Morphology and corrosion resistance of hybrid plasma electrolytic oxidation on CP-Ti

In this research, double plasma electrolytic oxidation coating in two different electrolytes on commercially pure titanium was done. First layer of PEO coating was synthesized in acidic solution of phosphoric/sulfuric acid with 4 levels of current density and the second layer was synthesized in the solution containing calcium acetate/sodium hypophosphite monohydrate. The role of current density on first layer's morphology, hybrid coating's morphology, and corrosion performance is investigated. Mott-Schottky analyses in combination with EIS test were used to reveal the underlying causes of corrosion resistance changes of hybrid PEO coatings. It was revealed that at low current densities grooved microstructure develops which showed high corrosion resistance, compared to porous microstructure. It was also found that grooved microstructure, as first layer, is not favorable for hybrid PEO coating and has detrimental effect on corrosion performance due to its effect on discharge behavior of the second layer. Comparison of corrosion performance of samples with porous morphology revealed that application of low current density improves corrosion resistance.Application of the hybrid-layered coating with the optimized process parameters led to 5 times increase of polarization resistance compared to single-layered coating. EIS studies revealed that corrosion resistance changes were mainly originated from changes in properties of the barrier layer. Analysis of Mott-Schottky plots showed that samples with more resistance has lower donor density which is ascribed to their compact microstructure.