Preparation And Properties Of Corrosion Resistant Surface Of Magnesium Alloy

Due to the advantages of low density,good electrical conductivity and electromagnetic shielding,magnesium and its alloys have been widely used in the fields of electronics,aerospace and automotive.However,magnesium alloys have limited their applications due to poor corrosion resistance.In recent years,researchers have proposed various surface modification techniques to improve the corrosion resistance of magnesium alloys,and significant results have been achieved.However,most of the surface modification techniques have limitations such as expensive equipment required and preparation process is hazardous to the environment and human body.In this paper,several simple and economical methods are proposed to prepare corrosion resistant coatings on the surface of magnesium alloy to improve the corrosion resistance of magnesium alloy with AZ91D magnesium alloy as the research object.The main research contents are as follows:（1）Deionized water is used as reagent to generate a corrosion-resistant coating on the surface of magnesium alloy by a two-step method.Firstly,the deionized water was heated to a certain temperature using a reactor,and after the deionized water was heated and evaporated,the steam generated reacted with the surface of magnesium alloy to grow the corrosion-resistant coating in situ on the surface of magnesium alloy.Then,the magnesium alloy after the initial steam treatment was put into a crucible and subjected to a secondary heat treatment in a horse boiling furnace.The chemical composition of the coating was analyzed by FT-IR,XRD and XPS mainly for Mg O and Mg-Al LDH,and the formation mechanism of the coating was further discussed.Electrochemical tests and immersion experiments were used to test the corrosion resistance of the coatings.The corrosion current of the prepared coating decreased by three orders of magnitude compared with the substrate,and the corrosion rate decreased by four orders of magnitude,which effectively improved the corrosion resistance of the magnesium alloy.（2）A hydrothermal reaction solution was prepared using silicon dioxide nanoparticles and sodium hydroxide,which was subsequently heated by an autoclave to grow an in situ Si O^2-doped magnesium hydroxide corrosion-resistant coating on the surface of magnesium alloy.The chemical composition of the coating was analyzed by FT-IR,XRD and XPS and the formation mechanism of the coating was discussed.The corrosion resistance of the coating was tested using electrochemical tests and immersion experiments.The corrosion resistance of the coating was significantly improved,with a decrease in corrosion current of four orders of magnitude and an increase in impedance of nearly five orders of magnitude.After seven days of immersion in 3.5 wt%Na Cl solution,the morphology of the coating on the sample surface was relatively flat and intact,providing long-term protective performance for the AZ91D magnesium alloy substrate.（3）A protective coating of magnesium hydroxide was prepared on AZ91D magnesium alloy substrate using hydrothermal method.A reaction solution was prepared using sodium-based montmorillonite and sodium hydroxide,and the magnesium alloy substrate was held in the reaction solution for 2 hours using the hydrothermal method to prepare the corrosion-resistant coating.The analysis of XRD and XPS test results showed that the chemical composition of the coating was magnesium hydroxide,and the corrosion resistance of the coating was tested by electrochemical test and immersion test.From the results,it is known that the prepared coating has four orders of magnitude lower corrosion current and four orders of magnitude higher impedance compared with the substrate,and the retardation efficiency has reached 97.7%,and the formation mechanism of the coating is discussed.After seven days of immersion,the surface of the sample remained basically intact and the immersion solution remained clear.The adhesion test showed good adhesion between the coating and the substrate.