Research On The Modification Process And Mechanism Of Galvanized Coating With CeO₂ Nano-particles

Galvanizing is the most efficient and widely used measure in protecting steel from corrosion. But with the increasing possibilities of acid rain and sand-dust, the galvanized coatings are required to provide not only a higher corrosion resistance, but also a good wear resistance. Therefore, it is significantly important to research and develop novel galvanized coatings with improved corrosion and wear resistances. Combining nano-technology with galvanizing technology, and based on the special effects of rare earth elements on the corrosion resistance of galvanized coating and the abundance of rare earth elements in China, it was proposed in this dissertation to modify the galvanized coatings with rare earth oxide CeO₂ nano-particles. A systematic research on the modification process and mechanism was carried out, and the results provide the fundamental for the application of nano-composite galvanized coatings. The main work and results are as follows:1. Zn-CeO₂ nanocomposite coatings with homogeneously distributed monodisperse CeO₂ nano-particle have been obtained by electrochemical codeposition. The electrochemical impedance spectroscopy technique was used to investigate the mechanism of codeposition. The result shows that the charge transfer resistances are lowered with the addition of CeO₂ nano-particle which is in favor of the deposition of zinc.2. Zn-CeO₂ and Galfan-CeO₂ nanocomposite coatings were fabricated successfully for the first time by hot-dip process with high intensity ultrasonic stirring, and then characterized using TEM, XRD, SEM and EDS methods. It was found that the monodisperse CeO₂ nano-particle was distributed homogeneously in the coatings. The mechanism of high intensity ultrasonic stirring on the dispersion of nano-particles in the hot-dip galvanized coatings was deduced as well.3. The method of preparing Zn-based hot-dipped nano-composite coating via"master alloy"was originally investigated in details. The CeO₂/Zn, CeO₂/Al and CeO₂/Zn-5%Al nano-composite powders were fabricated by high-energy ball milling. The morphology and microstructure were investigated by means of TEM, XPS, XRD, SEM and EDS, respectively. The results show that nano-CeO₂ particles are distributed homogeneously in the composite powders, which was described using formation models of composite powders. The nano-composite bulks were produced by hot-press sintering under vacuum with as-prepared nano-composite powders, and the processing parameters were optimized by ways of orthogonal design. Adding the as-sintered CeO₂ /Zn nanocomposite bulks into the hot-dip galvanizing and Galfan alloy bath as"master alloy", Zn-CeO₂ and Galfan-CeO₂ nanocomposite coatings were obtained with homogeneously distributed monodisperse CeO₂ nano-particle.4. The effects of CeO₂ nano-particle on the corrosion and wear resistances, hardness, adhesion strength of the Zn-based coatings were investigated, as well as the tensile properties of the steel wire coated with the Zn-CeO₂ and Galfan-CeO₂ nanocomposite. It was found that the properties of the coatings, such as corrosion and wear resistances, microhardness, and so on, are improved obviously as the result of adding CeO₂ nano-particles. The results of corrosion weight loss, anodic polarization and electrochemical impedance spectroscopy analysis indicate that the improving effect on corrosion resistance is relevant to the size, content and dispersion of CeO₂ nano-particles in galvanizing solution. Erosion test shows that the erosion resistance increases with the increasing content of CeO₂ nano-particles within the concentration range used.5. The improving mechanisms of CeO₂ nano-particle on the corrosion resistance of the electrodeposited galvanized coatings, hot-dip galvanized and Galfan alloy coatings were investigated by combining the electrochemical impedance spectroscopy technique with morphology and microstructure analyses. Furthermore, the mechanisms of CeO₂ nano-particle on improving the erosion resistance and strengthening of composite coatings were discussed. All provides a theoretical basis for improving the whole properties of Zn-based composite coatings.