| Chromium plating has been widely used for functional and decorative coatings because of hardness, corrosion performance and aesthetic appearance, etc. In the electroplating industry, chromium plating occupies a pivotal position and is one of the world's three major platings. Currently, chromium plating process is conventionally performed by aqueous electrolytic system, which can be divided into hexavalent chromium and trivalent chromium according to the existing forms of chromium. In China, the traditional hexavalent chromium plating process has been widely used. But hexavalent chromium plating bath has environmental pollution problem, governments have strengthened the management to reduce or even a totally ban on the use of hexavalent chromium. Comparing with the hexavalent chromium, trivalent chromium plating process has many incomparable advantages, such as low toxicity, good dispersion capacity and coverage, etc. However, due to the limitation of aqueous solution, both hexavalent chromium plating process and trivalent chromium plating process exist problems such as low current efficiency (8 to 20%) and hydrogen evolution. Ionic liquids, comparing with water solution, include common properties:wide liquid range and thermal stability, lower melting points and wide electrochemical windows. The properties can avoid of hydrogen evolution reaction in the deposition process, greatly improve the current efficiency and reduce the energy consumption. In order to resolve the hexavalent chromium plating environmental pollution problem and low current efficiency problem in trivalent chromium plating, the technology and correlative theories of trivalent chromium plating in choline chloride (ChCl)-CrCl3·6H2O system were investigated.Electrolyte conductivity have studied to determine the appropriate ratio which was the 67% mol ratio CrCl3·6H2O. On this basis, the influence of temperature, current density and deposition time on the current efficiency, thickness, deposition rate, surface appearance and cell voltage have been obtained respectively. The results show that the temperature was directly proportional to the electrode reaction rate and the corrosion rate. The film quality deteriorated with the current density and deposition time increasing. Under the optimum conditions, the adherent chromium coating with thickness up to 1.65μm was obtained without using brighterner and leveling agent. Adopting the complex system, the current efficiency can be up to 84.44%, which increased by 71 to 76 and 64 to 74 percentage points comparing aqueous solution. The electrochemical reduction process of chromium in ChCl-CrCl3·6H2O system have been investigated by modern electrochemical methods, such as cyclic voltammetry, chronopotentiometry, chronoamperometry, alternating current impedance etc. Tafel curves indicated that the number of electron transferred was 3 in Cr(III) electrochemical reduction process. Chronopotentiometry and cyclic voltammetry indicated that Cr(III)/Cr is a one step irreversible process with diffusion controlled. It was found that existence of the adsorption process before reaction, and electrochemical impedance spectroscopy in this case were been theoretically derived. The experimental results agree very well with the theoretical spectroscopy. Chronoamperometry showed that the electrodeposition nucleation mechanisms of Cr(Ⅲ) is 3-D nucleation and related to overpotential. At lower overpotential, the electrocrystallization of chrome follows a 3-D progressive nucleation mechanism with a diffusion coefficient of (2.56±0.33)×10-11m2·s-1. While at higher overpotential, it follows a 3-D instantaneous nucleation mechanism. And in the latter case, diffusion coefficient and nucleus density with values of (3.04±0.3)×10-11m2·s-1 and (4.47±0.05)×1011·m-2 are independent of overpotential. However, in the both cases, the normal direction growth rate constant is directly proportional to the overpotential, and the crystal growth in normal direction undergoes no block.The structure, vibrational spectra, the frontier orbitals, natural population analysis (NPA) and other structure information of related complex in ChCl-CrCl3·6H2O system were studied by ab initio method. It was found that the existence of CrCl(H2O)52+ and CrCl3(H2O)3 in ChCl-CrCl3·6H2O system expect choline chloride cations, through the comparing of infrared spectral data and theoretical vibrational spectra. The relationships between the structure of trivalent chromium complexes and their redox activity were been discussed. In several chromium complexes, CrCl(H2O)52+ has maximum reactivity and has the largest proportion of chromium in the frontier orbital. |
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