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COrrosion PROtective Coating on Light Alloys by Micro-arc oxidation

Final Report Summary - CO-PROCLAM (COrrosion PROtective Coating on Light Alloys by Micro-arc oxidation)

Executive Summary:
To realize and achieve the objectives, the CO-PROCLAM project is built within a consortium of two partners whose facilities, know-how and competences are described hereafter in section 2.2.
- CNRS, the French national center for scientific research (http://www.cnrs.fr) who manages the Institut Jean Lamour (http://www.ijl.nancy-universite.fr)
- Galvanoplastie Industrielle Toulousaine, a French SME specialized in surface treatments and painting for industrial applications (http://www.git.fr)
The work program has been organized around 5 work packages (WPs) for which task(s) has(have) been defined. Deliverables will attest the realization of task(s). As a help in decision making, each WP has been assigned milestones. WPs, deliverables and milestones are summarized in tables 1, 2 and 3 respectively. A detailed description of each WP is given in the next section 1.3.3.
Management of the project will be done by the project coordinator (G. Henrion – CNRS) who will assume the overall coordination of the CO-PROCLAM project and work in close connection with the administration units – namely the regional representative administration of CNRS in Nancy for the financial and administrative issues.
The scientific and technical management committee (STMC) will assist the project coordinator with the scientific and technical management and running of the project as well as the quality assurance and the risk management.

The STMC will comprise each WP leader in addition to the project coordinator. As far as possible, each institution/partner should be represented in STMC
The STMC will meet as necessary and agreed, usually every six months, and as a minimum at the key milestones of the project.

Project Context and Objectives:
Nowadays, aeronautic industry accounts for about 4.1 million jobs and represents €228 Bn for the economy of the European Union (EU) [1]. If the importance of air transport on the tourist and economic development is not to be anymore demonstrated, its environmental cost is also a crucial factor in the development of this activity.
Indeed, air traffic strongly contributes to the increase in the global warming by emitting huge quantities of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. For the EU, the part of air transports in the global emission of greenhouse gases (GHG) is estimated to be between 2 and 4 %. If this amount might seem reasonable, it increases however faster than the contribution of all other sectors. Considering the doubling of air traffic every ten years, this figure can reach between 10 to 20 % in 2050. To attain the objectives fixed by the international community in reducing GHG emissions (the Kyoto protocol plans an 8 % reduction of GHG emissions for the EU compared to 1990) it is important to reduce the part due to air transports.

Within this context of high economic and social impact, one solution to reach these objectives of GHG reduction consists in strongly increasing the use of light materials in the construction of aircrafts so as to reduce fuel consumption. Aluminium or magnesium based alloys could efficiently replace iron base material for many parts of the aircraft structure or engine. However, these light alloys suffer from poor mechanical properties and are particularly sensitive to corrosion. Therefore such materials need specific surface treatments to improve their resistance and overcome their weaknesses. As reminded in the CfP, in the aeronautic domain, 95 % of Al-alloy made parts undergoes surface treatments.

For aeronautic needs, anodising process is widely used to grow thin ceramic layers onto Al- and Mg surfaces thus improving wear resistance and corrosion protection of the as treated parts. Depending on the future use of the parts, anodising can be achieved by means of different techniques: sulphuric acid anodising (SAA), phosphoric acid anodising (PAA), hard acid anodising (HAA), chromic acid anodising (CAA), the latter being well adapted to increase the corrosion resistance of the processed work pieces. However, anodised layers are generally porous oxides and supplemental further treatments such as sealing, painting and/or varnishing, are often needed to fully comply with the industrial requirements. Moreover anodising electrolytes and sealing bath contain pollutant, toxic and/or CMR compounds (especially hexavalent chromium) which use is nowadays either forbidden or at least strongly restricted by international rules (European directive 2002/95/CE, Kyoto protocol, REACH).
In order to comply with industrial requirements for what concerns light alloys surface treatment, micro-arc oxidation (MAO) also known as plasma electrolytic oxidation (PEO) is a promising alternative to anodising. Indeed, this environmental friendly innovative technique allows ceramic layers to be grown getting the pieces high level mechanical properties and good corrosion protection in a single step processing.

Regarding the call for proposal, the main objectives of the project may be summarized as follows:
i) Carrying a deep investigation into the influence of the process parameters in order to get a strong knowledge in the mechanisms that govern the PEO process; it is thus expected to achieve a control of the layer growth through the management of the process parameters. A particular attention will be paid on the electrolyte composition and electrical parameters
ii) Defining the best parameters sets that will allow the user to grow oxide layer onto light metallic alloys (Al, Mg) compliant with the industrial requirements.
iii) Implementing a demonstrator PEO unit which will make possible the processing of large size part, typically real industrial parts.

Obviously, objectives (i) and (ii) are closely connected except that whereas objective (i) will consider parameters independently, objective (ii) will aims at defining the best compromise in between all investigated parameters with respect to their respective influence on the process.
Achievement of these objectives will be check through the elaboration of oxide layers and analyses of their properties with respect to the call for proposal.

