Interfacial Reactions To Control The Corrosion Resistance Of Chemically Transformed Films Of Magnesium Alloys

Magnesium alloy has a light mass,better specific strength,specific stiffness,electrical conductivity and biocompatibility and other excellent characteristics,widely used in automotive,military,aerospace and other fields.However,magnesium alloy is prone to corrosion,making it far less resistant than steel in industrial production.It is necessary to treat magnesium alloy to enhance corrosion resistance,but most of these methods are applied to industrial production,and few are considered for biomedical applications.In this paper,we tried to optimize the process of chemical conversion film of magnesium alloy from two aspects,in the biomedical aspect,the organic nature of tannic acid conversion film was introduced to improve the corrosion resistance of magnesium alloy.The calcium phosphate film was prepared on the basis of tannic acid conversion film of magnesium alloy at the same time,so that the corrosion resistance of magnesium alloy was further improved,and it has osteoinductive and osteoconductive properties.In the actual industrial production,in order to understand the influence of different additives on the corrosion resistance of phosphate film,improve the corrosion resistance of magnesium alloy manganese phosphate film,the use of chemical transformation method in AZ91D magnesium alloy surface preparation of manganese phosphate film.The study of the phosphate solution components and content of the phosphate film corrosion resistance performance.The tannic acid conversion film was prepared on AZ91D magnesium alloy by chemical transformation method,and the effects of time,temperature and p H on the corrosion resistance of the film were investigated by single-factor test method and response surface method to optimize the process conditions."The SEM morphology of the cross-section shows that the thickness of the film is 4μm,and the corrosion resistance of the tannic acid film is weakly protected against the substrate by dot-drop and electrochemical characterization,and the corrosion resistance is low,and more cracks exist on the surface of the film.As shown by the polarization curve analysis,the corrosion current density of the film layer is 1.58×10^-6A/cm²,which has a greater enhancement than the substrate,but still cannot meet the requirement of corrosion resistance on biomedical treatment.In order to further improve the corrosion resistance of magnesium alloy and enhance its application in biomedical applications,tannic acid-calcium-based composite conversion films were prepared on the basis of tannic acid conversion films,and the experimental scheme was optimized by response surface method,and the thickness was about 17μm as shown by the cross-sectional morphology,and the corrosion resistance was characterized by spot drop test,hydrogen precipitation test and salt spray test.The results showed that the corrosion resistance of the tannic acid-calcium system composite conversion film was much greater than that of the single tannic acid conversion film.The Mn-system phosphate films were prepared on AZ91D magnesium alloy by chemical transformation method.The process conditions of each component of the phosphate solution were determined by single-factor and comparative experiments as follows:35 g/L Mn SO₄,30 g/L Na H₂PO₄,0.5 g/L EDTA-4Na,1 g/L Na F,2 g/L Na NO₃,p H=2.5（p H adjusted by phosphoric acid）.The microscopic morphology of the cross sections observed that the magnesium alloy phosphate film had an obvious bilayer structure and the thickness of the film layer was about 24μm.The optimized phosphate film had a thicker thickness,a denser surface and a lower number of cracks.Observed in EDS,the addition of Na F,the composition of the film layer changes,there is a small amount of element F involved in film formation.Comparing the basic phosphate film,the optimized phosphate film increased corrosion resistance,corrosion current density was reduced from 8.912×10^-7A/cm²to 5.56×10^-7A/cm²,the radius of capacitive arc resistance was also significantly increased,and the corrosion resistance was improved.