knowledge of these mechanisms will then help controlling the process through the management of the micro-discharges. The work is planed to be performed by a consortium of two partners – one research institution, one supplier of micro-arc coatings – and organized around five work packages. It is intended that a part of the work will be subcontracted for investigating some process parameters unavailable within the partner's facilities.'

Introduzione (Teaser)

One effective way to reduce fuel consumption and emissions is to decrease aircraft weight using lightweight materials. Scientists developed an eco-friendly corrosion protection process that could make that more feasible in the near future.

Descrizione progetto (Article)

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.