Zinc impregnation of the anodic oxidation layer of 1050 and 2024 aluminium alloys

The porous oxide layer obtained by phosphoric anodic oxidation (PAO) of 1050 and 2024T3 aluminium alloys is modified by impregnation with zinc under alternating voltage. The resulting current against applied voltage relationship shows that a threshold voltage is required to deposit the zinc. Beyond a low critical voltage, V _{c L}, zinc electrocrystallization starts near the barrier layer and grows with time through the porous oxide layer whatever the alloy used as substrate. For the 2024T3 alloy, beyond a high critical voltage V _{c H}, S.I.M.S. analysis shows that zinc is also present on top of the oxide layer. The distribution of zinc particles depends on the porous layer morphology: formation of zinc needles in a columnar form (1050 alloy) and dispersion of zinc particles in a disorganized structure (2024T3 alloy). The polarization curves obtained in a 3 wt % NaCl solution show a decrease in anodic and cathodic currents indicating a protective effect of zinc impregnation, confirmed by electrochemical impedance analysis.     

The influence of the electrochemical treatment on Cu-Al-Ag alloys in deaerated 0.5 M NaOH

The effect of consecutive cyclic polarization in de-aerated 0.5 M NaOH solutions on the surface microstructure of mechanically polished Cu-Al-Ag alloys of different compositions and heat treatments has been studied using optical microscopy, SEM and EDS. The current peaks of the cyclic polarization curves do not depend on the alloy composition in the composition range studied. The repetitive potential scans between H₂ and O₂ evolution in alkaline media lead to preferential dissolution of aluminium, the roughness and phase composition of the surface of the alloys changing significantly. The quasistationaryI-E curves of the different Cu-Al-Ag alloys studied consist in the superposition of the quasistationaryI-E curves of high-purity Cu and Ag, the EDS microanalysis showing that aluminium is not present on the surface of the alloy in these conditions.     

Electrochemical impedance spectroscopic study of barrier layer thinning in nanostructured aluminium

The electrochemical behaviour of electropolished and anodised aluminium was studied by electrochemical impedance spectroscopy (EIS). Freshly electropolished aluminium behaves as a pure capacitor exhibiting Warburg impedance at low frequencies. Storage of the electropolished aluminium, even in an air-tight bottle, results in the reconstruction of a uniform compact barrier layer. The impedance response of a stored electropolished aluminium as well as anodised aluminium after oxide removal, done by chemical etching, exhibits only a capacitive loop in the complex plane. The effect of the oxide layer thickness on the impedance data was investigated for layers formed during anodising at a cell potential of 15 or 23 V. Impedance measurements carried out over a wide range of frequencies gave useful information on the efficiency of the thinning of the barrier layer at the bottom of porous aluminium oxide layers. The rate of thinning of the barrier layer was estimated for samples anodised at different voltage.     

Characterization of PTFE-bonded porous carbonelectrodes tested in a 100W phosphoric acid fuel cell(PAFC) stack using XPS and ICP–AES techniques

A 100W PAFC stack with 12 cells was assembled using in-house developed PTFE-bonded gas diffusion porous carbon electrodes, graphite bipolar plates and aluminium external gas manifolds. The stack was operated for 1000h continuously with acid management, using H2 and air at 1bar and at 175°C. After completion of the test the stack was disassembled and the electrodes were characterized using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma–atomic emission spectroscopy (ICP–AES) techniques. The XPS and ICP–AES results revealed the presence of platinum in the electrolyte matrix layers (SiC+PTFE) and carbon mat layers (carbon+PTFE), which were applied on the cathodes and the anodes, respectively. This clearly indicates that there was a migration of platinum from cathodes to anodes during the stack operation. This may have occurred when operating the stack at it's open circuit voltage (OCV) while taking measurements of stack voltage and curre nt for the I/V curves.     

Characterization of corroding aluminium alloys with electrochemical noise and electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) and spontaneous voltage fluctuation measurements were made on three orthogonal faces of 7039-T64 and 2519 aluminium alloy specimens during 10 day exposure periods into aerated 3% NaCl solutions. The spontaneous voltage fluctuations generated by the specimens were analysed with two techniques designed to reveal the stochastic character of these signals and their fractal dimension to establish practical correlation between noise measurements and the degree of pitting of the corroding aluminium specimens. Attempts were made to correlate the results of these analyses with parameters measurable by microscopic examination of the specimens or calculated from EIS measurements. During this study it was found that the slope of the voltage fluctuations and the depression angle of EIS results were both good indicators of the pit density observable on the corroded aluminium specimens.     

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.     

Influence of water in NaH2PO4ethylene glycol on the type and the formation kinetics of anodic films of lanthanum

The influence of water on the type, the formation behavior and the kinetic parameters of anodic oxide films of lanthanum in NaH₂PO₄ethylene glycol solutions containing various amounts of water was investigated by transmission electron micrographs, voltage—coulomb curves at constant current, current decay curves at constant voltage and the formation kinetics. The formation kinetic parameters such as the average field strength and Tafel slope, which were determined by the galvanostatic technique, were plotted against the metal—oxide bond energy. These results indicated that the film behaved as a barrier type when it was formed in a solution containing small amounts of water; however, it resembled a porous type in the higher water content solutions. It was suggested that the role of water varied with its content in the solution.     

Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries

Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 μm located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking.

The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference.

Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, K_IC, the critical energy release rate, G_IC, the J-Integral and the work of fracture, γ_WOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance.     

影响多孔阳极氧化铝膜结构特性因素的研究

酸性电解液中采用电化学方法可在铝表面形成多孔阳极氧化铝膜。研究了影响膜孔径和膜厚的关键因素,包括电解液、氧化电压、温度和时间。此外,通过X-射线衍射和扫描电镜,分析了形成氧化膜的微观形貌和晶体结构。实验结果表明多孔阳极氧化铝膜结构特性依赖于采用的氧化条件、电解液、氧化时间和电压的选择。     

Electrochemical behaviour of Al-In alloys in chloride solutions

The present work is concerned with the study of the activation mechanism of Al in alloys produced by indium. The electrochemical behaviour of aluminium in NaCl solutions containing In ³⁺ ions and the dissolution of Al-In and In-Al alloys were studied using potentiostatic, galvanostatic and potentiodynamic techniques, complemented by SEM. It was concluded that the aluminium activation is obtained only when indium comes into a true metallic contact with aluminium within an active pit and in the presence of chloride ions. Polarization curves for dissolution of Al-In and In-Al alloys were compared. The results suggest that the initial step in the dissolution mechanism of the Al-In alloy can be interpreted through chloride ion adsorption on a surface In-Al alloy. This adsorption occurs at more electronegative potentials than that of pure aluminium, thus avoiding repassivation.     

The role of iron(iii) and tartrate in the zincate immersion process for plating aluminium

The zincate immersion process is a commercial process used to treat aluminium prior to electroplating. Superior zinc coatings are obtained through modifications to the original process. One such modification involves the addition of ferric chloride, complexed with potassium sodium tartrate, to the plating baths. This paper describes a detailed investigation into the function of these additives. It was found that the addition of tartrate alone did not significantly affect the rate of zinc deposition, the deposit morphology or the position of the aluminium or zinc polarization curves. This suggests that the role of tartrate is solely as a complexing agent to maintain iron (iii) in solution. In contrast to this, the addition of iron(iii)/tartrate had a marked effect on the reaction. It was found that iron(iii) functioned primarily by reducing the size of the zinc crystals as they formed. This produces a thinner and more compact zinc immersion coating.     

Deposition of diamond-like carbon films on aluminium substrates by RF-PECVD technique: Influence of process parameters

The properties of diamond-like carbon (DLC) films are influenced by both the process parameters and the properties of the substrate on which they are deposited. Deposition of DLC films on aluminium and its alloys has drawn increasing attention owing to its potential applications as wear resistant coatings in automobile pistons, bores, VCR heads, copier machine drums and textile components. In the present study, DLC films have been deposited on commercial pure aluminium (98.9% purity) in a 200 kHz RF glow discharge sustained by methane gas in an asymmetric and capacitively coupled deposition system. Influence of various process parameters such as power density or bias voltage, methane gas pressure and flow rate on deposition kinetics, hardness and elastic modulus of the films has been assessed. Interrelationships between independent process variables like power density, methane gas pressure and flow rate, and dependent process variables like bias voltage and temperature have also been evaluated on the basis of available models.     

Investigation of fluoroacid based pretreatments on aluminium

Conclusions  Effective alternatives to chromium phosphate are available for the pre-treatment of extruded aluminium. Blends of fluoroacids and polymers may be selected to provide equivalent corrosion protection and facilitate the adhesion of paint. Polymer choice is critical in assuring good performance. Electrochemical evaluation of unpainted, fluoroacid/polymer treated aluminium provided some clues concerning the effectiveness as corrosion inhibitors. Polarisation curves showed a pseudo-passive region in the anodic curve that was not observed for untreated aluminium. Polarisation resistance was sensitive to choice of fluoroacid and polymer, with the combination of fluorozirconic acid and 7:3 AA/AM polymer providing the lowest corrosion rate. EIS demonstrated that the addition of polymer improved both the stability and the corrosion resistance provided by fluoroacid treatments. This behaviour was retained for long immersion times (up to 6 days). Microscopic examination of the treated aluminium surface, showed a heterogeneous coating when treated without polymer. Fluorotitanic acid was especially non-uniform, with titanium-enriched 0.5 μm particules on the metal surface. Incorporation of polymer into the pre-treatment enhanced surface uniformity. FTIR spectroscopy (and to a limited extent, XPS) gave evidence of poly(acrylic acid) ionisation on fluoroacid/polymer treated aluminium. Acid ionisation was not observed on evaporation of either treatment or polymer solution. Other workers have proposed that PAA complexation with zirconium enhances pre-treatment performance.     

