Plasma electrolytic oxidation (PEO) process was employed in a galvanostatic regime on Zr-2.5Nb alloy in NaAlO2 aqueous electrolyte solutions within a concentration gradient of 5-30 g/L in order to produce Al2O3/ZrO2 composite coatings. The composition and microstructure of the oxide layers were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Vickers microhardness, scratch adhesion and potentiodynamic polarization measurements were used to assess the mechanical and electrochemical properties of the PEO coatings. The experimental findings suggest a strong correlation between the electrolyte concentration and the coating layers. By increasing the concentration of NaAlO2 in the electrolyte solution, significantly increased tetragonal crystal structure, particle size, hardness, adhesion strength and corrosion resistance occur with pore size decrease. Best values for the hard and adherent coating were similar to 1500 HV and similar to 60 N critical load. The corrosion current densities for the plasma electrolytic oxidized samples are lower than the black oxide coated Zr alloy. (C) 2018 Elsevier B.V. All rights reserved.