XPS and EIS study of the passive film formed on orthopaedic Ti-6Al-7Nb alloy in Hank's physiological solution

The composition and structure of the passive film formed on Ti-6Al-7Nb alloy by electrochemical oxidation in Hank's physiological solution were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The oxide layer was predominantly TiO₂, but contained small amounts of suboxides TiO and Ti₂O₃ at potentials more negative than 0.75 V. At more positive potentials, TiO₂ was the only form. The formation of suboxides in the lower potential range is less pronounced than in Ti-6Al-4V alloy. The passive range in Hank's physiological solution is broad and extends up to 6.0 V. Aluminium oxide Al₂O₃, and niobium oxides, Nb₂O₅, and NbO and/or NbO₂, are incorporated in the passive layer. Angular resolved XPS analysis confirmed that they are located mainly at the outer oxide/solution interface of the TiO₂ matrix. The thickness of the oxide layer was dependent on the oxidation potential and, after oxidation at 5.75 V, it reached 9.4 nm. EIS measurements correlate well with the XPS data. The incorporation of the oxides of alloying elements into the TiO₂ layer is reflected in the increase in the outer layer resistance at high anodic potentials and longer immersion times. The consequences of this process are beneficial for the overall stability and high corrosion resistance of the Ti-6Al-7Nb alloy under physiological conditions.     

The distribution of antimony in the oxide layer formed by potentiostatic oxidation of Pb-Sb alloy

The distribution of antimony within the oxide films on Pb-Sb alloy prepared by potentiostatic oxidation in H₂SO₄ solutions was examined by SIMS. The study of oxide films prepared by applying different potentials for three hours showed that two types of film were obtained depending on whether the potential was more negative or more positive than 1·5 V. Antimony profiles were obtained for films at several stages in the initial growth. It was found that antimony was retained in the oxide film at 1·5 V during both nucleation and two- or three-dimensional growth of PbO₂ and at 1·6 V during the lateral overlaps of three-dimensional centres of PbO₂. Relationships between the antimony distribution profiles and the oxide film growth are discussed.     

The anodic polarization characteristics of the graphite in alkaline solution

The anodic behaviour of a graphite electrode in alkaline solution was studied by means of the linear potential sweep method, the controlled potential electrolysis and the colourimetry. Linear sweep polarization curves indicated that the surface of graphite electrode is oxidized in the anodic region from ?0.3V.The numbers of electron participating in the oxidation of a carbon atom was calculated to be 1, 2, and 2 at 0.2, 0.3, and 0.4V, respectively, based on the quantity of electricity which was measured by the controlled potential electrolysis and the numbers of carbon atom on the surface of the graphite electrode, which was calculated with the double layer capacity.The structure of oxide layer formed on the graphite electrode was examined by the colourimetry with HgBr₂ and NaOEt. It was found that the oxide layer on the electrode contained carbonyl groups.     

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—III. identification of corrosion products after long-term polarization

During the oxidation of pure antimony (Sb) in sulphuric acid (H₂SO₄) solutions a passivating layer is formed on the electrode surface. In the early stage of oxidation the layer consists of amorphous Sb oxide and/or hydroxide, but with longer oxidation times aggregates of whisker-like crystals of a trivalent oxidation product, Sb₄O₄(OH)₂SO₄, are formed. The identification of this compound was done by ex situ X-ray powder diffraction, and the result was confirmed by electron microprobe and X-ray photoelectron spectroscopic analysis. By means of an extraction procedure succeeded by atomic absorption spectrometric analysis it was shown that the Sb ions dissolved during the oxidation exist in H₂SO₄ in the trivalent state even at potentials well above the equilibrium potential of the Sb(V)/Sb(III) couple.     

Oxidation states of lead in anodic oxide films formed in sulphuric acid solutions containing antimony (III) species

The oxidation states of lead in anodic oxide films formed in sulphuric acid solutions containing antimony(III) species were investigated by means of X-ray photoelectron spectroscopy. The presence of divalent lead ions in the anodic oxide films was confirmed. The content of-PbO₂ in these anodic oxide films was found to be higher than that in films prepared in an antimony-free solution. The formation of the complex oxide, containing divalent lead ions and-PbO₂, was studied.     

