Oxygen reduction reaction on electrodeposited zinc oxide electrodes in KCl solution at 70 °C

The reduction of oxygen was studied in 0.1 M KCl at 70 °C using the rotating disk electrode (RDE) technique on platinum and electrodeposited ZnO thin film electrodes deposited on platinum substrates. In the absence of Zn²⁺ ions in solution, a Tafel slope of 139 mV dec⁻¹ was obtained, a value close to that measured on bare platinum electrode (133 mV dec⁻¹) and ascribed to the limitation of the reaction rate by the first electron transfer. The main difference between the noble metal and the oxide electrode was a shift of the curves towards more negative potentials. In the presence of Zn²⁺ ions, the current density decreased significantly and the Tafel slope was measured at 282 mV dec⁻¹ showing that the electrode was partially blocked by zinc oxide formation reaction intermediates.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Insights into the simultaneous chronopotentiometric stripping measurement of indium(III), thallium(I) and zinc(II) in acidic medium at the in situ prepared antimony film carbon paste electrode

The simultaneous measurement of microgram per liter concentration levels of indium(III), thallium(I) and zinc(II) at the antimony film carbon paste electrode (SbF-CPE) is demonstrated. The antimony film was deposited in situ on a carbon paste substrate electrode and employed in chronopotentiometric stripping mode in deoxygenated solutions of 0.01 M hydrochloric acid (pH 2). The chronopotentiometric stripping performance of the SbF-CPE was studied and compared with constant current chronopotentiometric stripping and anodic stripping voltammetric operation. In comparison with its bismuth and mercury counterparts, the SbF-CPE exhibited advantageous electroanalytical performance; namely, at the bismuth film electrode, the measurement of zinc(II) was practically impossible due to hydrogen evolution, whereas the mercury film electrode exhibited a poorly developed signal for thallium(I). The SbF-CPE revealed favorable calculated LoDs (3σ) of 1.4 μg L⁻¹ for thallium(I) and 2.4 μg L⁻¹ for indium(III) along with good linear response in the examined concentration range from 10 to 100 μg L⁻¹ with correlations coefficients (R²) of 0.992 for thallium(I) and 0.994 for indium(III) associated with a 120 s deposition time. The chronopotentiometric stripping performance of the SbF-CPE was characterized also by satisfactory reproducibility of 1.62% for indium(III), 3.96% for thallium(I) and 2.11% for zinc(II) (c = 40 μg L⁻¹, n = 11).     

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.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Factorial experimental design for biosorption of iron and zinc using Typha domingensis phytomass

Typha domingensis phytomass was used as a biosorbent for metal ions removal from wastewater. A full 2³ factorial design of experiments was used to obtain the best conditions of biosorption of Fe³⁺ and Zn²⁺ from water solutions. The three factors considered were temperature, pH, and biosorbent dosage. Two levels for each factor were used; pH (2.5 and 6.0), temperature (25 and 45 °C), and phytomass loading weight (0.5 and 1 g/50 ml). Batch experiments were carried out using 50 ml solutions containing 10 mg/l Fe³⁺ and 4 mg/l Zn²⁺ simulating the concentration of those metals in a real wastewater effluent. The removal percentages of iron and zinc after 120 min of contact time were then evaluated. The results were analyzed statistically using the Minitab 15 statistical software to determine the most important factors affecting the metals removal efficiency. The pH was found to be the most significant factor for the two studied metal ions.     

Zinc oxide quantum dots synthesized by electrochemical etching of metallic zinc in organic electrolyte and their electrochemiluminescent properties

A universal and facile electrochemical etching method for synthesizing nanosized metal oxide semiconductors has been presented by taking the synthesis of ZnO quantum dots (QDs) from metallic zinc as an example. By applying an appropriate potential, metallic zinc was controllably oxidized to Zn²⁺ ion, the latter (Zn²⁺) was released into an organic electrolyte and hydrolyzed by trace dissolved water therein, giving rise to ZnO QDs. The electrochemically synthesized ZnO QDs were uniform particles with an average diameter of 5.0 nm, and exhibited good photoluminescent and electrochemiluminescent activities. The effects of applied potential window, amount of water and dissolved oxygen on the formation of ZnO QDs were investigated and discussed in detail. The presently proposed electrochemical etching method not only has provided a facile, low-cost and controllable way to obtain ZnO QDs, but also would be applicable as a universal method in synthesizing other kinds of nanosized metal oxide semiconductors from related metals.     

Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction (?_pt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60-70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn²⁺ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ?_pt ≥ 0.1. The efficiency of in-coating Zn²⁺ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.     

