【作者】 谢治辉；

【导师】 余刚； 胡波年；

【作者基本信息】 湖南大学 ， 物理化学， 2013， 博士

【摘要】 镁合金具有密度小、散热性好、强度质量比高、电磁屏蔽能力强等诸多优点而广泛应用于航空、电子、计算机及汽车等领域。但是镁合金的耐蚀性和耐磨性差，为了充分发挥镁合金的优势，克服这两种缺点对其使用范围的阻碍限制，在工业应用中，尤其是一些特殊行业，化学镀镍已经成为镁合金使用前必经的工艺步骤。然而，镁合金化学镀镍还存在诸多问题，多数研究者都致力于一次性化学镀液条件下镀层耐蚀能力的提高。本论文一方面研究了镁合金化学镀镍的界面反应机理和镀层性能，另一方面重点对镀液的性质变化进行了系统的研究，得到了一种稳定性高、缓冲能力强的化学镀镍溶液配方。论文通过扫描电子显微镜（SEM）、能量色散X射线光谱（EDX）、X-射线衍射仪（XRD）、原子力显微镜（AFM）及电化学测试等现代检测技术对镁合金化学镀镍层的表面形貌、微观结构、沉积速率、沉积电势（Edep）、沉积电流（idep）等参数的表征和测试，研究了镁合金化学镀镍的初始沉积过程、前处理工艺、动力学方程和镀液的性能。主要的研究成果如下：1．开发了一种无铬低氟绿色环保的的镁合金化学镀镍酸洗活化前处理新工艺。鉴于镁合金传统化学镀镍前处理工艺中六价铬对环境造成的严重污染，通过对比研究几种不同酸洗活化工艺对基底的刻蚀效果及对镀层性能的影响发现，使用（H₃PO₄+HNO₃）酸洗，K₄P₂O₇和NH₄HF₂分别活化的二次活化工艺可以使基底形成特殊的腐蚀形貌及合适的氟氧元素比（F/O），从而在保证镀层耐蚀性的同时大大提高镀层的结合力。同时，研究还发现，化学镀镍沉积初期包括两类重要反应：（1）置换反应触发的镍初始沉积；（2）镍的自催化还原沉积。2．研究了化学镀镍液中硫酸根离子的积累对化学镀镍的影响。通过添加硫酸铵的方法模拟了硫酸根离子累积对镀液沉积速率、次磷酸钠效率、镀液稳定性及镀层质量的影响。结果发现，一定浓度的硫酸铵可以提高沉积速率(<12g·dm^-3)，但会使镀液的稳定性和次磷酸钠效率下降。浓度的进一步增大对镀液稳定性影响较小，同时有利于次磷酸钠效率的提高。低浓度硫酸铵的引入不会对镀层造成不利影响，相反，在一定的浓度范围内，还可以细化晶粒，改进镀层的耐蚀性能。但是，硫酸铵浓度过高(>16g dm^-3)则导致镀层应力增加，降低镀层结合力。3．研究了镀液pH值对镀层质量的影响及稳定镀液pH值的方法。研究结果表明，pH值不但会影响镀层的致密性，同时还会改变镀层中的P含量，使得镀层的微观结构发生变化，表现出不同的耐蚀性能。添加适量浓度的醋酸铵可以大大提高镀液缓冲能力，得到的镀层耐蚀能力更强，镀液的pH值和稳定系数在周期（MTO）循环中不会明显下降，镀液的使用寿命可以由原来的4个MTO延长至9个MTO。4．讨论了镁合金化学镀镍液的电化学行为。针对镁合金化学镀镍电化学行为还存在诸多争议的现状，研究了镀液组成、pH值及温度变化对Edep和idep的影响；同时，还考察了老化时间对镀液电化学阻抗谱（EIS）及镀层形貌的影响。极化曲线试验说明，在组成完整的化学镀镍液中获得的idep才能代表真实的化学镀镍沉积速率。试验结果与根据巴特勒-伏尔默公式（Butler-Volmer Equation）推导得到的理论值具有较好的一致性，分析结果还说明镁合金化学镀镍过程受阴、阳极反应共同控制。EIS实验结果表明，镀液老化一定时间后，其表面界面电荷传递电阻（Rd）明显增加，镀层沉积速率降低，镀层变得粗糙。以上结果说明通过电化学方法对镁合金化学镀镍沉积速率和镀液性能进行在线监测是可能的，这为得到合格质量的镀层提供了有力的保证。5．确定了镁合金化学镀镍的反应速率方程及表观活化能。讨论了pH值、温度和镀液中不同组分浓度变化对沉积速率的影响，建立了主盐在不同浓度范围条件下，镁合金酸性化学镀镍的速率动力学方程。由速率方程可知，主盐浓度增加，沉积速率不会持续增大，当镍离子浓度高于一定值(4.69g·dm^-3)后，沉积速率反而会有所下降；而还原剂、pH值（4⁶）和温度增加，沉积速率则相应增大；配合剂浓度增大，沉积速率降低。根据阿仑尼乌斯（Arrhenius）公式计算可得到本体系中化学镀镍的表观活化能为38.03kJ·mol^-1,这说明镁合金化学镀镍是一种比较容易实现的化学反应。本论文在如下方面有创新性研究工作：（1）得到了一种环保型的无铬酸洗和二次活化前处理工艺，克服了铬酸酸洗带来的环境污染问题，避免了氢氟酸的易挥发特性给车间操作人员构成的潜在威胁；（2）剖析了施镀过程中硫酸根离子积累对化学镀液和镀层质量的影响，分析了硫酸根可能对化学镀镍造成的各种有利或不利作用；（3）揭示了施镀过程中镀液pH值的变化对镀层质量的影响，找到了稳定镀液pH值的缓冲剂，确定了化学镀镍液的最佳pH值，获得了高稳定性、长寿命的镁合金化学镀镍溶液。（4）对不同条件下镀液的极化和EIS行为进行了研究，为工业上能够快速在线监测镀液的变化，成功控制镀层质量提供了一种有效的测定方法。（5）建立了新开发的镁合金化学镀液的沉积速率方程，为有效控制镀液温度、pH值及各组分浓度变化对镀层质量的影响提供了理论依据。更多还原

【Abstract】 Magnesium alloys have many desirable features, including low density, highthermal conductivity, high strength to weight, good electromagnetic shieldingcharacteristics, etc., which make it valuable in a number of structural applicationsincluding aerospace, electronics, computer parts and automobile fields. However, therelatively poor abrasion and corrosion resistance of magnesium alloy limit itsextensive utilization. Electroless nickel （EN） deposition is a viable option to improvethe wear and corrosion characteristics of Mg alloy, but challenges exist in thedeposition process due to the corrosion of Mg in the EN bath. Moreover, mostresearchers are committed to improve the corrosion resistance of the coatings whichdeposited in a once-plating bath only, rather than in cycles. On one hand, this