Preparation And Corrosion Resistance Of Micro-arc Oxidation Composite Coating On Magnesium Alloy Surface

Compared with other metal materials,magnesium alloy material have better material properties,such as better castability,electrical shielding,fatigue resistance,machinability,and high heat dissipation.Therefore,magnesium alloy materials have broad development prospects and are known as the most vital new material in the 21st century.Magnesium is relatively active and is easily oxidized and corroded in general media.Therefore,the corrosion resistance of magnesium alloys is the biggest factor restricting its industrial applications.Surface coating technology is an effective anti-corrosion method,but most of the traditional surface coating technology is a single film layer,which has many shortcomings,such as thin film layer and incomplete coverage,so therefore,the preparation of composite coatings with better corrosion resistance is the main research direction.In this paper,through the influence of different micro-arc oxidation systems and self-assembled molecules on the performance of the prepared composite coating,the preparation and corrosion resistance of the micro-arc oxidation composite coating on the surface of AZ91 magnesium alloy are studied.Firstly,the micro-arc oxidation process uses four systems:basic system,silicate system,silicate+additive system,and composite salt+additive to prepare different micro-arc oxidation films,and self-assemble with Dodecafluoroheptyl-propyl-trimethoxysilane(G502B)on different micro-arc oxidation film layers,and electroless nickel plating is performed on the outer layer of the self-assembled film to prepare a composite coating.SEM and EDS were used to analyze the micro morphology and composition of the different coatings prepared,and MATLAB software was used to simulate and calculate the porosity and pore size distribution of the coatings at different stages.The experimental results show that the porosity of the micro-arc oxide film prepared by different systems is different.After the subsequent self-assembly and electroless nickel plating,the porosity of the composite film will decrease.The different porosity of the underlying micro-arc oxidation film has little effect on the formation of the micro-arc oxidation/self-assembly/nickel composite film.Secondly,the film thickness,roughness,and hydrophobicity of the composite coatings prepared by different systems were characterized,and the electrochemical polarization curve and electrochemical impedance spectroscopy were used to determine the characteristics of the MAO/SAM/Ni composite coatings prepared by different MAO films.The corrosion resistance was tested.The contact angle of the MAO/SAM/Ni composite coating prepared by the silicate+additive system is 113.62°,with a certain degree of hydrophobicity,the lowest roughness,and the minimum corrosion current density of the composite coating is 6.910×10^-7A/cm².It shows that the MAO/SAM/Ni composite coating prepared by the silicate+additive system has the best performance and the highest corrosion resistance.Finally,using silicate+additive system as the micro-arc oxidation electrolyte for micro-arc oxidation to prepare the MAO layer,Using 3-mercaptopropyltriethoxysilane and dodecyl mercaptan as self-assembled molecules,self-assembled into films on the surface of MAO,finally,electroless nickel plating prepares composite coating respectively,test the performance of the composite coating prepared by two self-assembled molecules,compare the corrosion resistance of the composite film prepared by the two molecules,and the salt spray corrosion experiment was carried out on the obtained three-layer composite coating,explore the corrosion propagation process of composite coatings,and test and analyze it's stability at different temperatures.The experimental results show that the performance of the composite coating prepared by different self-assembled molecules is different,which is related to the functional group and the film-forming mechanism.The corrosion current density of the composite coating prepared by the two self-assembled molecules has reached 10^-7,The corrosion current density of the composite coating prepared by using 3-mercaptopropyltriethoxysilane as the self-assembled molecule is lower than that of the composite coating prepared by dodecyl mercaptan,therefore,the corrosion resistance of the composite coating prepared by 3-mercaptopropyltriethoxysilane as a self-assembled molecule is better than the composite coating prepared by dodecyl mercaptan.