Study On Preparation And Properties Of Superhydrophobic Coatings On Hot-Dip Galvanized Steel

Hot-dip galvanizing is an effective method to improve the atmosphere corrosion resistance of steel.However,the hot-dip galvanized coating is prone to corrosion and white loose corrosion products will be easy to form the surface in a humid environment.It is necessary to further protect galvanized steel after hot dipping.In the past,Chromate passivation is a common after-treatment process for galvanized steel.However,chromate as a highly toxic carcinogen is strictly restricted to use,it is necessary to develop a chromate-free passivation process instead of chromate passivation.The superhydrophobic surface technology is of great significance to the protection of metal because of its excellent water-proofing ability,and the superhydrophobic surface technology is introduced into chrome-free passivation technology on zinc,and the preparation of chromate-free superhydrophobic film with high corrosion resistance is one of the new research directions of chromate-free passivation technology.In this dissertation,a simple and non-toxic solution immersion method was used to construct a roughness structure on the surface of hot-dip galvanized coating,and two modifiers of stearic acid?STA?and perfluorooctyltrichlorosilane?PFTS?were used to modify the surface energy of the roughness surface on zinc,and the effects and the modification mechanism of the two modifiers on the hydrophobicity of the film were compared and analyzed.The optimum anti-corrosion and hydrophobic process of the film was determined by orthogonal experiment,and the superhydrophobic film was successfully constructed on zinc.The morphology,composition and hydrophobicity of the superhydrophobic film were characterized by field emission scanning electron microscope?FESEM?,X-ray photoelectron spectroscopy?XPS?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,3D morphology analysis,differential display scanning calorimetry?DSC?,contact angle meter.The corrosion resistance of the film was studied by neutral salt spray test?NSS?,potentiodynamic polarization curve and electrochemical impedance spectroscopy?EIS?.The delay-icing property was tested by vacuum freeze-drying machine.The effects of ZnCl₂ concentration and cetyltrimethylammonium bromide?CTAB?concentration in roughening solution,roughening time and modification time on the corrosion resistance of the film were studied by orthogonal experiment.The optimum process conditions were obtained:an ethyl alcohol-ammonia?volume ratio 4:1?mixed solution contained 0.4mol/L ZnCl₂ and 0.5 g/l CTAB as the rough solution,roughening time 4 h,STA modified time20 min.The resulting film has a water contact angle of 156.0°,and the resistance to neutral salt spray test is 3 cycles.The rigid hexagonal flake roughness structure of Zn₅?OH?₈Cl₂·H₂O were obtained on zinc by Roughening 4 hours.Using PFTS and STA to modify the surface energy of rough surface respectively,it is found that the microstructure and surface roughness of the roughness surface have no changes by PFTS modified with different time,and the maximum water contact angle of the modified film is only 135°;STA modification has obvious influence on the microstructure and surface roughness of the rough surface.With the prolongation of the STA modification time,the original rigid hexagonal flake structure of the rough surface will gradually become thinner,edge-smoothed,bent,the surface of the layer appears small folds,and finally turn to the petal flake structure with small folds on the surface.At the same time,the surface roughness of the film appears to decrease rapidly and slowly increase with the increase of STA modification time.The composition analysis of the film shows that the modified film mainly consists of zinc stearate and ZnO/Zn?OH?₂,and some free STA molecules.Because the roughness structure of the zinc layer is alkaline zinc chloride,which is easy to react with stearic acid and generate zinc stearate deposited on the surface of the coating.Therefore,STA modifer not only provides the hydrophobic group to the film layer and reduces the surface energy of the film,but also reconstructs the roughness structure on zinc and makes it suitable for forming the superhydrophobic film.The results of electrochemical test of the superhydrophobic film show that the polarization current density of the film is 0.158?A/cm²,two orders of magnitude lower than that of the untreated hot-dip galvanized coating,and the initial AC impedance of the film tested by EIS is up to 7x10⁶?·cm²,which is 4 orders of magnitude higher than that of the untreated coating.With the prolongation of corrosion time,the AC impedance of the film decreases gradually,and the impedance value of the film is still 10⁵?·cm² at the corrosion time of 10 h,which shows that the superhydrophobic film has good and stable corrosion resistance.The results of freezing experiments show that the superhydrophobic film has good slow condensation and anti-freezing abilities.The experimental results of acid and alkali resistance show that the superhydrophobic film has good alkali resistance and weak acid resistance.