Growth of plasma electrolytic oxidation coatings on Nb and corresponding corrosion resistance

Plasma electrolytic oxidation (PEO) coatings were formed, respectively in three silicate-based, phosphate-based and aluminate-based electrolytes on niobium, with the aim to explore the relationships between electrolyte composition, dielectric breakdown behavior and corresponding corrosion resistance. The initial growth stages of the anodic coatings were studied, and result shows that an impedance falling stage (from about 200 V to 400 V) starts at the emergence of spark discharge accompanied with the surface morphology evolution from compact to porous. The PEO coatings were characterized through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods. Results show the coatings fabricated in (NaPO₃)₆ electrolyte provide the best corrosion resistance. It attributes to the effective thickness of the compact inner layer rather than the entire thickness of the coating that decrease diffusion of corrosion medium.