Valve metal behaviour of bismuth in NaOH

Galvanostatic anodization of bismuth in different media indicates that the formation of the oxide film depends simultaneously on both pH and the anions present in the anodization medium. Dissolution of these films in NaOH supports this observation. The film is formed of two layers. Open circuit impedance and potential measurements in NaOH indicate film growth. The anodic film formed in NaOH dissolves, however, in NaOH solutions following a zero order mechanism. A number of factors including formation voltage, NaOH concentration, current density and temperature are investigated. The activation energy of the oxide film dissolution is calculated. It may be concluded that the outer layer, in addition to being of a more defective structure, is probably of a higher oxidation state than the inner layer.     

Micro-electrochemical characterization and Mott–Schottky analysis of corrosion of welded X70 pipeline steel in carbonate/bicarbonate solution

The electrochemical corrosion behavior of welded X70 pipeline steel in a bicarbonate/carbonate solution was studied by scanning vibrating electrode technique and localized electrochemical impedance spectroscopy as well as Mott–Schottky analysis. The effect of applied stress on corrosion of various zones in welded steel specimen was investigated. Results demonstrated that passivity can be established on various zones, i.e., weld metal, heat-affected zone (HAZ) and base steel, of welded X70 steel. There is a higher passive current density for the passivated HAZ than that for weld metal and base steel. Applied stress enhances the anodic dissolution of steel, as indicated by the decreasing charge-transfer resistance and increasing dissolution current density of steel with the increase of stress. A maximum current density is observed in HAZ at all stressing levels. With the positive shift of anodic potential, the stability of passive film is enhanced, and the dissolution current density decreases. When the applied potential approaches the water oxidation potential, the current density increased significantly again due to the oxidation of water. The passive film formed is an n-type semiconductor. There are higher donor density and thinner space-charge layer in passive film formed at HAZ than those formed on weld metal and base steel. Moreover, the donor densities in all zones increased significantly with the applied stresses. The metallurgical transformations and formation of low temperature hardening phase transformation products during welding result in the enhanced electrochemical activity in HAZ.     

An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium

In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide.     

Influence of the electrolyte composition and temperature on the transpassive dissolution of austenitic stainless steels in simulated bleaching solutions

The transpassive corrosion of highly alloyed austenitic stainless steels—UNS N08904, UNS S31254 and UNS S32654—was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.     

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—I. Corrosion processes and anodic dissolution of antimony

The corrosion processes at open circuit conditions and in the active dissolution region of antimony, at low anodic overpotentials, have been studied in sulphuric acid solutions. It was established that antimony dissolution at open circuit is coupled to oxygen reduction, which occurs via the hydrogen peroxide route. The rate of antimony dissolution was found to increase linearly with increasing acid concentration. Based on potentiodynamic, galvanodynamic and steady-state measurements as well as ac impedance data possible mechanisms for the corrosion processes at open circuit as well as for the dissolution of antimony in the active region are proposed and discussed.     

汽车用6061铝合金柠檬酸阳极氧化工艺的研究

对汽车用6061铝合金进行柠檬酸阳极氧化处理。研究发现:铝合金的阳极氧化过程伴随氧化膜的生成和溶解。适当升高氧化温度,有利于增加氧化膜的厚度和硬度。但氧化温度过高,会使得氧化膜的溶解速率加快。铝合金阳极氧化膜呈现典型的多孔结构。40℃下制备的氧化膜表面平整,具有优良的耐蚀性。当氧化温度高于40℃时,氧化膜表面的孔径增大,表面疏松,耐蚀性下降。     

汽车用5052铝-镁合金阳极氧化及封闭工艺的研究

使用草酸阳极氧化技术对汽车用5052铝-镁合金进行阳极氧化处理,并研究了阳极氧化对铝-镁合金耐蚀性的影响。铝-镁合金的阳极氧化属于氧化膜的生成和溶解的动态过程。氧化膜主要由多孔层和阻挡层构成,厚度约为8μm。经过阳极氧化处理后,铝-镁合金的耐蚀性显著提高。重铬酸钾封闭后,腐蚀电流密度下降为原来的10%,膜电阻显著增大,进一步提高了铝-镁合金的耐蚀性。     

A theoretical approach of impedance spectroscopy during the passivation of steel in alkaline media

A theoretical impedance function is deduced for a proposed mechanism of passive film formation of steel in contact with alkaline aqueous media involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite and Fe(III)-oxides. The reduction reaction of dissolved oxygen is considered as the only cathodic reaction compensating the anodic current induced by the formation of iron oxides at open circuit potential. The iron dissolution takes place through a chemical dissolution of ferric oxide. A two-layered passive film with 3D structure is considered. A satisfactory agreement between the digital simulations on the basis of the theoretical impedance function and experimental spectra validates the proposed model.     

