【作者】 穆松林；

【导师】 李宁；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2010， 博士

【摘要】 化学镀Ni-P镀层因其具有良好的均镀性、优良的耐蚀、耐磨和可焊性,在航空航天、石油化工、国防、能源以及电子微电子等领域得到广泛的应用。但由于金属镍具有较强的自钝化特性,使镀层表面容易形成镍的氧化物而失去光泽甚至变色,严重影响镀层外观。而且该膜层易遭破坏,也会使镀层的可焊性与耐蚀性受到严重影响。因此,有必要对Ni-P镀层进行有针对性的后处理,以提高镀层抗氧化变色性能与耐蚀性。传统的六价铬钝化处理可满足上述要求,但六价铬对人体与环境都具有很大的危害,加之人们对生态重要性的认识,研究无六价铬的钝化工艺已势在必行。本文通过无六价铬钝化处理来提高Ni-P镀层的抗氧化变色性能和耐蚀性。基于绿色环保的指导思想,系统研究了化学镀Ni-P层三价铬钝化膜和无铬钝化膜的制备工艺与成膜机制。为便于对比,同时也对六价铬钝化膜的耐蚀性与成膜机制进行了探讨。本研究的实验发现,酸性钝化体系易使镀层变色,尤其当pH值低于2.0时,镀层极易发黑,较难实现镀层的有效钝化。采用碱性三价铬工艺成功实现了化学镀Ni-P层的钝化,并以单因素实验确定了钝化体系主盐用量,体系操作温度,pH值等工艺参数。采用硝酸点滴法以及在3.5%NaCl溶液中的Tafel测试方法快速检验镀层抗氧化变色性能与耐腐蚀性能。结果表明,三价铬钝化膜可使镀层耐氧化性以及耐Cl-腐蚀的性能得到明显提高。以月桂酸为成膜剂,实现了Ni-P镀层的无铬钝化,并通过单因素实验确定了月桂酸用量及钝化体系的操作工艺。快速检测表明,该无铬钝化膜对镀层抗氧化能力提高有限,但可显著提高镀层耐Cl-侵蚀的能力。对硅烷偶联剂采用适当比例进行水解可得到稳定性超过一年的镀层封闭试剂。使用该试剂对Ni-P镀层进行封闭处理,可在镀层上获得一层致密的自组装膜,该膜层可使镀层抗氧化能力提高一个数量级,耐Cl-侵蚀能力可提高两个数量级以上。采用电化学方法研究了各种钝化膜在3.5%NaCl溶液中的腐蚀特性。使用扫描电子显微镜（SEM）考察了六价铬与三价铬钝化处理对镀层形貌的影响。使用X射线光电子能谱（XPS）方法研究了钝化膜的元素组成,并通过拟合分析了膜层中各元素的化合物形态。采用Mott-Schottky曲线分析了各膜层的半导体特性。电化学测试表明,EIS结果与Tafel测试结果一致性较好,硝酸点滴结果显示几种钝化膜抗氧化能力强弱顺序为:六价铬钝化膜>硅烷膜>三价铬膜>月桂酸膜。SEM观察显示,钝化处理后的表面与空白表面无明显差异。XPS分析表明六价铬钝化膜中可检测到O,Cr,Ni元素,而三价铬膜层可检测到C、N、O、Cr、Ni、P等六种元素。对六价铬钝化膜中Cr 2p精细XPS谱图分析发现,膜层中的Cr元素仅以三价铬形式存在,其化合物形式为Cr₂O₃和Cr（OH）3,其中Cr₂O₃占87.0at%。三价铬钝化膜中Cr元素也以Cr₂O₃和Cr（OH）₃形式存在,但Cr₂O₃仅占21.4at%。半导体特性分析表明,镀层表面的六价铬钝化膜、三价铬钝化膜和月桂酸钝化膜在较宽的频率区间内均表现为n型半导体特征;而硅烷自组装膜则主要表现为低电位p型,高电位n型的特征。根据六价铬钝化膜的XPS测试结果,探讨了六价铬钝化过程中镀层Ni元素与P元素的作用。研究表明,在采用K₂Cr₂O₇钝化处理的过程中,Ni-P镀层中的Ni与P均参与了反应,但二者的反应产物均未参与钝化膜的成膜过程。根据三价铬钝化膜的元素组成分析,钝化液中的芳香族化合物参与了成膜过程,以吸附态的形式参与成膜。由于其吸附过程消耗镀层附近的OH^-,从而促使碱性条件下以Cr（OH）4-状态存在的Cr³⁺转化为Cr（OH）₃沉积到镀层表面形成钝化膜。通过分析月桂酸钝化膜的Tafel测试结果发现,月桂酸在镀层上形成的钝化膜是化学吸附膜,其吸附满足Langmuir等温模型,即为单分子层吸附膜。硅烷自组装膜通过硅醇分子中的羟基与镀层表面吸附的羟基缩合而与镀层结合,同时硅醇分子间也通过羟基脱水缩合形成致密的膜层,从而起到对镀层的保护作用。更多还原

