ABSTRACT
Microarc oxidation (abbreviated as MAO) coatings were prepared on AZ91 Mg alloy and SiCP /AZ91 Mg matrix composite in order to enhance their corrosion resistance. The effects of electrolyte composition, current density, pulse frequency and oxidation time on microstructure and corrosion resistance of the MAO coatings were studied. Sealing treatment of MAO coatings was also studied. And then, the influences of SiC particles to micro-arc oxidation process and corrosion resistance of the coating were analysised. Surface morphology, composition of the MAO coatings were observed and analyzed by SEM, EDS and XRD. Corrosion resistances of the coatings were evaluated by potentiodynamic polarization curve and electrochemical impedance spectroscopy. The results showed that the corrosion resistance of MAO coatings can be improved when (NaPO 3)6 or NaAlO2 was added into the silicate-based electrolyte. The MAO coating with excellent corrosion resistance can be prepared under the conditions of current density of 6A/dm2 , pulse frequency of 1000Hz and oxidation time of 30 minutes. MgO and Mg2SiO4 are the main constituent phases of the MAO coatings which were prepared in silicate-based electrolyte. After sealing treatment by stearic acid, the pores with minisize in the coating can be sealed; therefore the corrosion resistance of the MAO coating could be improved by sealing treatment.
SiCP/AZ91 Mg matrix composites exhibit the different oxidation behaviors comparing to AZ91 Mg alloy due to the existence of SiC particles. When the volume fraction of SiC particles in the composite is increased, the current density for occurrence of microarc discharge also needs to be increased, that is，the growth efficiency of MAO coating will be reduced with the increase of volume fraction of SiC particles. The observation on the existence state of SiC particles shows that SiC phase can not exist steadily during microrac oxidation and it will be oxidized gradually. Microarc discharge of the composite would be delayed by SiC phase.
EIS test shows that corrosion resistance of SiCp/AZ91 composite can be highly improved by microarc oxidation treatment.
Key words: Mg alloy; Mg matrix composite; microarc oxidation; SiC particle; corrosion resistance
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