氯化胆碱-CrCl₃·6H₂O体系中电沉积铬的研究

【作者】 崔焱；

【导师】 华一新；

【作者基本信息】 昆明理工大学 ， 有色冶金， 2011， 博士

【摘要】 铬镀层因具有显微硬度高,耐磨耐蚀性能优良和外表美观等优点,在世界电镀行业中占据着举足轻重的地位,成为世界上三大镀种之一,并被广泛用于功能性和装饰性镀层。目前,获得工业应用的镀铬工艺普遍采用水溶液电解体系,根据铬的赋存价态又可分为六价铬和三价铬两大类别,我国电镀行业普遍采用传统的六价铬镀铬工艺。由于六价铬镀液对人体的危害和对环境的污染,各国政府已加强了立法管理,以期减少乃至完全禁止六价铬的使用。相对而言,三价铬镀铬工艺具有六价铬镀铬无法比拟的优点,如毒性低,分散能力和覆盖能力好等。但由于水溶液体系的限制,不论是三价铬体系还是六价铬体系,均存在电流效率不高(8-20%),析氢现象严重等问题。离子液体作为一种新型电解质,与水溶液相比,具有电化学窗口宽、可室温操作、热稳定性高、导热导电性良好等优点,能避免电沉积过程中的析氢反应,极大地提高电流效率并降低能耗。因此本文针对当前电镀铬体系六价铬污染严重和三价铬电镀电流效率低的问题,系统研究了氯化胆碱(ChCl)-CrCl3·6H2O体系中三价铬电沉积的工艺及其电化学机理。通过电解液电导率的研究,确定了合适的电解液配比为CrCl3·6H2O的摩尔百分含量为67%。在此基础上,通过单因子实验考察了温度、电流密度和时间对电沉积过程的影响。实验结果表明：温度升高,提高了电极反应速率,但同时也会加快腐蚀速度；增大电流密度,会使副反应增加,电流效率降低；延长电沉积时间,会导致镀层质量下降。在综合外观质量、电流效率等因素后得到了氯化胆碱(ChCl)-CrCl3·6H2O体系三价铬电沉积的最优实验条件。在未添加任何光亮剂和整平剂的条件下,电沉积获得光滑致密的铬镀层,厚度为1.66μm达到了装饰性镀层的要求。电流效率相比水溶液六价铬体系和三价铬体系分别提高了71-76和64-74个百分点。采用循环伏安法、电流阶跃法、电势阶跃法及电化学阻抗法等多种电化学方法,首次系统研究了三价铬在ChCl-CrCl3·6H2O体系中的电化学还原过程。稳态极化曲线证明了Cr(Ⅲ)还原过程的电子转移数为3。电流阶跃法和循环伏安法证明Cr(Ⅲ)还原是单步骤三电子的不可逆过程,反应受扩散控制,反应不存在前置反应,但有吸附的情况。理论上推导了存在吸附情况下的电化学阻抗图谱,实验结果与理论推导相吻合,吸附速率相对较快,整个反应受扩散控制。电势阶跃法的研究结果表明,Cr(Ⅲ)的电化学成核机理是三维成核并与过电位有关。当过电位较小时,Cr(Ⅲ)的电结晶过程为三维连续成核,其扩散系数为(2.56±0.33)×10-11m2·s-1。当过电位较大时,Cr(Ⅲ)的电结晶过程趋向于三维瞬时成核过程,其扩散系数和晶核数密度均与过电位无关,分别为(3.04±0.3)×10-11m2·s-1和(4.47±0.05)×1011m-2。两种成核方式的法向生长速率常数均与过电位成线性关系,晶体法向的生长没有受到任何阻滞。采用量子化学从头计算法,研究了ChCl-CrCl3·6H2O体系中各个相关配合物的稳定结构、振动光谱、前线轨道及自然布居分析(NPA)等电子结构信息。通过理论光谱信息与实验测定的三价铬电解液红外光谱数据比对,确定了所研究体系中三价铬的主要存在形态为CrCl(H2O)52+和CrCl3(H2O)3。探讨了各三价铬配合物结构与其氧化还原反应活性的关系,从微观结构讨论了反应可能发生的途径,在几种铬配合物中,CrCl(H2O)52+的反应活性最大,前线轨道中铬所占的比重最大,最有可能在阴极上还原。更多还原

【Abstract】 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.更多还原