Study On Microarc Oxidation Process Control Of Magnesium-based Materials And Sealing Of Coating Defects

In many areas, the application of magnesium-based materials has been limited due to its susceptibility to corrosion. After microarc oxidation(abbreviated as MAO), in-situ growth of a protective coating on the surface of magnesium-based materials can improve the anti-corrosion performance. But due to the limitation of magnesium-based materials characteristics, the density of coating is lower than other valve metal MAO coating, resulting in poor anti-corrosion performance. This paper aims to controll the sparking discharge in MAO process and increase the MAO coating density of magnesium-based materials by selecting electrolyte additives and changing the mode of power. Then on this basis, applying silanization treatment to repair defects of coating, further improving the anti-corrosion performance of MAO coating on magnesium-based materials.This study selects AZ91D alloy, ZnO@Al18B4O33w/AZ91 composite and AI18B4O33w/AZ91 composite as basis materials by MAO. First find effects of electrolyte additives for MAO behavior on AZ91D alloy so as to select the type and concentration of additives which can significantly improve the anti-corrosion performance of MAO coating.After that study the influence of oxidation time and power control for anti-corrosion performance of MAO coating on AZ91D alloy. Optimized process achieved from above experiment is applied to investigate the anti-corrosion performance of MAO coating on magnesium matrix composite. At last repair the structure defects including micropore and microcrack of MAO coating by silanization treatment in order to further improve the anti-corrosion performance.The surface morphology of the MAO coatings under different process was observed and the sealing effect of silanization treatment was investigated by scanning electron microscope(abbreviated as SEM). The hydrophobicity of MAO coating was judged under contact angle test. Corrosion performance and corrosion behavior of MAO coating was surveyed using potentiodynamic polarization curve test, neutral salt spray test and electrochemical impedance spectroscopy (abbreviated as EIS). The study results show:When suitable additives were added into basic solution of silicate system, sparking discharge became more dense and uniform in MAO process on AZ91D alloy, meanwhile the size of sparks and extent of discharge were reduced. Dense and uniform sparking discharge was in favor of improving the density of MAO coating, increasing diffusion resistance of corrosive medium and corrosion products, finally enhancing the anti-corrosion performance.This research selected the effective additives including 1H-Benzotriazole, Potassium hydrogen phthalate, Propylene glycol, Sodium citrate, Polyethylene oxide, Sodium aluminate and Sodium tetraborate, thereinto Potassium hydrogen phthalate, Sodium aluminate and Sodium tetraborate were best.The MAO coating thickness increased with the extension of oxidation time, but the density of coating decreased, the anti-corrosion performance increased first and then decreased under unrestrained voltage mode. However, when the voltage was restrained, the density of coating did not decrease with thickness of coating increased, and the anti-corrosion performance increased.The anti-corrosion performance of magnesium matrix composite is inferior to magnesium alloy due to introduction of reinforcement phases. But the corrosion performance of composite was higher than alloy by the same method of MAO, meanwhile the additives can better improve the anti-corrosion performance of MAO coating on magnesium matrix composite than magnesium alloy. This may be due to the Al18B4O33 whisker does not react in MAO process, and has been covered within the MAO coating when the MAO coating growth towards substrate, reducing volume shrinkage of coating. Thus, there was better density and anti-corrosion performance of MAO coating on magnesium matrix composite as compared with MAO coating on magnesium alloy.The surface of MAO coating on magnesium-based materials could form a layer of silane film with good hydrophobicity by silanization treatment. Besides, the hydrolysate of silicane can spill over into defects of coating to seal inner cavities and gaps, further perfecting the anti-corrosion performance of MAO coating.