Antimony Remediation Using Ferrate(VI)

This research investigates a remediation technique for antimony involving the adsorption and co-precipitation of aqueous antimony by in-situ formed ferric hydroxide. Flocculation is initiated by the oxidation of iron(II) with potassium ferrate(VI), K₂FeO₄, along with oxidation of the more toxic Sb(III) to Sb(V). A 3/1 mole ratio of Fe(II)/Fe(VI) and a total Fe/Sb mole ratio of 300/1 was needed to achieve total antimony concentrations below the maximum contaminant levels for drinking water (6 μg/l).     

Oxidation and adsorption of arsenic species by means of hybrid polymer containing manganese oxides

The aim of this work was a comprehensive study of the oxidative and sorptive properties of a hybrid polymer containing manganese oxide toward As(III) and As(V). A poly(styrene‐divinylbenzene) copolymer containing oxidative functional groups (? SO₂NBrNa) was used as the supporting material for MnO₂. The inorganic component was deposited as a result of the oxidation reaction of Mn(II) with oxidative groups of the host polymer. The surface of the polymer matrix was evenly covered with a thin layer of manganese oxide. The obtained product (R/S/Mn) exhibited high oxidative capacity over a wide pH range (2–12); however, under acidic and neutral conditions, the reaction ran significantly faster. The studied material shows some sorption properties but its sorption capacity is much lower than its oxidation capacity. The treatment, in a column regime, of the arsenic solution containing 1 mg As(III) dm^?3 and coexisting ions in concentrations similar to those in natural waters, confirmed the excellent oxidation capacity of the obtained product. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39489.     

Preliminary investigations on the anodic oxidation of Ti–13Nb–13Zr alloy in a solution containing calcium and phosphorus

Investigations on the surface modification of Ti–13Nb–13Zr alloy by anodic oxidation are reported here. The oxidation process was carried out in a solution containing 4 mol dm⁻³ H₃PO₄ and 100 g dm⁻³ Ca(H₂PO₂)₂. The anodising was realised at voltages of 80 V and 150 V. Moreover, a portion of the samples that had been oxidised at 150 V were held at a temperature of 500 °C. It was found that the morphology of the sample surface did not change during the oxidation of the alloy at 80 V. An application of 150 V led to the incorporation of calcium and phosphorus into the formed oxide layer and to significant modification of the surface morphology. The electrochemical characteristics of the modified alloy was also determined in Ringer's solution. It was shown that the electropolishing and anodising result in a considerable increase in the corrosion resistance of the Ti–13Nb–13Zr alloy.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Production of nanoparticles of copper compounds by anodic dissolution of copper in organic solvents

The anodic behaviour of copper was investigated in ethanol solution containing LiClO₄, LiCl electrolyte and water. The type of electrolyte and the water content influences the mechanism of the anodic process and the formation of anodic products. In LiClO₄ electrolyte the dissolution of copper is related to the oxidation of Cu(I) to Cu(II). In solutions of LiCl the etching of copper begins with the creation of soluble complexes of Cu(I) with chloride ions and solvent molecules. At potentials above 0.4 V the formation of alkoxides was observed in both solutions, characterized by a yellow tint. On the other hand, above 0.8 V (i.e. above the equilibrium potential of alcohol oxidation) copper dissolution is accompanied by the formation of a blue colloidal suspension of Cu (II) copper salt. Anodic etching of copper in solutions containing 3% H₂O at potentials higher than 0.4 V leads to the formation of colloidal suspension of copper oxide nanoparticles.     

