Influence of oxysalts on plasma electrolytic oxidation (PEO) of titanium performed in NaOH solution for corrosion protection

The present paper discusses the influence of oxysalts, namely tungstate, zirconate, and silicate, on the plasma evolution during the PEO process on titanium, performed in a sodium hydroxide solution. The morphology, thickness, and corrosion resistance strongly depend on the specific type of oxysalts added to the electrolytic bath. Scanning electron microscope (SEM) imaging reveals that zirconate promotes the production of thicker coatings than tungstate, while when silicate is used more porous oxides with a very rough morphology are obtained. Coatings processed with silicate also show a high level of amorphism, while the other types of oxides are mainly crystalline, as highlighted by X-ray diffraction (XRD) testing. The anti-corrosive performance is evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analyses performed in a hot sulfuric acid solution. All the PEO samples display higher polarization resistances and lower corrosion current densities than the bare titanium showing an improved corrosion resistance.