Study On Service Life Of Electrolyte For Micro-arc Oxidation On AZ91D Magnesium Alloys

Micro-arc oxidation(MAO) of magnesium alloys is a plasma discharge that occurs at the metal/electrolyte interface when the applied voltage exceeds a certain critical breakdown value.The process produces thick,hard and well-adhered ceramic-like coatings to be obtained with high corrosion resistance,thermal stability and dielectric properties.Therefore,MAO is one of the most prospective methods of surface treatment for magnesium and its alloys.Service life of electrolyte of micro-arc oxidation on AZ91D magnesium alloy in silicate solutions were probed into.First,effect of experiment periodicity on start voltage,final voltage,conductivity and concentration of electrolyte,microstructure and properties of micro-arc oxidation coating were studied,and the reason of electrolyte aging was discussed by analysis electrolyte.The microstructures of micro-arc oxidation coating were studied by means of TT260 digital coating thickness gauge,JSM-6700F scanning electron microscope(SEM) and 2206 surface roughness measuring instrument.Concentrations of electrolyte were studied by means of analytical chemistry.Magnesium alloy of anode dissolve into electrolyte and react with element in the electrolyte,the reactant is amorphous precipitate mostly.Stainless steel of cathode dissolve into electrolyte in the double pole pulse power,the reactant is hydrate of Iron. Hydrate of Iron is amorphous precipitate,as well.Amorphous precipitate decrease the Speed of ion moving and decrease conductivity of electrolyte.Resistance of electrolyte increases with the reducing of conductivity.Accordingly, voltage of electrolyte becomes smaller and start voltage increases,as well. Conductivity decreased gradually and voltage of sample becomes smaller,leading to energy decreased.The size of micro-pore in the micro-arc oxidation coating becomes smaller,but the amount of micro-pores increases.Accordingly,the roughness of coating becomes smaller.Finally,the macro-quality of MAO coating becomes rougher. Accordingly,the roughness of coating becomes larger quickly.Corrosion resistance of coating reduces with the increasing of roughness(the macro-quality of MAO coating descend).Before electrolyte failure,thickness and increase weight of coating steadied. The phase of the coatings was hardly effect,the coatings was mainly composed of MgO,MgSiO₃,Mg₇F₂(SiO₄)₃. Filtrating Precipitate can quicken the rate of film forming and increase the service life of electrolyte.Conductivity of electrolyte becomes larger than without filtrating and becomes smaller than forepart.Simultaneity,start voltage declined than without filtrating.OH^-'s concentration in the electrolyte increases in evidence,OH^-'s concentration has effect on forming and properties of micro-arc oxidation coating.