【Abstract】 The electroless Ni-P coating （ENPC） has been widely applied in field of petrochemical, aerospace, national defense, energy source and micro-electron because of its high throwing power, excellent corrosion and wear resistance as well as solderability. Despite its excellent properties, the strong tendency of self-passivation of Ni in coating makes the outermost layer of the coating be easily oxidized and thus lose its brightness even discolor, which will seriously affect the appearance of the coating. And this Ni oxide film is susceptible to breakage, and its presence will deteriorate the solderability and weaken the corrosion resistance of the Ni-P coating. Thus it is necessary that the coating must be treated by some appropriate post treatments to improve its corrosion performance and anti-discoloration （anti-oxidization） ability. The traditional chromate （Ⅵ） treatment can meet the demand of corrosion resistance and anti-discoloration, but chromate （Ⅵ） will bring very serious damage to both human beings and environment. Since the importance of eco-enviroment has been awared, the urgent action should be taken to research chromate（Ⅵ）-free passivation technics for ENPC.In this work, chromate（Ⅵ）-free methods were used to solve the mentioned problems on Ni-P coating. To avoid pollution, the preparation technics for environmental friendly chromate（Ⅲ） and chromium-free passive films were systematically researched. The forming mechanisms of the passive films were also proposed. At the same time, for the purpose of comparison, the forming process of chromate（Ⅵ） passive film was also discussed.A series of experiments found that the ENPC is apt to discolor and difficult to be effectively passivated at acid condition, especially when the pH value of passive bath is lower than 2.0, the coating is likely to darken. By a alkaline bath, a chromate（Ⅲ） passive film was obtained on ENPC. The content of main salt, working temperature and pH value for the bath were determined by single-factor experiments. Taking nitric acid （50vol.%） exposure test and Tafel test in 3.5%NaCl solution as fast-checking methods, the corrosion resistance and anti-discoloration ability of the chromate（Ⅲ） passive film were checked. The test results indicated that the chromate（Ⅲ） passive film can improve the anti-oxidation ability of the coating and obviously enhance the corrosion performance in Cl--containing solution.At alkaline condition, with dodecanoic acid （DA） as film-forming agent, a chromium-free passive film was prepared on ENPC, the content of DA and working pH value were determined by single-factor experiments. Fast-checking tests showed that although the improvement of anti-oxidation ability of the DA film in nitric acid exposure test was almost negligible, this chromium-free passive film could dramatically impove the corrosion resistance of ENPC in 3.5%NaCl solution.. A sealing agent for ENPC, whose storage time is more than one year, can be obtained by hydrolyzing SCA at an appropriate proportion between SCA, ethanol and water. A compact self-assembled film would form on ENPC by treated in the sealing agent, and this compact film could improve the corrosion resistance of EPC in 3.5%NaCl solution by two orders of magnitude, while the anti-oxidation in nitric acid was only impoved by one order of magnitude.The nitric acid exposure test, Tafel test and electrochemical impedance spectroscopy （EIS） were employed to analyze the anti-oxidation ability and corrosion resistance of the researched passive films, including chromate（VI） passive film. The scanning electronic microscopy （SEM） was utilized to observe the effect of Cr3+- and Cr6+-treatment on the appearance of ENPC, the elemental composition of the Cr3+ and Cr6+ passive films were studied by X-ray photoelectron spectroscopy （XPS） and the chemical states of these elements were analyzed by high resolution XPS spectra. The research on the semiconductor properties of the films was conducted by Mott-Schottky （M-S） method. The electrochemical tests indicated that the result of Tafel test was in good agreement with that of EIS, and the order of the anti-oxidation ability for the various films was: chromate（Ⅵ） passive film > SCA self-assembled film > chromate（Ⅲ） passive film > DA passive film. The SEM result showed that no obvious difference could be observed between the blank Ni-P coating and passivated coating （both Cr3+- and Cr6+-treated coatings）. The XPS analysis on the Cr6+-treated coating illustrated that the passive film comprised O,Cr and Ni, while the chromate（Ⅲ） passive film was made up of C, N, O, Cr, Ni and P. The high resolution XPS analysis indicated that the Cr in chromate（Ⅵ） passive film was only in the state of trivalence, its existence form was Cr₂O₃ and Cr（OH）₃, 87.0at% of the total Cr was in the form of Cr₂O₃. The element Cr in chromate（Ⅲ） passive film was also in the form of Cr₂O₃ and Cr（OH）₃, but only 21.4at% of the Cr was Cr₂O₃. The M-S test results illustrated that Cr⁶⁺ passive film, Cr³⁺ passive film, and DC passive film exhibit n-type characteristic at lower potential and p-type at higher potential at various frequencies; while the SCA film shows a contrary p/n characteristic, namely p-type at lower potential and n-type at higher potential.Based on the XPS analysis of Cr6+ passive film formed on the ENPC, the function of Ni and P in the forming process of passive film was discussed. It could be inferred that both Ni and P in coating will react during the chromate（Ⅵ） passivation process, but the reaction products of Ni and P will not involve in the formation of passive film. In the light of the XPS analysis on the Cr³⁺ passive film, the aromatic compound in chromate（Ⅲ） passive bath would participate in the formation of Cr³⁺ passive film by chemisorption on Ni-P coating. Because the consumption of OH- during the adsorption leads to decrease of pH value in the solution layer very clear to Ni-P coating, Cr（OH）₄^-particles will change into Cr（OH）₃ and deposit on the surface of Ni-P coating and eventually form Cr³⁺ passive film. The research on the DA passive film found that the DA molecules adsorb on Ni-P coating by chemisorption, its adsorption process meet the Langmuir isothermal, namely monolayer adsorption film. At cure temperature, the SCA film was formed through a self-assembled process by condensation reaction between hydroxyl in silanol molecule and hydroxyl on Ni-P coating, at the same time the condensation reactions will also take place between the hydroxyl groups of silanol molecules and form a compact SCA film.更多还原