Understanding the protective role of a gradient titanium oxide ceramic layer on Ti6Al4V against corrosion via analyses of Mott-Schottky curve and electron work function (EWF)

Thermal oxidation (TO) process was employed to generate a gradient titanium oxide ceramic layer for improving corrosion performance and service safety of Ti6Al4V alloy. The semiconductor characteristic of the TO layer was evaluated in CO₂-saturated simulated oilfield brine. The generated TO layer with a thickness of about 20 μm was dense and continuous without cracks or spalling characteristics. The TO layer mainly comprised of an oxide ceramic layer (rutile TiO₂ ceramic phase, minor anatase one, and Al₂O₃) and an oxygen diffusion layer. The conducted electrochemical analysis suggested that the corrosion resistance of Ti6Al4V alloy was improved using TO surface strengthening process. It was demonstrated that the TO layer with semiconductor characteristics showed a transition from n-type (donor) to p-type (acceptor) with the increasing applied electric potential. The electron work function of the TO layer was higher than that of Ti6Al4V alloy with a naturally formed passive film. The improvement in corrosion properties was attributed to the excellent chemical stability and semiconductor properties of the metal oxide ceramic phases (TiO₂, Al₂O₃) in the TO layer.     

The electrochemical oxidation of alkaline copper cyanide solutions

A systematic investigation of the electrochemical oxidation of copper cyanide was carried out. At low pH, cyanide destruction is believed to be catalyzed by the heterogeneous reaction involving adsorbed [Cu(CN)₃]²⁻ and possibly [Cu(CN)₄]³⁻. At high pH, rapid oxidation of cyanide was observed around 0.75 V versus Hg/HgO with the formation of a black copper oxide film. This enhanced electrocatalytic activity is believed to be related to the formation of an active copper(III) species. The transition point between low and high pH as a function of cyanide and copper concentrations is discussed. Bulk electrolysis of a copper cyanide solution at 0.90 V oxidized most of the cyanide to cyanate. Prolonged electrolysis further oxidized the cyanate to nitrate. The copper oxide film is found to be catalytic, capable of electro-oxidizing hydroxide to oxygen and cyanate to nitrate.     

Correlation between the electrochemical properties of colloidal oxides and supported oxides on metallic substrates

Mixed oxide electrodes of Ti(IV) and Cr(III) were prepared by calcining Cr(OH)₃ layers deposited on metallic titanium supports. This treatment produced a mixed oxide film of TiO₂–Cr₂O₃ covered with a layer of pure Cr₂O₃. The electrochemical response (cyclic voltammetry) shows the presence of two or three oxidation peaks depending on the electrode preparation conditions. One peak may be interpreted as the oxidation of Cr(III) to Cr(VI) species and the appearance of other peaks is due to the presence of chromium atoms in oxidation states higher than (III). The results of chemical analyses, electrophoretic mobilities and acid-base potentiometric titrations on calcined Cr(OH)₃ powders shows that the calcination step in air produced the decomposition of the Cr(III) hydroxide to the Cr(VI) oxide. Soluble Cr(VI) compounds were found in equilibrium with the suspended powder oxide which markedly affected the shape of the titrations curves. From the amount of Cr(VI) present in solution it was possible to correct the experimental ₀-pH curves. These corrected data indicated that Cr(VI) soluble species adsorbed at the Cr₂O₃/electrolyte interface.     

Preparation and effect of Mo-V-Cr-Bi-Si oxide catalysts on controlled oxidation of methane to methanol and formaldehyde

Mo-Cr-V-Bi-Si multi-component oxide catalysts were synthesized by three different coprecipitation methods and used in the controlled oxidation of methane to methanol and formaldehyde. It was shown that Mo content in Mo-V-Cr-Bi-Si oxides and the performance of these catalysts were strongly influenced by different coprecipitation methods. The highest methanol and formaldehyde selectivity of 80.2% could be achieved at a methane conversion of 10 % for the catalyst prepared by a particular method. The results of XRD indicated that the crystalline phase structures of catalysts were sensitive to Mo, V and Bi loadings. Bi(III) could combine with V(V) and Mo(VI) to form BiVO₄ and γ-Bi₂MoO₆, whereas Cr seemed to form a single Cr₂O₂ crystalline phase in the presence of Bi. The effects of Mo and Cr loading on controlled methane oxidation were also investigated. Mo(VI) oxide appears to favor the formation of partial oxidation products and Cr(III) oxide seems to enhance the conversion of methane.     

