Preparation and characterization of tungsten nanopowders from WC scrap in molten salts

Tungsten carbide (WC) scrap was used as a consumable anode to prepare tungsten powder in NaCl–KCl melt at 1023 K for the first time. The electrolysis process was investigated. Results showed that the tungsten component in WC anode was dissolved as W^2 + into the NaCl–KCl melt. The cathode reaction was controlled by the diffusion of tungsten ions. The effect of electrolysis parameters, including the anode–cathode distance, cathode current density and different electrolysis ways (galvanostatic and potentiostatic electrolysis), on the purity and grain size of the cathode deposits was studied. It showed that a large anode–cathode distance was beneficial to the formation of pure tungsten powder at the cathode. With increasing the cathode current density, the tungsten grain size first decreased and then increased. When the cathode current density was 0.125 A cm^− 2, tungsten powder with a diameter of smaller than 100 nm was obtained. Deposits prepared through galvanostatic and potentiostatic electrolysis was compared in the end.     

Influence of anodic oxidation at low potential on the Corrosion resistance of hafnided zirconium in concentrated sulphuric media

In the context of increasing applications of zirconium in the chemical processing industry the influence of anodic oxidation at low potentials on the corrosion resistance of zirconium has been studied. Experiments were performed in concentrated sulphuric acid media, using electrochemical methods. A transition range of potentials between ?65 and ?50 mV/SCE has been observed. On each side of this transition range hafnided zirconium covered with passive film exhibits different corrosion resistance. Alteration of the passive film, which can be assumed from nuclear microanalysis experiments, has been hypothesized to take account for the modification of the electrochemical behaviour.     

Effect of applied voltage and fluoride ion content on the formation of zirconia nanotube arrays by anodic oxidation of zirconium

Anodised zirconium (Zr) foil at 20 V for 1 h in Na₂SO₄ with 0.5 wt.% NH₄F resulted in 6 μm thick anodic oxide film with nanotubular structure. At 50 V the anodic layer is consisted of an irregular inner layer with the nanotubular structure only at the top of the layer. Nonetheless, the anodic oxides formed are crystalline and photocatalytic ability of the anodic oxide was assessed by photodegradation of methyl orange.     

The nature of anodic gas at plasma electrolytic oxidation

The output of electro- and thermochemical reactions has been estimated quantitatively on the basis of analyses of the products obtained by plasma electrolytic oxidation (PEO) such as volume and composition of anodic gases evolved on aluminium, weight of oxide and metal passed in to solution. It was shown, that the main component of anodic gases is hydrogen (about 90 vol %). This testifies to preferential occurring of thermochemical processes such as vapor conversion of aluminium by water. Estimation of the hydrogen ‘energy yield’ has provided evidence of an excellent correlation between calculated and experimental results. The efficiency of thermal aluminium conversion in PEO processing under sparking conditions has been found to be approximately 50% and of water decomposition approximately 12%.     

A phenomenological approach to the mechanical breakdown of anodic oxide films on zirconium

A phenomenological theory of the mechanical breakdown of films growing on valve metals during galvanostatic oxidation is presented and discussed in detail for ZrO₂ anodic films. It is shown that the mechanical breakdown voltage, V_mb, can be linearly related to the logarithm of the anodizing current density both in the case of constant and variable critical thickness, L_c, at which the breakdown occurs. It is also shown that the A_mb and B_mb parameters of the relationship: V_mb = A_mb + B_mb log i are strictly related to the kinetic parameters of growth of the films in the different solutions. The expressions of A_mb and B_mb parameters are derived for films grown in the presence as well as in the absence of space-charge effects. A quantitative check of the equations obtained is presented for different ZrO₂/electrolyte systems.     

