Ellipsometric study of anodic oxide films formed on niobium surfaces

Anodic oxide films formed potentiostatically on niobium surfaces, from open circuit potential (OCP) to 10 V, were studied by performing in situ and ex situ ellipsometric measurements. The kinetics of the film thickness growth in 1 M H₂SO₄ and complex indices of refraction of these films were determined. A strong influence of the surface preparation conditions on the complex refractive indices of the metal substrate and anodic oxide films was shown. By steady-state measurements at OCP, a small thickening of the natural air-formed oxide film with chemical composition Nb₂O₅ in 1 M H₂SO₄ solution was detected. With cathodic pre-treatment, only partial reduction and small thinning of the natural air-formed oxide film was possible. The thicknesses of the natural air-formed oxide films on fine mechanically polished and electropolished Nb surfaces were determined. The build up of the natural air-formed oxide film, at ex situ conditions, on the already formed anodic oxide films was confirmed. It was shown that electropolishing gives more similar optical surface properties to the bare metal than the fine mechanical polishing. Electronic Publication     

Growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions

The present work was conducted to explore the growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions by using a.c. impedance spectroscopy and a beam deflection technique. From the analyses of a.c impedance data, it was found that the reciprocal capacitance of anodic oxide film on pure aluminium increased linearly with increasing film formation potential in both acidic and alkaline solutions, indicating a linear increase in the film thickness with film formation potential. However, as the film formation potential increased, the resistance of anodic oxide film decreased in acidic solution, while it increased in alkaline solution. From the measurements of the deflection, the deflection was observed to move towards only a compressive direction with time in acidic solution, but it showed a transition in the direction of movement from compressive to tensile in alkaline solution. Based upon the above experimental results, it is suggested that the movement of oxygen vacancy through the oxide film contributes to the growth of anodic oxide film on pure aluminium in acidic solution, but the movement of both aluminium vacancy and oxygen vacancy accounts for that oxide film growth in alkaline solution. Received: 12 August 1997 / Accepted: 9 October 1997     

Mechanism of anodic dissolution of copper in aqueous acidified solutions of different anions

The potentials of copper anodes were measured as a function of current density in aqueous acidified solutions of different anions. Tafel lines were obtained, whose slopes depend on the nature of the anion. This denoted the participation of the anion in the anodic dissolution of the metal. Hence, mechanisms were suggested based on the formation of intermediate cuprous compounds, which are further oxidized electrochemically, or undergo chemical disproportionation to cupric salts.The results in phosphate solutions indicated that the second electrochemical oxidation is the rate determining step. In chloride, nitrate and sulphate solutions, both the second electrochemical step and the disproportionation reaction govern the overall reaction rate.Measurements in acidified copper salt solutions showed that Cu²⁺ ions affect the reaction mechanism. Thus, in chloride and sulphate solutions, the disproportionation reaction becomes predominating, whereas in nitrate solutions the intermediate is mainly oxidized electrochemically. An interpretation is provided, based on the adsorption of Cu²⁺ ions.     

Ellipsometric and microwave reflectivity studies of current oscillations during anodic dissolution of p-Si in fluoride solutions

Periodic current oscillations during anodic dissolution of monocrystalline p-Si(100) in buffered ammonium fluoride solutions (0.1 mol dm⁻³ fluoride, pH 4.5) were investigated using a flow cell in order to eliminate mass transport limitations. The flow cell was designed to permit simultaneous in-situ ellipsometry, impedance and potential modulated microwave reflectivity measurements. Analysis of the ellipsometric response showed that the current oscillations are accompanied by a synchronous variation of the overall oxide thickness with an amplitude of 4.5±0.1 nm. Analysis of the relationship between the total oxide thickness and the current during the oscillation cycle shows that to a first approximation the rate of chemical dissolution of anodic oxide remains constant. Oscillations of the electrode admittance and potential modulated microwave reflectivity were also measured. The imaginary component of the admittance is related to the oscillation in thickness of a narrow inner region of ‘dry’ oxide and to changes in the accumulation capacitance. The oscillation in the potential modulated microwave reflectivity is interpreted in terms of the changes in the density of holes accumulated at the p-Si SiO₂ interface.     

The mechanism of pulse anodic activation of iron dissolution in acid solutions

Basing on the model of spatial separation of the dissolution and passivation of hydrophilic metals, formation of stationary surface morphology during pulse anodic activation of iron dissolution in sulfuric acid solutions is analyzed.     

Kinetics and mechanism of anodic dissolution of gold in aqueous solution of 1,4-diaminobutane

Russian Chemical Bulletin - The corrosion of the Au anode in aqueous solution of 1,4-diaminobutane (DAB) was studied by gravimetric analysis. The corrosion products were found to reduce at a steel...     