A flow model of porous anodic film growth on aluminium

The development of pores in a classical porous anodic film formed on aluminium in phosphoric acid solution is investigated. The study employs a tungsten tracer layer that is incorporated into the anodic film from the aluminium substrate, followed by detection of the tracer by transmission electron microscopy and Rutherford backscattering spectroscopy. Distortions of the tungsten layer on entry into the film and retention of tungsten species in the film are compatible with porosity arising mainly from flow of anodic oxide beneath the pore bases towards the cell walls. The behaviour is contrary to expectations of a dissolution model of pore formation.     

铝阳极氧化膜多色彩电解着色工艺

简述了多色彩电解着色工艺的发展和特点,介绍了2种达到多色化显色的方法:阳极氧化膜底部扩孔处理和调整阳极氧化膜阻挡层。给出了扩孔处理过程中阳极氧化膜横断面的显微结构变化示意图,并给出了“阻挡层调整”实现多色化的工艺参数。描述了多色化阳极氧化膜的性能,并对比了电泳涂漆前后多色彩电解着色样品的颜色。     

铝合金阳极氧化膜的性能研究

在硫酸电解液中加入适量由羧酸和有机化合物组成的添加剂,制得铝合金阳极氧化膜。研究了温度对所得氧化膜厚度和硬度和影响,并利用扫描电镜观察了氧化膜的结构。结果表明,高温下形成的氧化膜结构松散,厚度和硬度低,而加入添加剂后,氧化膜溶解减慢,在高温下所形成的氧化膜的厚度和硬度大大增加。     

Local deposition of polypyrrole on aluminum by anodizing, laser irradiation, and electrolytic polymerization and its application to the fabrication of micro-actuators

Polypyrrole was deposited at selected areas on aluminum by anodizing, laser irradiation, and electrolytic polymerization, and the application of the technique for fabricating micro-actuators was attempted. Aluminum specimens covered with porous type anodic oxide films were irradiated with a pulsed Nd-YAG laser to remove the oxide films locally, and then thin Ni layers were deposited at areas where film had been removed. Polypyrrole could be successfully deposited only on the Ni layer by anodic polarization of the specimens in pyrrole monomer solution, and a polypyrrole/Ni bilayer structure could be obtained by dissolution of the aluminum substrate and anodic oxide film in NaOH solutions. The bilayer structure was found to be inactive to doping and dedoping of ions during anodic and cathodic polarization. A three-layer structure, nitrocellulose/Ni/polypyrrole, fabricated by electrolytic polymerization after nitrocellulose coating on a Ni layer detached from the aluminum substrate, showed ion-doping and -dedoping activity, suggesting the possibility of fabricating micro-actuators in this manner.     

Investigation of the growth processes of coatings formed by AC plasma electrolytic oxidation of aluminium

The formation of alumina-based coatings on aluminium by AC plasma electrolytic oxidation (PEO) has been investigated using a silicate electrolyte with selective additions of fine zirconia particles. The coatings comprised an amorphous barrier layer, a relatively dense intermediate layer and a more porous outer layer that contained silicon species. Zirconia was incorporated non-uniformly into the outer layer and, to a limited extent, the intermediate layer, as both particles and a component of cellular microstructures. Following treatments firstly in zirconia-containing electrolyte and secondly in zirconia-free electrolyte, the zirconia did not extend beyond about the middle of the intermediate layer, indicating its limited inward mass transfer during microdischarges. The coating efficiency decreased at oxidation times in excess of 40 min due to dissolution of either the substrate or the coating, or physical loss of coating material. The oxidation of aluminium consumed on average ∼29% of the anodic charge; the remainder was used mainly in generation of oxygen gas.     

