Preparation And Post Sealing Treatment Of Micro-Arc Oxidation Ceramic Coating On AZ31B Mg Alloy

The corrosion resistance of magnesium alloy is a key factor for its wildly application. In this work, ceramic coatings characteristed with corrosion resitance were prepared in-suit in Na₂SiO₃ solution by micro-arc oxidation (MAO) process on magnesium alloy (AZ31B) substrate. The technology parameters, Ce doping and sealing treatment, microstructure and corrosion resistance of the ceramic coatings were studied in detail. Besides, the corrosion mechanism of the coating in 3.5%NaCl solution was discussed preliminarily.The phase composition, morphology of the coatings suface and section, the element composition was studied by X-ray diffraction (XRD) and the scanning electron microscopy (SEM) and EDS. The thinkness of the coatings was measured by an eddy current based thickness gauge. The adhesion between the coating and the substrate was measured by scratch tests and thermal shock tests. The corrosion resistance properties of the coatings were evaluation by CHI660 electrochemical analyzer and salt spray corrosion test.It was found that, the pore structured coatings were mainly conposition by MgO and Mg₂SiO₄. The proper increase of the reaction time and current density is helpful for the thickness and roughness of the coatings. However, with the increase of frequenc, the thickness and the surface roughness of the coatings decreased. The electrochemical measurments of point corrosion and corrosion were selected for optimizing technology parameters. The results reveled that the corrosion current density of the coated samples prepared with 300 Hz, 45% duty ratio, 2A/dm² current density was decrease by three quantity orders, compared with AZ31B substrate. In order to bring Ce element into the ceramic coatings, Ce solution treatment or added into electrical solution after or before the coatings were prepared respectively. The results indicated that Ce atoms introducted before MAO process, Ce atoms can not only reduce the reaction voltage of MAO process but also enhance corrosion resistance ability of the product ceramic coatings.In the present work, the pore-sealing treatemt for the ceramic coatings was excuted by boiling sealing, dip-coating paint sealing, spray sealing. The results showed that a paint layer with thickness of 2⁵μm was formed on the ceramic coatings surface by dip-coating paint sealing, which sealed most of the coatings pores and enhanced the corrosion resistance ability of ceramic coatings. 15μm thicked paint layer was formed by spray sealing, which sealed all of the coatings pores and greatly improve the corrosion resistance of the coatings. After spray sealing treatment, the ceramic coatings were almost the same as newly prepared after 144 h salt spray corrosion test.