机械镀锌无结晶形层的研究

【作者】 王胜民；

【导师】 史庆南； 何明奕；

【作者基本信息】 昆明理工大学 ， 材料学， 2010， 博士

【摘要】 机械镀锌层的形成过程,金属锌粉仅发生固—固变化,没有明显的结晶现象。传统的镀层形成结晶理论难以解释机械镀锌的形成过程,而国外的“冷焊”(cold welding)理论也不能从根本上解释机械镀锌层的形成过程。尤其是近几年来,随着欧盟在电子电器领域RoHS指令的执行,机械镀工艺在表面处理领域发展迅速,但因其形层机理发展缓慢,大大制约了机械镀工艺的发展及应用。本文以目前国内普遍应用的机械镀锌少锡盐沉积工艺为研究对象,制备了机械镀锌层、锌—铝层、锌——RE层、锌—镍层试样；以机械镀锌镀液环境、镀层结构及结合界面、镀层物理化学性能为主要研究内容；采用ICP-AES、XRD、体视显微镜等分析了镀层形成时锌粉的吸附、沉积过程,采用OM、SEM、EDS、XRD、XPS等分析了镀层的组织结构及镀层—基体界面间的结合机理,通过孔隙率实验、致密度计算、拉力实验、中性盐雾试验、电化学极化等方法分析了镀层的物化特征；结果表明：机械镀锌层形成过程不发生电沉积结晶和高温液态金属结晶,镀层是由锌粉颗粒、锡、金属M(如Mn、Fe、Cd等)、空隙构成的多相混合体系,镀层的结合强度为2-3MPa,镀层的结合机制主要为机械咬合。基层建立阶段所添加Sn2+参与的Zn+Sn2+→Zn2+Sn↓反应和镀层增厚时所添加Fe2+参与的Zn+Fe2+→Zn2+Fe↓反应是机械镀锌形层过程的主要反应；形层过程需要的能量较低,为电镀锌形层能耗的1/10-1/20,热浸镀锌形层能耗的1/2-1/5。锌粉形成镀层的致密化过程主要包括锌粉颗粒和空隙的位移及变形。致密化过程锌粉颗粒发生了位移、转动、变形,导致空隙发生位移、体积压缩、小尺寸锌粉颗粒在空隙的填充,致使镀层致密度提高,锌粉形层过程不产生固溶体或化合物,机械镀锌层的形成是有置换沉积存在下的无结晶过程。锡盐和铁盐可分别在基层建立阶段和镀层增厚阶段作为先导金属,它们的作用机制分别是置换反应Zn+Sn2+→Zn2+Sn↓和Zn+Fe2+→Zn2+Fe↓。镀层形成后,先导金属残留在镀层中,或分布在空隙中,或分布在锌粉颗粒的接触界面部位。机械镀锌基复合镀层的结构与机械镀锌层相似,复合镀层形成过程没有因结晶而产生固溶体、化合物等相,但锌—镍复合镀层中存在Ni3Sn4合金相。采用片状锌粉制备的机械镀锌层中锌粉颗粒呈片状层叠排列,镀层由片状锌粉、空隙和夹杂构成,形层过程锌粉颗粒没有发现明显塑性变形,也没有产生化合物或固溶体等新相。机械镀锌形层过程产生的锡和铁不影响镀层内锌粉颗粒之间的结合。机械镀锌过程添加非锌微粉、稀土等不影响镀层／基体间、镀层内锌粉颗粒间的结合机制。镀后加热至100℃-300℃不改变镀层／基体间、镀层内锌粉颗粒间的结合机制。强化0 min—60 min不改变镀层／基体间、镀层内锌粉颗粒间的结合机制,延长强化时间时镀层／基体界面及镀层内不会发生扩散、冶金反应等合金化现象。镀层的拉伸破坏发生在镀层／基体界面,镀层越厚,结合强度越低,镀层厚度由20μm增加至60μm时,镀层结合强度由2.68 MPa降至2.31 MPa。机械镀锌层的密度约为5.67g/cm3,厚度对镀层的致密度影响不明显。锌粉粒径越小,镀层的致密度越高,当锌粉粒径为-800目、-1000目和-1200目时,镀层的致密度分别为34.73%、60.08%和77.85%。强化时间越长,镀层的致密度越高,当强化时间由0 min延长至60min时,镀层的致密度由76%提高到94.5%。分别采用球状和片状锌粉制备的镀层中均不存在连通空隙。添加非锌微粉或其它金属盐制备锌基复合镀层(锌—铝、锌—镍、锌—RE等)对镀层的孔隙率没有影响。机械镀锌层对钢基体可提供牺牲阳极保护,在35g/L的NaCl溶液中呈活性溶解特征,当镀层厚度达到60 gm时可耐中性盐雾腐蚀600小时。强化时间长短(0 min至60min)和镀后干燥加热温度高低(100℃至300℃)对镀层的耐腐蚀性能没有影响。片状锌粉制备的镀层与球状锌粉制备的镀层相比自腐蚀电位正移2.9 mV,腐蚀电流密度前者不到后者的1/3,前者的耐电化学腐蚀性能优于后者。铝、镍、RE的添加均提高了镀层的耐盐雾腐蚀性能,使镀层的自腐蚀电位正移,极化电阻增大,腐蚀电流密度减小,提高了镀层的耐腐蚀性能。更多还原