Regularities of gold anodic dissolution in thiocarbamide solutions

It is shown that the gold dissolution in thiocarbamide electrolytes containing sulfide ions in the pH range from 1 to 4 proceeds with a current efficiently of almost 100%, and a change in solution acidity has a weak effect on the process. The oxidation of thiocarbamide to formamidine disulfide proceeds at the potentials around 1.1 V and depends on the pH of solution. When the pH value is raised, the thiocarbamide oxidation potential shifts in the negative direction and approaches the potential of gold dissolution. It is found that, in the absence of catalytically active species, the gold dissolution rate in thiocarbamide solutions in the pH range from 6 to 10 is vary low. At these pH values, as well as in the acidic solutions, an addition of less than 10⁻⁴ M sulfide ions to thiocarbamide electrolyte accelerates the gold dissolution at the potentials about 0.6 V. It is shown that sulfide ions in the concentration above 10⁻⁴ M inhibit the process. At pH 11, the gold dissolution proceeds also with no special addition of sulfide ions. This is associated with the formation of sulfide ions as a result of accelerated decomposition of thiocarbamide and formamidine disulfide with increasing pH value. It is found that in the pH range from 12 to 13, the processes of gold dissolution and thiocarbamide oxidation are inhibited as a result of the formation of passive film on the electrode surface. Probably, the film consists of elemental sulfur.     

The anodic dissolution of tin in acidic chloride solutions

The anodic dissolution of Sn in acidic chloride solutions (pH = −2.0 to 2.9) was studied at 25 °C. The apparent valence of the dissolving ions varied from about 0.4 to 2.4, and was a function of both electrolyte and c.d. The complexing of Sn²⁺ by Cl⁻ had an important influence, with SnCl⁻₃ apparently being the dominant product. A reaction sequence is proposed involving the stepwise oxidation of Sn, accompanied by reaction with Cl⁻.     

The anodic dissolution of mercury in sulphide ion solutions

The anodic dissolution of mercury in sulphide ion solutions has been studied by the faradaic impedance method, using in situ, generation of the dissolving species. The mercury passes into solution as the soluble complex ion HgS₂²⁻. The rate-determining step in the dissolution reaction has been identified as a heterogeneous single-electron transfer process which is first order in [S²⁻] and a reaction mechanism-is-proposed on this basis. Double-layer capacity measurements have been made in the presence of the dissolution reaction and these indicate the absence of any strong specific anion adsorption at the electrode. The shape of the electrocapillary curve for sulphide solutions is discussed in the light of these observations.     

Ellipsometric study of nonionic polymer solutions

The thickness and refractive index of adsorption films of poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) were determined by null-ellipsometry at the air-aqueous solution interface. Both parameters, in the same way as the earlier studied dynamic surface elasticity and surface tension, exhibit rather abrupt changes when the concentration approaches the range of semidilute solutions. This behavior can be explained by the worsening of the solvent quality with increasing PEG concentration and by the PVP displacement from the surface by a contamination of high surface activity.     

Chemical dissolution of iron in aqueous solutions

By anodic oxidation of hydrogen on the Pt ring of a metal-disk-Pt-ring rotating electrode in aqueous acid (pH 0.5–3) sulfate solutions, it is shown that during the polarization of the metal disk hydrogen coevolves by two parallel reactions. One is the well known electrochemical charge transfer process, while the other one is a direct chemical reaction of the metal, i.e. iron, with water molecules from the electrolyte. This process was proposed a long time ago by Kolotyrkin and co-workers, but their experimental results were subjected to serious criticism which is taken care of in the present paper. The chemical reaction is potential- and pH-independent and, depending on the actual conditions during the corrosion processes (actual potential, pH, etc.), can be of greater or smaller importance in the overall process. The consequences of the existence of this direct chemical reaction of metal with water molecules (i.e. H₂O-induced corrosion or chemical corrosion) on pitting and stress corrosion cracking of metals is discussed. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. This article was submitted by the authors in English.     

The anodic dissolution of chalcopyrite in water + acetonitrile solutions

The characteristics of chalcopyrite anodic electrodissolution in aqueous acetonitrile solutions have been studied. Experiments with a high-grade sample, of massive chalcopyrite have been carried out at 25°C in solutions containing 50 g l^?1 NaCl and different concentrations of sulphuric acid and acetonitrile. Galvanostatic and potentiostatic conditions were employed and the results obtained in solutions with and without acetonitrile compared.The characteristics of the mechanism involved in the chalcopyrite electrodissolution precluded acetonitrile having an influence at low potentials. However its influence is more noticeable at higher potentials where solvent decomposition reactions are involved. Chronopotentiometric experiments in acetonitrile aqueous solutions indicate the existence of diffusional (in the solid) and coulombic phenomena in originating the potential jump.     

