Effect of synthesized ZnO nanoparticles on the corrosion and tribological performance of PEO coatings on AM50 magnesium alloy

Plasma electrolytic oxidation (PEO) coatings were fabricated on AM50 magnesium alloy using a phosphate–silicate electrolyte containing synthesized ZnO nanoparticles to investigate their influence on coating formation, corrosion resistance, and tribological performance. Phase-pure spherical ZnO nanoparticles were synthesized via a green chemical route and added to the electrolyte at concentrations of 0–3 g/L. Electrochemical results revealed that incorporation of ZnO nanoparticles significantly enhanced corrosion resistance, with the corrosion current density decreasing from 3.96 × 10 -4 A·cm -2 for the nanoparticle-free coating to 6.11 × 10 -7 A·cm -2 at an optimal ZnO concentration of 1 g/L, corresponding to nearly three orders of magnitude improvement. Electrochemical impedance spectroscopy confirmed this enhancement, showing an increase in coating resistance (R 2 ) from 3.7 × 10 4 to 7.3 × 10 5 Ω·cm 2 . Morphological analyses demonstrated that ZnO addition reduced coating porosity from 6.6% to 3.5% and increased coating thickness from ∼11 μm to ∼14 μm through discharge modulation and pore sealing. Nanoindentation and wear tests indicated that the PS-1Zn coating exhibited the highest hardness (5.52 GPa) and lowest wear rate, while excessive ZnO addition (3 g/L) led to microcrack formation, increased friction, and degraded corrosion–wear performance. These results demonstrate that controlled incorporation of synthesized ZnO nanoparticles effectively improves the corrosion and tribological behavior of PEO coatings on AM50 magnesium alloy, while excessive nanoparticle loading is detrimental. • Green-synthesized, phase-pure ZnO nanoparticles were incorporated into PEO coatings on AM50 alloy, allowing controlled evaluation of their functional role. • A systematic concentration study (0–3 g/L) identified the optimal ZnO loading that maximizes corrosion resistance and tribological performance while avoiding crack-inducing excess. • ZnO nanoparticles modulated discharge behavior—delaying breakdown, reducing discharge intensity, refining pores, and increasing coating thickness. • Coatings containing the optimal ZnO concentration showed significant improvements in hardness, friction coefficient, and wear rate compared to ZnO-free PEO layers. • The balance between nanoparticle incorporation, coating compactness, and surface integrity is critical for optimizing PEO coatings on Mg alloys.