| Aluminum and its alloys with high specific strength and low bulk density are widely used in automotive,aerospace and other fields.However,aluminum and its alloys also have the disadvantages of low surface strength and poor corrosion resistance.Reasonable surface modification treatment of aluminum and its alloys can effectively overcome the above shortcomings and prolong the service life.Micro-arc oxidation(MAO)technology can in-situ prepare an oxide ceramic coating that is firmly bonded to the substrate and has stable properties on the surface of the aluminum and its alloys,so that the corrosion resistance and wear resistance of aluminum and its alloys are significantly improved.Due to the occurrence of the plasma discharge during the MAO process,the resulted MAO coating is generally porous,which is not conducive to the improvement of the corrosion resistance of the coating.The type and intensity of discharges will be affected by MAO parameters such as current,voltage,processing time and electrolyte.Studying the effects of the above conditions on the plasma discharge and coating growth process can provide theoretical guidance for the optimization of coating corrosion resistance.The coating prepared in the electrolyte containing sodium hexametaphosphate((NaPO3)6)generally has the characteristics of fast growth and good uniformity,but it takes a long time for the first dielectric breakdown to occur.To study the growth law of the coating and the characteristics of plasma discharge in alkaline(NaPO3)6 electrolyte,the AA1060 aluminum was used as the substrate(because of the less alloying elements in substrate),and the alkaline(NaPO3)6 was used as the basic electrolyte.This paper took the growth process of aluminum-based MAO coating as the starting point to evaluate the influence of the current density distribution during the MAO process on the growth process,micro structure and corrosion resistance of the coating.Through comprehensive analysis of the changes in electrical signals and spark morphology during the MAO process,and the coating micro structure,element and phase compositions,electrochemical information,the formation-discharge-growth process of the coating from the anodic oxidation stage to the micro-arc oxidation stage,and the influence mechanism of the electrolyte parameters and voltage parameters on the coating formation-discharge-growth process were studied systematically.And it provides theoretical guidance for the adjustment of corrosion resistance of MAO coating.The research results show that,unlike the dense anodic oxidation film prepared in the anodic oxidation stage of traditional MAO treatment,the film formed in the anodic oxidation stage in alkaline(NaPO3)6 electrolyte is a porous anodic oxidation film.The difference in current density distribution leads to the transition of the porous film to the MAO coating in the edge region earlier than that in the central region.And the crystalline aluminum oxide phase is easier to form in the coating in the edge region.During the MAO process,the surface roughness of the coating in the edge region is always greater than that of the central region,and its carrier concentration value is lower than that of the central region.With the prolonging of the MAO process,the difference in the corrosion resistance of the MAO samples between the edge region and the center region decreases gradually.The formation of porous anodic oxide film is related to the incorporation of phosphorus-containing ions from the electrolyte into the film.The transformation of the porous anodic oxide film to the MAO coating is produced by the initial plasma discharge that occurs at the "wrinkles" and annular "barb" structures of the pore walls of the porous film.Plasma discharge locally accelerates the diffusion of oxygen ions to the side of the substrate,leading to rapid growth of the coating in a local area,and finally forming the basic unit of the coating/substrate interface—the hemispherical cap structure.The P element from the electrolyte is present in the oxide coating in the form of aluminum phosphate and sodium phosphate.The MAO samples all exhibit the properties of n-type semiconductors,and the carrier concentration inside the coating decreases with the extension of the MAO treatment time.The anions in the electrolyte can enter the MAO coating assisted by plasma discharge and substance deposition,and repair the anion vacancies in the coating,which increases the impedance of the coating and decreases the carrier concentration,thereby shortening the time for the first dielectric breakdown of the coating.The application of negative voltage causes high-energy discharge(B-type discharge)to be suppressed,and low-energy discharge(A,C and D-type discharge)to be strengthened.The breakdown voltage of the MAO coating decreases with the increase of the electrolyte concentration,and increases with the increase of the applied negative voltage;When the applied positive voltage decreases,the breakdown of the coating becomes difficult and even disappear.The application of negative voltage improves the compactness of the coating,but has no significant effect on the phase composition of the coating;?-Al2O3 phase is not detected in the coating prepared under lower positive voltage.In the study of the influence of(NaPO3)6 concentration and positive/negative voltage on the structure and corrosion resistance of the coating,it is found that the MAO coated sample,which prepared in 40 g/L(NaPO3)6 electrolyte and under 5/5 positive/negative voltages,has the best corrosion resistance.Based on the research results of the semiconductor characteristics of the MAO sample,Cu has been deposited into the cavity and discharge channel of the MAO coating by AC deposition treatment,and it is completely converted into CuO during the secondary MAO treatment.Electrochemical testing showed that the composite coating prepared with an AC deposition time of 5 min has the best corrosion resistance.When the secondary MAO treatment is maintained for 1 min,a continuous CuO layer that is closely connected to the MAO coating can be formed on the surface of the coating and blocks the discharge channel in the MAO coating. |
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