Reaction mechanism studies on isoquinoline with hydroxyl radical in aqueous solutions

The reaction mechanism between isoquinoline and ·OH radical in aqueous dilute solutions under different conditions was studied by pulse radiolysis. The main characteristic peaks in these transient absorption spectra were attributed and the growth-decay trends of several transient species were investigated. Under neutral or alkaline conditions, the reaction of ·OH radical and isoquinoline produces OH-adducts with respective rate constants of 3.4 × 10⁹ and 6.6 × 10⁹ mol⁻¹ · dm³ · s⁻¹ while under acidic conditions, the isoquinoline was firstly protonated and then ·OH added to the benzene ring and produced protonated isoquinoline OH-adducts with a rate constant of 3.9 × 10⁹ mol⁻¹ · dm³ · s⁻¹. With a better understanding on radiolysis of isoquinoline, this study is of help for its degradation and for environmental protection. __________ Translated from Journal of Fudan University (Natural Science), 2006, 45(6): 774–778 [8BD1;81EA: 590D;65E6;5B66;62A5;(自然科学版)]     

Cellulose dissolution in aqueous lithium bromide solutions

A new all-aqueous process of the dissolution/regeneration of cellulose was developed. Cellulose was completely dissolved in the 54–60 wt% lithium bromide aqueous solutions in the temperature range of 110–130 °C within a dissolution time of 1 h. Then, the cellulose was directly regenerated from the solution by cooling down to approximately 70 °C and removing the salts with water, yielding a translucent gel. The cellulose gel was not significantly chemically decomposed even though some decrease of the degree of polymerization occurred during the dissolution/regeneration of cellulose. The X-ray diffraction analysis demonstrated that the dissolution/regeneration of cellulose induced a crystalline structural change from cellulose I to cellulose II, confirming the complete loss of the original cellulose structure. The cellulose gel had highly porous three-dimensional networks composed of fairly long cellulose fibrils interconnected with one another. The dissolution/regeneration of cellulose in aqueous lithium bromide solutions offers new and important options for cellulose-based materials.     

Influence of the electrolyte composition and pH on the anodic dissolution of p-type Si in aqueous HF solutions

The influence of the electrolyte composition and pH on the anodic currents obtained during electrochemical etching of p-type silicon in aqueous HF solutions has been investigated. Original and accurate pH measurements were performed to characterize the exact composition of the HF + H₂O electrolytes commonly used. It is shown that for these very acid solutions (pH < 2) almost all fluoride is in the form of the non-dissociated HF species which appears to play a preponderant role in the silicon dissolution reaction kinetics. The effect of pH can be restricted to its influence on the modification of the different concentrations by shifting the equilibria.     

Polymerization of trimethylene carbonate in aqueous solutions: Reaction mechanism and characterization

Polymerization of a trimethylene carbonate (TMC) in an aqueous solution was investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. The polymerization reaction proceeded rapidly in the aqueous solution and high conversion was achieved in a relatively short time. 1,3‐Propanediol (PPD) formed by hydrolysis of TMC was used as the initiator. The TMC oligomer obtained by ring‐opening polymerization had a TMC unit backbone with terminal 3‐hydroxypropyl groups at both chain ends. The oligomer underwent transesterification reaction with elimination of PPD, resulting in a gradual increase in the molecular weight of the product. The molecular weight was affected by the concentration of TMC. The thermal properties of the polymers were investigated by differential scanning calorimetry. Polymers within the molecular weight (M_n) range from 6.0 × 10³ to 2.3 × 10⁴ g/mol crystallized, and endothermic peaks corresponding to the melting temperature were observed. The glass transition temperature increased with the molecular weight of the polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1485–1492, 2010     

Formation of As2O3 during anodic dissolution of GaAs

The formation of As₂O₃ particles on GaAs surfaces, which have been anodically polarized at potentials <1 V versus SCE, has been studied using SEM, EDX, AES and XPS. Selective dissolution of GaAs occurs resulting in the formation of an As-rich surface layer. The As layer agglomerates and oxidizes on exposure to air forming As₂O₃ particles. The particle formation is dependent on test conditions, with p-GaAs forming As₂O₃ in both dark and daylight conditions; As₂O₃ only forms on n-GaAs when polarized in daylight. Polarization at corrosion potentials does not lead to particle formation, as GaAs dissolution rates are too low for surface enrichment of As.     

Rapid anodic growth of TiO2 and WO3 nanotubes in fluoride free electrolytes

In the present work we report on the formation of bundles of high aspect ratio TiO₂ nanotubes and WO₃ nanopores structures with very thin tube or pore walls using anodization under “high voltage” conditions in perchlorate or chloride containing electrolytes. The bundles of TiO₂ nanotubes consist of separated tubes with diameters in the range of approximately 20–40 nm and the WO₃ nanopores consist of pores with diameters in the range of 30–50 nm. Growth occurs locally at specific surface locations. Both the TiO₂ and the WO₃ structures can be grown up to several dozens of micrometers in length within few minutes. We suggest that the growth of these high aspect structures is initiated by localized anodic breakdown event, triggered by a sufficiently high applied anodic field.