The Research On Corrosion Resistance And Preparation Of Low Voltage With Sealing And Micro-arc Oxidation Coatings Of AZ91D Magnesium Alloys

Magnesium alloy corrosion could be improved by micro-arc oxidation,but the traditional micro-arc oxidation coatings commonly exhibited porous structure,which was considered to be main reason for failure of corrosion resistance.The corrosive liquid was easily penetrated into the coatings through the micropores.In order to improve the corrosion resistance of ceramic coatings,the lower voltage were used to sealing the ceramic coating formed on a voltage of a higher micro-arc oxidation treatment.In this paper,the sealing microstructure ceramic coatings were prepared by adjusting the sealing voltages and times in the zirconium salt system electrolyte solution,which reduced the corrosive liquid penetrating into the ceramic coatings through the micropores and improved the corrosion resistance of the ceramic coatings.The growth of ceramic coatings and micropores closure were completed at one time.In addition,the mechanism of the sealing process in the experiment was discussed.The surface and cross-sectional morphology of the ceramic coatings was observed by scanning electron microscopy,and the effects of sealing voltages and times on ceramic coatings were investigated.The relationships between sealing voltages and ceramic coating characteristic such as thickness,pore size and porosity were investigated,respectively.The relationships between sealing times and ceramic coating characteristic were also discussed.The results showed that the fraction of micropores on the surface was reduced after low voltages sealing treatment,which had insignificant effect on ceramic coating thickness.Surface and cross-sectional morphology,porous morphology and structure of the ceramic coatings were different by using the sealing voltages and times.In addition,the sealing effect was very obvious with 160 V sealing voltage and 5-8min sealing time,where the surface micropores were almost invisible.The phase composition of ceramic coatings that were obtained by using the sealing voltages and times was studied by X-ray Diffraction.The results showed that the ceramic coatings mainly consisted of MgO,MgF2,ZrO2 and Mg2Zr5O12,which were no affected by the sealing voltages and times.It turned out that the sealing material was consistent with the composition of ceramic coatings.5% NaCl immersion corrosion tests were carried out and polarization curve were used to analyze the corrosion resistance of different samples before and after low voltages sealing treatment,and the main results showed that the porous structure of the ceramic coatings formed in the micro-arc oxidation process was the main channel of corrosive medium,and the corrosion resistance of the ceramic coatings could be improved remarkably by sealing.In addition,corrosion resistance exhibited significant dependence on porosity,structure and morphology of micropores.It is known from this paper,the corrosion resistance could be improved by sealing microstructure obtained by applying sealing voltages and times.