| Aluminum and aluminum alloys have the advantages of low density,high specific strength and recyclability,and are widely used in the fields of aerospace,automobile and high-speed railway trains.However,aluminum and aluminum alloys have high chemical activity,and are prone to corrosion in humid environments.Hydrotalcite with unique interlayer intercalation and anion-exchange,has been widely prepared into a conversion film for corrosion resistance of aluminum and aluminum alloys.In recent years,many researchers have developed many corrosion inhibitors modified hydrotalcite conversion coatings,such as sodium 8-hydroxyquinoline modified magnesium-aluminum hydrotalcite conversion coating,vanadate modified zinc-aluminum hydrotalcite conversion coating,all of which show good corrosion resistance for the aluminum alloy.However,when preparing these hydrotalcite conversion coatings modified with corrosion inhibitors,the reaction temperature is too high and the reaction time is too long,which violates the current concept of sustainable development.In addition,the corrosion inhibitors used in the preparation of these hydrotalcite conversion coatings are not environmental-friendly.For example,sodium 8-hydroxyquinoline are toxic to some extent,which will cause cancer in people,and vanadate compounds will pollute the natural environment,which will destroy the balance and development of the ecosystem.Therefore,it is necessary to prepare a hydrotalcite conversion coating modified with an environmental-friendly corrosion inhibitor in a low temperature and a short time.In this paper,Schiff base was used as a corrosion inhibitor,and the lithium-aluminum hydrotalcite conversion coating modified with a Schiff base was prepared on the surface of aluminum alloy by one step in-situ synthesis,and its properties were studied.This research work is mainly divided into the following two parts:?1?Study on the synthesis and corrosion inhibition of Schiff base—Disodium vanillin L-aspartic acid.Vanillin and L-aspartic acid were used as the main synthetic materials,and anhydrous methanol was used as the reaction solvent.Disodium vanillin L-aspartic acid was prepared by dehydration and condensation reaction.In 1H NMR,a single peak of H of H-C=N-was observed at 9.25 ppm,while no single peak of H of H-C=O in vanillin was observed at 9.92 ppm.Fourier transform infrared spectroscopy?FT-IR?shows that there is a stretching vibration absorption peak of-H-C=N-at 1656 cm-1,but there is no stretching vibration and bending vibration absorption peaks of-NH2 and stretching vibration absorption peaks of carbonyl at 3020 cm-1,1510 cm-11 and 1670 cm-1,respectively,confirming that disodium vanillin L-aspartic acid was successfully prepared.The results of field emission scanning electron microscopy?FE-SEM?and potentiodynamic polarization test?Tafel?showed that disodium vanillin L-aspartic acid had the excellent corrosion inhibition for the surface of A6N01-T5 aluminum alloy in 3.5wt.%NaCl solution.When the concentration of disodium vanillin L-aspartic acid in the solution was 120 mg·L-1,icorrorr of the aluminum alloy was 2.37×10-77 A·cm-2,and Ecorrorr was-0.837 V,?reached 73.8%.Langmuir adsorption isotherm was used to study the adsorption of vanillin L-aspartic acid anions on the surface of aluminum alloy.This result showed that the Kadsds of vanillin l-aspartic acid anions was 12513.24 L·mol-1,the?Gadsds was-33884.79 J·mol-1,and the vanillin l-aspartic acid anions had both physical and chemical adsorption on aluminum alloy surface.Vanillin L-aspartic acid anions can adsorb on the surface of aluminum alloy both physically and chemically.Based on the above results,the corrosion inhibition mechanism of vanillin L-aspartic acid anions in 3.5 wt.%NaCl solution was analyzed and summarized.?2?Lithium nitrate and lithium hydroxide are used as the main raw materials and disodium vanillin L-aspartic acid are used as a corrosion inhibitor.Lithium-aluminum hydrotalcite conversion coating modified with disodium vanillin L-aspartic acid?VLDH conversion coating?was prepared on the surface of aluminum alloy by one step in-situ synthesis.X-ray diffraction?XRD?showed that?0 0 3?characteristic diffraction peak moved toward direction of low angle,causing the spacing of lithium-aluminum hydrotalcite?NLDH?increased to a certain extent.The stretching vibration absorption peak of C=C of benzene ring in the Fourier transform infrared spectroscopy?FT-IR?spectrum of VLDH conversion coating.The energy spectrum peaks of C-C/C=C/C-H,C-O-C and C=O were found at 284.79 eV,286.32 eV,and 288.30 eV in the X-ray photoelectron spectroscopy?XPS?spectra of VLDH conversion coating,respectively.Compared with the NLDH conversion coating,the thickness of nanosheets in the VLDH conversion coating was increased in the images of field emission scanning electron microscopy?FE-SEM?.The energy spectrum peak of C was found in the EDS spectrum of VLDH conversion coating.The above results indicated that vanillin L-aspartic acid anions were successfully intercalated into the interlayer of Lithium-aluminum hydrotalcite,and VLDH conversion coating was formed on the surface of aluminum alloy.For the performance study,the results of elemental analysis?EA?and EDS spectra confirmed that NLDH and VLDH conversion coatings had the excellent anion-exchange property for the interlayer of NLDH and VLDH in 3.5 wt.%NaCl solution.FE-SEM,electrochemical impedance spectroscopy?EIS?and potentiodynamic polarization test?Tafel?were used to investigate the corrosion resistance of NLDH and VLDH conversion coatings for the aluminum alloy in 3.5 wt.%NaCl solution.According to FE-SEM images,after immersion for 5 d in 3.5 wt.%NaCl solution,the NLDH conversion coating was seriously damaged,while the VLDH conversion coating remained intact.Based on the results of EIS and Tafel,the aluminum alloy coated with VLDH had larger Rctt and lower icorrorr than the aluminum alloy coated with NLDH at the same immersion time.The above results showed that the corrosion resistance of the NLDH conversion coating for the aluminum alloy was significantly improved after the modification of disodium vanillin L-aspartic acid. |
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