Preparation And Corrosion Resistance Of Chromium Free Composite Coating Formed On AZ91D Magnesium Alloy

In recent years, magnesium alloy is widely used in automobile, aviation, electronics and other fields because of its low density, high specific stiffness and strength ratio, good ductility and high thermal conductivity and so on. However, active chemical properties as a fatal weakness of magnesium alloy which is extremely easy to oxygen in the various circumstance.Therefore, the development of protection technology of magnesium alloy to promote the industrialization development of magnesium alloy has very important significance and application value.In this paper, AZ91 D magnesium alloy,by using orthogonal experiment and single factor experiment confirmed the molybdate conversion solution system: sodium molybdate?NaMoO₄? 30g/L, potassium permanganate?KMnO₄? 8g/L, the temperature is 50 degrees centigrade, processing time is 40 min with pH value of 5.5.Surface morphology of conversion film was based of wide and narrow crack, EDS analysis indicated that the concentration in the zone of wide cracks distributed more than narrow crack zone; resistance spot corrosion experiments showed that corrosion time of conversion film was longer nearly 11 minutes than magnesium matrix in the corrosion liquid; electrochemical test showed that corrosion current density of the conversion film which reached 1.32×10-7A/cm2 was lower two orders of magnitude than magnesium substrate and polarization resistance for 1368.4?·cm2 that was seven times higher than polarization resistance of the magnesium matrix.Based on the conversion film, combining the chemical conversion with sol-gel technology in magnesium alloy, hybrid organic coating were prepared on the conversion film forming a composite film.The results showed that combination ranking of the composite film was up to 1 level, surface morphology of coatings had no crack that was dense and uniform,the corrosion current density was lower 3 orders of magnitude than the magnesium substrate,the polarization resistance reached 3.03×104?·cm2. Corrosion behavior ofconversion film and composite coating in NaCl solution was investigated by the polarization curves and electrochemical impedance spectroscopy. The results indicated that the conversion film and composite coating in the late immersion would generate inductive reactance arc,which showed similar electrochemical corrosion behavior.The difference was immersion time of composite film reached 204 hours but conversion film only 42 hours.