An insight into the evolution of corrosion resistant coatings on titanium during bipolar plasma electrolytic oxidation in sulfuric acid

Abstract

This work describes recent progress related to bipolar pulsed plasma electrolytic oxidation (PEO) applied to grade 2 titanium in 0.5 M H2SO4. Corrosion resistance of oxide coatings was characterized by anodic polarization, immersion tests in acidic environment and electrochemical impedance spectroscopy (EIS). Scanning electron microscope (SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES) and visual observation were employed to describe oxide microstructure and composition. The influence of frequency and amount of cathodic polarization were rationalized in terms of modification of the electrical double layer (edl) formed at the oxide-electrolyte interface. Depending on its characteristics, different plasma regimes are established, leading to the growth of oxides optimized for corrosion resistance in different environments. It is here shown how the selection of high frequency (1000 Hz) and a negative-to-positive charge ratio (R) of 0.78 favors the establishment of a strong plasma regime, promoting the growth of a coating optimized for corrosion resistance in concentrated H2SO4. It is also reported that low frequency (20 Hz) and cathodic polarization (R = 0.07) should be used to enhance localized corrosion resistance.