【Abstract】 In the formation process of zinc coating by mechanical plating, zinc powder remains solid state without obvious crystallization. It is difficult to explain zinc coating’s formation either by traditional crystallization theory, or by foreign cold welding theory fundamentally. Especially in recent years, with the implement of ROHS order in European electronics industry, mechanical plating has undergone rapid development in surface technology field. However, the imperfect mechanism of coating formation has greatly restricted the growth and application of mechanical plating. Based on the technology of mechanical plating with little tin salt, which is widely used in China, the subject prepared samples of zinc coating, zinc-aluminum coating, zinc-RE coating, zinc-nickel coating by mechanical plating, did research on the bath environment, coating’s structure, bonding interface and coating’s physical and chemical properties. The thesis analyzed the adsorption and deposition of zinc powder in the formation process through testing methods like ICP-AES, XRD, and stereomicroscope, and studied the microstructure of zinc coating and the interface between the coating and steel substrate by OM, SEM, EDS, XRD and XPS. The physical and chemical properties of zinc coating were tested and analyzed by ferroxyl test, density calculation, tensile test, neutral salt spraying test(NSS), and electrochemical polarization. The results show that:In zinc coating’s formation process, there is no electrocrystallization or liquid metal’s crystallization under high temperature. Zinc coating is a multiphase composite, composed by zinc particles, tin, M metal(such as Mn, Fe, Cd) and interstices. Mechanical interlocking is the main adhesion mechanism of zinc coating, whose adhesion strength is 2-3 MPa. The reactions Zn+Sn2+→Zn2++Sn↓and Zn+Fe2+→Zn2++Fe↓are two main ones in zinc coating’s formation. The former happens in seed coating building process, and the latter coating thickness increasing process. The energy consumption of zinc coating’s formation in mechanical plating accounts for 1/10～1/20 of that in electroplating and 1/2～1/5 in hot-dip galvanizing. The densification process of zinc coating by mechanical plating mainly includes displacement and deformation of zinc particles and interstices. Displacement, rotation and deformation take place in densification process, which leads to the displacement of interstices, compact of volume, filling in interstices by small-sized zinc particles and the improvement of zinc coating’s density. Without solid solutions or compounds, zinc coating’s formation is a noncrystallization process under the existence of replacement deposition.Sn2+and Fe2+respectively play the role of "driving metal" in seed coating building process and coating thickness increasing process. The mechanisms of their actions reflect in the two displacement reactions Zn+Sn2+→Zn2++Sn↓and Zn+Fe2+→Zn2++Fe↓After the formation of zinc coating, the "driving metal" remains in interstices, or on the contact interface of zinc particles.In mechanical plating, the structures of zinc-based composite coatings are similar with that of zinc coating. There are no formation of new phases like solid solution and compounds caused by crystallization. But the alloy phase Ni3Sn4 exists in Zn-Ni composite coating. Zinc particles array in zinc coating, which is composed by lamellar zinc powder, interstices and inclusions. There is neither obvious plastic deformation of zinc particles found in coating formation process, nor new phases like solid solution or compounds.Sn and iron have on effects on the adhesion of zinc particles. The adhesion mechanism of zinc coating/steel substrates, and zinc particles cannot be influenced by the additions of non-zinc powder or RE. Moreover, neither the heating temperature 100℃-300℃nor the strengthening time 0 min—60 min can change the adhesion mechanism. There are no alloying phenomena like diffusion or metallurgy reaction in zinc coating and on the interface between zinc coating and the substrate caused by the extension of strengthening time. The tensile failure happens on the interface between zinc coating and the substrate. With the improvement of zinc coating’s thickness, the adhesion strength reduces gradually. When the coating’s thickness increases from 20μm to 60μm, the adhesion strength reduces from 2.68 MPa to 2.31 MPa.The density of zinc coating by mechanical plating is about 5.67 g/cm3. Its thickness has no obvious effect on the density. The decrease of zinc particle’s diameter can cause the improvement of the coating’s density. With the diameters of 800 mesh、1000 mesh and 1200 mesh, zinc coating’s relative densities are 34.73%、60.08% and 77.85%. The extension of strengthening time improves the coating’s density. When the strengthening time extends from 0 min to 60 min, the coating’s relative density improves from 76% to 94.5%. No connected interspaces exist in coatings which are prepared with spherical and lamellar zinc powder. The additions of non-zinc powder or other metal salt to prepare zinc-based composite coatings(such as Zn-Al、Zn-Ni and Zn-RE) have no influence on the coating’s porosity.Acting as the sacrificial anode that protects steel substrate, zinc coating by mechanical plating shows active dissolution property in NaCl solution with the concentration of 35 g/L. It can endure 600 hours in neutral spray corrosion test. The strengthening time(0 min—60 min) and heating temperature(100℃-300℃) have no effects on the coating’s corrosion resistance performance. The electrochemical corrosion resistance of zinc coating prepared with lamellar zinc powder is slightly superior to that with spherical zinc powder. Compared with the latter, the former’s free corrosion potential moves 2.9 mV to positive direction, and its corrosion current density is less than 1/3 of the latter. The additions of Al、Ni and RE can respectively improve the coating’s salt spray corrosion resistance by making its free corrosion potential move to positive direction, increase the polarization resistance and reduce the corrosion current density.更多还原