| The traditional phosphating and chromating can meet the technical requirements of metal coating, but both technologies have their own defects. Both processes are complex and not environmental protection and chromating has been forbidden. Silane treatment and nano ceramic zirconium salt treatment are economic, environmental and efficient processes before coating which many people are looking for to replace phosphating. But both technologies also have their limitations. The hydrolyzation of silane coupling agents is unstable; the corrosion resistance of silane film is poor and silane films don’t have self-healing capability. The adhesion between zirconium salt treating film and organic coating is not as good as silane film phosphating film, its corrosion resistance is poor too. Therefore, both new technologies had been modified or combined each other through a variety of ways by many researchers to eliminate respective disadvantages. But the domestic researches and explorations of the complex processing about both technologies are mostly focus on the property of the treatment fluid, the advantage of complex processing and the microcosmic structure of membrane layer. The research on anticorrosion mechanism and concrete experimental data about the silane hydrolysis and complex processing is few.In this article, the best hydrolysis process of KH550 silane which include the best ratio of hydrolysate, pH, hydrolysis temperature and time is studied by conductivity test on the basis of abundant experiments. The best ratio and the preparation route of silicon zirconium compound processing fluid is determined with the method of orthogonal experiment. The best film-forming process conditions including pH, soaking time, temperature, stop time and drying methods are determined by using the method of single factor experiment. The microcosmic structure of silane film, zirconium film and complex processing film are compared by using metallographic microscope and SEM. The corrosion resistance, adhesion and thermal performance of films above are researched and compared with the methods of copper sulfate spot test, electrochemical analysis test, immersion test, the neutral salt spray test, coating adhesion test and thermal performance test. Besides, the formation and anticorrosion mechanisms of complex processing film are also discussed.The best hydrolysis process of KH550 silane is studied on the basis of abundant experiments. The best ratio of hydrolysate V (KH550):V (ethanol):V (deionized water) is 15:20:65. The hydrolysis time is 12 hours and control within 24 hours, pH can control between 8 and 10 and hydrolysis temperature can control between 20℃ and 40℃.The preparation route of silicon zirconium compound processing fluid is that the hydrolyed KH550 is dropwised add to the zirconium fluoride acid system until the mixture solution pH value is 3, then adjust pH with hartshorn. This study indicates that the basic compositions of zirconium fluoride acid system are:50ml/L zirconium fluoride acid; 5g/L tartaric acid; 3g/L sodium nitrate; lOml/L hydrofluoric acid; 15ml/L hexafluorotitanic acid; lOml/L fluoboric acid. The optimum process parameters is:pH 3.5, film-forming temperature 35℃, dipping time 5 minutes, stop time 15 s, hair dryer dry.It can be seen that silane treatment and silicon zirconium compound treatment can form amorphous films on the surface of cold rolled steel, but zirconium salt treatment forms a structure of nano crystalline coating from microstructure. The complex film without any crack has the best appearance quality, because silane film and zirconium film have micro crack. Si and Zr are the main compositions of complex film, which indicate that silane and zirconium fluoride acid are involved in film-forming reaction. The results of spot test, electrochemical analysis test, immersion test, the neutral salt spray test, coating adhesion test and thermal performance test indicate that complex film has the best corrosion resistance, adhesion and thermal performance of all.In this article, the formation process and corrosion mechanism of complex coating are discussed and analyzed. We can know that the ZrO2 nanoparticles which come from zirconium fluoride acid hydrolysis are packaged by three-dimensional mesh structures (Si-O-Si) which come from KH550 silane hydrolysis to deposited collaboratively on the surface of metal. The film-forming process shows that the structure of complex film is that inorganic amorphous oxide mixtures mix with organic three-dimensional mesh structures. The corrosion mechanism shows that the pores of three-dimensional mesh structures and microscopic cracks are filled with nanometer ceramic particles, which can improve the integrity and comprehensive performance of the complex film. |
