Thicker the better or thinner: Addressing the dichotomy in the case of MAO coated AZ91 Mg alloy for corrosion protection

Abstract

Magnesium alloys, owing to their high specific strength and low density, are promising materials for lightweight structural applications. However, their poor corrosion resistance severely restricts broader industrial use. The present study developed micro-arc oxidation (MAO) coatings on AZ91 magnesium alloy at a constant current density of 0.3 A cm⁻² using silicate–KOH-based electrolytes containing K₂ZrF₆ and NaAlO₂ additives to modulate the ionic conductivity, consequently the coating voltage. The incorporation of K₂ZrF₆ promoted the in-situ formation of ZrO₂, while aluminate addition facilitated the development of hard MgAl₂O₄ and Al₂O₃ phases and stabilized the high-temperature tetragonal ZrO₂ (t-ZrO₂). The synergistic effect of these phases enhanced coating compactness, reduced microcracks, and improved hardness and corrosion resistance, particularly in thinner coatings. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution revealed that all the MAO coatings outperformed the corrosion resistance of the bare substrate. Among them, the optimized aluminate-containing electrolyte produced the most compact coating (A3–11), exhibiting approximately six orders of magnitude reduction in corrosion rate and a four-order increase in impedance relative to the bare substrate. The findings demonstrate that tailoring electrolyte composition enables the formation of dense, defect-minimized MAO coatings with superior protection performance on AZ91 magnesium alloy.