【作者】 张军军；

【导师】 李文芳；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2010， 博士

【摘要】 使用六价铬转化膜来提高铝合金的耐腐蚀性能,是最常用的表面处理技术之一。然而,六价铬有毒,在电器及电子工业中,欧盟已经禁止使用,因此对环境友好的替代铬技术的研究与开发成为当今铝合金表面急需解决的问题。稀土转化技术就是其中最有希望替代铬化处理的技术。目前大多数稀土转化成膜工艺复杂,处理的温度较高,时间较长等限制了其规模化的应用。本文针对上述问题研究了一种高效、环保、金黄色的稀土转化膜室温制备技术,并对其成膜机理和耐腐蚀性能进行了系统而深入的研究。首先确定了以Ce（NO₃）₃为主盐和KMnO₄为氧化剂的常温转化液基础工艺。采用正交实验和单因素实验研究了四个变量（包括沉积时间、槽液pH及硝酸铈和高锰酸钾的浓度）对Ce-Mn转化膜的防腐蚀性能的影响,获得了该体系的优化工艺条件:时间30min、pH=2.0、Ce（NO₃）₃和KMnO+4的浓度分别为10g/L和2g/L。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,转化膜中铈元素的价态为三价和四价,锰元素的价态为四价。为了提高Ce-Mn转化膜的常温成膜速度和耐腐蚀能力,以前述基础工艺为前提选择添加H₃BO+3、Zr（SO₄）₂、NaF、HF、NaBF₄和Na₂ZrF₆为成膜促进剂。实验发现,NaF和NaBF+4能有效地提高成膜速度,9min就可以在铝合金表面生成金黄色的转化膜,耐腐蚀能力明显得到改善。无论是极化曲线还是交流阻抗结果都显示,NaF是其中最好的成膜促进剂。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,结果表明添加NaF后,转化膜膜层更均匀,铈和锰的含量增加,铈元素的价态为三价和四价,锰元素的价态为四价。选择NaF为成膜促进剂,采用正交实验和单因素实验,进一步优化了制备Ce--Mn转化膜的工艺。获得了最佳工艺参数为10 g/L Ce（NO₃）₃ + 2 g/LKMnO₄ + 0.6g/L NaF, 9 min。通过大量实验发现,前处理对转化膜的成膜过程和性能至关重要。通过优化获得了一种新的前处理工艺:即在混合酸溶液（HNO₃（100mL/L）、Na₃PO₄（10mL/L）、HF（10mL/L）、OP-10（1.5g/L）和硫脲（0.5g/L））中室温下浸泡3分钟→1%NaOH溶液中浸渍30秒。就前处理对转化膜成膜过程的影响规律进行了深入研究,发现应用混酸处理后,没有以往前处理工艺中铁,铜和锌的溶解--再沉积过程的发生,表面阴极点分布均匀细密,沉积速率大幅提高,生成的膜厚且致密均匀,使Ce-Mn转化膜耐腐蚀能力得到较大提高。深入研究了转化膜的成膜原理。对各元素沉积的临界pH值进行了理论计算,采用极化曲线及交流阻抗技术研究了成膜时间、转化液pH值以及硝酸铈、高锰酸钾和氟化钠的浓度对成膜过程的影响;建立了转化膜成膜机理模型,采用光电子能谱（XPS）验证了机理模型的合理性。对Ce-Mn转化膜处理后的样品进行热喷涂实验,结果显示本研究提出的稀土转化技术符合国标GB 5237.4-2004第四部分,粉末喷涂型材的要求,达到国家标准。本课题研究的Ce-Mn膜转化技术,可以使6063铝合金表面生成金色的化学转化膜,具有良好的耐腐蚀性能。该处理工艺简单,无毒副作用,有望成为铝型材铬酸盐化学转化处理的替代技术。更多还原

【Abstract】 As one of the most popular surface treatment technologies, chemical conversion coating based on Cr⁶⁺ is widely used to improve the corrosion resistance of aluminium alloys. However, Cr⁶⁺ is toxic and its application has been prohibited in electrical and electronics industries by the European Union. There is an emergent need to develop an environmentally friendly chromate-free technology for the surface treatment of aluminium alloys. Rare-earth metals conversion coating is one of the most promising green alternatives.Currently most rare-earth metals conversion coating technologies are complicated, time consuming and elevated temperature is needed for the treatment. Its industrial application is thus limited. In this thesis, a high efficient, environmentally friendly conversion coating technology based on rare-earth metals is presented. The treatment is carried out at room temperature and the coating is of golden color. The detailed mechanism of coating formation and the reason for corrosion resistance enhancement are discussed.The room temperature treatment solution is based on Ce（NO₃）₃ and KMnO4. Four parameters （deposition time, pH value, the concentration of Ce（NO₃）₃ and KMnO4） were investigated for the influence on corrosion resistance of Ce-Mn conversion coating by orthogonal and single factor experiments, respectively. The solution composition and treatment parameters were optimized as follows: treatment time is 30 min, pH = 2, and the concentrations of Ce（NO₃）₃ and KMnO4 are 10g/L and 2g/L respectively. Scanning electron microscopy （SEM）, energy dispersive spectrometry （EDS） and X-ray photo-electron spectroscopy （XPS） were used for the conversion coating characterization. Ce was found in the coating with the valence state of +3 and +4, while the valence state of Mn was +4.In order to improve the corrosion resistance and room temperature formation speed of Ce-Mn conversion coatings,H₃BO₃、Zr（SO₄）₂、NaF、HF、NaBF₄和Na₂ZrF₆ were selected as additives in to the above mentioned treatment solution. Both NaF and NaBF4 can effectively increase the coating formation speed and in dued golden conversion coating with exellent corrosion resistance. As indicated by both potentiodynamic polarization curves and electrochemical impedance spectroscopy （EIS）, NaF is the best additive for the forming of the coating of high quality. The solution composition was further optimized as: 10 g/L Ce（NO₃）₃ + 2 g/LKMnO₄ + 0.6 g/L NaF, the coating process only 9 min.It is interesting to note that pre-treatment process is crucial to the formation speed and performance of conversion coating. A new pre-treatment was developed: 3 min soaking in home made solution （100mL/L HNO₃ + 10mL/L Na₃PO₄ + 10mL/L HF + 1.5g/L OP-10 + 0.5g/L thiourea） at room temperature, followed by 30 seconds soaking in 10 wt.% NaOH. It was found that after such pre-treatment, there was no dissolution and re-precipitation of iron, copper and zinc during conversion coating process. Cathodic area was small and distributed closely and uniformly on the aluminium alloy surface. The coating formation speed was raised and the resultant coating was compact and uniform. The coating corrosion resistance was enhanced.The formation mechanism of conversion coating was investigated. The critical pH value for the deposition of different elements was calculated. The effects of treatment conditions （time, pH value, concentration of Ce（NO₃）₂, KMnO₄ and NaF） on the growth of conversion coating was studied by electrochemical techniques. A model of conversion coating formation mechanism was put forward and justified by XPS.Thermal spray was carried out on aluminium alloy after Ce-Mn conversion coating treatment. The conversion coating technology developed in this study was shown to meet the Part 4 of National Standard GB 5237.4-2004 as required by powder coating profiles. A golden conversion coating with good corrosion resistance was achieved on 6063 aluminium alloy. Such conversion coating treatment on aluminium profiles is simple and environmentally friendly, and might be used as an alternative for the treatment that is based on toxic chromate.更多还原