Microstructure and corrosion resistance of yellow MAO coatings

Yellow ceramic coatings were prepared on ultra-fine grained (UFG) AZ91 magnesium alloy using micro-arc oxidation (MAO) process in the alkaline-silicate electrolyte with different KMnO ₄ addition. Microstructure and phase composition were investigated by SEM, EDS (energy-dispersive spectrometer) and XRD (X-ray diffraction). With 2 gL ⁻¹ KMnO ₄ addition, the extreme sparking discharge was inhibited and the coating exhibited more compact and uniform morphology. However, adding 3 gL ⁻¹ KMnO ₄ in electrolyte extremely woke the discharge intensity and caused deleterious influence such as micro-crack. The generation of complex oxide Mg ₆ MnO ₈ phase resulted in the yellow colour of the coatings. Electrochemical measurements including potentiodynamic polarisation and electrochemical impedance spectroscopy tests were carried out in 3.5% NaCl solution. The UFG alloy with the MAO coating produced with 2 gL ⁻¹ KMnO ₄ addition exhibited superior corrosion resistance, which was owing to the more compact and uniform microstructural features.