Ultrasonic Induced Rapid Formation and Crystal Refinement of Chemical Conversed Hopeite Coating on Titanium

Hopeite coating on metals by the phosphate chemical conversion (PCC) method has received more attention for its potential biomedical use. It is difficult to get a PCC phosphate coating due to the presence of a passive oxide layer on the surface of titanium. In this research, we report on effects of ultrasonic irradiation (UI) on formation, crystal size, microstructure, and corrosion resistance of the PCC coatings on Ti. It is shown that both coatings formed with and without UI are composed of hopeite (Zn3(PO4)2·4H2O) with similar crystal shape. FE-SEM observation demonstrates that UI can significantly decrease crystal size from 50–100 to 5–20 μm within a duration time of 30–60 min. Short period PCC treatment of 5 min shows that UI can enhance the formation of coating with the increase of nucleation rate. And the nucleation rate with UI of 250 W is significantly higher than that of 50 W. The electrochemical analysis reveals that the corrosion resistance of the coatings can also be improved by ultrasonic ir...