氨基酸类有机添加剂对镁合金阳极氧化的影响及其作用机制研究

为了进一步改善镁合金表面阳极氧化膜的质量和性能,在Na OH+Na_2SiO_3基础电解液中添加氨基酸类有机添加剂对AZ31镁合金进行阳极氧化,研究了氨基酸类有机添加剂对镁合金阳极氧化过程及氧化膜厚度、表面形貌、结构及耐蚀性的影响。探讨了有机添加剂在镁合金阳极氧化中的作用机制。结果表明:不同的氨基酸对镁合金阳极氧化过程及氧化膜性能影响不尽相同,其中乙二胺四乙酸和L-鸟氨酸醋酸盐可显著提高阳极氧化的击穿电压,起到明显的抑弧作用,大幅度提高了氧化膜的厚度、致密性和耐蚀性能。有机添加剂主要通过缓蚀、抑弧和表面活性剂3方面的综合作用来影响镁合金阳极氧化的过程及氧化膜的性能。     

Electrochemical behavior of zirconium in the LiCl-KCl molten salt at Mo electrode

The electroreduction process of Zr(IV) was studied at molybdenum electrode in LiCl-KCl-K₂ZrF₆ molten salt. The transient electrochemical techniques, such as cyclic voltammetry and chronopotenimetry were used. The experimental results showed that the electrochemical reduction of Zr(II)/Zr and Zr(IV)/Zr(II) were both diffusion-controlled process. In the 773-973 K range, the diffusion coefficients of Zr(ii) and Zr(IV) were determined: D_Zr(II) = 0.15567exp{−69.65 × 10³RT(K)} cm²/s, D_Zr(IV) = 1.09 × 10⁻⁴ exp{−44.39 × 10³RT(K)} cm²/s. The activation energy values for the diffusion process were 69.65 kJ/mol and 44.39 kJ/mol, respectively.     

Effect of anodic oxidation parameters on the titanium oxides formation

The aim of this paper is to evaluate the effects of titanium anodic oxidation in a sulphuric acid electrolyte on the crystallinity of the oxide layer and to adjust the process parameters, in order to maximize the TiO₂ crystalline phase, specially for what concerns the anatase form. In particular, the relationship between anatase formation and anodization parameters, such as current density and applied potential, will be evidenced. From XRD analysis two opposite trends emerged: oxide conversion to anatase is promoted either by an increase in current density or by a decrease in sulphuric acid concentration.     

Kinetic mechanism of anodic oxidation of tantalum in nitrate melts

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The formation of thin oxide films on metals

It is shown that one may not expect simple rate laws between the limiting case (i) of transport of ions across a thin oxide film by virtue of an electrical field due to electrical charges of opposite sign at the metal/oxide and the oxide/gas interface and the limiting case (ii) of diffusion of ions and electrons across a thick oxide layer in accord with classical diffusion equations.     

In-situ synthesis of silicide coatings on molybdenum substrates by electrodeposition in chloride-fluoride molten salts

Silicide coatings including MoSi₂ and Si-MoSi₂ were prepared on molybdenum substrates by electrodeposition in NaCl-KCl-NaF-K₂SiF₆ molten salt. The experimental was conducted under different cathodic current densities at the temperature of 1073 K. The effect of the current density on the microstructure, phase composition, cross-section morphologies and elemental distribution of the as-prepared coatings were investigated by means of SEM, XRD and EDX. The results revealed that the type of the silicide coatings was strongly dependent on the current density and the relevant deposition mechanism was discussed. Besides, the electrochemical behavior of silicon ion in the chloride-fluoride molten salts was also studied using cyclic voltammetry and chronopotentiometry to reveal the electroreduction mechanism. The reduction of Si(IV) to Si was proved to be a quasi-reversible or irreversible diffusion-controlled single-step reaction and the diffusion coefficient of Si(IV) calculated from chronopotentiograms was (1.28 ± 0.25) × 10⁻⁵ cm²·s⁻¹.     

The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature

For a better understanding of the atmospheric rusting of iron and steels, the present work is aimed to explore the mechanism of formation of green rusts, Fe₃O₄, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide in aqueous solution at room temperature. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The systematic diagram of formation processes of iron oxide and oxyhydroxides has been presented, in which both dissolved and solid species of iron are included.     

The kinetics and mechanism of molten copper oxidation by top blowing of oxygen

The rate of liquid copper oxidation plays an important role in the separation of impurities from copper during fire refining. However, the importance of factors affecting the rate of oxygen uptake by liquid copper is not clear. Top blowing oxidation of molten copper shows that the rate of oxidation is almost independent of the average oxygen concentration of the copper and proportional to the oxygen flow rate of the gas mixtures used. A pressure monitoring technique was used to determine the effect of a carrier gas such as N₂, helium, and CO₂, and the effect of the lance distance from the liquid surface on the partial pressure of oxygen in the gas phase. A numerical model was developed to help understand the mechanism of oxygen mass transfer into the molten copper. For more information, contact T. Marin, University of Toronto, Department of Materials Science and Engineering, 184 College St., Toronto, Canada M5S 3E4; e-mail tanai.marin@utoronto.ca.     

