Observation of barrier layer during ac electrolytic colouring of anodized aluminium

The relationship between the interfacial impedance change and the morphological change during ac electrolytic colouring of anodized aluminium has been studied. The cross-section of electrolytically coloured film was observed under an electron microscope and the interfacial impedance was measured in a mixed aqueous solution of nickel sulphate and boric acid, immediately after ac electrolytic colouring.It was found that the interfacial impedance of electrolytically coloured films was larger than that of uncoloured ones. From microscopic observation, it was known that the barrier layer thickness was more than twice as large as the initial barrier layer thickness during ac electrolytic colouring. It was confirmed that the increase in interfacial impedance during ac electrolytic colouring was due to the growth of a new barrier layer.     

Evaluation of corrosion resistance of aluminium anodic oxide film by Hunter's highly sensitive method

The effect of impurities or alloy component in aluminium on the formation and corrosion resistance of the barrier layer was studied by techniques for producing highly sensitive i—V (current: 10^?9 A/cm²) curves, a dipping test, scanning electron microscopy and transmission electron microscopy. Results were obtained as follows: (a) Hunter's highly sensitive method, it was demonstrated that aluminium of 99.99%, 99.91%, 99.46%, 991.61% and 99.15% purity provided more defects in the same order as the purity; (b) measurements of i—T curves and the dipping test demonstrated that the corrosion resistance of the samples increased in the same order as the purity; and (c) observations by scanning and transmission electron microscopy suggested that the cells decreased in number and were uniform in size, in the same order as the purity noted above. These defects appeared to contribute to pitting corrosion.As noted above, it is concluded that pitting corrosion on aluminium anodic films is dependent on the defects in the barrier layer and the highly sensitive Hunter's method, which has a high sensitivity, is an appropriate means of measuring the defects quantitatively.     

Synthesis and electrochemical evaluation of polypyrrole coatings electrodeposited onto AA-2024 alloy

Polypyrrole coating was successfully deposited on anodized 2024 unclad aluminum alloy showing that the presence of the anodic film is the key factor to ensure better adhesion to the polymer coating. Corrosion resistance of the conductive polymer layers grown for three and five cycles were evaluated in a 3 wt.% NaCl solution using polarization curves and impedance spectroscopy. It was found that the thermally treated polymer coating with higher thickness (five cycles) exhibited the best corrosion performance. This best coating shifts the corrosion potential towards nobler values, about 650 mV, and the exchange current density decreases two orders of magnitude regarding the anodic layer. The partial reduction of the structure of the polymer promoted by the thermal treatment plays a key role in the corrosion resistance of the coating allowing to the thermally treated polymer layer to act as a physical barrier against corrosion.     

电压对7N01铝合金阳极氧化膜结构及耐蚀性的影响

采用160 g/L硫酸溶液在17°C下对7N01铝合金阳极氧化30 min,氧化电压分别选取14、15、16、17和18 V。用扫描电镜观察所得阳极氧化膜的形貌,用能谱仪和电化学测量分析了它的成分、厚度和耐蚀性。结果表明,7N01铝合金经过不同电压下的阳极氧化处理后,表面均能形成凹凸不平并有孔洞的阳极氧化膜,电压为17 V时所制膜层致密、均匀,厚度约为7.6μm,耐蚀性最佳,在3.5%NaCl溶液中浸泡1440 h后没有明显的腐蚀。     

Corrosion behavior of 2024 aluminum alloy anodized in presence of permanganate and phosphate ions

In this article, the effects of two inorganic corrosion inhibitors, permanganate and phosphate anions, on morphology, composition, thickness, and corrosion resistance of the anodic film formed on 2024 aluminum alloy in sulfuric acid were investigated. Surface morphology of the oxide films were studied by field emission scanning electron microscopy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests were conducted to assess the corrosion resistance of the coatings. These analyses showed that the presence of the mentioned inhibitors changed the coating morphology, especially in the case of phosphate ions. Corrosion results indicated that using inhibitors during the anodizing process increased the corrosion resistance of the anodized samples. The increases were approximately 81% and 97% for 0.05 M of permanganate and phosphate ions, respectively, compared with the anodized coatings in the absence of additives. An increase in the inhibitor concentration results in better corrosion resistance of the aluminum anodic layer.     

Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et₄NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current-time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br⁻ anions. The pitting potential (E_b) decreased with the increase of solution temperature and Br⁻ concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current-time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.     

Nature and stability of anodic oxide films formed on molybdenum in chloride solutions

The formation and stability of anodic oxide films on molybdenum in chloride solutions was tested using impedance and polarization measurements. The efficiency of oxide formation increases as the acidity of the formation medium increases. The film is highly stable and resistive to attack and dissolution regardless of the chloride concentration, pH or film thickness. The corrosion potential did not vary with variation of immersion time, solution composition or film thickness and recorded 0.000V vs SCE indicating the high insulating properties of the film. Polarization measurements on previously anodized molybdenum electrode showed that the electrode is ideally polarized over a potential region not less than 2V. The magnitude of that potential region increases as the film thickness increases. The anodic film cannot be reduced or removed by galvanostatic cathodic polarization. The impedance behaviour of the anodized molybdenum electrode was found to be purely capacitive and the oxide film may be treated approximately as a perfect dielectric material.     

Electrochemical studies of the pitting corrosion of tin in citric acid solution containing Cl

The electrochemical behavior of a tin electrode in citric acid solutions of different concentrations was studied by electrochemical techniques. The E/I curves showed that the anodic behavior of tin exhibits active/passive transition. Passivation is due to the formation of Sn(OH)₄ and/or SnO₂ film on the electrode surface. Addition of NaCl to citric acid solution, enhances the active dissolution of tin and tends to breakdown the passivity at a certain breakdown potential. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second stages involve pit nucleation and growth and third stage involve repassivation. The impedance spectrum of pure Sn is found to consist of three intersecting capacitive semicircles at the high and medium frequencies with an inductive loop at low frequencies. The capacitive semicircles occurring at the high and medium frequency are due to the dielectric properties of surface oxide film and dissolution of underlying metal, respectively. The inductive loop at low frequencies results from Cl⁻ adsorption at the pitting region. By increasing the potential the pitting corrosion and the fractal dimension of surface due to pitting increase.     

汽车用铝合金阳极氧化及双重封闭工艺的研究

对汽车用2024铝合金进行了硫酸阳极氧化处理,并使用硫酸镍结合铬酸钾的双重封闭技术对阳极氧化膜进行了封闭处理。结果表明:在2024铝合金表面制备的阳极氧化膜属于典型的多孔膜。经过硫酸镍一次封闭处理后,大量镍的氢氧化物填充于膜孔内,有效地降低了阳极氧化膜的自腐蚀电流密度。经过铬酸钾二次封闭处理后,铬酸镍进一步覆盖在膜孔表面,大大提高了阳极氧化膜的均匀性和致密性。经过双重封闭处理后,阳极氧化膜的阻抗明显增大,耐蚀性大大提高。     

Formation of Al2O3–Nb2O5 composite oxide films on low-voltage etched aluminum foil by complexation–precipitation and anodizing

Niobium pentaoxide (Nb₂O₅) thin films were deposited on etched aluminum foils by complexation–precipitation followed by heat treatment. Then the Al₂O₃–Nb₂O₅ (Al–Nb) composite oxide films were formed by anodizing to increase the capacitance of anodized aluminum foils which are used in aluminum electrolytic capacitors. The composition and structure of niobium deposition layer were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), and the microstructures and dielectric properties of anodic oxide films were investigated by scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS) respectively. The results show that the niobium deposition layer after heat treatment existed in the form of crystalline Nb₂O₅. The aluminum foil with Nb₂O₅ coating can be anodized with higher efficiency and energy saving. Compared with that of normal anodized aluminum foils, the effective area of the anodized aluminum foils with Al–Nb composite oxide films had no apparent change. The specimens with Al–Nb composite oxide films anodized at 30 V exhibited about 20% higher specific capacitance than that of those with pure aluminum oxide films. It suggests that the method of complexation–precipitation is an effective way to increase the specific capacitance of anodized aluminum foils used in aluminum electrolytic capacitors.     

