【作者】 孙道明；

【导师】 李劲；

【作者基本信息】 复旦大学 ， 物理电子学， 2009， 博士

【摘要】 点腐蚀（Pitting Corrosion）是钝化态金属表面一种常见的腐蚀形态。根据点蚀发展过程的特点,可以将点蚀分为点蚀萌生（钝化膜的溶解破裂）、亚稳态点蚀、稳态点蚀生长（或者点蚀区域金属表面再钝化）三个阶段。工程应用中,铝合金表面常发生点蚀,并可能引发其他恶性腐蚀形式。另一方面,由于稀土离子对铝合金点蚀的有效抑制以及稀土元素的环境友好特性使铝合金的稀土表面改性工艺广受关注。为理解铝合金表面点蚀发生发展过程中的重要阶段和稀土离子抑制点蚀发展机理两方面的问题,本文进行了以下两个主要方面的工作。一,点蚀过程研究:采用Ex situ TEM研究铝合金萌生点蚀的位置,并用铅笔电极（pencil electrode）模拟稳态点蚀在NaCl溶液中的生长。二,稀土沉积研究:采用Ex situ TEM研究稀土离子在铝合金表面沉积的机理,并在铈盐的乙醇溶液中对铝合金进行阴极电泳沉积稀土改性膜。采用Ex situ TEM研究了铝合金表面诱发局部腐蚀的金属间化合物相颗粒的晶体结构、EDS成分、以及在含氯离子溶液中的腐蚀行为三者的关系。结果表明铝合金表面的异质相颗粒（如金属间化合物相颗粒）确为局部腐蚀的诱发位置。一般而言,成分不同的异质相颗粒在溶液中电化学电位不同,因此导致其在溶液中的腐蚀行为也不相同。而Ex situ TEM观察表明成分相似的金属间化合物相颗粒也会在溶液中表现出不一致的局部腐蚀行为,进而对有此类现象的颗粒进行晶体结构分析发现两者的结构相差甚大。因此,腐蚀行为的差异可能源于成分相同但结构相异的异质相颗粒具有不同的电化学电位。采用直径为0.076cm的铝丝制成铅笔电极模拟铝在NaCl溶液中点蚀生长动力学。电极测试过程中点蚀电流表现出暂态和准稳态两个阶段。暂态过程电极表面的反应过程可以近似地用半无限扩散模型来解析。准稳态过程电流-时间曲线为I正比于t^-0.5,这说明铅笔电极的点蚀生长是由某种反应产物的扩散/迁移控制。用显微镜精确聚焦技术测量点蚀坑深度校核用法拉第定律积分计算点蚀电流获得的点蚀坑深度发现金属铝丝铅笔电极点蚀生长的驱动力并非和铁基/镍基合金的生长驱动力（由蚀坑内外盐浓度梯度驱动）一样。铝稳态点蚀生长是点蚀坑内化学反应析氢形成气相在点蚀通道中的迁移速率控制。Ex situ TEM研究了铝合金在0.1MCe（NO₃）₃无水乙醇溶液中的行为,实验确定铝合金表面的异质相是稀土离子吸附/沉积的优先位置。在分析了铝合金表面优先吸附异质相的性质、沉积膜层的结构和成分,以及铈盐乙醇溶液的特点后,认为稀土离子乙醇溶液中铝合金表面的微原电池区域阴极反应不同于水溶液中的阴极反应。在水溶液中反应优先生成Ce₂O₃膜,而在乙醇溶液中是形成四价的氢氧化铈后脱水生成CeO₂膜。Ex situ TEM观察稀土离子沉积确定Ce离子表面沉积的影响因素有电位、pH值、和氧化剂等。采用阴极电泳对铝合金快速沉积稀土改性膜。实验结果表明在0.1MCe（NO₃）₃乙醇溶液中用12V电压阴极电泳60秒能获得保护性能优异的稀土改性膜。EIS测试结果表明沉积稀土膜层改性的铝合金试样在0.1MNaCl溶液中31天浸泡之后表面仍然未见明显腐蚀。电化学阻抗谱（EIS）分析表面沉积稀土试样的钝化膜电阻和表面电容在各个浸泡周期中都维持在10^-7ohm cm^-2和10^-6F cm^-2的量级。由于稀土改性处理的铝合金在溶液中阴极反应由于受稀土元素的抑制,局部腐蚀倾向显著降低。更多还原

【Abstract】 The evolution of corrosion pits on aluminum alloy immersed in chloride solution occurs in three distinct stages: nucleation, metastable growth and stable growth. This paper describes the initiation of corrosion pits and stimulates the pitting growth on aluminum alloy immersed in chloride solution. The rate of growth of stimulated individual corrosion pits is found to be controlled by transfer rate of the dissolving hydrogen which is produced on the electrode interface.Because current environmental legislations moving towards total exclusion of Cr^6- and because of tightening regulatory pressure to reduce the hazardous waste of chromium, many attempts are being made to develop non-toxic alternative methods of corrosion protection. Another object of present research has focused on rare-earth （RE） surface modification with emphasis on elimination of hazardous chemicals used in corrosion protection of aluminum alloy.Ex situ TEM was used in conjunction with EDS to monitor evolution near the preselected inclusions during initial stages of localized dissolution on the AA 6061 immersed in 0.1M NaCl solution. On the alloy TEM foil, Mg₂Si phase particles were observed to preferentially dissolve after 3 minutes immersion. Nevertheless, different dissolution behaviors were observed to form in the vicinities of the different structure Fe-rich precipitates; the trenches were observed nearby the anorthic phase particle, but no obvious priority dissolution was detected in the vicinity of hexagonal phase particles. The preferential sites of alloy dissolution were found to depend on both the component and structure of the intermetallics. The shift of electrochemical potential was found to relate the dissolution of the heterogeneous phases.The importance of pitting corrosion was investigated under potentiostatic dissolution conditions with pencil electrodes contained in inert supports. The artificial cavities created simulated localized corrosion conditions. Current-time behavior at voltages in excess of the critical pitting potential （-300mV vs. SCE） was examined for pure aluminum specimens in concentrated chloride solutions. The effect of changing the concentration of the dissolving metal cations within the artificial cavity was studied by altering the composition of the bulk solution. Solutions of NaCl ranging from 0.01 to 4M were used. Mass transfer models were developed for the observed transient and quasi steady-state periods of dissolution.Ex situ TEM technique was also used to monitor deposition behavior of cerium oxide near the pre-selected inclusions on the AA 5083 immersed in 0.1 M cerous ethanol solution. On the 5083 aluminium alloy TEM foil, inclusions were observed to preferentially depositing sites after 30 minutes solution treatment. Nevertheless, different deposition rate behaviors were observed to produce in the vicinities of precipitates; and the cerium oxides were observed in open cavities area near Si-rich inclusions, but no cerium oxide was detected on the substrates of aluminum. The preferential sites of cerium oxide dipositon were found to depend on both the corrosion potential the heterogeneous phases; also on the oxidizer, and pH of the solution.In the research devoted to the elimination of toxic materials in methods of surface modification for corrosion protection, deposition cerium oxide by cathode electrophoresis processes have been developed to improve the pitting resistance of aluminum alloys without the use of chromates. Impedance data collected in 0.1 M NaCl remained capacitive for 31 days, which is indicative of the lack of localized corrosion. Apparently local cathodes are eliminated during the surface RE modification process, thereby reducing the driving force for pitting.更多还原