Effects of Electrolytic MgO Coating Parameters on Corrosion Resistance of AZ91D Magnesium Alloy.

The electrolytic MgO coating on Mg alloy has been carried out in 0.1 M Mg(NO₃)₂·6H₂O aqueous solution to improve its corrosion resistance. The as-coating was Mg(OH)₂ which was formed by the electrolysis, Mg²⁺ + 2OH¯ → Mg(OH)₂, and finally transformed into MgO at 350°C. The surface morphology and phase structure were analyzed using scanning electron microscope and grazing angle X-ray diffraction. Polarization curves and immersion tests were used to evaluate the corrosion resistance of the coated specimens. More OH¯ ions were produced, and the migration of Mg²⁺ was enhanced at a more negative applied potential, leading to a higher nucleation rate of Mg(OH)₂ on the specimen and resulting in the (001) preferred orientation. The weight gain resulted from coatings increased with increasing deposition time for the initial 40 min, and the increasing rate slowed down after 1 h due to the precipitation of Mg(OH)₂ in the solution. A coating deposited at -1.8 V (Ag/AgCl) for 40 min and annealed at 400°C for 1 h was suggested to provide a more uniform and dense MgO protective coating, which results in a reduction of corrosion current density from 99.5 down to 0.2 μA/cm² and the enlarged passivation region between -1.52 and -1.25 V (Ag/AgCl) in 3.5 wt % NaCl aqueous solution, with respect to the uncoated specimen. Compared with AZ9ID magnesium alloy substrate, the corrosion resistance was improved nearly 2 orders by the electrolytic MgO ceramic coating. However, the bonding strength, 10 MPa, should be further improved to satisfy the minimum requirement, 35 MPa, for the orthopedic implants. [ABSTRACT FROM AUTHOR]