Formation mechanism and properties of fluoride–phosphate conversion coating on titanium alloy

A uniform grey conversion coating with the thickness of 4-5 mm was prepared on titanium alloy by a fluoride-phosphate treatment. Scratch testing results indicated that the conversion coating greatly improved the adhesion strength of the paint coat to the titanium substrate. The growth mechanism of the conversion coating was analyzed comprehensively by E-t curve, SEM, EDS, XPS and XRD. In the conversion process, the titanium substrate was first dissolved (0-45 s), then Na3TiF6 nucleation occurred via a series of reactions (45-120 s), whereby the conversion coating grew steadily with the nucleation and growth of Na3TiF6 grains (120-600 s); after 600 s, the conversion coating reached dynamic equilibrium of growth/dissolution. In the drying process at 100 +/- 5 degrees C, some Na3TiF6 grains reacted with O-2 to form Na3TiOF5. The components of the conversion coating were Na3TiF6, Na3TiOF5, a little TiO2, TiF4 and phosphate derivatives.