【作者】 赵惠；

【导师】 崔建忠；

【作者基本信息】 东北大学 ， 材料加工工程， 2008， 博士

【摘要】 本文针对镁合金耐蚀性低这一限制镁合金广泛应用的重要问题,采用新型化学镀方法,显著提高镁合金的腐蚀抗力。在对直接化学镀镍工艺详细分析的基础上,开发了新型化学镀镍方法；突破了直接化学镀的传统工艺,采用先涂敷后化学镀的复合镀新工艺路线,成功获得了表面致密均匀的银、铜、镍镀层,使镁合金的耐蚀性大幅度提高。本文首先系统分析了传统镁合金直接工艺镀镍工艺存在的问题。酸洗对样品的侵蚀比较严重,表面呈疏松多孔,这种疏松层易脱落,污染镀液,降低镀液使用寿命。更重要的是酸洗液中含有六价铬等对环境污染较严重的物质。化学镀时,镍在裸露的基体处沉积长大,快速连接形成镀层,过早地封闭了试样表面的疏松层,从而使基体与镀层之间存在大量孔洞。针对传统化学镀镍工艺中存在的不足,开发了一种新的直接化学镀镍工艺。设计了新的镀液体系,采用正交实验确定了新化学镀最佳工艺配方为：14g/L柠檬酸钠,10g/L氟化氢氨,10g/L氟化钾,pH=6,并分析了各个实验因素对沉积速度和耐蚀性的影响。与传统工艺相比,新工艺避免了使用氢氟酸和六价铬等对环境污染较严重的物质,简化了预处理步骤。新工艺镀层的自腐蚀电流比传统工艺的要低一个数量级。化学镀镍层对镁合金来说属于阴极性镀层,易与基体形成腐蚀电偶对,针对直接化学镀镍层的这一缺点,开发了一种新型复合化学镀工艺,即用先涂膜然后再化学镀的新方法对镁合金表面进行复合保护。经比较,涂膜材料选用了有机硅耐热漆。为了提高镀层与涂层的结合力,涂膜经过了粗化处理。为了使有机涂膜表面具备催化活性,进行了活化处理,以保证后续的化学镀能够顺利进行。本文首先对镁合金进行了新型复合化学镀银。确定了复合镀银的最佳条件,观察表明样品表面化学镀银的沉积过程属于岛状生长模式。极化曲线测试结果表明,镀后样品的自腐蚀电位比基体合金提高了约300mV。随后,对镁合金进行了新型复合保护化学镀铜处理,经过反复实验确定了化学镀铜的最佳反应条件：8～10g/L硫酸铜,30-35g/L次亚磷酸钠,10-20g/L柠檬酸钠,1g/L硫酸镍,pH=9,30-40g/L硼酸,温度70℃。化学镀60min后,镀层厚度可达7-8μm。镀覆后试样的自腐蚀电位比基体合金高约200mV。最后,本文对镁合金进行了新型复合保护化学镀镍处理,化学镀前要对试样进行胶体钯活化处理。化学镀后,样品表面沉积了了致密的镀镍层,它在3.5% NaCl溶液中腐蚀电流密度比基体合金低了约两个数量。将具有相同厚度镀层的直接化学镀镍工艺处理过的试样与新型复合化学镀工艺处理过的试样进行比较,浸泡腐蚀实验结果表明后者具有更好的耐蚀性。其主要原因是涂膜的存在将基体与镀层完全隔开,避免了基体-镀层形成腐蚀电偶对。新开发的复合化学镀工艺除了可以对镁及其合金进行化学镀以外,还适用于其它活性较高的材料,如钛及其合金,铝及其合金,锌及其合金等。更多还原

【Abstract】 This paper developed some new methods to improve corrosion resistance of magnesium alloy and solve its poor corrosion resistance the problem in application. After analysing the mechanisms of pretreatment and deposited process of electroless Ni plating, a new electroless Ni plating process was developed. After the tranditional electroless plating method was broken, a new idea-clad electroless plating process was proposed by using organic coating and electroless plating, the surfaces of magnesium alloys were deposited respectively compact Ag, Cu and Ni films so that the corrosion resistacne of the substrates increased greatly.The process of traditional electroless Ni plating on magnesium alloy was investigated first. The Chromic acid etching could remove metal oxides and rusts. However it resulted in the surface corrosed severely, which needed careful control of the etching time. After aicd eching, the surface was losse and porous, which would pollute the plating bath and decrease the using period. When the sample was immerged in the solution, Nickel was firstly deposited on the bare substrate, and then the nickel coating rapidly covered the whole surface, which resulted in closing early the loose layer on the surface and there were lots of holes between the coating and substrate.In order to avoiding these disavantages, this paper provided a new electroless plating Ni process. The optimum bath of the new electroless Ni plating was proposed by the orthogonal experiment,14g/L C6H8O7·H2O, 10g/L NH4HF2, 10g/L KF, pH=6. And the effects of the bath components on deposition rate and corrosion resisitance were discussed. Compared with traditional process, new process uses no poisonous materials, such as hydrofluoric and chromic acid and the process of pretreatment was simplifies. Moreover, the current density of coated sample decreased one order of magnitude than that of AZ31 magnesium alloy.However, the potentiometry of plating coating is higher than that of magnesium alloy, thus the coating and magnesium form easily corrosive couple. To avoid this disadvantage, new process-clad electroless plating was developed by electroless plating after organic coating (organicsilicon heat-resisting varnish), in which organic coating acts as interlayer between the substrate and coating. Compared with epoxide resin, the organic-sillicon varnish coated magnesium alloy more perfectly. The result of the cross-cut test indicated that the adhesion between the substrate and the interlayer reached level 2. In order to increase the adhesion between the interlayer and coating, the surface of interlayer was roughed, and then was activated for subsequent plating.Firstly, Ag was coated on the AZ31 magnesium alloy by clad electroless plating. X-ray diffraction and SEM analysis showed that growth of the silver film was like "island"model. The potentiodynamic polarization showed that, the potential of the sample clad electroless plating increased by 300mV compared with AZ31 magnesium alloy.Secondly, Cu was coated on AZ31 magnesium alloy by clad electroless plating. After plating, a compact Cu film was deposited the surface of the substrate. The tests showed that the optimum bath contains 8～10g/L CuSO4·5H2O,30～35g/L NaH2PO2,10～20g/LNa3C6H5O7, lg/L NiSO4·6H2O,30～40g/L H3BO3, pH=9, T=70℃The polarization curves showed that the sample clad electroless plating increased by 200mV.Lastly, Ni was coated on AZ31 magnesium alloy by clad electroless plating. A Ni-P film with fine and dense structure was obtained on the substrate. The electrochemical measurements showed that the sample clad electroless plating exhibited lower corrosion current density and more positive corrosion potential than that of AZ31 magnesium alloy. Furthermore, the Ni-P coating on the AZ31 magnesium alloy exhibited high corrosion resistance in the accelerated corrosion test.Clad electroless plating process developed in this paper was environmentally friendly, no fluoride or hexacalent chromium compounds. In addition, it provided a new concept for plating the metals, such as Al alloys and Ti alloys which were considered difficult to plate due to high reactivity.更多还原