【作者】 余会成；

【导师】 陈白珍；

【作者基本信息】 中南大学 ， 电化学工程， 2009， 博士

【摘要】 纯铝表面上有一薄层天然氧化物,它在空气及水中一般比较稳定,但纯铝缺乏某些必要的机械及物理性能,如较高的强度系数、弹性模量、抗蠕变强度、抗疲劳强度、硬度、耐磨性及较低的热膨胀系数。因此,纯铝在作为工程材料应用之前,通常与其它元素形成合金。一旦当铝与其它元素形成合金,铝表面上的氧化物的保护性能往往降低。为了提高铝合金的防腐蚀能力,必须进一步开发出有效且经济的表面改性技术。传统上使用六价铬转化膜来提高铝及其它金属如锌及钢铁的防腐蚀性能。然而,六价铬有毒,在电器及电子工业中,欧盟禁止使用六价铬化合物。因此对环保友好工艺的需求导致开发不同种类的无六价铬转化膜。最近的研究涉及到钛（Ti）、钼（Mo）、钨（W）、钴（Co）、锡（Sn）、锌（Zn）及稀土盐等新型转化膜,所有这些元素被认为低毒,在自然界也相对丰富。最近有专利报道过在铝合金上制备三价铬转化膜,然而进一步对其性能及机理研究的报道却很少。鉴于上述情况,作了以下详细的研究:在制备转化膜之前,使用单因素实验及正交实验系统地研究了添加剂(如铝离子(Al³⁺)、硼酸（H₃BO₃）、硫酸镍（NiSO₄）、缓蚀剂A及硫酸钴（CoSO₄）)在三酸（硝酸、硫酸及磷酸）中对6063铝合金光泽度及外观的影响。铝离子(Al³⁺)、硼酸（H₃BO₃）、硫酸镍（NiSO₄）、缓蚀剂A及硫酸钴（CoSO₄）的适宜浓度范围分别为:6至10 g L^-1、0.9至1.2 g L^-1、0.8至1.1 g L^-1、0.5至0.8 g L^-1及0.8至1.2 g L^-1。最优工艺条件:铝离子(Al³⁺)的浓度为6 g L^-1,硼酸（H₃BO₃）的浓度为1.2 g L^-1,硫酸镍（NiSO₄）的浓度为1.1 g L^-1,缓蚀剂A的浓度为0.5 g L^-1,硫酸钴（CoSO₄）的浓度为1.2 g L^-1。采用扫描电镜（SEM）对处理过和未处理过的铝合金表面形貌进行了分析。研究了一种新的6063铝合金三价铬转化膜工艺,采用单因素实验及正交法研究了五个变量（如沉积温度、时间、槽液pH及硫酸铬钾与磷酸的浓度）对三价铬转化膜的防腐蚀性能的影响,获得了比较适宜的成膜条件及最佳条件。适宜的条件如下:温度为30-40℃,沉积时间为9 min,pH为2.0-3.0,硫酸铬钾与磷酸的浓度分别为15-25 gL^-1及10-20 g L^-1。最优工艺条件:温度为40℃,pH为2,硫酸铬钾的浓度为（KCr（SO₄）₂）20 g L^-1,磷酸的浓度为（H₃PO₄）20 g L^-1。采用扫描电镜、能谱及光电子能谱对转化膜的形貌、成份及价态进行了表征,结果表明铬元素已沉积在铝合金表面上,价态为三价。采用极化曲线及交流阻抗技术研究了不同条件下形成的转化膜的电化学特性。极化曲线研究表明在适宜的条件下形成的转化膜具有更正的腐蚀电位(E_corr)、小孔腐蚀电位(E_pit)及较低的腐蚀电流(i_corr)。因此在适宜的条件下形成的转化膜具有较大的腐蚀阻力。为了解释转化膜的电化学特性,建立了一个简单的表面模型。实验发现预测的模型与实验结果能较好地相吻合。通过对交流阻抗谱的拟合,获得表面电阻、电容、荷转移电阻及双电层电容等电化学参数。扫描电镜（SEM）分析了未处理过和处理过的电极,发现其结果支持提出的表面模型。采用扫描电镜、能谱、极化曲线及交流阻抗技术研究了转化膜浸泡在氯化钠溶液中的腐蚀行为。按照不同的腐蚀时期及其特性,提出并解析了不同过程的等效电路模型,对交流阻抗谱进行了拟合。为了改善三价铬转化膜的电化学性能,尿素、硫脲、乙醇胺,二乙醇胺及三乙醇胺分别加入镀液中。采用极化曲线及交流阻抗技术研究了加入不同浓度的尿素、硫脲、乙醇胺、二乙醇胺及三乙醇胺之后的转化膜的电化学行为。结果表明加入少量的上述缓蚀剂后,防腐蚀性能明显得到改善,然而过度的添加,腐蚀阻力减少。实验还发现添加硫脲的效果好于添加尿素的转化膜,添加三乙醇胺的效果好于二乙醇胺,添加二乙醇胺的效果好于乙醇胺。为了解释转化膜的交流阻抗谱,使用了等效电路对交流阻抗谱进行拟合,并获得了相关参数。无论是极化曲线还是交流阻抗结果都显示加入缓蚀剂的效果要好于未添加的转化膜。采用扫描电镜、能谱及光电子能谱分析了转化膜的形貌、成份及价态。采用极化曲线及交流阻抗技术研究添加缓蚀剂尿素及硫脲后的转化膜在不同浸泡时间下的腐蚀行为。结果表明含缓蚀剂的转化膜能经受较长时间的浸泡。为更加深入理解和提高膜层的防腐蚀效果,研制了三种复合转化膜。研究了在不同硫酸镍（NiSO₄）、硫酸钴（CoSO₄）及硫酸锆（Zr（SO₄）₂）浓度下制备的复合转化膜的电化学性能。为了更好地解释转化膜的交流阻抗谱,也对交流阻抗谱进行了拟合,并获得了相关参数。实验发现Cr-Zr复合转化膜的防腐蚀性能大于Cr-Co复合转化膜,Cr-Co复合转化膜的防腐蚀性能大于Cr-Ni复合转化膜,Cr-Ni复合转化膜又大于非复合转化膜。提出了三种复合转化膜的缓蚀机理,并与含有机缓蚀剂的转化膜的缓蚀机理作了比较,缓蚀机理显然不同。采用扫描电镜、能谱分析了转化膜的形貌、成份,其结果表明三种复合转化膜已制备。最后,对三价铬化学转化膜的沉积机理进行了探讨,并采用光电子能谱（XPS）对提出的沉积机理进行了验证。应用分布系数及迭代法对铝及铬元素沉积的临界pH值进行了理论计算。更多还原

【Abstract】 Pure aluminum has a thin surface oxide layer which is generally stable in air and aqueous solution,but it lacks some necessary mechanical and physical properties,such as high specific strength,specific modulus of elasticity,creep strength,fatigue strength,hardness,wear resistance and low thermal expansion.Therefore pure aluminum is often alloyed with other elements for engineering applications.The protection imparted by the surface oxide to the Al substrate tends to decrease when Al is alloyed with other elements.This leads to further advancements in the development of effective and economic use of surface modification technologies with the aim of improving the corrosion resistance.Traditionally,chromate(Cr⁺⁶) conversion coatings have been successfully used on Al alloys and other metals such as Zn and steel to improve