| Hot-dip galvanized sheet is widely used in automobiles,home appliances,construction and other industries due to its high corrosion resistance,low cost and good processing performance.However,hot-dip galvanized sheet is prone to corrosion in a humid environment,so in order to increase the service life of the hot-dip galvanized sheet,passivation treatment is essential before serving.The traditional passivation process is mainly chromate passivation,due that hexavalent chromium is a toxic substance,the use of the chromate passivation process has been severely restricted in recent years,and a non-toxic,environmentally friendly chromium-free passivation has been developed.The process has become the main research focus for hot-dip galvanized steel sheets.In this paper,the inorganic-organic composite chromium-free passivation process of hot-dip galvanized sheet is taken as the research object,and the chromium-free passivation process using inorganic-silane-resin composite is determined through preliminary research and screening.The formulation of the passivation solution and the corresponding passivation process parameters were optimized by orthogonal test design and single factor test method.The optimal formulation of the passivation solution is(100 g solution):the mass of the silane coupling agent KH-151 is 2.5 g,the mass of KH-560 is 3.5 g,the mass of fluorotitanic acid(H2TiF6)is 0.9 g,The mass of ammonium metavanadate(NH4VO3)is 0.07 g,the mass of the resin styrene-acrylic emulsion is 30 g,the mass of the organophosphate polymer is 0.25 g,the mass of the SDS is 0.05 g,and the mass of the op-10 is 0.4 g.The optimum passivation process parameters are:the pH of the passivation solution is 5,the drying temperature is 150℃,and the drying time is 60 s.The performance of the inorganic-organic composite chromium-free passivation film was tested according to the method of detecting the passivation film performance in the actual industrial production process,the results show that the corrosion area of the chromium-free passivation film after 96 h neutral salt spray test is less than 5%,which is in line with industrial application standards,moreover,and the passivation film’s paint adhesion,heat and humidity resistance,heat resistance,solvent resistance,boiling water resistance,fingerprint resistance,acid and alkali resistance can meet the requirements of the market and related manufacturers for the performance of the passivation film.The surface morphology of the passivation film was observed by scanning electron microscopy(SEM)and atomic force microscopy(AFM),it was found that a dense and uniform passivation film is formed on the surface of the hot-dip galvanized sheet treated by the inorganic-organic composite chromium-free passivation solution.X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)were used to detect the passivation film,and it was found that there were ZnO,Zn(OH)2,SiO2,etc.in the passivation film,and there are also groups such as C=O,Si-O,C-Si,CH,Si-O-Zn,Si-O-Si.Through various tests on the inorganic-organic composite chromium-free passivation film,many reactions occurred during the passivation film formation,mainly including the polycondensation reaction between the silane coupling agents and the dehydration reaction between the silane coupling agent and the hot-dip galvanized sheet,the occurrence of these reactions causes the passivated hot-dip galvanized steel sheet to form a uniform,dense and internally stable passivation film on the surface.The presence of the passivation film hinders the penetration of the corrosive medium into the hot-dip galvanized sheet,suppresses the occurrence of the corrosion reaction,and significantly improves the corrosion resistance of the hot-dip galvanized sheet. |
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