Study Of The Layered Double Hydroxides And Magnesium Hydroxide Modified Coatings On Magnesium Alloys

Magnesium alloys are widely used in automotive,aerospace,and electronic products due to their low density,high specific strength,and excellent processing properties,but their excessively fast corrosion rates have limited their applications on a large scale.In order to improve the corrosion resistance of magnesium alloys,surface treatments are generally adopted on magnesium alloys prior to application.In this work,two kinds of composite coatings were prepared to improve the corrosion resistance of magnesium alloys:(1)The magnesium hydroxide coating was prepared on the surface of AZ31 magnesium alloys by hydrothermal method,and then composite coatings,including different concentrations of ceria/polymethyltrimethoxysilane and magnesium hydroxide coating,were obtained by soaking.(2)Magnesium hydroxide coating was prepared on the surface of AZ31 magnesium alloy by hydrothermal method,followed by soaking in different concentrations of dodecanethiol solution to obtain a modified composite coating.In addition,this article also explored the corrosion inhibition effect of vanadium hydrotalcite as a corrosion inhibitor on magnesium alloys.X-ray diffractometer(XRD),Fourier transform infrared spectroscopy(FT-IR),Raman spectroscopy(Raman),field emission scanning electron microscopy(FE-SEM),contact angle tester(CA),and X-ray photoelectron spectrometer(XPS)were used to characterize the morphologies,microstructure and elemental compositions of the composite coatings;and the corrosion resistance and corrosion behavior of the coatings were studied by electrochemical tests,hydrogen evolution tests,and immersion tests.The results indicated that the structure of the magnesium hydroxide coating prepared by the hydrothermal method is compact and effectively improves the corrosion resistance of the AZ3 1 substrate in 3.5 wt.%NaCl solution.The ceria/polymethyltrimethoxysilane modified magnesium hydroxide composite coating has hydrophobic properties.When the additive amount of cerium nitrate is 10-3 M,the contact angle of the composite coating is 151°,achieving superhydrophobic effect.At the same time,electrochemical tests show that the corrosion current density of the composite coating is reduced by three orders of magnitude compared to substrate.Hydrogen evolution and immersion experiments showed that the composite coating can provide long-term effective protection for the substrate.The coating formation mechanism and corrosion resistance mechanism of the composite coating were thus constructed.The 1-dodecanethiol modified magnesium hydroxide composite coating also has a hydrophobic effect.When the concentration of 1-dodecanethiol solution is 10 mM,the contact angle is 1050 and corrosion current density is 9.01×10-8 A·cm-2,which is three orders of magnitude lower than the matrix.At the same time,the corrosion resistance mechanism of 1-dodecanethiol modified film was discussed.In addition,vanadate layered double hydroxide has obvious corrosion inhibition effect on magnesium alloys.Compared with magnesium aluminum and zinc aluminum vanadate layered double hydroxides,zinc aluminum vanadate hydrotalcite had better sustained release effect,and its corrosion inhibition mechanisms were explored.ZnAl-VOx-LDH could provide a longer and better protection for AZ31 Mg alloys due to the synergism of Zn2+ and VOx-.