Construction And Performance Study Of Superhydrophobic Surface On Light Alloys

As the most widely used light alloy structural metals,aluminum and magnesium alloys are chemically active and easy to corrode.In order to improve the corrosion protection of light alloys,appropriate surface treatment is necessary.Among surface treatment technologys,Superhydrophobic protective coating,as a promising surface technology,has opened up a new platform for corrosion protection of light alloys by minimizing the contact area between the corrosive medium and metal substrates.However,these fabrication methods are somewhat complicated or using more chemical reagents,even harmful to the environment,in addition,the durability and stability of as-prepared protective coatings is not ideal.To solve the above problems,two simple,less chemical reagents required and environment-friendly processes,rare-earth-salt-solution boiling bath-stearic acid modification and F/Zr chemical conversion-boiling water sealing-stearic acid modification,are proposed to facilely construct superhydrophobic coating on light alloys.The surface wettability,morphology and chemical composition of superhydrophobic cotings were characterized by video contact angle measurement(OCA),field-emission scanning electron microscopy(FE-SEM),energy dispersive spectroscopy(EDS),fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD),according to these characterization results,and the forming mechanism of superhydrophobic coatings was explored.The corrosion resistance of the superhydrophobic coatings was investigated by electrochemical methods and corrosion medium immersion test;the high-speed droplet impact test to verify its water-impact resistance;the simulated contaminants test to explore the self-cleaning property;and laboratory simulation(towards chemical medium,UV irradiation,high temperature,abrasion,boiling water)tests and natural atmospheric storage to demonstrate the high stability and durability of the as-prepared superhydrophobic coatings.At last,the inorganic fluoride/stearate superhydrophobic composite coatings were found to achieve reversible wettability conversion in circular"UV irradiation-vacuum storage",and the wettability conversion behavior and mechanism of the composite coating in 5 cycles experiments were explored.The main research results are as follows:(1)The rare-earth-salt-solution boiling bath-stearic acid modification method is used to facilely construct superhydrophobic protective film on high strength aluminum alloy.The as-prepared superhydrophobic surface exhibits a high WCA of 157.5°;the FE-SEM images of the superhydrophobic film presents hierarchical nanostructures with nanofibers and dandelions-shaped nanospheres;the XPS and FTIR characterization results confirm the superhydrophobic film is mainly composed of aluminum stearate and samarium stearate,which greatly reduces surface energy and promotes superhydrophobicity of the surface.Electrochemical Impedance Spectroscopy results show that the resistance of the superhydrophobic film is more than 400 k?cm~2,indicating the strong protection for metals.Various laboratory simulation tests toward chemical medium,UV irradiation,high temperature and abrasion demonstrate the high stability of the as-prepared film as well as their robustness to harsh environments.(2)A F/Zr chemical conversion-boiling water sealing-stearic acid modification process was developed to construct an inorganic fluoride/stearate superhydrophobic composite coating on high strength aluminum alloy.The as-prepared coating exhibits a high WCA of 159.3°and low WSA of 1.0°;the FE-SEM images show that the superhydrophobic surface has a layered structure of“rice leaf”top layer and nanosphere bottom layer.The process has a certain universality and can be extended to the magnesium alloy.The as-prepared superhydrophobic coating exhibits a high WCA of160.4°and low WSA of 1.0°on magnesium alloy;the FE-SEM images show a dense nanofiber network structure.The XPS,FTIR characterization results reveal that the stearic acid is successfully bonded to the composite coating to form stearate,which greatly reduces surface energy and promotes superhydrophobicity of the surface.Corrosion medium immersion and simulated pollutants tests demonstrate the excellent anti-corrosion and self-cleaning properties of superhydrophobic composite coatings;the high-speed droplets can completely rebound and not destruct the composite coating,which indicates its water-impacting resistance;a series of laboratory simulation tests(including chemical media,high temperature,boiling water)and the natural atmospheric environment dispaly the high stability and durability of superhydrophobic composite coatings.(3)We have proposed and adopted a“UV irradiation-vacuum storage”stimulating method to achieve a reversible superhydrophobicity to superhydrophilicity transition on the as-prepared inorganic fluoride/stearate superhydrophobic composite coating surface.After 5 cycles of experiments,the surface can return to a superhydrophobic state.According to the analysis results for the surface wettability,morphology and composition of the composite coating in periodic“UV irradiation-vacuum storage”process,we speculated the mechanism of reversible wettability conversion:in the case of keeping its initial morphology unchanged,the wettability conversion of the superhydrophobic composite coating is mainly regulated by degradation and adsorption of low surface energy substances.In addition,after UV irradiation,the surface may generate oxygen vacancies and the surface polarity of the inorganic fluoride,which is more conducive to the adsorption of water molecules and promotes the hydrophilicity of surface;after vacuum storage,oxygen adsorption gradually replaces water adsorption,which facilitates the surface back to a hydrophobic state.The two synergistic effects promote the reversible wettability conversion of the inorganic fluoride salt/stearate superhydrophobic composite coating.