MODélisation ELectrochimique du comportement des phases Intermétalliques dans les alliages d'Aluminium: approche multi-echelle de matériaux polyphasés – MODELIA

Aluminium alloys of 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg-Cu) types are widely used in mechanical industry, and especially in aeronautic domain, because of their low density coupled with high mechanical properties. In these alloys, the hardening of aluminium is due to the coherent precipitation of Cu-based intermetallic phases. However, their use is restricted because of their high content of copper and the presence of Cu-rich intermetallic phases. Indeed, some previous projects have shown that these intermetallic phases induce a great number of defects in the protection layers on the surface. So the presence of intermetallic phases makes very complex the development of efficient protection on this kind of poly-phased metallic materials. So, our project concerns the development of a methodology to model the electrochemical and chemical behaviour with the aim of build up new anti-corrosion protections for complex metallic alloys. The scientific program is based on the development of a multi-scale approach to understand and forecast the chemical and electrochemical behaviour of intermetallic phases in aluminium alloys in the surface treatment conditions. The methodology will consist in synthesizing the pure phases and model alloys to record thermodynamic and kinetic data at a macro- and meso-scale, required for modelling and fitting the chemical and electrochemical behaviour of these polyphased materials. So the scientific gaol can be divided in three parts in function of the scale and the system complexity:  Synthesis of pure intermetallic phases at a macroscopic scale (few millimetres to few centimetres) to measure the thermodynamic data necessary to the build-up of potential-pH diagram of each phases. These phases are also required to study the kinetic of the different chemical and electrochemical processes by impedance spectroscopy (EIS) and atomic emission spectroelectrochemistry technique (AESEC).  synthesis and study of bi-phasic and tri-phasic model alloys at a mesoscopic scale in order to evaluate the galvanic coupling and the kinetic of these couplings by electrochemical techniques and AESEC method. The mesoscopic scale of the phases in model alloys will be useful to facilitate the observations and analysis.  the data recorded during the macroscopic and mesoscopic study of the phases will be used to simulate the behaviour of the commercial alloys in function of their composition and the surface treatments conditions. After, the technical and industrial goal of the project will consist to apply these modelling tools to imagine new surface treatment for these polyphased metallic materials. The practical objective concerning the anti-corrosion characteristic is to reach 1000 h without pits in the salt spray test, without applying an organic topcoat (paint). This multi-scale approach to explain the chemical and electrochemical behaviour of polyphased metallic alloys could be extended to other complex alloys for which the chrome(VI) prohibition requires the research of alternative. The advanced surface treatments of complex materials need the development of modelling tools to predict the material behaviour in complex electrochemical conditions (various electrolyte, high voltage …).