Study Of Silicate Conversion Coating On Hot-dip Zn-5%Al Steel

The hot-dip Zn-5%Al alloy coating has more excellent performances than the HDG?hot-dip galvanizing?,such as better processing performance,brighter appearance quality and stronger corrosion resistance.Its brilliant corrosion resistance,2 to 3 times that of the HDG in the atmospheric environment,makes possible to be applied in a wider field.However,the Zn-5%Al alloy coating still has the same reaction characteristics as the galvanized zinc coating,so that it is prone to corrosion during shippage and storage in a wet and poorly ventilated enviroment.In order to improve the corrosion resistance of the coating,passivation treatment is usually used.At present,the main treatment method for zinc,aluminum,magnesium and their alloys are passivated by hexavalent chromate passivation.However,the chromate treatment on the surface of HDG is severely restricted due to the serious harm to the environment and highly carcinogenicity of Cr???.There have been a lot of studies on silicate conversion coatings on the surface of zinc,aluminum,magnesium and their alloys due to its friendly environment and rich resource.However,very few works have reported on the study of silicate conversion coating prepared on the hot-dip Zn-5%Al alloy coatings.In this paper,silicate conversion coatings were prepared on hot-dip Zn-5%Al coatings.The effects of passivation time,passivation temperature,solution modulus?SiO₂:Na₂O molar ratio?and drying temperature on the corrosion resistance of the coatings were analyzed by orthogonal tests.Scanning electron microscopy?SEM?and energy dispersive spectroscopy?EDS?,Fourier transform infrared spectroscopy?FT-IR?,and X-ray photoelectron spectroscopy?XPS?were used to study the morphology and composition of silicate conversion coating prepared on hot dip Zn-5%Al coatings with different SiO₂:Na₂O molar ratio?SiO₂:Na₂O=2.0,3.0,3.5,4.0,4.5?.Neutral salt spray test,Tafel polarization curve and electrochemical impedance were used to study the corrosion resistance of silicate conversion coatings with different SiO₂:Na₂O molar ratio.The Erosion mechanism of silicate conversion coating on hot-dip Zn-5%Al was discussed.The orthogonal results show that modulus and drying temperature have major influence on the film formation among all the processing factors.The optimum process conditions for obtaining the silicate conversion film by single factor analysis are:4.0of solution modulus,60 s of passivation time,80°C of passivation temperature,and125°C of drying temperature.The SEM results show that as the solution modulus increases,the surface of the film layer tends to be more uniform.When the SiO₂:Na₂O molar ratio is 2.0,the surface of the film layer appears a large number of cracks.At a modulus of 3.0,the surface of the film is relatively flat,but there are a few local protrusions.The surface morphology of the film obtained at 3.5,4.0,and 4.5 are similar with the feature of compactness,uniform and thinness.The XPS and FT-IR results show that the silicate conversion film on hot-dip Zn-5%Al coating is mainly composed of zinc silicate,aluminosilicate and silica,aluminum oxide/hydroxide and zinc oxide/Hydroxide.The NSS test results show that after hot dip Zn-5%Al is treated with silicate,the salt spray corrosion resistance of the coating is greatly improved.When the modulus is in the range of 2.0⁴.0,the salt spray corrosion resistance of the film increases with the increase of the modulus.When the modulus is 4.0,the corrosion resistance of the film is the best,and then decreases with the increase of the modulus.Tafel polarization results show that silicate conversion coating obtained by 4.0 of SiO₂:Na₂O molar ratio has the most effect due to suppressing the cathodic corrosion process and the anodic corrosion process.At this time,the polarization resistance R_p of the film is as high as134.17 k?·cm²,and the corrosion current density i_corr also reaches the minimum value of 0.251?A·cm^-2.EIS results show that the corrosion resistance of the hot dip Zn-5%Al coating with silicate conversion coating is greatly improved.The corrosion resistance of the film is the worst when the modulus is 2.0.As the modulus increases,the corrosion resistance gradually increases.When the modulus is 4.0,the film resistance reaches206.19 k?,and the corrosion resistance is the highest.However,the corrosion resistance of the conversion coating began to decline as continiuing to increase the solution modulus.By studying the corrosion process of the conversion film,the corrosion of the silicate conversion film on the surface of hot-dip Zn-5%Al coating in 5 wt.%NaCl solution is mainly divided into three stages:at early stage of corrosion,the film surface is smooth and can effectively block the erosion of corrosive media.At the middle stage of corrosion,there is a sharp decline in the impedance of the film.Electrochemical resistance plays a leading role to the corrosion resistance of the conversion coating.At the last stage of corrosion,Warburg diffusion resistance occurs in the electrochemical impedance of the film and plays a leading role in the corrosion resistance of the film.During this period.The substrate is corroded and corrosion products appear on the surface of the conversion coating.After immersion in 5 wt.%NaCl solution for 24 hours,the film loses the protective effect of the matrix,the surface is covered with the corrosion products.