Effects Of Electrolyte On Microstructure And Properties Of Micro-arc Oxidation Coatings Based On Formula Experiment Design

In this study,the A356 aluminium alloy widely used in many industries is as the substrate to be treated in different electrolytes containing Na₂SiO₃,NaOH and KF under two voltages.The composition of electrolytes is designed by formula experiment to discover the rulers among the compostion of electrolytes and the electrolyte’s characteritics during preparings MAO（micro arc oxidationcoatings）coatings and the properties of coatings,and in this way,it can be feasible to reduce the number of experiment as much as possible.Firstly,by analyzing the relationships between the compostion of electrolytes and its characteristics including the feasibility of electrolytes and the quality of coatings obtained in these electroytes,it can be shown that the compositions of electrolytes have a significant influence on coatings’preparing process,and the affect of NaOH concentration dramaticly exceeds the NaOH and KF on the conductivity and pH value of electrolytes.And the intuitive method is adopted to find the available ranges under two voltages.Under 400 V,the practicable ranges of electrolytes’conductivity,pH and the concentration of NaOH are 41.7⁶7.3 ms/cm,12.69¹2.94,2.72⁹.92 g/L,respectively.Under 480 V,the usable ranges of electrolytes’conductivity,pH and the concentration of NaOH are<41.7 ms/cm,<12.69,<2.72g/L,respectively.Then,the relationships between the compositions and the properties of MAO are researched by regression analysis.The results indicate that properties of coatings changes with the electrolytes’compostions.First,the concentration of NaOH plays the most important role in increasing the coatings’thickness,but the Na₂SiO₃ hardly affects that of coatings.Second,with the enhancing of NaOH concentration,the compactness of coatings reduces.Third,Na₂SiO₃ concentration is the best for improving the corrosion resistance,and that of KF is second,but that of NaOH is least.What’s more,the coatings’corrosion resistance prepared under two voltages both can be increased remarkedly.Especially,under 400 V,the corrosion current density of MAO coatings obtained in 2#scheme reduces almost four orders of magnitude.With the increasment of voltage,the thickness of coatings will grow,and the phase composition of that doesn’t change,but the coating’s microstructure may be destroyed in different values.When the concentration of NaOH exceeds 3 g/L in the electrolyte,increasing the voltage will have a negative affect on the coatings’corrosion resistance.Eventually,it can be concluded that considering the feasibility of electrolytes,the quality of coatings,the thickness and microstructure and the stain resistance of coating at the same time,under 400 V,2#electrolyte scheme is optimal and its concentration,conductivity and pH value are Na₂SiO₃:26.4 g/L,NaOH:9.92 g/L,KF:3.64 g/L 66.3 ms/cm and 12.93;under 480 V,3#electrolyte scheme is optimal and its concentration、conductivity and pH value are Na₂SiO₃:22.44 g/L,NaOH:0.68 g/L,KF:16.88 g/L 35.6 ms/cm and 12.53.