环境友好型镀锌硅酸盐钝化工艺、机理及应用研究

【作者】 董鹏；

【导师】 张英杰；

【作者基本信息】 昆明理工大学 ， 冶金物理化学， 2011， 博士

【摘要】 长期以来,镀锌钢板的钝化处理往往采用六价铬酸盐钝化液,但铬酸盐毒性高、易致癌,对人体及环境造成极大的危害。随着人们环保意识的增强,铬酸盐的使用受到严格的限制,无铬钝化工艺的应用已成为发展的必然趋势。硅酸盐钝化是一种无毒、无污染的钝化技术,但其工艺不稳定、成膜效率不高。为解决这一问题,本论文开发了一种新型成膜促进剂,在提高成膜速率的同时有效改善了产品的耐蚀性。论文对硅酸盐钝化工艺进行了系统的研究,深入探讨了镀锌层硅酸盐钝化膜的成膜机理与耐蚀机理,并在电镀厂中进行了使用,取得了一定的经济效益。首先确定了镀锌硅酸盐钝化工艺的最佳工艺参数,钝化液组成为SiO32-4g/L, H2O215ml/L, NO3-20g/L, SO42-9g/L,成膜促进剂6g/L,工艺条件为钝化液pH值1.5-2.5,钝化时间10-30s,钝化温度15～40℃,空停时间5s左右,出光时间5s左右,得到的钝化膜表面光亮、外观均匀、耐蚀性好,且钝化液性能稳定、适应性强。钝化液组成对钝化膜耐蚀性的影响程度由大到小为SiO32-,成膜促进剂,H2O2, NO3-, SO42-。钝化液pH值、钝化时间、钝化温度对耐蚀性影响较大,而出光时间和空停时间为次要因素,只要不做极端操作,对钝化膜的耐蚀性没有明显影响。通过热力学和量化计算,结合电化学、SEM及XPS等测试手段分析了硅酸盐钝化膜的成膜机理。钝化膜的形成包括二氧化硅胶状物的生成、镀锌层的溶解、碱性薄层的形成和钝化膜形成四个过程。由于镀锌层表面存在晶体缺陷,这些晶体缺陷属于能量较高的电化学不均匀的区域,在含有H2O2的酸性钝化液中易形成众多微电池,阳极发生锌的溶解,生成Zn2+,阴极发生H2O2的极化还原反应,以OH-的形式吸附在两相界面上,引起镀锌层表面pH值上升,形成碱性膜层,为成膜反应提供必要条件。在有OH-吸附于界面的情况下,溶液中的Zn2+可与OH结合生成Zn(OH)2,继而脱水形成ZnO; pH值在1.5-2.5的钝化液中硅酸根首先生成Si02胶状物,在OH-的作用下,SiO2、OH-和Zn2+共同形成ZnSiO3。生成的ZnO、ZnSiO3等含锌化合物沉积在镀锌层表面,微细的胶态Si02粒子填充膜层的孔隙,最终形成硅酸盐钝化膜。钝化膜成膜过程分为三个阶段：反应初期(0-30s)钝化膜快速成长,但膜层还不完整,表面有小孔分布；反应中期(30-120s)钝化膜生长速率降低,钝化膜的结构不断完善,表面平整光滑,表现出良好的外观和耐蚀性；当钝化时间大于120s,膜层出现堆积现象其耐蚀性降低。耐蚀机理研究表明,腐蚀过程中硅酸盐钝化膜是通过机械隔离和电化学缓蚀作用而对基体产生保护。采用场发射电镜观察硅酸盐钝化膜的微观形貌发现,钝化膜由无数细微粒子紧密排列而成,均匀致密的结构可将镀锌层表面与腐蚀介质隔离开来,有效地阻挡外界氯离子和氧等腐蚀介质对镀锌层的侵蚀。电化学研究表明,在NaCl腐蚀溶液中,高频区硅酸盐钝化膜的电化学反应阻抗远远大于镀锌层,达到780Ω·cm2,较大的交流阻抗可以有效地阻碍电荷自由传输,腐蚀电流密度由2.0062×10-5Amp/cm2降至6.1561×10-6Amp/cm2,腐蚀速率仅为0.0183g·m-2·h-1。采用扫描电化学显微镜技术对钝化膜的电化学特性研究也表明,硅酸盐钝化膜的电化学活性不高,能够有效地降低镀锌层表面电子的传递速率,微电池的腐蚀反应发生倾向明显降低,从而抑制了腐蚀过程,显著提高了镀锌层的耐腐蚀性能。硅酸盐钝化膜对腐蚀过程阴极、阳极过程均有不同程度的控制,阳极极化度为180.663mV,明显大于阴极极化度98.579mV,其腐蚀过程表现为阳极控制型。对镀锌硅酸盐钝化工艺进行了日处理量1.5吨左右的工业生产应用,生产所得各种形状零部件的外观光亮,膜层均匀,无脱膜现象产生,其耐蚀性完全达到生产的要求。钝化液的稳定性好,方便维护,在长时间连续使用过程中,钝化液的pH值变化不大；随着生产量的增加,钝化液中硅酸根、硫酸根、硝酸根含量有所减少,为了保证产品的质量,在生产一段时间后,可以通过添加硅酸钠、硫酸和硝酸钠的方式来调节钝化液的组成。操作过程中采用合适的钝化液配制方法、搅拌方式、钝化温度、浸渍时间、干燥温度等,可以有效地延长钝化液使用寿命。综上所述,本论文所开发的镀锌硅酸盐钝化工艺环境友好、成本低廉,其产品性能优良,工艺性能稳定,具有十分广阔的应用前景。更多还原

