Study Of Titanium Conversion Coating On Hot-dip Galvanized Coating

Chromates are widely used in anticorrosive treatments for zinc and zinc coating. However, hexavalent chromium presents very high toxicity and carcinogenic nature, and has a bad impact on environment. For this reason, an intense research effort is being undertaken to replace chromates passivation by new more environmentally friendly non-chromate treatments. In this paper, Ti(SO₄)₂ is chosen as a main salt of the titanium salt solution and the optimal process parameters of the titanium salt conversion coating for hot-dip galvanized (HDG) steel has been systematically studied. Then phytic acid was added into the titanium salt solution as a compositon of the coating. The HDG samples were dipped into the improved titanium salt to obtain the improved titanium conversion coating, which has a better corrosion resistance. The process technologies, microstructure, growth and corrosion resistance of the single titanium salt conversion coating and improved titanium salt conversion coating on HDG steel were systematically investigated by SEM, EDS, XPS, IR, electrochemical polarization, electrochemical impedance spectroscopy (EIS), neutral salt spray (NSS) and immersion test. Moreover, the self-sealing abilty of the improved titanium salt conversion coating were studied by NSS test after scratching.The optimal process parameters of the single titanium salt conversion coating are as follows: Ti(SO₄)₂ 1g/L,H₂O₂ 60mL/L(w=30%),pH 0.5～1.0,T 25～30℃,t 10min。The optimal concentration of phytic acid additives to the titanium salt solution is 20 mL/L.It is shown that the thickness of the single titanium salt conversion coating increase with the increase of treatment time, microcracks are visiable in the conversion coating which is treated for 10min. After treated for 10 min, the microcracks will gradually occupy on the whole surface of the conversion coating and warps will appear, the cracks and warps will lead to the decrease of the corrosion resistance of the conversion coating. The improved titanium salt convertion coating may grow faster with the addition of phytic acid. The continuous, compact and complete convertion coating is formed on the surface of zinc when treatment time is 1 min. After treated for 10 min, there are cracks in the coating, which broaden with increasing treatment time. The growth of the improved titanium convertion coating is similar with the single titanium convertion coating.The results of XPS analysis show that the single titanium conversion coating is composed of Ti(OH)₄/TiO₂ and Zn(OH)₂/ZnO which contains crystal water, and Ti exists as TiIII in the coating. IR and XPS analysis results show that phytic acid inhibitor engages in the formative process of the conversion coating after the phytic acid inhibitor has added into the titanium salt solution, the improved titanium conversion coating is composed of phytic acid salt and Ti(OH)₄/TiO₂ and Zn(OH)₂/ZnO which contains crystal water.The results of NSS analysis show that the corroded area of the improved titanium salt conversion coating reaches 5% in the 8th cycle while the single titanium salt conversion coating reaches the same in the 6th cycle and the HDG in the 1st. Therefore, the corrosion resistance of the improved titanium salt conversion coating is more outstanding than that of the single titanium salt conversion coating.The results of electrochemical test show that the two coatings can control anode and cathode reaction. The corrosion resistance of the improved titanium salt conversion coating is greatly improved. Compared with HDG, the corrosion current density of the improved titanium salt conversion coating decrease from 9.020μA·cm^-2 to 0.118μA·cm^-2, the polarization resistance increase from 1.043 kΩ·cm² to 66.83 kΩ·cm², the impedance resistance increase from 1 kΩ·cm² to 41 kΩ·cm², and its corrosion resistance is superior to the single titanium salt conversion coating.The corrosion process of the improved titanium salt conversion coating is divide into three stage: electrolyte solution diffuse into the conversion coating as the dominate reaction at the early stage; the conversion coating is destroyed and electrolyte solution diffuse into the coating/zinc layer interface during the middle stage; the corrosion of zinc become the dominate reaction at the last stage.The scratching corrosion test shows that the improved titanium salt conversion coating have good self-heal ability.