A Research Of Galvanized Layer Sellac Inorganic Salt Chromium-free Passivation Proess

Iron and steel materials are widely used in various areas of human life,but steel materials are susceptible to corrosion during it long-term exposure to the air.So,human coat steel materials with protective layers.The hot-dip Zinc is of great significance for the protection of steel materials,but zinc is more active and easily reacts in the air,the coating to generate a layer of corrosive substances such as"white rust",which not only greatly reduces the protective of steel,but also affects the coating.So the coating surface protective treatment of passivation process,among them,hexavalent chromium passivation with good corrosion resistance and low price is popular,but hexavalent chromium is toxic,which not only affects health but also pollutes the environment.Domestic and foreign teams are committed to research and development of chromium-free passivation instead of chromium-passivation.This subject has developed a composite passivation film that uses silicate as the main film-forming agent and adds shellac and other film-forming accelerators.Based on the research of silicate passivation technology,the silicate-shellac composite passivation film was studied by adding shellac solution.According to the dissolution of shellac,the optimal amount of shellac was selected,and the compound passivation process was optimized by single factor test and orthogonal test.The paper uses Infrared Spectrum,Scanning Electron Microscope（SEM）and X-ray Photoelectron Spectroscopy（XPS）,the microstructure and composition of the passivation film were analyzed.With the help of the lead acetate stop test,neutral salt spray test,salt water immersion test,electrochemical test and adhesion test to study the performance of the passivation film.The study of the paper reached the following conclusions:According to the research on the dissolution of shellac in acid,alkali and alcohol,and the corrosion resistance of the composite passivation film prepared by adding it to the composite passivation solution,the Infrared Spectrum analysis was used to determine the When the shellac dissolving solution was prepared by adding 500 ml n-butanol+5 g shellac to the shellac dissolving solution,the composite passivation film had better corrosion resistance.According to the test results of single-factor test and orthogonal test,the best formula of passivation solution was finally determined as:sodium silicate 22 g/L+nitric acid 7 ml/L+hydrogen peroxide 70 ml/L+n-butanol shellac solution 1.5a ml/L.After studying the influence of the passivation process conditions on the passivation film a fter studying the passivation process conditions using a single factor test,the passivation p H of the composite passivation film is set to 1.5,the passivation temperature is 55°C,the passivation time is 50 s,and the drying method is a blow dryer.It can be known from the Scanning Electron Microscope（SEM）that the surface of the composite passivation film is smoother and denser.According to the Infrared Spectroscopy,it is known that the shellac-added composite passivation film has the same hydroxyl group,aldehyde group,and-CH₂ group as shellac,which indicates that the shellac resin is present in the film component of the complex passivation film.The X-ray Photoelectron Spectroscopy（XPS）on the surface of the composite passivation film added shellac shows that the passivation film layer contains bonds such as Zn O,Si O₂,Zn（OH）₂,Si-O-H,Si-O bonds,and Si-C.The corrosion resistance test of the composite passivation film was conducted.After 72hours of the neutral salt spray test,the white rust corrosion area of the composite passivation film was 5%.White rust appeared on the composite passivation film and chromate passivation film after immersion in Na Cl salt water for 60 hours,while white rust appeared on the blank sample and single passivation film after 24 hours.Electrochemical experiments have shown that the corrosion potential of the composite passivation film is significantly positive compared to the blank sample and the single passivation film,and the corrosion current density of the composite passivation film is reduced by one order of magnitude compared to the blank sample.The polarization resistance of the passivation film is 52.6 times that of the blank sample.At the same time,the impedance data and the lead acetate spot test also show that the composite passivation film has good corrosion resistance.The silicate-shellac composite passivation film has excellent corrosion resistance,because the cross-linking synergy between the organic component shellac and the inorganic component silicate forms-Si-O-Si-Zn and-Si-O-Si-CH₂ structure,so as to form a three-dimensional network structure,and Zn O and Si O₂ monomers are filled in the voids of the network to form a dense,uniform,and excellent passivation film.