Project Results:
All results/foreground achieved along the project duration are reported with details in the intermediate, mid-term and final reports (deliverables D1.1 D1.2 D1.3 D1.4) and also along the periodic reports. Therefore the reader is kindly asked to refer to these documents for further detail. The attached pdf file (COPROCLAM_MOMs-and-Final-Report.pdf) contains all minutes of all meetings that were held durind the project and the final sientific report as well.

Briefly, the main achievement along the project duration may be summarized as follows:
Along the project duration, aluminium and magnesium alloy samples have been PEO processed using the Ceratronic® process which allows us:
- to control independently the positive and negative charge quantity delivered to the processed part,
- to match the best parameter set (in terms of charge quantity for instance) while optimizing the energy consumption.

Many PEO parameters were investigated: electrolyte composition, electrical parameters, treatment time, electrode configuration, electrolyte temperature. For each parameter set, the grown ceramic layer will be carefully analysed by means of different tools: optical microscope, SEM, EDX. Additional characterization by means of X-ray microtomography, 3D profilometry and X-ray diffraction were carried out on some chosen samples.
Standardized aeronautical neutral salt spray testing procedure (ISO 9227) have been used to check the sample behaviour regarding corrosion.
Whatever the process conditions, the PEO coating have a high level of porosity and due to the layer breakdown. Therefore, thin layers (< 5-10 µm) exhibit open porosities and defects that are detrimental to the corrosion protection. It was shown that increasing the layer thickness to some tens of µm strongly improves the corrosion resistance.
Nevertheless results of this project allow us to define trends to improve the process.
Regarding the effect of the electrolyte composition on the PEO coatings, it was found that the best additive in KOH or NaOH-base electrolyte is silicate salts (sodium silicate) and that an increase in the silicate content improves the corrosion resistance of the Al-alloy processed parts. In the case of Mg-alloy (EV31) it was shown that the silicate concentration should be less than 0.05 mol/L while addition of phosphate improves the coating adhesion and reduces the pulverulence of the coating. Phosphate also reduces the edge effect and leads to a more homogeneous coating with quite no thickness variation between the edge and the centre of the sample. According to electrochemical and EIS measurements, the addition of fluoride improves the corrosion resistance of Mg-alloy parts.

Regarding the effect of the process parameters (especially electrical parameters) the following conclusions have been drawn:
Within the range of low values (< 50°C), the electrolyte temperature has quite no effect on the layer growth and layer properties.
The use of a current pulse frequency in the medium range (typ. 500 Hz < F < 1000 Hz) improves the coating quality in terms of porosity and compactness. Regarding corrosion, best results were obtained within this frequency range.
The current density should be adjusted so that strong arcs do no develop at the sample surface (typ. < 20 A/dm2).
Although it slightly influences the edge effect, the gap distance between the work electrode and the counter electrodes has quite no effect on the coating properties.


Meanwhile, the physics of the micro-discharges was investigated to get a better understanding on both the mechanisms that sustain the micro-discharge formation and behaviour, and the mechanisms that are responsible for the oxygen transport inside the material to grow the oxide layer. The behaviour of micro-discharges was studied using optical emission spectroscopy and fast video imaging. It was thus possible to get a wide variety of information on micro-discharges, such as their number, location, lifetime, size, distribution, etc… This permits us to show that some particularities of PEO micro-discharges only occur at the early beginning of the process (e.g. edge effect) and to describe the evolution of the micro-discharges with the process time.


Potential Impact:
The project CO-PROCLAM is perfectly in line with the Clean Sky JTI objectives, especially within the "Eco-Design" domain since it aims at suppressing the use of CMR compounds and other pollutants in the surface treatment of light alloys of use in aircraft structure and engines, and at developing and implementing in an industrial scale the micro-arc oxidation process, a green technology which meets the European regulations such as the REACH regulation.
In this project, the defined methodology which consists in closely coupling the fundamental investigations of the PEO process with the coating characterization (microstructure, composition and corrosion properties) will allow us to gain knowledge on the phenomena which occur at the solid/liquid, gas/liquid and gas/solid interfaces. Consequently the information obtained will helps to adjust and improve the PEO process conditions so as to make it possible a perfect control of the layer growth. It is thus expected to achieve thin protective oxide layers (< 5 µm for aluminum ; < 15 µm for magnesium) directly usable in the processing of current parts without any change in drawings. The application to aircraft component will then be developed, complying with the REACH requirements but also answering CAA requirements regarding corrosion for both aluminium and magnesium.
Thus, the project has a long-term scientific, industrial and ecological benefit and will increase our competitiveness and so the EC's one.
In addition to the benefits in the particular case of micro-arc oxidation, fundamental results obtained during this project (in terms of micro-discharge / surface interaction) will find applications in the emerging field of micro-plasma sources for surface treatment at the nano-scale.

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