Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol-gel coatings

This study evaluated the corrosion resistance of AZ91D magnesium alloy coated by composite coatings which consisted of a molybdate conversion coating and three layers of silicon sol-gel coatings. For molybdate conversion treatment, various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements. The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters (7.3 g/L (NH₄)₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30 °C). In order to get a further improvement of corrosion protection for AZ91D magnesium alloy, three layers of silicon sol-gel coatings were successfully deposited on the molybdate conversion coating pre-applied to AZ91D alloy. The surface morphology and the corrosion protection performance of the composite coatings were investigated in detail using scanning electron microscope, electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. The results demonstrated that the composite coatings could greatly improve the corrosion protection performance of the AZ91D magnesium alloy.