Preparation And Properties Of Corrosion Resistant And Hydrophobic Composite Protective Layer On Magnesium Alloy Surface

Magnesium alloy is known as the most promising environmental engineering material in this new century,which has the characteristics of light weight and high strength and stiffness,and magnesium alloy could realize electromagnetic shielding function.These outstanding features made Mg alloy applicated in many products,such as aircraft and spacecraft,armored vehicles,self-propelled guns,medical equipment and so on.However,the corrosion potential of magnesium alloy was low and it was easy to be damaged by external corrosion,which has become the obstacles of the development of magnesium alloy industry.On the one hand,the existing single coating technology had limited protection effect,so composite coating is the development trend of magnesium alloy protection in the future.On the other hand,when the corrosive medium adheres to the protective layer,the durability of the coating is difficult to guarantee,so improving the hydrophobicity and corrosion resistance of materials is the development trend of magnesium alloy protection.Inspired by the characteristics of lotus leaves,the corrosion-resistant and hydrophobic composite protective layer with micro nano binary structure was prepared on the surface of magnesium alloy by femtosecond laser modified composite electrophoresis and plasma electrolytic oxidation combined with hydrophobic nano-SiO2 modified electrophoresis.Effects of femtosecond laser modification and plasma electrolytic oxidation on magnesium alloy surface microstructure was observed by scanning electron microscope(SEM),3D profilometer,Origin three-dimensional drawing and Image Pro Plus software calculation.The wettability of femtosecond laser modified electrophoretic composite layer and PEO composite electrophoretic layer was measured by contact angle measuring instrument.The corrosion resistance of composite electrophoretic protective layer modified by femtosecond laser,PEO layer on Mg alloy and PEO composite electrophoretic layer were evaluated by electrochemical test and immersion test.Finally,the change mechanism of corrosion resistance and hydrophobicity of the two composite coatings was discussed based on the change of microstructure,wettability and corrosion resistance.It was concluded that the lotus leaf like micro nano binary structure was formed on the surface of magnesium alloy by femtosecond laser modification,and the microstructure of magnesium alloy could be improved by increasing the spot diameter and impact times.Raising the overlap ratio significantly increased the number of nanostructures,and made the ordered microstructure transform into disorder.Surface and cross section morphology of femtosecond laser composite electrophoresis layer showed that the surface of the protective layer still had micro convex structure.The smaller the diameter of femtosecond laser spot,the less the number of single point impact,and the higher the coverage of the composite electrophoretic coating.With the decrease of femtosecond laser spot diameter and the increase of single point impact times,the hydrophobicity of the material surface increased,and the corrosion resistance of composite film improved.The results of AC impedance showed that the Rf impedance mode value of the femtosecond laser-electrophoretic composite layer was nearly five times as large as the substrate.The less the impact times,the higher the impedance and the lower corrosion rate of composite layer.Furthermore,micro nano binary structure was prepared on the surface of magnesium alloy by PEO technology,the results showed that micro pores and nano pores were formed on the surface of magnesium alloy after PEO treatment.With the increase of positive voltage and oxidation time,the porosity of magnesium alloy surface presented an upward trend,the micropore size gradually increased and the film thickness became thicker.With the increase of the negative oxidation voltage,the porosity ratio of the surface ceramic layer first decreased and then increased,and the film first thickened and then thinned.When the negative oxidation voltage is 90V,the thickest film and the minimum porosity ratio can be obtained.With the increase of positive voltage and,the corrosion tendency showed a decrease first and then increased.With the increase of negative voltage,the film resistance Rf first decrease and then increase,and the corrosion resistance first fell and then rose.At the same time,the passivation platform appeared on the polarization curve under negative voltage.The composite protective layer was constructed by hydrophobic nano SiO2 modified electrophoresis on the surface of PEO film.The results showed that The introduction of Nano-SiO2 further improved the size of micro convex structure on the surface of magnesium alloy.With the increase of Nano-SiO2 content,the size and quantity of micro convex structure increased.The hydrophobicity of the film increased first and then decreased.When the content of SiO2 was 10 g/L and electrophoresis time was 3 min,contact angle reached the best 111.5°.Electrochemical test showed that,when the electrophoresis time was prolonged,the polarization voltage and resistance increased,and the corrosion resistance improved.With the increase of electrophoresis voltage,the film resistance showed a trend of first increasing and then decreasing,and when the electrophoresis voltage at 50V,protective layer had the best corrosion resistance.When the electrophoresis time increased,the impedance modulus continuously increases,and then reached the maximum at 4min,and the corrosion resistance was the best.