paperstudied the characteristics of coatings, interfacial reactions between the magnesiumalloys and the EN bath; on the other hand, a more systematic work about the changesof plating bath in the plating processes was also studied, and thus an EN bath formulawith high stability and good buffering capacity was obtained.By using scanning electron microscopy （SEM）, energy dispersive X-rayspectroscopy （EDX）, X-ray diffraction （XRD）, atomic force microscopy （AFM）, et al,the surface morphologies and microstructure of the coatings obtained by variouspretreatments and in different plating baths were characterized. The deposition rate,deposition potential （Edep） and deposition current （idep） in different pH value,temperature, and concentration ranges of compositions of EN bath were determinedby electrochemical techniques. The main research results are as follows:1. Influences of three different pickling and activation processes on direct ENplating on Mg alloys have been studied in order to obtain an environment-friendlyprocess of pretreatment. A chromium-free pretreatment has been finally establishedby the procedure of pickling with （HNO₃+H₃PO₄）, twice activations with K₄P₂O₇andNH₄HF₂. It is found that a coarse surface and proper F/O ratio were produced on theMg substrate via the new pickling-activation process. The coatings coated by this newprocess exhibit excellent adhesion and corrosion resistance. The results also foundthat two important types of chemical reactions in the initial deposition on Mgsubstrate were occurred. They are:（1） replacement reaction between the magnesiumsubstrate and nickel ions;（2） autocatalytic deposition reaction of nickel itself. 2. The effects of ammonium sulfate （AS） in bath on the deposition rate,efficiency of hypophosphite, bath stability, and characteristics of EN deposits werealso studied. The results show that the rate of EN can be improved by adding AS whenits concentration is lower than12g·dm^-3. The bath stability test indicates thatstability of the EN bath can be significantly reduced by a low concentration of AS,whereas little impact on bath stability was observed with an increased ASconcentration. The results also indicate that a low accumulation of AS in the platingbath did not adversely affect the adhesion and corrosion resistance of the coatings. Anoptimum concentration of AS can decrease grain size, refine microstructure, andimprove corrosion resistance. However, a very concentrated AS (>16g dm^-3) platingbath may increase stress, hence creating cracked cross-section morphologies.3. The effects of complexing agents on the bath life, plating rate and coatingquality were studied in detail. The results show that the changes of pH value not onlychanged the nature of the bath solution, but also had a strong impact on the P content,microstructure and electrochemical properties of the EN coatings. The bufferingcapacity of the EN bath can be improved obviously with the addition of optimumconcentration