Microstructural tuning of AZ91 magnesium alloy for improved morphology and corrosion protection of electrodeposited hydroxyapatite coatings

The corrosion stability of magnesium alloys can be substantially enhanced through surface modification and coating techniques. However, the relationships between the substrate microstructural aspects and resulting electrodeposited coatings remains largely unexplored. To address this, the present study investigates the development of hydroxyapatite coatings with varying morphologies on thermally treated AZ91 Mg alloy substrates. Development of an intermediate Mg(OH) ₂ layer at the AZ91 substrate during immersion in the coating electrolyte, under the influence of the micro-galvanic corrosion of the secondary phases / α-matrix coupling in the AZ91 microstructure, plays a critical role in determining the final coating morphology. Notably, the coating exhibits a finer morphology when applied to substrates with finer, intragranular β-phase precipitates resulting from aging treatment. The corrosion rate of the uncoated casting alloy reduced by 22 % and 12 % after solution treatment and aging, respectively, due to reduced cracking in the corrosion layer and enhanced passivation as a result of CaCO ₃ formation. The solution-treated AZ91 substrate yielded a nearly defect-free, uniform hydroxyapatite coating with the lowest corrosion current, while the as-cast sample produced a hydroxyapatite coating with defects, particularly over the α + β sites, leading to higher corrosion currents. The polarization resistance (Rp) of the initial as-cast alloy increased about 7-fold after solution treatment followed by hydroxyapatite coating. Overall, the Al 8 Mn 5 and β -phases in AZ91 alloy contribute to micro-galvanic corrosion by acting as cathodic sites, significantly influencing both the resistance of bare alloy to corrosion and the morphology and protective efficacy of the hydroxyapatite layer.