the corrosion resistance.However,Cr⁺⁶ species are toxic,and European Union has prohibited them in the electrical and electronic industries.Therefore the need for more environmentally friendly processes has led to the development of various Cr⁺⁶-free candidates. Recent research covers a wide variety of new chemical conversion coatings,including Ti,Mo,W,Co,Sn,Zn compounds and rare earth metal salts.All these elements are thought of low toxicity and are relatively abundant in nature.Recently,patents for trivalent chromium coatings on Al alloys have been reported.However,further study on the mechanism has rarely been reported.Considering the above situation,the following investigations are carried out in details:Before the conversion,the influence of the additives(i.e.Al³⁺ ion, H₃BO₃,NiSO₄,inhibitor A and CoSO₄) on the gloss and appearance was investigated systemically using the orthogonal and single factor experiments in three acids（HNO₃,H₂SO₄ and H₃PO₄）.The suitable concentrations of Al³⁺ ion,H₃BO₃,NiSO₄,inhibitor A and CoSO₄ were 6 -10gL^-1,0.9-1.2gL^-1,0.8-1.1 gL^-1,0.5-0.8gL^-1,0.8-1.2gL^-1, respectively.The optimal quantities of the additives were as follows:Al³⁺ ion 6 g L^-1,H₃BO₃ 1.2 g L^-1,NiSO₄ 1.1 g L^-1,inhibitor A 0.5 g L^-1, CoSO₄ 1.2 g L^-1.SEM was used to analyse the untreated and treated Al alloy surfaces. A new process for trivalent chromium conversion coatings on Al 6063 alloy has been developed by us,and the influence of five variables(i.e. deposition temperature,time,bath pH,and concentrations of Cr³⁺ compound（KCr（SO₄）₂） and H₃PO₄) on the corrosion resistance of Cr³⁺ coatings on Al 6063 alloy was investigated using the orthogonal and single factor experiments.The optimal conditions and the suitable ranges of the above variables were determined.The suitable conditions were obtained as following:temperature was 30 - 40℃,deposition time 9 min, the pH was adjusted to 2.0 - 3.0,the KCr（SO4）2 and H3PO4 concentrations were controlled within 15 - 25 g L^-1 and 10 - 20 g L^-1, respectively.The optimal conditions were as follows:temperature 40℃, Bath pH 2,KCr（SO₄）₂ 20 g L^-1,H₃PO₄ 20 g L^-1.The morphologies and composition and valence state of the coatings were analyzed by scanning electron microscopy（SEM） and energy dispersive spectrometry（EDS） and X-ray photoelectron spectroscopy（XPS）,respectively.The result indicates that the trivalent chromium chemical conversion coatings were made on aluminum alloy surface.Electrochemical properties of trivalent chromium coatings on 6063 aluminium alloy were studied.The polarization result showed that the coatings had more positive corrosion potential（Ecorr） and pitting corrosion potential（Epit）,and lower corrosion current density（icorr）, indicating that the coatings formed under the optimum conditions had better corrosion resistance.To explain the electrochemical properties of the coatings,A simple surface model was derived and experimentally tested in terms of an equivalent circuit.Good agreement was found between the model predictions and the experimental results. Electrochemical parameters of EIS,such as the