of acetate ammonium, and thus the pH value and bath stability wouldnot be changing quickly in MTO cycles, the operational life can be extended from4MTO to9MTO.4. In order for obtaining more kinetics information of the EN plating processes,the polarization behavior of EN plating in different baths was investigated withpolarization curves. Influence rules of every process parameters on the polarizationcurves and EN deposition in a complete bath were also described. From thesevariations in the deposition potential （Edep） and current density （idep）, a kineticexpression employing the Butler-Volmer equation and mixed potential theory was setup and was in good agreement with the experimental findings. It was confirmed thatthe EN deposition process in the present researches was under a mixed control.Electrochemical impedance spectroscopy （EIS） was proposed to measuring the agingability of the EN bath. With prolonged aging time, the charge transfer resistance （Rd）of samples increased obviously, the deposition rate decreased, and the surfacemorphology of the coatings observed became coarser. These results show that theelectrochemical measurement may be effectively in monitoring EN deposition rateand characteristics of EN bath in the future.5. Lastly, the dynamical equation of EN plating on Mg alloy in an acidic bath wasderived by studying the effects of concentrations of metal salt, reducing agent, pH value, and temperature on the deposition rate. The equation shows that the depositionrate increased with increasing temperature, concentration of H－2PO2, pH value （4⁶）,and decreased with increasing concentration of complexing agents. For nickel ions,the rate increased gradually with increasing concentration (x, g·dm^-3) when x<4.69.However, it decreased when the concentration exceeded4.69. The apparent activationenergy （Ea） calculated from the Arrhenius equation is approximately38.03kJ·mol^-1,this could explain the reason why the deposition reaction did take place readily.The innovations in the dissertation as bellow:（1） An environmentally friendlypickling and activation process with chromimu-free and only low hydrogen fluoridewas exploited;（2） The effects of accumulation of sulfate ion on the plating bath anddeposits were studied by simulate method;（3） The influences of pH value changes inthe deposition process on the characteristics of the coatings were analyzed, and aplating bath with an optimum concentration of pH value buffering agent wasascertained;（4）Electrochemical behavior of EN plating in different baths was studiedby polarization curves and EIS methods. A monitoring technique may be developedand applied to industrial production lines based on this new measurement;（5） Anempirical deposition rate equation for EN on Mg alloys in an acidic bath was obtained.This is favorable for better control over the quality of EN coatings by varying thetemperature, pH value and composition concentrations of the plating bath in thedeposition process.更多还原