光圆钢筋与混凝土界面粘结滑移本构模型研究

对配置光圆钢筋的既有混凝土结构进行承载性能评估时,钢筋与混凝土界面粘结性能优劣是评价该类混凝土结构的重要内容.为此,制作了4组光圆钢筋的中心拉拔试件和2组变形钢筋对比试件,选取粘结长度、钢筋表面形式为主要变量对粘结性能进行试验研究,并通过4组铝合金拉拔试件对比分析光圆筋与混凝土界面的粘结机理.研究结果表明:光圆钢筋的粘结强度显著小于变形钢筋的粘结强度,平均约为变形钢筋粘结强度的18.3%;光圆钢筋的粘结强度主要由化学粘着力和界面摩擦力构成,而铝合金的粘结强度则由化学粘着力构成,平均约为光圆钢筋粘结强度的10%;增加粘结长度后,光圆钢筋的粘结强度随之增大,但铝合金筋的粘结强度无明显变化.进一步基于试验结果及理论分析,建立了光圆钢筋与混凝土界面的粘结-滑移本构模型,并通过已有试验数据对建议模型进行了验证,吻合较好.     

Analytical solution for the voltage distribution in one-dimensional porous electrode subjected to cyclic voltammetric (CV) conditions

An analytical solution for the voltage distribution in one-dimensional porous electrode subjected to cyclic voltammetric (CV) conditions was developed by considering the resistivity of both electrode and electrolyte. Voltage distribution inside the electrode as a function of dimensionless distance was generated at different electrode phase resistances. The analytical solution of voltage distribution for cyclic voltammetric conditions was employed in deriving the charging currents. Capacitance studies were conducted on carbons derived from catechol-formaldehyde gels prepared at pH conditions of 3, 6, and 7.5 in 30 wt.% H₂SO₄ electrolyte using CV and constant current charge/discharge studies. Charging currents derived from voltage distribution were fitted with experimental anodic part of CV curves and equivalent series resistance (ESR) was generated. Thus, generated ESR values were compared with that obtained using constant current charge-discharge curves. The model fitted resistance values matched closely with resistance values obtained from galvanostatic constant current charge-discharge method.     

Investigations of a large-capacity medium-power saline aluminium-air battery

This paper describes the development and operation of a medium-power (24 W) aluminium-air battery with saline electrolyte or sea water. The battery consisted of two packs of 10 cells, each connected in series with a projected aluminium anode capacity of 2590 Ah. The anodes were made of a Ga–Sn–Mg alloy capable of giving 89% material efficiency, with an open circuit voltage of 13.4V. The specific energy based on aluminium was calculated at 577 Wh kg^–1.     

Using bent carbon nanotubes for the fabrication of electromechanical switches

An electromechanical switch based on bent carbon nanotubes was fabricated. The shape and structure of the bent carbon nanotubes allows one to produce a low cost and low working voltage switch. The fabrication process is free of any nanolithography. The electrical characteristics of the fabricated device were investigated, both experimentally and theoretically. Actuation of the fabricated device shows hysteresis behavior in the measured I–V curves depending on the structural parameters of the bent nanotubes. The relationship between the pull-in voltage and the morphology of the bent nanotubes was studied by the obtained hysteresis curves. A scanning electron microscope was used for structural analysis. This study introduced an easy way to fabricate electromechanical switches with controllable on/off states.     

Influence of power ultrasound on the corrosion of aluminium and high speed steel

The influence of power ultrasound on the corrosion of aluminium and high speed steel in chloride medium has been investigated. Open circuit potential, polarization curves and electrochemical impedance spectroscopy were employed to study the corrosion process before, during and after ultrasonic irradiation as well as scanning electron microscopy of the corroded surfaces. Cavitation led to formation of pits, erosion and increased mass transport. The effect of ultrasound power, chloride concentration, horn tip–metal distance and of tip diameter were assessed. The principal effect of ultrasound on aluminium is destruction of the oxide film and on high speed steel removal of corrosion products and increased solution mass transport; the influence of ultrasound is greatest at low chloride concentrations. For both metals it was shown that for chloride ion concentrations up to 0.1 M the influence of ultrasound power is most evident, followed by the distance between probe tip and metal, then [Cl^–] and lastly the size of the probe tip. The potentialities of using ultrasound as a rapid diagnostic test of corrosion resistance are indicated.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

The effect of preparation method on the catalytic activity of amorphous aluminas in ethanol dehydration

The known heterogeneous catalytic dehydration of ethanol on amorphous aluminium oxide samples was adopted as a measure of catalytic activity and selectivity. The aluminium oxide samples were prepared by hydrolysis of aluminium chloride using propylamine, as proton acceptor, in pure and mixed aqueous ethanolic and acetonic media, freeze dried and activated. The catalytic activity and selectivity of the samples was measured in the temperature range 623–698 K for various flow rates of ethanol. The apparent activation energies were calculated (53–78 kJ mol^?1) and depended on the solvent used in the preparation of the aluminas. The effect of solvent medium on the activity and selectivity was discussed. © 2001 Society of Chemical Industry