Preparation And Corrosion Resistance Research Of Sol-gel Coating That On Magnesium Alloy

As the lightest structural metal materials, Magnesium alloys which are compared as the "21st Green Engineering Material" have a broad development prospects in the automotive,aerospace and electronic industry. But so far, the lower standard electrode potential of magnesium and the oxidation film on the surface of magnesium that was loose and porous make magnesium and its alloys corrosion easily in most environmental medium, as hindered application of magnesium alloys materials. Surface treatment technology is an effective means to improve the corrosion resistance of magnesium alloys. Micro-arc oxidation technique which developed from the traditional technology of anodic oxidation is a kind of new type of methods about surface treatment, and it is a very promising technology about magnesium alloy surface treatment. However, there are a large micron grade distribution holes on the surface of micro-arc oxidation magnesium alloy, even some of the pores extends to the surface of substrate, as makes the channel between corrosion media and the matrix, to a certain extent, it will reduce the corrosion resistance. In view of this, it is quite important to follow-up seal.In order to further improve the corrosion resistance of micro-arc oxidation protective layer on AZ91D magnesium alloys surface, according to the inherent defect of arc-oxidize film, the transparent and stable Al₂O₃ sol and SiO₂ sol hole sealing agent were prepared in this paper with inorganic salt Al (NO₃)₃·9H₂O, organic alcohol salt aluminum isopropanol (C₉H₁₂AlO₃) and tetraethyl ortho-silicate (TEOS) as precursor, nitric acid as catalyst by sol-gel technique. Meanwhile, the Al₂O₃ and SiO₂ sol sealing coating which have the good corrosion resistance were prepared on the micro-arc oxidation of AZ91D magnesium alloy by dip-process.This paper mainly discussed the influence of the ratio of raw material, coating times and heat treatment temperature on the properties of SiO₂ layer. The results shows that, the coats are best when R = 4, viscosity was 8mPa/s, parallel coating twice and sintered at 350℃.In this paper, the corrosion behaviors of the coated samples were also studied by anodic polarization curves test. Surface morphology of the layers was characterized through scanning electronic microscopy(SEM). The composition was analyzed by electron diffraction spectrum(EDS). The micro-arc oxidation coating of AZ91D magnesium alloy were sealed by silica and alumina sol. It can be seen that uniform and light coating was achieved and the corrosion resistant performance of magnesium alloy was improved. Al₂O₃ sol coating not only greatly improved corrosion resistance performance of matrix, but also increased the microhardness of surface.