| To meet the requirements of high quality and performance of electrogalvanized steel sheet, a new surface treatment which is called anti-fingerprint treatment has been developed in recent years. The anti-fingerprint plate has been widely used in automotive, aerospace, electrical and domestic areas due to its high added value. However, the anti-fingerprint treatment baths are usually prepared with hexavalent chromium, which are carcinogenic and harmful to human health and environment. Nowadays , there is no non-chromium anti-fingerprint treatment which can substitute chromium treatment. Therefore, new alternative and more environmentally friendly anti-fingerprint treatments need to be developed.Anti-fingerprint treatment baths are composed of inorganic and organic parts. The research idea was to study the components of friendly anti-fingerprint treatment for electrogalvanized steel (EG) plates separately. Firstly, inorganic oxidants were studied. The vanadate conversion coating as chromate replacement was prepared on EG plates previously treated in a solution mainly composed of vanadate in this paper. Influences of pH value, film forming temperature, film forming time and vanadate concentration on the conversion coating were studied by the single factor experiments respectively. Next, the preparation process was determined through the orthogonal experiments. The optimum processing parameters of vanadate conversion coating were confirmed as follows: vanadate solution concentration is 30g/L, pH value of solution is 6, treating temperature is 20℃, and treating time is 10 min. The morphology, composition, structure and corrosion resistance of the coating were investigated by using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), X-ray photoelectron spectroscopy (XPS) and neutral salt spray test (NSS) respectively. The film mainly consists of vanadium and oxygen, which mainly exist as the oxides or its hydrates of V5+ as well as a small amount of oxide or its hydroxide of V4+ and Zn2+ according to the XPS results. The formation process and mechanism of the conversion coating were proposed by using the results combining the electrochemical measurements. The formation process can be divided into three steps: dissolution of zinc, stable film formation and dissolution of film. The possible formation mechanism was proposed that the formation of the vanadate conversion coating is cooperated with the dissolution of zinc substrate and the deposition of conversion coating. The corrosion behavior and failure mechanism of vanadate conversion film were in depth investigated by using EIS with the film immersed in 3.5wt% NaCl solution for different times.Performances of the coating were further optimized by using vanadate/organic resin composite system (namely anti-fingerprint treatment system). Water-based acrylic resin was selected as the film-forming agent, mixture of L-ascorbic acid and vanadate as a mixed oxidant, nano-SiO2 as a barrier agent, silane coupling agent KH560 as an interface modifier and phosphoric acid as acidity regulator. The surface morphology, composition and corrosion resistance of the complex coating were studied respectively. The results showed that there was a more flat morphology for the vanadate/organic resin composite coating and corrosion resistance of the complex coating was much better than that of the similar products. Inorganic additives such as nano-SiO2 and vanadate were connected to the water-based acrylic resin by the molecular bridge of silane coupling agent; the methods of NSS and quantum chemical calculation were used to describe this process. Finally, the morphology, structure, corrosion resistance and its formation process of the composite coating were investigated by SEM, XPS, NSS and EIS.Silane coupling agent has been widely used in the the field of surface treatment of metals, based on its unique structure. Firstly, researches on the hydrolysis of three silane coupling agents (KH171, KH570 and KH560) were carried out in this paper. The hydrolysis process of silane was characterized using the method of conductivity, which indicated that the process was comprised of two hydrolysis steps. The reasonable concentration of silane solution was determined by the electrochemical tests. The results of hydrolysis showed that the silane solution was more stable after the second hydrolysis, and corrosion resistance of the coating prepared from KH560 solution was the best among three solutions. Secondly, a two-step method was used to prepare the vanadate/silane composite coating. The silane film was firstly made on the surface of zinc substrate, and then the sample was immersed in vanadate treatment solution. The morphology, composition and structure of the composite coating were investigated by SEM, XPS and infrared spectra (IR), and corrosion resistance of the coating was analyzed by NSS and electrochemical tests. The surface micro-cracks of composite coating were significantly reduced compared with that of simple silane treated film. XPS and IR results showed there were some chemical bonds between the silane film and the zinc substrate. Corrosion test results showed that the better microstructure of the composite silane film had more excellent corrosion resistance. The corrosion resistance results of films with different immersion time in 3.5wt% NaCl solution showed that the failure process can be divided into three stages: chloridion absorption, conversion coating dissolution and penetration into the coating. Different equivalent electrical circuits were used to explain the structure of the composite film.
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