Sonia R. Biaggio

Steady‐State Passive Films Interfacial Kinetic Effects and Diagnostic Criteria

This paper reports that the point defect model for steady-state passive films formed anodically on metal s in aqueous environments has been extended to include irreversible dissolution of the film and the irreversible generation and annihilation of cation and oxygen vacancies at the metal/film and film/solution interfaces. THe model yields a number of diagnostic criteria that can be used to identify the majority (vacancy) charge carrier and to characterize the kinetic nature of the interfacial vacancy generation and annihilation processes. We use these criteria to show that the steady-state passive film that forms on nickel in acidic phosphate buffer solutions is a cation conductor and that cation transport from the metal to the solution involves irreversible ejection of cations from the film. On the other hand, the passive film that forms on tungsten in the same environment under steady-state conditions is found to be an oxygen ion conductor with the passive current being determined by kinetically controlled film dissolution at the film/solution interface. These different behaviors are rationalized in terms of the relative energies for generation of cation and anion vacancies in the passive films.

Electrochemical degradation of a real textile effluent using boron-doped diamond or β-PbO2 as anode

Constant current electrolyses are carried out in a filter-press reactor using a boron-doped diamond (Nb/BDD) or a Ti-Pt/β-PbO(2) anode, varying current density (j) and temperature. The degradation of the real textile effluent is followed by its decolorization and chemical oxygen demand (COD) abatement. The effect of adding NaCl (1.5 g L(-1)) on the degradation of the effluent is also investigated. The Nb/BDD anode yields much higher decolorization (attaining the DFZ limit) and COD-abatement rates than the Ti-Pt/β-PbO(2) anode, at any experimental condition. The best conditions are j = 5 mA cm(-2) and 55 °C, for the systems optimized hydrodynamic conditions. The addition of chloride ions significantly increases the decolorization rate; thus a decrease of more than 90% of the effluent relative absorbance is attained using an applied electric charge per unit volume of the electrolyzed effluent (Q(ap)) of only about 2 kA h m(-3). Practically total abatement of the effluent COD is attained with the Nb/BDD anode using a Q(ap) value of only 7 kA h m(-3), with an energy consumption of about 30 kW h m(-3). This result allows to conclude that the Nb/BDD electrode might be an excellent option for the remediation of textile effluents.

Lead recovery from a typical Brazilian sludge of exhausted lead-acid batteries using an electrohydrometallurgical process

Abstract Lead recovery from the nonmetallic portion of exhausted lead-acid batteries, also called sludge, was investigated using an electrohydrometallurgical process. Among 13 aqueous solutions studied in solubility tests, only the following three were chosen for the whole process (leaching and electrowinning steps): tetrafluoroboric acid (200 g/L), glycerol (92 g/L)+sodium hydroxide (120 g/L) and sodium potassium tartrate (150 g/L)+sodium hydroxide (150 g/L). The tetrafluoroboric acid showed an attractive performance as leaching electrolyte due to its low cost and reasonable leaching strength. In the electrowinning process using the solution obtained from the leaching of a desulfated sludge with this acidic electrolyte, compact, adherent and highly pure lead deposits were produced at 250 A/m2. Scanning electron micrographs (SEM) of lead deposits obtained at different current densities in the range of 250–500 A/m2 revealed a marked influence of the current density on the deposit morphology.

A study of thin anodic WO3 films by electrochemical impedance spectroscopy

The properties of thin anodic passive films potentiostatically formed (1 V ≤ Ef ≤ 5 V vs sce) on polycrystalline tungsten in aqueous H3PO4, H2SO4, HNO3 and HClO4 solutions (pH ≈ 1.3) were studied using electrochemical impedance spectroscopy at room temperature. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the W(0) → W(VI) reaction, with these in parallel with the capacitance of the metal/passive film/electrolyte system. The relative permittivity (er) of the films was found to depend on the acid solution used. Except for the case of films grown in HNO3, the values of er, are smaller than those of thicker films grown galvanostatically. The film grown in H3PO4 solutions presented the lowest value of er, which could be due to the incorporation of phosphate ions into the film. The number concentration of donors in the films (ND) was found to decrease with Ef (ie increasing film thickness) and to be quite dependent on the electrolyte used: ND(H3PO4) < ND(HClO4) < ND(HNO3) < ND(H2SO4). The values for ND(H3PO4) are, nevertheless, 103 times larger than the ones for much thicker WO3 films and are of the same order of magnitude as the ones for annealed WO3 films. The flat band potential of the films grown in H3PO4 was found to be more than 0.1 V smaller than those for films grown in the other solutions.

