Effect of pH on the Phase Composition and Corrosion Characteristics of Calcium Zinc Phosphate Conversion Coatings on Titanium

Abstract

Calcium and/or zinc phosphates (Ca-Zn-P) coatings are adopted to improve the bio-performance of titanium (Ti) using the phosphate chemical conversion (PCC) method. Although solution pH is well known in affecting PCC coating formation rate and properties, little is known about its effect on the phase composition of Ca-Zn-P coatings. In this study, we have prepared different types of Ca-Zn-P PCC coatings on Ti from solutions at various pH levels. The chemical composition, microstructure and elemental distribution of the coatings were examined via X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy equipped with an energy dispersive spectrometer. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy in the simulated body fluid. It is shown that pH can significantly affect the phase composition of the coatings. At pH 2.50–3.25, plate-like hopeite (Zn3(PO4)2·4H2O) coatings were formed on Ti, and at pH 3.50–4.25, flower-like scholzite (CaZn2(PO4)2·2H2O) coatings were deposited. Electrochemical analyses revealed that hopeite coatings enable better corrosion resistance than scholzite coatings. The phase change of Ca-Zn-P coatings can be attributed to the different pH-dependent activities of Ca2+ and Zn2+ ions, and the effect of zinc on the formation of Ca-P phases depends on the solution pH.