Surface Properties of Mg–3.5Y Alloy Treated by Micro-Arc Oxidation Using Eggshells and Ammonium Hydrogen Phosphate-Added Electrolyte

Magnesium–yttrium (Mg–Y) alloys are promising biodegradable orthopedic implant materials because of their mechanical compatibility with bone tissue; however, their degradation rate in physiological environments requires further control. In this study, the surfaces of gravity-cast Mg–3.5Y alloy were treated with micro-arc oxidation (MAO) in Na2SiO3-based electrolytes supplemented with diammonium hydrogen phosphate ((NH4)2HPO4), eggshell powder, or a mixture of both, to form corrosion-resistant ceramic coatings containing calcium and phosphates. The MAO process utilized a bipolar pulsed DC power supply for 8 min, and the resulting coating characteristics were analyzed via FESEM, EDS, XRD, Vickers microhardness, electrochemical test (potentiodynamic polarization), and simulated body fluid immersion tests. Cross-sectional SEM and EDS revealed that all MAO-treated samples contain Mg, Na, O, Si, and elements derived from the electrolyte additives. Electrolytes containing diammonium hydrogen phosphate not only produced thicker films but also facilitated the deposition of calcium-related species derived from eggshells; in contrast, the deposition of calcium is limited when only eggshell powder is added to the electrolyte. X-ray diffraction analysis confirms the formation of MgO, Mg2SiO4, and calcium-containing compounds within the coating. Energy-dispersive X-ray spectroscopy spectrum further verified the presence of these compounds in the deposited films. Electrochemical tests in simulated body fluid revealed that the electrolyte containing only diammonium hydrogen phosphate yields the highest open-circuit potential and corrosion current; further immersion tests demonstrate better corrosion resistance. Overall, the results show that combining diammonium hydrogen phosphate with eggshell-derived calcium compounds can be an effective method for producing multifunctional MAO coatings on Mg–Y alloys. For microhardness, adding diammonium hydrogen phosphate and eggshell to Na2SiO3-based MAO electrolytes yields a harder coating, whereas Na2SiO3-based electrolytes with only diammonium hydrogen phosphate would lead to a better corrosion-resistant coating. These differences are caused by differences in composition and microstructure in the deposited films. This study offers a viable production method for application in biodegradable materials.