surface resistance（Rcoat） and capacitance（Qcoat）,the charge transfer resistance（Rct） and double layer capacitance（Qdl）,were obtained by fitting the EIS plots.The morphologies of the coated and uncoated electrodes were examined by scanning electron microscopy（SEM）,and the results also support the proposed surface model.The corrosion behavior of the coatings was studied by polarization curves and electrochemical impedance spectroscopy at the different immersion times and temperatures.The different equivalent circuit models were also designed and explained according to the different corrosion periods and their properties and carried out simulating.SEM and EDS were used to confirm the mechanism.In order to improve the electrochemical performance of trivalent-chrome coatings,urea,thiourea,monethanolamine（MEA）, diethanolamine（DEA） and triethanolamine（TEA） inhibitors were added into the deposition bath.The electrochemical behaviors of coatings formed with different concentrations of urea,thiourea, monethanolamine（MEA）,diethanolamine（DEA） and triethanolamine （TEA） were investigated in 3.5wt%NaCl solution at 25 oC,using potentiodynamic polarization curves and EIS.The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea,TEA better than DEA, DEA better than MEA.To explain the EIS results of the coatings,a simple equivalent circuit was applied.The EIS parameters were obtained by fitting the EIS plots.The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor.The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS,respectively.The results indicated that the trivalent chromium coating was developed on Al 6063 alloy,inhibitors were also deposited on the substrates,respectively.A noticeable chemical shift was also observed.The corrosion behaviors of the inhibitor-containing（urea and thiourea） coatings were studied by polarization curves and electrochemical impedance spectroscopy at the different immersion times,the results show that the inhibitor-containing coatings remained unchanged after a longer immersion.To deeply understand and improve performance of the coatings,three complex coatings were prepared.The electrochemical behaviors of the complex coatings formed with different concentrations of NiSO4, CoSO4,Zr（SO4）2 were also investigated.To better explain the EIS results of the coatings,the EIS parameters were also obtained by fitting the EIS plots.The corrosion resistance of Cr-Zr complex coating is larger than that of Cr-Co,the Cr-Co larger than Cr-Ni,and Cr-Ni larger than the basic coating.A mechanism was also proposed and compared with the inhibition mechanism of organic inhibitor-containing coatings,the mechanism is obviousely different.The morphology and composition of the conversion coatings were examined by SEM and EDS,respectively. The results showed that the three complex coatings were prepared.At last,a deposition process model for trivalent chromium conversion coatings was proposed,the XPS was used to confirm the model.In theory, the critical pH of Al,and Cr deposition was calculated using the distributing coefficient and the method of simple iteration.更多还原