Research On Preparation And Corrosion Resistance Of Micro-arc Oxidation Coatings Of Magnesium Alloy And Magnesium Matrix Composite

Microarc oxidation (abbreviated as MAO) coatings were prepared on AZ91Mg alloyand SiCP/AZ91Mg matrix composite in order to enhance their corrosion resistance. Theeffects of electrolyte composition, current density, pulse frequency and oxidation time onmicrostructure and corrosion resistance of the MAO coatings were studied. Sealing treatmentof MAO coatings was also studied. And then, the influences of SiC particles to micro-arcoxidation process and corrosion resistance of the coating were analysised. Surfacemorphology, composition of the MAO coatings were observed and analyzed by SEM, EDSand XRD. Corrosion resistances of the coatings were evaluated by potentiodynamicpolarization curve and electrochemical impedance spectroscopy.The results showed that the corrosion resistance of MAO coatings can be improvedwhen (NaPO3)6or NaAlO2was added into the silicate-based electrolyte. The MAO coatingwith excellent corrosion resistance can be prepared under the conditions of current density of6A/dm2, pulse frequency of1000Hz and oxidation time of30minutes. MgO and Mg2SiO4arethe main constituent phases of the MAO coatings which were prepared in silicate-basedelectrolyte. After sealing treatment by stearic acid, the pores with minisize in the coating canbe sealed; therefore the corrosion resistance of the MAO coating could be improved bysealing treatment.SiCP/AZ91Mg matrix composites exhibit the different oxidation behaviors comparing toAZ91Mg alloy due to the existence of SiC particles. When the volume fraction of SiCparticles in the composite is increased, the current density for occurrence of microarcdischarge also needs to be increased, that is，the growth efficiency of MAO coating will bereduced with the increase of volume fraction of SiC particles. The observation on theexistence state of SiC particles shows that SiC phase can not exist steadily during microracoxidation and it will be oxidized gradually. Microarc discharge of the composite would bedelayed by SiC phase.EIS test shows that corrosion resistance of SiC_p/AZ91composite can be highlyimproved by microarc oxidation treatment.