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

Abstract

A uniform grey conversion coating with the thickness of 4–5 μm 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 °C, some Na₃TiF₆ grains reacted with O₂ to form Na₃TiOF₅. The components of the conversion coating were Na₃TiF₆, Na₃TiOF₅, a little TiO₂, TiF₄ and phosphate derivatives.