Study On The Effect Of Electrolyte On Micro-arc Oxidation Coatings Based On Simplex-centroid Experimental Design

In this paper,the simplex-centroid experimental design was applied to change the concentration proportion of four components—Na₂Si O₃（₁x）、KF（x₂）、Na OH（₃x）and Na Al O₂（₄x）,and micro-arc oxidation（MAO）coatings were prepared on AZ91D magnesium alloy and pure magnesium.The regressive equations were established about coating thickness₁y（μm）,corrosion resistance corresponding to spot testy₂（s）,corrosion current densityy₃（A/cm²）and percentage of each component,to optimize the electrolyte for mulation and study the effects of various components and their collocation in different electrolyte systems on microstructure and property of MA O coatings.The results showed that the regression equations established from the experimental results based on the simplex-centroid were very significant.When there was only KF in electrolyte,the thickness of coating on AZ91D reached the maximum value of 38μm;when the ratio of KF was 0.48 and the ratio of Na OH was 0.52,the thickness of coating on pure magnesium reached the maximum value of 22.341μm.When the ratio of Na₂Si O₃ was 0.482 and the ratio of KF was 0.518,corrosion resistance corresponding to spot test of coating on AZ91D was the best,the drip liquid whiten time reached 166.67 s;when the ratio of KF was 0.59 and the ratio of Na OH was 0.41,corrosion resistance corresponding to spot test of coating on pure magnesium was the best,the drip liquid whiten time reached 162.41 s.When the ratio of KF was0.322,the ratio of Na OH was 0.309 and the ratio of Na Al O₂ was 0.369,corrosion resistance corresponding to electrochemical experiments of coating on AZ91D was the best,corrosion current density was the lowest about 4.283?10^-88 A/cm²;when there was only KF in electrolyte,corrosion resistance corresponding to electrochemical experiments of coating on pure magnesium was the best,corrosion current density was the lowest about 5.15×10^-88 A/cm².It was found that the main film-forming material and KF in electrolyte had a great influence on film-forming ability of AZ91D and pure magnesium.In order to get a good coating,for AZ91D,there should be only a main film-forming material in electrolyte,in addition,the concentration of KF shouldn’t be more than 15 g/L in Na Al O₂ system.for pure magnesium,there should be the main film-forming material and the concentration of KF can not be higher than or equal to the concentration of main salt,in addition,adding a little Na OH in Na ₂Si O₃-Na Al O₂ system.Although KF and Na OH can significantly increase the thickness of coating,film-forming ability was poor,coating was rough and loose,and cracks were more if there wasn’t the main film-forming material in electrolyte,which can lead to the poor corrosion resistance.The combination of Na₂Si O₃ and Na Al O₂ can form gel-like substances,which increased the viscosity of electrolyte and resulted in more micro cracksin uneven coating,leading to the average corrosion resistance.It revealed that the MAO coatings were composed of Mg O phase.The phase composition of coatings on AZ91D and pure magnesium was changed by various components in electrolyte.Mg₂Si O₄ was observed in coatings because of Na₂Si O₃ in electrolyte;Mg Al₂O₄ phase was observed in coatings because of Na Al O₂;the presence of KF in electrolyte resulted in the deposition of Mg F ₂ phase;the phase composition of the coatings was unchanged by Na OH.According to the principle of both giving p riority to corrosion resistance and thickness of coating performance,the optimal electrolyte formulation was chose:For AZ91D,the pore size of coating was the most uniform and the minimum,the coating was more compactand containing spinel phase Mg Al ₂O₄ which obtained in Na Al O₂system,therefore,the optimal electrolyte formulation was that the ratio of Na Al O₂,KF and Na OH were 0.369,0.322 and 0.369 respectively.For pure magnesium,the compact coating was stacked closely by scale-like substances,no obvious pores,and there were no obvious micro cracks with Mg F₂ phase which obtained in only KF,therefore,the optimal electrolyte formulation was only existing KF in electrolyte.