Preparation And Anticorrosion Application Of Tungstate Micro/Nanopowders

The synthesis and preparation of nanomaterials is the base of the application and the future development of nanoscience and nanotechnology, and how to develop new way and novel method for preparing nanomaterials is still an important task. Tungstate micro/nanopowders have good properties such as chemical stability, hard wearing, anticorrosion and nontoxicity, etc. But their applications were limited by disadvantages of the existing synthesis techniques such as high temperature, high energy consumption, high cost and high requirements for instruments. In this dissertation,two simple and convenient routes with low-cost and high yield , i.e. low temperature molten salt synthesis and electrochemical synthesis were proposed to realize the control synthesis of tungstate micro/nanopowders. The products were characterized by XRD, SEM, TEM, UV-VIS and PL spectra techniques, and were then used to modify epoxy resin paint. The anticorrosion property of modified epoxy coatings was initially investigated by electrochemical impedance spectroscopy (EIS) technique. Main points are summarized as follows:1. Nickel, cobalt and manganese tungstate nanopowders were prepared by low temperature molten salt method, respectively. The effects of temperature, salt quantity and holding time on the crystallization, development and optical properties of nanocrystals were studied.Experimental results showed that nickel tungstate and cobalt tungstate nanopowders with ca. 30 nm and 45nm in diameter could be prepared at 270℃for 8h with 6:1 mass ratio of the salt to precursor. UV-VIS spectrum revealed that nickel tungstate nanopowders had better light absorption properties in ultraviolet light region than that of cobalt tungstate nanopowders. Similarly, MnWO4 nanopowders with ca. 50nm in diameter were prepared at 270℃for 4h with 3:1 mass ratio of the salt to precursor, exhibiting absorption in UV and visible light region, and with the increase of molten salt quantity and holding time, the morphology of some nanoparticles changed from spherical particles to stripe-like particles.In molten salt system, the growth of tungstate nanopowders followed the dissolvation-crystallization mechanism, and the formation of ball-shape nanoparticles was determined by the diffusion process, whereas the formation of strip-like nanoparticles was determined by the interface mechanism.2. Nickel tungstate (NiWO4) nanopowders were modified using stearic acid in order to change its surface performance from hydrophilicity to hydrophobicity. After that, the NiWO4 nanopowders were adopted to modify epoxy paints. The corrosion protection of modified epoxy coatings on Q235 steels was investigated by utilizing EIS technique. The continuous evolutions of impedance of coated-metal systems under different deterioration condition were measured, and four electrical equivalent circuits corresponding to different immersion stages were established to fit the EIS data. The coating performance as a function of immersion time was obtained from the fitted results. The results indicated that NiWO4 nanopowders could not only improve the anticorrosion performance of the coatings, but also have the ability of shielding UV to some extent, which slowed down the aging of modified epoxy coating. The optimal dosage is 3% mass percentage of epoxy resin.3. Barium tungstate (BaWO4) micro/nanopowders were successfully synthesized via electrochemical method. Effects of electric current value, pH value of electrolyte solution, Ba2+ concentration and electrolyte component on the crystallization, morphology, size, development and optical properties of BaWO4 crystallites were studied.Experimental results showed that the growth of flower-, fishbone- and taper-like crystallites were inhibited effectively under a condition of 20℃, 0.5A, with Ba2+ concentration of 0.05M and pH of 12.0, which is beneficial to the formation of well-crystallized BaWO4 crystallites with uniform octahedral morphology and particle size of 2μm. With the increase of absolute ethanol in the electrolyte solution, the growth of octahedral BaWO4 crystallites were inhibited, and with the presence of KOH in the electrolyte solution, the preferred orientation growth of BaWO4 crystallites would be promoted. In addition, BaWO4 nanopowders with diameter of ca. 60nm was prepared at 1A in 3 minutes at 20℃, with Ba2+ concentration of 0.05M and pH of 12.0, and the volume ratio of absolute ethanol to distilled water is 2:1. The nanopowders exhibited an excellent PL property and displayed a very strong PL peak at 440nm with the 380nm excitation wavelength, showing a little blue-shift compared with that of micropowders.4. Epoxy coating modified by BaWO4 powders was prepared and its anticorrosion mechanism was initially studied by EIS technique. Results showed that the anticorrosion behavior of the coatings modified by nanopowders is better than that of the coatings modified by micropowders. And the optimal dosage of BaWO4 nanopowders is 5% mass percentage of epoxy resin.