汽车用2024铝合金阳极氧化膜性能的研究

对汽车用2024铝合金板材进行酒石酸阳极氧化处理,并研究了阳极氧化对铝合金的成分、结构、表面形貌及耐蚀性的影响。研究发现,铝合金阳极氧化膜是由表面多孔层和内部无孔层构成的。铝合金阳极氧化过程是一个氧化铝生成和溶解的动态过程。阳极氧化膜由刚玉结构的α-Al_2O_3和八面结构的γ-Al_2O_3构成,α相和γ相大大提高了阳极氧化膜的硬度和耐蚀性。阳极氧化膜为典型的多孔结构,孔洞分布均匀,孔径为50nm左右。     

On the electrochemical behaviour of the anodic oxide film on tungsten in HCOOH and CH3COOH solutions

The electrochemical behaviour of the anodic oxide film on tungsten during, or after, interruption of current flow was studied in HCOOH and CH₃COOH solutions by galvanostatic and capacitance techniques. The results show the conditions under which enhancement of film growth occurs as revealed from its formation and dissolution characteristics. The results also show the possibility of the electrochemical oxidation of formic acid by the thick oxide film on tungsten. The importance of this electrocatalytic process is the occurrence of the oxidation process without appreciable oxygen evolution. The galvanostatic oxidation of tungsten in HCOOH as a reducing agent can be considered as a novel method for the preparation of a class of oxides; oxidation with simultaneous partial reduction.     

A mechanism for the corrodability of hafnium and the anodic oxide film thereon in HCl solutions

A study is made to account for the corrodability of each of the naturally developed and anodically formed oxide films on hafnium in HCl solutions. The behaviour of these oxide films was investigated by capacitance and potential measurements. The naturally developed oxide film grows spontaneously according to a direct logarithmic law. In diluted HCl (< 1 N), the rate of growth is small but the resultant film has better electrochemical insulating properties than that rapidly grown in the more concentrated solutions. On the other hand, the anodic oxide film is more liable to chemical (currentless) dissolution in HCl as the initial thickness increases. The rate determining step in the dissolution process is determined mainly by the oxide and is almost independent of the acid concentration. The behaviour of each of the two films has been correlated to changes in the fine structure of the parent oxide during its growth.     

pH-dependent anodic reaction behavior of tungsten in acidic phosphate solutions

Potentiodynamic and potentiostatic polarization experiments, and the electrochemical impedance spectroscopy technique were used to study the pH dependent anodic behavior of tungsten (W) in acidic phosphate solution. At very low pH values (pH < 2.6) the dissolution of tungsten was H⁺-assisted and as the pzc (pH 2.6) was approached H₂O-assisted dissolution became main dissolution pathway. Above pH 2.6, however, tungsten dissolution was OH⁻-assisted. The thickness and dielectric properties of the W-oxide barrier layer were observed almost pH independent at corrosion potential. The oxygen vacancy transport across the oxide film caused a capacitive response at very acidic solutions (pH ≤ 3.5) and as the dissolution rate increased (pH > 4.5) the capacitive response turned into the inductive one due to the accelerating effect of negative surface charge in Tafel region. The inductive response in the tungsten impedance spectra shifted to a very low frequency range as the tungsten dissolution rate decreased in the pseudo-plateau and potential independent regions. Fitting of the tungsten impedance data according to the surface charge approach showed that the resistance to the defect migration increased as the pzc was approached and the film capacitance decreased above pH 3.5 due to the accelerated formation rate of the non-protective loosely bound hydrated layer on the metal oxide surface.     

Kinetics of the dissolution behaviour of anodic oxide films on niobium in NaOH solutions

The dissolution behaviour of the anodized niobium electrode in NaOH solutions was investigated as a function of alkali concentration, formation voltage, formation current density and temperature using potential and impedance measurements. The rate of dissolution is dependent on the alkali concentration. In dilute NaOH solutions (1 N) the anodic oxide film formed in 0.5 M H₂SO₄ is reasonably stable. On the other hand, at higher concentrations of NaOH (2 M), the anodized electrode is subject to continuous dissolution depending on the alkali concentration. Also, the dissolution process is considerably affected by temperature; at temperatures greater than 320 K the oxide film is destroyed in less than 30 min. The results show that the current density used during the formation of the oxide film has no effect on its dissolution rate.     

汽车用铝合金阳极氧化及钵转化膜封闭技术

采用阳极氧化和钵转化膜封闭技术提高汽车用2036铝合金的耐蚀性。研究发现:铝合金阳极氧化膜由外部的多孔层和内部的阻挡层构成,多孔层孔径均匀,约为30 nm0经过阳极氧化处理后,铝合金的自腐蚀电位正移,自腐蚀电流密度下降,耐蚀性提高。经过钵转化膜封闭处理后,大量钵的氢氧化物覆盖阳极氧化膜表面,进一步提高了铝合金的耐蚀性。     

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α-Al₂O₃和γ-Al₂O₃,大大提高了铝合金的硬度。阳极氧化膜由高电阻的阻挡层和多孔层构成,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。