LD10连接座表面铬酸阳极氧化白斑原因分析

通过体视显微镜观察、扫描电镜及X射线能谱仪分析、点滴实验等检测手段对送检的LD10铝合金连接座表面铬酸阳极氧化膜层出现的白斑现象进行原因分析.表面白斑区域的阳极氧化膜层呈现出龟裂、破碎、脱落、点蚀坑洞等形貌特征,并含有异常质量分数的氯元素;连接座螺纹深孔内存在典型的点蚀特征,并含有外来的腐蚀介质元素.分析结果表明:螺纹...     

Evidence for formation of metallo-siloxane bonds by comparison of dip-coated and electrodeposited silane films

It has been amply demonstrated that thin films of organofunctional silanes deposited by dipping or spraying on metals, such as aluminum, can provide protection against various forms of corrosion. In this paper we show that denser films with higher pore resistance and better corrosion protection performance can be obtained if the silane film is produced by electrodeposition rather than by dipping. In such a process the silane reacts with the metal oxide in a different way, and in the case of aluminum, aluminate ions seem to be incorporated into the silane film. The resulting films can protect aluminum alloys against uniform and pitting forms of corrosion in a salt solution for more than 1000 h, which is comparable to the standard chromate-based treatments. This paper discusses the protection mechanism in some detail. An important aspect of this work is that the use of XPS and TOFSIMS to analyze electrodeposited silane films provided direct evidence for the presence of O₂Al(O–Si–O) and OAl(O–Si–O)₂ groups at the silane–aluminum oxide interface. Such bonds have been suggested, but they have largely remained elusive over the years.     

排球挂钩用铝合金基材阳极氧化处理及耐蚀性研究

使用铝合金挂钩对装入网兜的排球进行悬挂和管理,具有取用方便、节约空间、美观等优点。使用硫酸阳极氧化技术对排球挂钩用5005铝合金基材进行了处理,并对阳极氧化膜的性能进行了研究。结果表明:阳极氧化膜主要由铝、氧、硫、碳元素构成,其中铝和氧的总质量分数超过80%,少量的硫来自硫酸。经过硫酸阳极氧化处理后,铝合金表面形成高硬度和高熔点的a-Al2O3和y-Al2O3,大大提高了铝合金的硬度。阳极氧化膜由高电阻的阻拦层和多孔层构成,可以有效地分散和降低自腐蚀电流密度,大大提高铝合金基材的耐蚀性。     

铝合金上锂盐抗蚀层的研究

由于名酸盐对环境的毒害，铝合金表面耐蚀处理采用锂盐转化膜代替铬酸盐转化膜。本文介绍了铝合金上制备锂盐抗蚀层的工艺过程。利用电化学阻抗叔谱技术结合ＸＰＳ、ＳＥＭ等表面技术来表征和评价抗蚀层在氯化钠沉吟液中的耐蚀性能及孔蚀的发生。采用破损函数Ｄ＝ｌｏｇ（Ｚｏ／Ｚｒ）０．１Ｈｚ亚有征抗蚀性能随时间的衰退变化。     

Fabrication of three-dimensional platinum microstructures with laser irradiation and electrochemical technique

Three-dimensional (3D) platinum microstructures were fabricated by successive procedures: aluminum anodizing, laser irradiation, nickel/platinum electroplating, and removal of the aluminum substrate, the oxide films, and the nickel metal layer. Aluminum plates and rods were anodized in an oxalic acid solution to form porous type oxide films. The anodized specimens were immersed in a nickel electroplating solution, and then irradiated with a pulsed Nd-yttrium aluminum garnet (YAG) laser beam to remove the anodic oxide film with a three-dimensional XYZθ stage. The specimens were cathodically polarized in the nickel and a platinum electroplating solution to form the metal micropattern at the laser-irradiated area. The electroplated specimens were immersed in NaOH solution to dissolve the aluminum substrate and the oxide films, and then immersed in HCl solution to dissolve the nickel deposits. A platinum grid-shaped microstructure, a microspring, and a cylindrical network microstructure with 50-100 μm line width were obtained successfully.     

