Effect of voltage on microstructure and corrosion resistance of microarc oxidation coatings on CP-Ti

Abstract

The surface modification of commercially pure titanium (CP-Ti) by microarc oxidation (MAO) under different voltages was investigated using 1%H₃PO₄ solution as an electrolyte. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM) and X-ray diffraction. The corrosion behaviour of the coating was also examined by potentiodynamic polarisation testing in a 3·5 wt-%NaCl solution. Micropore oxide films were formed on all the sample groups by MAO. The thickness and micropore size of the MAO coating increased with the increasing voltage. Energy dispersive X-ray spectroscopy results indicate that Ti, O and P became incorporated into the MAO coatings. At a low voltage of 250 V, the MAO coatings were composed of amorphous, P₂O₅, TiP₂O₇ and titania phases (rutile and anatase). Variation of treatment voltages increased the ceramic coatings from an amorphous structure to a phase structure, and the P₂O₅ phase disappeared. The corrosion potential Φ_corr of the MAO sample shifted towards nobler directions, and the corrosion density I_corr fell significantly compared with that of the bare CP-Ti. Corrosion testing showed that the sizes of the micropore of the MAO samples obviously decrease, and the MAO surface becomes smooth.

Keywords:

• Commercially pure titanium
• Microarc oxidation
• Corrosion

Acknowledgements

The authors acknowledge the financial support of this work from the National Science Council of Taiwan under project nos. NSC 101-2622-E-020-006-CC3 and NSC 101-2221-E-020-011. The authors also thank S. F. Ping for the useful discussion and sample preparation. Scanning electron microscopy was performed by the Precision Instrument Center of National Pingtung University of Science and Technology, Taiwan.