Microstructure and anodic dissolution mechanism of brazed WC–Ni composite coatings

WC–Ni composite coatings were prepared on the mild steel substrates by high temperature sinter brazing. Potentiodynamic polarisation was performed on the coated sample immersed in 3%NaCl solution. Two limited current plateaus appear in the anodic polarisation curve. The first limited current plateau is attributed to the formation of Ni and Cr oxide, while the second corresponding to W oxide. The surface morphology observation and composition analysis after anodic polarisation were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. The elemental concentrations in the immersion solution were studied with inductively coupled plasma (ICP) spectroscopy. It shows that the selective dissolution of Ni base binder alloy as well as the electrochemical dissolution of WC exists during the anodic polarisation process.     

CsCl对NaCl-KCl-CsCl熔盐物理性质及铌涂层电沉积行为的影响

因其优异的综合理化性能,铌（Nb）被广泛应用于航空航天、核能和超导领域。其涂层制备方法中,熔盐电沉积技术沉积速率快、阴极电流效率高、绕镀性好,有望实现大规模工业化生产和应用。当前广泛使用的氟化物支持电解质体系毒性大、环保性差,亟待开展更环保的全氯化物支持电解质体系的开发工作。为实现支持电解质熔盐物性的调控和络合离子的稳定化,本文在NaCl-KCl体系中添加CsCl,制备全氯化物支持电解质体系,研究CsCl对支持电解质熔盐物理性质及Nb涂层电沉积行为的影响。结果表明,NaCl-KCl-CsCl三元混合熔盐的共晶温度约为485℃,随CsCl含量增加,熔盐的初晶温度先降低后增大,密度增大,电导率和表面张力减小。CsCl通过改变熔盐的初晶温度和电导率影响熔盐中离子的传质速度,进而影响电沉积Nb涂层的表面质量,其优选含量约为60wt.%。CsCl的添加可使熔盐中含氧络合离子NbOF63-的还原电位负于NbF72-,有助于获得不含氧杂质的Nb涂层。     

On the mechanism of formation of zinc pack coatings

The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 °C) is endothermic and involves the transformation of α-NH₄Cl to β-NH₄Cl and the NH₄Cl decomposition to NH₃ and HCl. During the second step (at 248.6 °C), which is exothermic, Zn²⁺ salts are formed and especially ZnCl₂. Finally at 264.1 °C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl₂. The as-cast Zn diffuses into the iron lattice forming the gamma (Γ-Fe₁₁Zn₄₀) and delta (δ-FeZn₁₀) phases. Al₂O₃ is not involved in the above-mentioned mechanism and acts only as filler.     

Multicycle chronoammetry of RRDE in the investigation of the anodic oxide formation

The method of multicycle chronoammetry of RRDE makes it possible to obtain separately the partial currents of metal electrode ionization, anodic oxide formation and chemical oxide dissolution. The method is tested for Ag∣Ag₂O∣OH⁻(H₂O) system. In the range of low anodic potentials (0.48 ÷ 0.51 V) the process of active silver dissolution prevails; the phase formation current rapidly drops. At higher potentials (0.52 ÷ 0.53 V) the phase formation current prevails and noticeably exceeds the rate of the chemical oxide dissolution. The thickness of Ag₂O film rapidly increases; and the net phase formation current is close to 100%.     

The mechanism of formation of composite microarc coatings on aluminum alloys

It is shown that heating of an internal layer of microarc coating up to temperatures at which insignificant rates of heat removal from microdischarges that possess a temperature of no less than 1200°C are ensured leads to realization of a sufficient time interval for the occurrence of almost 100% transformation of low-temperature aluminum-oxide modifications into high-temperature ones. Insufficient heating of a number of areas of the coating to these temperatures is the main reason for formation of a composite microarc coating that contains both low- and high-temperature aluminum-oxide modifications in its internal layer.