【Abstract】 Traditionally, hexavalent chromate was used in passivation of the galvanized steels. However, chromate is not environmentally friendly due to its toxicity and carcinogenicity, which greatly restricts its application when the people’s health is of great concern. Therefore, it is impeding to explore the chromate-free passivation techniques, and, among them, silicate is one of the most potential alternatives. Unfortunately, passivation process in the silicate is inefficient and unstable. In order to solve this problem, a new promoting agent of film-formation was explored in this thesis, and this agent can accelerate the deposition rate and can improve the corrosion resistance of the formed passive film. Meanwhile, the process of passivation and mechanisms of film-formation in the silicate solvent have been discussed in detail, and also discussed is the anti-corrosion performance of this film. Furthermore, industrialized application of this passivating technique is successfully effected and a great economic benefit is obtained.At the beginning parts of the thesis, the optimized compositions and the operating conditions of the passivating solution have been determined as follows:SiO32-is4g/L, H2O215ml/L, NO3-20g/L, SO42-9g/L, film promoting agent6g/L, pH value1.5to2.5, passivation time10～30s, temperature15～40℃, slot time5s, light time about5s. It was found that pH values, immersion time and temperature all influence markedly the corrosion resistance of passive film.The mechanism of silicate passive films formation was studied. In the formation of silicate passive films, four processes took place, which include the formation of silicon dioxide gels, the dissolution of zinc coating, the formation of alkaline thin film and the formation of silicate passive films in succession. Thermodynamics and quantum chemical calculations showed that there exist crystal defects on the surface of zinc coating, which are the electrochemically heterogeneous parts with high energy and tend to form the anode areas of micro-cell corrosion compared with the rest parts of the coating. Metal zinc was oxidized to zinc ion at the anode, and hydrogen peroxide was reduced at the cathode. Hydrogen peroxide adsorbed on the galvanized zinc surface and breaked into OH" which stayed the two-phase interface. Subsequently, the surface pH value increased because of the formation of alkaline film, which is a prerequisite condition for the formation of passive film. The hydroxyl could combine with the zinc ion to produce zinc hydroxide, immediately followed by dehydration to zinc oxide. On the other hand, silicate hydrolyzed to silicon dioxide gels in the acidic passivating solution in pH values of1.5～2.5. And then, silicon dioxide gel reacted with zinc ion and hydroxyl to form zinc silicate. As the whole, ZnO, ZnSiO3, other zinc compounds and the filling colloidal SiO2fine particles co-deposited on the surface of galvanized zinc, and the silicate passive film formed.Electrochemical tests indicate that the passive films formation process undergoes three stages. In initial stage (0～30s), the passive film grows rapidly, but the film is not complete, and some holes are distributed in the film. In the subsequent stage (30～120s), the film growth rate decreases, but membrane performance is improved continuously, the surface of film become smooth and its corrosion resistance increased. When the passivation time exceeds120s, corrosion resistance decreases with time instead due to the films’ overlap.Because of silicate passive film’s structural and electrochemical character which was able to significantly improve the corrosion resistance of galvanized zinc. It was found that silicate passivated films formed by the accumulation of small particles, which can effectively isolated the corrosive medium due to its structural integrity, finesse and uniform distribution. Electrochemical researches indicated that the electrochemical impedance of silicate passivated films is780Ω·cm2in the NaCl medium, which is larger than zinc coating. As the electrochemical corrosion inhibitor, the passive film hinders the charge transfer process. The corrosion potential and current are-0.8828V and6.156×10-6Amp·cm-2, respectively. Especially, the silicate passive films corrosion rate is0.0183g·m-2·h-1. By the scanning electrochemical microscopy characterization of silicate passivated film was researched. Compared with galvanized zinc and chromate passivation film, no regions of high electrochemical activity exist on silicate passivated film surface. The rate of electron transfer across the substrate/electrolyte reduced, and as result the dissolution of the substrate is significantly inhibited. Parameters from Tafel curve slope showed that, compared with the zinc coating, silicate passivated film polarized both the cathodic process and the anodic process to the greater extent, and the ba is180.663mV, significantly greater than the98.579mV of bc, indicating that the corrosion processes are controlled mainly by the anodic polarization.In industrial application of this silicate passivating technique, galvanized steel parts in complicated shapes can be well passivated, and the passivation film looks mirror-like, uniform, without stripping phenomenon; Results of corrosion resistance test of this passive film fully reach the industrial standards. pH values of the passivating solution in the long-term continuous operation change little, indicating of good stability and easy maintenance; As production volume increases, the content of silicate, sulfate and nitrate in passivation solution decrease, and in order to ensure product quality, these can be supplied by adding sodium silicate, sulfuric acid and sodium nitrate after some operation time to regulate the compositions of the passivating solution. During operation, the appropriate method for the preparation of passivating solution, stirring, passivation temperature, dipping time, drying temperature, etc, can effectively extend the life of passivating solution.In summary, the technique of silicate-contained passivating solution in the present study shows environmentally friendly, low cost, excellent product performance, wide application, and great market competitive advantage, and it is expected to produce enormously economic and social benefits.更多还原