Structure and improved corrosion properties of MAO coating on Mg-5Gd-1Zn-0.4Zr formed under various current densities

Abstract

To investigate the structure and corrosion properties of micro-arc oxidation (MAO) coating on Mg-5Gd-1Zn-0.4Zr (GZ51K) alloys for biomedical application, the alloy was MAO-treated using silicate electrolyte system under current density of 400, 600 and 800 mA/cm² respectively. The microstructure, surface analysis and corrosion resistance of samples were evaluated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiodynamic polarisation, electrochemical impedance spectroscopy in SBF solution. The results indicate that the main phases of the MAO coatings were MgO, Mg₂SiO₄ and MgF₂. With the increase of MAO current density, the thickness of the MAO coating decreased from 16.5 to 10.4 μm. When MAO current density is 400 mA/cm², the specimen shows the best corrosion resistance with a four-order-of-magnitude decrease on I_corr ((1.95 ± 0.01) × 10⁻¹⁰ A/cm²), a four-order-of-magnitude increase on R_P((7.67 ± 0.02) × 10⁷ Ω cm²). Even though the thickness of the MAO coatings obtained with 600 and 800 mA/cm² are very close, the corrosion resistance of the coating prepared under 800 mA/cm² is better than that under 600 mA/cm². Generally, MAO treatment can greatly slow down corrosion rate of the GZ51K alloy and keep uniform corrosion mode which is desirable for biodegradable implant applications.