Plasma Electrolytic Oxidation (PEO) Process on Commercially Pure Ti Surface: Effects of Electrolyte on the Microstructure and Corrosion Behavior of Coatings

The current study investigates the ceramic oxide coatings formed on commercially pure Ti substrate via plasma electrolytic oxidation (PEO) technique in electrolytes with different chemical compositions. The effects of electrolyte on the discharge characteristics, macro- and micro-structure, thickness, phase compositions, and corrosion behaviors of coatings were evaluated. Five kinds of electrolytes based on the sodium salt compounds including aluminate, phosphate, silicate, aluminate-phosphate, and aluminate-silicate were used. The PEO processes were carried out at fixed voltage (420 V) for 180 seconds. The morphology and phase composition of samples were investigated using scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. The corrosion behavior of specimens was analyzed in a 3.5 wt pct NaCl solution using the electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The results showed that the most uniform and dense structure with less porosity was obtained in the coating prepared in aluminate-phosphate-based electrolyte. This coating consisted of TiO2 (Rutile and Anatase) and TiAl2O5 phases. Also, it was confirmed that the noblest corrosion potential and lowest corrosion current density, and consequently, the best corrosion behavior (22.96 × 107 Ω cm2), were observed for aluminate-phosphate-based electrolyte. The Mott–Schottky analysis exhibited that all samples had n-type semiconducting behavior. The coating prepared in aluminate-phosphate-based electrolyte possessed the lowest donor densities.