Characterization of porous aluminium oxide films from a.c. impedance measurements

An equivalent circuit (EC) that reproduces the a.c. impedance of porous aluminium oxide films in a highly approximate manner is proposed. The results reveal that electrochemical impedance spectroscopy (EIS) is a powerful tool for obtaining detailed information on the electrochemical properties of both the porous and barrier layer on which the corrosion resistance of aluminium depends. The impedance at a given frequency can be used for accurate calculation of the electrochemical parameter for the oxide film represented by each element of the EC. In this way, the effects of any factor on sealing and ageing of anodized aluminium oxide films can be precisely analysed. The EIS technique provides an effective, advantageous alternative to existing seal quality control tests.     

Ellipsometric measurements of the barrier layer in composite aluminum oxide films

The composite oxide film (hydrous + anodic) formed on aluminum foil was chemically stripped to remove only the outer hydrous layer. Ellipsometry of the remaining barrier film showed it to be thinner and have a higher refractive index than conventional anodic barrier films grown to the same voltage. Reanodization to improve film stability gave a further increase in refractive index which indicated that the barrier film had contained some voids. The barrier film is almost entirely crystalline γ-Al₂O₃ and the higher field strength compared with conventional amorphous anodic films on aluminum is believed to be a characteristic of the denser oxide.     

Complex anticorrosion coating for ZK30 magnesium alloy

This work aims at developing a new complex anticorrosion protection system for ZK30 magnesium alloy. This protective coating is based on an anodic oxide layer loaded with corrosion inhibitors in its pores, which is then sealed with a sol–gel hybrid polymer. The porous oxide layer is produced by spark anodizing. The sol–gel film shows good adhesion to the oxide layer as it penetrates through the pores of the anodized layer forming an additional transient oxide–sol–gel interlayer.The thickness of this complex protective coating is about 3.7–7.0 μm. A blank oxide–sol–gel coating system or one doped with Ce³⁺ ions proved to be effective corrosion protection for the magnesium alloy preventing corrosion attack after exposure for a relatively long duration in an aggressive NaCl solution.The structure and the thickness of the anodized layer and the sol–gel film were characterized by scanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with the complex coating was tested by electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET), and scanning ion-selective electrode techniques (SIET).     

Inhibition of CuNi10Fe corrosion in seawater by sodium-diethyl-dithiocarbamate: an electrochemical and analytical study

The efficiency of sodium-diethyl-dithiocarbamate (NaDDTC) as corrosion inhibitor for CuNi10Fe alloy was studied in natural seawater by means of immersion tests, polarization measurements, electrochemical impedance spectroscopy (EIS) and Auger electron spectroscopy (AES). After immersion of the alloy for 24 h in various concentrations of NaDDTC, layers were formed which act as a physical barrier to the corrosion attack. The polarization curves obtained with electrodes in seawater indicate retardation of both the anodic and cathodic processes. From EIS measurements equivalent circuits, which illustrate the behaviour of the reaction product film, were proposed and individual circuit elements at the open circuit potential and anodic polarization were defined. The results obtained for inhibitor efficiency from weight losses, corrosion currents and from impedance measurements are in fair agreement. The inhibition film does not lose efficiency even at anodic polarization, although the AES measurements indicate a change in the organic molecule structure. The hypothesis proposed is that a possible reconstruction of the organic molecule takes place, with release of CS₂.     

重金属离子对5052铝合金耐蚀性能的影响

采用70℃脱氧人工海水模拟低温多效蒸馏海水淡化设备内部的特殊腐蚀环境,分析了不同含量的重金属Cu2+、Fe3+对5052铝合金腐蚀情况的影响。极化曲线结果表明,溶液中的微量重金属离子(10-9级)也会在铝合金表面沉积,使铝合金自腐蚀电位正移,但却不会破坏铝合金表面自然氧化膜,使点蚀电位保持不变。而腐蚀失重结果表明,随着溶液中氯离子对氧化膜的侵蚀、破坏,表面沉积金属与铝基体的耦合将加快铝合金腐蚀过程。