Optimization Of Micro - Arc Oxidation Process And Corrosion Resistance Of Aluminum Alloy

Aluminum and its alloys have been widely used in many fields due to many advantages such as scalability, light weight, ease to work and high specific strength. However, their application have been largely limited by a number of disadvantages like poor abrasive and corrosion resistance, low hardness and so on. The micro-arc oxidation technology as a new surface treatment technology has been developed based on the anodic oxidation. Good dense ceramic coating is formed when the surfaces of aluminum alloys treated with the micro-arc oxidation technology which has good wear and corrosion resistance, high hardness and excellent adhesion to substrate, therefore, the technology is widely used in the field of aerospace, machinery, electron, automobile, national defense industry and so on.In this study, Aluminum alloy6063was chosen to be treated by micro-arc oxidation. First, under the initial process parameters (current density of15A/dm2, oxidation time of20min, frequency of3500Hz, temperature of20℃), using the single factor analysis method to explore the different concentration of sodium silicate, sodium hydroxide and disodium EDTA on the arc voltage of micro-arc oxidation, the film thickness, the hardness and corrosion resistance. Finally, the best compositions of electrolyte was optimized as follows:the concentration of sodium silicate of9g/L, the concentration of sodium hydroxide of2.0g/L, the concentration of disodium EDTA of0.7g/L.Using the optional of electrolyte compositions, the four factors and the three levels of orthogonal experiments were chosen to optimize the processing parameters in the process of micro-arc oxidation, the respective of the four factors and the three levels were as follows: current density of10,15,20A/dm2, temperature of20,30,40℃, oxidation time of10,20,30min, frequency of1500,2500,3500Hz. The optimal process conditions were determined by the above orthogonal test and the range analysis as follows:the current density of15A/dm2, the oxidation time of20min, the frequency of3500Hz, the temperature of20℃A good dense ceramic coating was formed under the optimal electrolyte composition and the optimal process parameters. The morphology of the film surface and the composition of the elements were determined by SEM and EDS. The uneven surface was observed with a number of micro holes and cracks. The films were mainly composed of the transition layer, dense layer and loose layer base on the cross-section. It also confirmed that the micro holes and cracks on the surface morphology inside the film was closed; The EDS analysis showed the main elements of Al, O, Si and a small amount of Na inside the film. Si and Na elements derived from the electrolytes, indicating that the components of the electrolyte were involved in the film formation. The XRD patterns confirmed the main compositions of the film are α-Al2O3,γ-Al2O3and SiO2, the α-Al2O3is higher.The corrosion resistance of the layer were analyzed by using the electrochemical measurements of tafel polarization curve and the electrochemical impedance spectrum. The corrosion potential and the corrosion current density were determined by the Tafel polarization curve; The charge transfer resistance, membrane electric double layer capacitance and membrane layer resistance were obtain by fitting the equivalent circuit diagram to use ZSimpWin software. The performance of the film layer was evaluation within different immerse time with the electrochemical methods, the corrosion potential down and the corrosion current density increased with the extension of soaking time. The decreasing charge transfer indicating the decrease in the corrosion resistance of film.