Oxidation of the copper alloy with pure copper plating

The oxidation failure of a copper alloy lead frame with/without a copper plating layer was investigated. The oxidation rate and adhesion strength of oxide films on copper alloy substrates were studied by measuring the thickness and by carrying out peel tests. The adhesion strength of the oxide film was mainly influenced by the composition but not the thickness of the oxide film. The highest adhesion strength was obtained when the oxide film was composed mainly of Cu₂O. When the thickness of the copper preplated layer was over 0.165?μm, the Cu atoms of the preplated copper were available for oxidation. Thus the oxidation process was within the copper preplated layer, and the main product of the oxidation was Cu₂O. It was found that the large column grain of the oxide film on the copper alloy with a copper plated layer, favored the diffusion of copper or oxygen atoms that led to the formation of Cu₂O, and lead to higher adhesion strength. This indicated that the oxidation resistance of a copper alloy lead frame can be effectively improved by electroplating copper.     

The activated electro-oxidation of sulphur dioxide on smooth platinum

The electrochemical behaviour of sulphur dioxide in sulphuric acid solutions at a platinum rde is characterized. Cycling the potential in these solutions between about ?0.10 and 1.2 V sce results in activation of the electrode so that diffusion-controlled SO₂ oxidation currents can be observed in the double layer region of platinum. Without activation, SO₂ oxidation proceeds noticeably only in the potenial region of surface oxide formation. Evidence is presented which indicates that activation results from formation of a catalytic layer of sulphur species. The catalytic activity of this layer decays with time in the course of SO₂ oxidation. The formation of sulphur oxides through oxidation of adsorbed sulphur and the formation of platinum oxides complicate the voltammetric behaviour of the system.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

Antimony/iron interactions in chlorinated polymer blends

The flammability properties of blends of acrylonitrile‐butadiene‐styrene (ABS) and chlorinated poly(vinyl chloride) (CPVC) have been investigated. Additive‐free ABS/CPVC blends, blends containing an iron(III) based smoke suppressant, basic iron(III) oxide (FeOOH) and blends containing equal amounts of the commercially available flame retardant, antimony(III) oxide (Sb₂O₃) and FeOOH were prepared. Char formation at 650°C, flammability (Oxygen Index) and smoke density (flaming‐mode‐NBS Smoke Density Chamber) data was obtained using standard procedures. Char formation in these systems was found to be linearly dependent on the chlorine content of the blends, even in the presence of antimony(III) oxide and basic iron(III) oxide. Oxygen index values for the additive‐free blends vary with blend composition (non‐linear) but the presence of FeOOH and Sb₂O₃/FeOOH combinations in the blends raise oxygen index values, especially at low (～20%) chlorine concentrations. Smoke density values for the additive‐free blends are linearly dependent on chlorine content, but the additives have different effects on smoke formation, effects that also depend on the chlorine content of the blends. Attempts to relate these changes in flammability parameters to chemical interactions occurring between polymers and additives during thermal decomposition are discussed. J. Vinyl Addit. Technol. 10:88–94, 2004. © 2004 Society of Plastics Engineers.     

A new ternary mixed oxide catalyst for ammoxidation of propane: Sn/V/Sb

The catalytic properties of a Sn/V/Sb mixed oxide for propane ammoxidation to acrylonitrile are studied in this paper. In particular the antimony and vanadium amounts were changed in order to optimize the relative atomic ratio. The ternary sample with the best catalytic performance was compared with the analogous binary samples (Sn/Sb and Sb/V). The results obtained indicate that vanadium is responsible for the paraffin activation, nevertheless much vanadium produces a lot of carbon oxides. Antimony is responsible for the insertion of nitrogen in the molecule (it can be considered as an acrylonitrile selectivity modulator); nevertheless, too much antimony deactivates the catalyst. Tin does not act only as a dispersive matrix for the active sites, but leads to the formation of a polyfunctional catalyst, increasing the rate of acrylonitrile formation from the intermediate propene. From a structural point of view, this catalytic system can be described as a homogeneous system, containing microfields of rutile type oxide (SnO₂), promoted with antimony and vanadium in substitutional solid solution, dispersed in an excess of amorphous antimony oxide. This revised version was published online in July 2006 with corrections to the Cover Date.