Study On Color Passivation Process And Coloration Mechanism Of Electro - Galvanized Chromium - Free Titanium Salt

Electro-galvanized process is one of the main means to protect metallic materials corroding, zinc layer was passivated with chromate can be obtained decorative superb conversion coating, and it have the better color and corrosion resistance. However, due to the passivation media and product components containing hexavalent chromium which harm human health and pollute the environment. Many countries of the world have promulgated a series of relevant regulations strictly to limit the use and emissions of hexavalent chromium. Color passivation process that are commonly used in industry is trivalent chromium passivation process, it is relatively low toxicity compared with hexavalent chromate color passivation film. But trivalent chromium passivation film has the poor stability and easily convert to hexavalent chromium in the process of transportation, placement and use, it does not solve the problem of pollution fundamentally which is caused by chromate color passivation process.This article select the titanium conversion coating which is cognate with chromium, and study its color passivation process. Optimized the concentration of the components in the basis formulation and the parameter of process conditions of the color titanium passivation process through designing the single factor experiments and orthogonal experiments, and finally obtained the best formula and process conditions parameters of the color titanium passivation process as follow:TiCl3 8mL/L, H2O2 10mL/L, NaN0315g/L, C6H11O7Na 4g/L, pH= 1.5, passivated time:25s, passivated temperature:30℃, drying method:drying at the constant temperature of 60℃. Color titanium passivation films obtained at the above formulation and process condition parameters visually present the color of red-green-doped, colorful and good brightness. The primary to the secondary effect which the chemical compositions to the appearance and corrosion resistance of the color passivation films as follows:TiCl3, H2O2, NaNO3, C6HnO7Na.The results of SEM show that the color passivation film formed on the galvanization surface present mesh structure, and the film surface smooth, dense and crack-free. It shows that the film is mainly composed of titanium dioxide. Zinc oxide and zinc hydroxide composition by XPS spectra analysis. Electrochemical test results confirmed that the passivation film can effectively protect the base material from corrosion. The results of Neutral salt spray test show that it’s corrosion resistance is closed to the color trivalent chromium passivation film.The theoretical analysis shows that the chemical elements in the film and external light interference produced the color of passivation film. The XPS and argon ion etching data analysis shows that the color of the film are mainly caused by the titanium atoms and it’s compounds occur the d-d transitions. Finally, the color film are quantified characterization preliminarily through the introduction of the RGB color model.