Hydrophobic Modification Of Mg?OH?₂ Coating Prepared By MAO/CED On Mg Alloy

Magnesium alloys are known as"green engineering materials"in the 21st century because of their conventional performance and biomedical advantages.However,the excessive corrosion speed have been restricting their larger applications due to the high chemical activity.In this paper,AZ91D magnesium alloy was studied to achieve hydrophobic modification based on micro-arc oxidation/cathodic deposition Mg?OH?₂ coating.The Mg?OH?₂ coating was successfully prepared which was even and uniform with sheet-like microscopic structure,close to superhydrophobic with great corrosion resistance and tighly bonded to the substrate.The elemental and phase composition,micromorphology,corrosion resistance and film adhesion of typical samples were tested and analyzed,and the related mechanisms were explained.The main research work and conclusions of this thesis are as follows:?1?Judging by the film appearance,the weight gain per unit area and the adhesion level,the formula of NaOH as single component was selected as the micro-arc oxidation electrolyte,and the electrical parameters were optimized.The obtained film was tightly bonded to the substrate,has a typical porous microscopic structure and can provide a good primer for the subsequent cathodic deposition reaction.The optimal micro-arc oxidation process was determined:100 g/L NaOH,constant direct current density of 30 mA/cm²,energization time of10 min.?2?Based on the micro-arc oxidation film,judging by the film appearance,the weight gain per unit area and the adhesion level,the electrolyte formula,reaction temperature and electrical parameters of the pulse cathodic deposition were optimized.The obtained Mg?OH?₂ film was even and uniform,tightly bonded to the micro-arc oxidation film,and has sheet-like microscopic structures with micro/nano-sizes.The optimal pulse cathodic deposition process was determined:complex solution of 25.6 g/L magnesium nitrate and 2 mL/L hydrogen peroxide,solution temperature of 40?,turn-off interval of 0.06 s,power-on interval of 5 s,peak current density of 2 mA/cm²,total conduction time of 40 min.?3?The micro-arc oxidation/cathodic deposition Mg?OH?₂ coating was modified using an ethanol solution of stearic acid and myristic acid under optimal micro-arc oxidation and cathodic deposition processes,respectively.The effects of stearic acid and myristic acid concentration,solution temperature and immersion time on the hydrophobic properties were systematically studied.The results show that immersed in 10 g/L stearic acid ethanol solution at 80? for 90 min,the Mg?OH?₂ coating can achieve a static contact angle of up to 148.8°,which is close to the superhydrophobic surface;its rolling angle is as low as 15°,meaning a good low viscosity.?4?Study on the corrosion resistance of typical samples shows that hydrophobic treatment of micro-arc oxidation/cathodic deposition Mg?OH?₂ coating can significantly enhance the corrosion resistance of magnesium alloy.The corrosion resistance ranking is as follows:hydrophobic micro-arc oxidation/cathodic deposition sample>micro-arc oxidation/cathodic deposition sample>micro-arc oxidation sample>blank sample.