Performance of an eco-friendly phytic acid conversion film on TC4 titanium alloy

An eco-friendly phytic acid (PA) conversion film was developed to enhance the marine durability of Ti-6Al-4 V titanium alloys (TC4) by improving galvanic corrosion resistance and organic coating adhesion. Optimal formation parameters were identified through orthogonal experiments. The film exhibited a dense, mud-cracked morphology (6 μm thick) with a composition of TiF₃ (6.7%), TiOF₂ (4.6%), TiO₂ (8.7%), TiF₄ (9.4%), AlF₃ (16.7%) and PA-Ti amorphous matrix (53.9%). The film-forming process was divided into three stages: dissolution of the substrate, growth of the film, and the dynamic equilibrium of the film growth/dissolution. After PA film treatment, the galvanic corrosion current density with coupled metals aluminum alloy (AA6061: 0.120 μA/cm 2 ), brass (H59: 0.036 μA/cm 2 ) and steel (Q235: 0.093 μA/cm 2 ) was reduced by 1–2 orders of magnitude, and the organic coating adhesion increased by 99.54%. Following saltwater immersion, adhesion decreased by only 9.95%. This work provides a practical green pretreatment strategy for titanium alloys. • Organo‑phosphorus film was prepared via a novel phytic-acid-based route. • Formation mechanism of phytic acid conversion film was studied. • The film structure was a blend of amorphous PA-Ti and fluoride crystal. • Good adhesion of phytic acid film to paint before and after saltwater immersion. • Excellent galvanic corrosion resistance of phytic acid film.