Relative hardness and corrosion behavior of micro arc oxidation coatings deposited on binary and ternary magnesium alloys

• Both Al and Zn added to Mg alloy significantly improve MAO coating kinetics.
• Al in Mg alloys promote MgAl2O4 phase and enhances coating integrity.
• Min. 3 wt.% Al is essential for achieving high relative corrosion resistance.
• 6–9 wt.% Al in Mg alloy improves MAO coating mechanical properties.

Magnesium alloys have attracted wide-spread industrial applications in automotive, space, mobile and telecommunication segments. Present investigation reports the influence of Mg-alloy composition (7-different substrate compositions: pure Mg, three binary Mg–Al alloys with varying wt.% Al such as 3%, 6% and 9% and three Mg–Al–Zn ternary alloys such as AZ31, AZ63 and AZ91) on the micro arc oxidation (MAO) coating formation kinetics, coating morphology, elemental and phase compositions, coating hardness and corrosion behavior. The substrate composition significantly influences the coating formation kinetics, coating hardness and corrosion behavior. Although Zn added to Mg alloys does not contribute directly toward the formation of any specific Zn-bearing phase like MgAl2O4 phase as contributed by the Al present in the Mg alloy, it provides dramatic reduction in corrosion rate. A minimum of 3 wt.% Al is essentially required in ternary Mg alloys to exhibit significantly improved relative corrosion resistance (RCR) while 6–9 wt.% of Al is preferred for enhanced relative hardness (RH). The RCR and RH were found to be beneficial criterions for effective design of new Mg-alloys with attractive combination of mechanical properties and corrosion resistance.

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