Design Of Electrolyte To Lower Micro-arc Oxidation Energy Consumption On Magnesium Alloy And Reaction Mechanism Of Electrolyte Additives

Magnesium alloys present a great potential in the field of aerospace,automobile, electronic and others for excellent properties such as low density, highspecific strength and specific rigidity. However, low hardness and poor corrosionresistance limit their further applications. So it is necessary for magnesium alloys tohave surface modification treatment. Micro-arc oxidation has been developedrapidly in recent years. However MAO is an anodizing process during which micro-discharges generate dielectric breakdown of the anodic oxide film under highelectric field. The question of high energy consumption limits its further applicationin industrial field. Developing low energy consumption MAO technology is the onlyway to solve the question. In this paper an idea of designing low energyconsumption electrolyte was brought out. A new method on predicting arc-initiatingof micro-arc oxidation by anodic polarization curve was given so as to choose theadditves which could lower the arcing voltage. The mechanism of reaction inducedby sodium carbonate additive to release carbon dioxide to modulate coatingformation was studied and the working voltage could be lowered.Coating formingvelocity was increased by complex reaction induced by sodium hexametahposphate.MAO coating was fabricated on magnesium alloys with low energy consumptionand the microstructure has been investigated by SEM, XRD, and TEM respectively.Meanwhile, the properties of MAO coatings formed on AZ31magnesium alloy havebeen studied. Ultimately a large-scale （12dm2） MAO coating with power less than1kW has been fabricated and the advantage of MAO with lower energy consumptionis demonstrated.It can be proposed from the method of designing low energy consumptionelectrolyte that the introduction of additives with the ability to lower the arcingvoltage or additives with modulation effect on increasing the porosity of the coatingis beneficial for lowering the energy consumption. And additives with complexeffect to increase the forming velocity of MAO coating is needed. Phosphate alkaliis suitable to be the basic electrolyte of low energy consumption electrolyte. Andsodium fluoride is chosen to lower the arcing voltage and sodium carbonate isutilized to modulate the structure of the coating. Sodium hexametahposphate isemployed to increase coating velocity.A new method on predicting arc igniting of micro-arc oxidation on magnesiumalloy by anodic polarization curve was proposed to choose additives to lower thearcing voltage. The results show that arcing voltage is much dependent on thestability of passivating films formed during anodic polarization process. The arcing voltage is higher with a larger passivation region of the polarization curve. Whenthe stability of passivating films is at about the same level, the arcing voltagebecomes lower in the solution possessing a smaller polarization current at the latersection of passivation curves. Microstructure of the MAO coating can be modulatedby gas evolution on anodes during micro arc oxidation process. The modulatationprocessed with a two-step reaction. Carbonate ions combine with magnesiumelement and then magnesium carbonate decomposes under the high temperature.Carbon dioxide escapes and micropores generate in the coating. Sodiumhexametahposphate is easy to hydrolyze to have hydroniums with complex ability inthe solution. During MAO process some Mg²⁺cations dissolved could becomplexed and the complex ions could return to anode region in the electrical fieldand take part in the reaction. And it is beneficial to the thickening of the coating.Effect of the three additives on MAO discharge has been studied. NaF hasadvantage in lowering the working voltage and increasing coating velocity. Thevoltage could be lowered with the introduction of sodium carbonate with themodulation effect on microstructure of coating and the coating formation velocitycoule be increased with sodium hexametahposphate introduced. Although the modeof reaction is different, the ECPUV could be lowered with introduction of the threeadditives. MAO process with low energy consumption coule be realized with all thethree additives in the electrolyte. In the electrolyte （50g/L Na₃PO₄+2g/L KOH+30g/L NaF+10g/L Na₂CO₃+30g/L （NaPO₃）₆） the ECPUV could be lowered to4.71kJ/dm2·μm. During lower energy consumption MAO a microarc lasts onlyhudrends of microseconds and fewer species of particles take part in the plasmadischarge compared to normal MAO.Magnesium fluoride generates with sodium fluoride introduced into theelectrolyte and the coatings become compact. It is beneficial for the corrosionresistance of the coating with a right amount of sodium fluoride in the electrolyte.The porosity of the coating increase without change the phase and composition ofthe coating when sodium carbonate is introduced into the electrolyte. It is beneficialfor the corrosion resistance of the coatings with a certain amount of sodiumcarbonate in the electrolyte and the corrosion resistance becomes weak when theamount is too large. Surface morphology will change with introducing sodiumhexametahposphate into the electrolyte but it has no negative effect on thecompactness of the coating. Phosphorus element is found in the coating formed inelectrolyte with sodium hexametahposphate addition. The coatings fabricated inelectrolyte exhibita good corrosion resistance.MAO coatings have been fabricated on magnesium alloys in the low energyconsumption electrolyte. The l coatings mainly are composed of MgO amorphousphase. Experimental results also demonstrate that the performance of the coatings is as good as that of the normal MAO coatings. Even the abending ability of thesample treated by MAO with low energy consumption is better than that treated bynormal MAO process.To investigate the feasibility of industrial application of ow energyconsumption MAO, a AZ31magnesium alloy plate with the area of12dm²has beentreated by MAO process with power less than1kW. This promotion the treatingability of the MAO power equipment and industrialization of this technology maybe realized.