Spectroelectrochemical and electrical characterization of poly(cobalt - tetraaminophthalocyanine)-modified electrodes: Electrocatalytic oxidation of hydrazine

Abstract Polymeric complex films of cobalt–tetraaminophthalocyanine (Co–TAPc) and tetraaminophthalocyanine without the metallic center (H2–TAPc) were grown potentiodynamically on either glassy-carbon or conducting-glass electrodes. The electrolyte used in the electropolymerization was a 0.1 M tetrabutylammonium perchlorate–DMF solution containing the monomer. The modified electrodes were characterized by UV–Vis spectroscopy, voltammetry and electrochemical impedance spectroscopy. The role of the Co(II) center and the performance of these polymeric electrodes as electrocatalysts for hydrazine oxidation in aqueous solutions of pH 13 are discussed. The results indicated that Co(II) is the species acting as the electronic receptor of hydrazine, thus enhancing the electrocatalytic effect of the poly(Co–TAPc) electrode. This was also confirmed by the charge transfer resistance values from electrochemical impedance spectroscopy obtained during hydrazine oxidation, i.e. about 6×103 Ω on the poly (H2–TAPc) electrode and about 500 Ω on the poly(Co–TAPc) electrode.

Influence of the first potential scan on the morphology and electrical properties of potentiodynamically grown polyaniline films

Abstract Polyaniline (Pani) films were grown potentiodynamically on Pt electrodes in 0.1 M aniline/0.5 M acid (H2SO4 or HClO4), or in a 0.1 M aniline/0.5 M acid/0.2 M salt containing an alkaline cation, between −0.25 and 0.69 V vs. SCE. A series of films were grown at a sweep rate of either 0.1 V s−1 or 0.002 V s−1 only for the first cycle, followed by 0.1 V s−1 for the remaining cycles up to a constant anodic charge of 40 mC cm−2. Changes in the morphology, as well as in the electrical properties of the films as a function of the initial potential perturbation and the alkaline cation in the electrolyte were followed, respectively, by SEM images and electrochemical impedance spectroscopy measurements.

Electrochemical degradation of the reactive red 141 dye on a β-PbO2 anode assessed by the response surface methodology

The electrochemical degradation of the Reactive Red 141 dye using a filter-press reactor with a β-PbO2 anode was investigated through the application of the response surface methodology. The charge required for 90% decolorization (Q90) and the chemical oxygen demand removal percentage after 30 min electrolysis (COD30) were used to model the system. The investigated independent variables were the current density, pH, NaCl concentration, and temperature. Low values of Q90 (0.2-0.3 A h L-1) were obtained at acidic conditions (pH 1-3) and high concentrations of NaCl (1.0-2.0 g L-1), when Cl2 and HOCl are the predominant oxidant species. The best values of COD30 were obtained at high current densities and acidic to neutral conditions (pH 5-7); however, the consequent energy consumption makes the process not economically feasible under these conditions. For strongly acidic solutions, specific energy consumptions associated to Q90 as low as 0.79 kW h m-3 were attained.

Studies on the stability of anodic oxides on zirconium biocompatible alloys

Studies on the stability of anodic oxides grown on zirconium and its biocompatible alloys Ti-50Zr at.% and Zr-2.5Nb wt.%, in aerated Ringer physiological solution, at 25 and 37 °C, were carried out by comparing their formation charge and their reconstruction charge after dissolution at open circuit in the physiological solution. The stability of oxides grown in the Ringer solution and in a 0.15 mol L-1 Na2SO4 solution was compared. The obtained results show that the stability of these oxides is increased by aging under potentiostatic conditions and can be decreased by the presence of chloride ions in the electrolyte during the anodization process.

A comparison of electrodeposited Ti/β-PbO2 and Ti-Pt/β-PbO2 anodes in the electrochemical degradation of the direct yellow 86 dye

The electrochemical performance of electrodeposited Ti/β-PbO2 and Ti-Pt/β-PbO2 anodes was galvanostatically evaluated (batch mode, 50 mA cm-2) to degrade the Direct Yellow 86 dye (100 or 200 mg L-1 in 0.1 mol L-1 Na2SO4 + 1.5 g L-1 NaCl), investigating the effect of pH and temperature. Similar results were obtained for both electrodes and the best conditions for removal of color and chemical oxygen demand are pH 7 and 40 °C, when 90% decolorization is attained by passing a charge of only ~0.13 A h L-1 and total mineralization is achieved with expenditure of ~5 kW h m-3.

Growth of aluminum-free porous oxide layers on titanium and its alloys Ti-6Al-4V and Ti-6Al-7Nb by micro-arc oxidation

The growth of oxides on the surfaces of pure Ti and two of its ternary alloys, Ti-6Al-4V and Ti-6Al-7Nb, by micro-arc oxidation (MAO) in a pH 5 phosphate buffer was investigated. The primary aim was to form thick, porous, and aluminum-free oxide layers, because these characteristics favor bonding between bone and metal when the latter is implanted in the human body. On Ti, Ti-6Al-4 V, and Ti-6Al-7Nb, the oxides exhibited breakdown potentials of about 200 V, 130 V, and 140 V, respectively, indicating that the oxide formed on the pure metal is the most stable. The use of the MAO procedure led to the formation of highly porous oxides, with a uniform distribution of pores; the pores varied in size, depending on the anodizing applied voltage and time. Irrespective of the material being anodized, Raman analyses allowed us to determine that the oxide films consisted mainly of the anatase phase of TiO2, and XPS results indicated that this oxide is free of Al and any other alloying element.