Microstructure and corrosion resistance of yellow MAO coatings

ABSTRACT

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 KMnO₄ addition. Microstructure and phase composition were investigated by SEM, EDS (energy-dispersive spectrometer) and XRD (X-ray diffraction). With 2 gL⁻¹ KMnO₄ addition, the extreme sparking discharge was inhibited and the coating exhibited more compact and uniform morphology. However, adding 3 gL⁻¹ KMnO₄ in electrolyte extremely woke the discharge intensity and caused deleterious influence such as micro-crack. The generation of complex oxide Mg₆MnO₈ 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 KMnO₄ addition exhibited superior corrosion resistance, which was owing to the more compact and uniform microstructural features.

KEYWORDS:

• Magnesium alloy
• yellow coating
• micro-arc oxidation
• microstructure
• corrosion resistance

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The study was supported by the Key Research and Development Project of Jiangsu Province of China [grant number BE2017148], the Fundamental Research Funds for the Central Universities [grant number HHU2016B10314], the Natural Science Foundation of Jiangsu Province of China [grant number BK20160869], the Public Service Platform and Science & Technology Support Program in the industrial field of Suqian City of China [grant numbers M201614 and H201615].