Electrochemical properties of boron-carbon-nitride films formed by magnetron sputtering

The electrochemical behavior of B_1.0C_2.4N_1.0 thin film was investigated in acidic, neutral and alkaline solutions. The anodic polarization curve of the film in 1 M NaOH showed the anodic dissolution of the film. The curve of the film in 1 M HCl showed no anodic dissolution. The cathodic polarization curve in 1 M NaCl showed shift to a negative potential side, but the anodic polarization curve was the same as that of Pt. The anodic dissolution in 1 M NaOH depended on potentials, that is, no anodic dissolution was recognized in a potential range of −0.2 to 0.1 V but the dissolution rate increased with increasing potential in a range of 0.1-0.6 V. The anodic current density of the film is directly proportional to the dissolution rate at potentials higher than 0.1 V. The dissolution rate of the film was increased with increasing solution pH.     

Investigation on the inhibition effect of aspartic acid and Zn2+ ions on carbon steel surface in aqueous solution

A protective film has been formed on the surface of carbon steel in aqueous environment using a synergistic mixture of an environment-friendly inhibitor, aspartic acid, and Zn²⁺. The synergistic effect of aspartic acid (AS) in controlling corrosion of carbon steel has been investigated by gravimetric studies in the presence of Zn²⁺. The formulation consisting of AS and Zn²⁺ has an excellent inhibition efficiency. The results of potentiodynamic polarization revealed that the formulations are of mixed-type inhibitor. Impedance studies of the metal/solution interface indicated that the surface film is highly protective against the corrosion of carbon steel in the aqueous environment. X-ray photoelectron spectroscopic analysis of the protective film showed the presence of the elements iron, nitrogen, oxygen, carbon, and zinc. The spectra of these elements in the surface film showed the presence of oxides/hydroxides of iron(III), Zn(OH)₂, and [Fe(III)/Fe(II)–Zn(II)-AS] complex. Further, surface characterization techniques such as Fourier transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy are used to ascertain the nature of the protective film formed on the carbon steel surface.     

The spatial distribution of Zn during galvanic corrosion of a Zn/steel couple

The spatial distribution of Zn²⁺ during galvanic corrosion of a model Zn/steel couple in 0.01 M NaCl was investigated using a scanning zinc disk electrode. The couple had a coplanar arrangement of a steel substrate with an electroplated zinc layer at the center. During galvanic corrosion, the marked changes in the Zn²⁺ concentration were confined to a thin solution layer ca. 1.0 mm thick above the couple surface. In this thin solution layer above the zinc layer, a higher concentration region of Zn²⁺ in the range of 5-18 mM extended around the zinc layer in the solution during galvanic corrosion. Conversely, above the steel surface distant from the zinc layer, the surface concentration of Zn²⁺ was almost zero during galvanic corrosion. On this surface, the precipitation of zinc corrosion products due to the hydrolysis reaction of Zn²⁺ was observed. The distribution of the Zn²⁺ concentration supported that Zn²⁺ acted as a buffer that suppressed the increased pH due to the cathodic reaction on the steel surface near the zinc layer and almost no corrosion products formed there. The spatial distribution of Zn²⁺ is discussed in relation to the distributions of potential and pH and the surface morphology of the galvanic couple.     

Protective mechanisms occurring on zinc coated steel cut-edges in immersion conditions

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn²⁺ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH)₂ film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH)₂ covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.     

Hydrothermal Synthesis of Mn–Fe Nano Oxides and Their Composite for Removal of Zn<Superscript>2+</Superscript>, Ni<Superscript>2+</Superscript> and Co<Superscript>2+</Superscript> from Simulated Radioactive Waste

A study on the sorption of Zn²⁺, Ni²⁺ and Co²⁺ onto mixed oxide of Mn and Fe obtained at different hydrothermal conditions and its organic hybrid film modified with polyacrylamide (Mn–Fe oxide/PAM) has been examined. The characterization of inorganic oxides and its composite samples were performed using XRD, SEM, FTIR, XRF and DTA-TGA techniques. The percent sorption of Zn²⁺, Ni²⁺ and Co²⁺ on Mn–Fe oxide at pH 4.5 was 97, 11.85 and 10 % respectively with selectivity order Zn²⁺ ? Ni²⁺ > Co²⁺. The sorption value of Zn²⁺ at pH 4.5 onto Fe–Mn oxide reached nearly the same value of Zn²⁺ onto its composite. So, the new compound of Fe–Mn oxide has promising uses for separation of zinc ions while its composite can be used for removal all of these cations.     

Chelation of chitosan derivatives with zinc ions. I. O,N‐carboxymethyl chitosan

The chelation between O,N‐carboxymethyl chitosan (ONCMCh) and zinc sulfate in aqueous solution was studied by kinetic experiments and characterized by inductively coupled plasma (ICP) and UV spectrophotometry. The experimental data indicated that the chelating processes were greatly controlled by the reaction conditions (i.e., reaction time, temperature, and Zn²⁺ ionic and ligand concentrations). The consequence of chelating Zn²⁺ onto ONCMCh was the formation of complexes with different solubilities. The favorable complexes for ONCMCh‐Zn²⁺ chelate were at the low zinc ionic and ligand concentrations, as well as at the appropriate temperature. The evidence provided by the kinetic parameters and the changes in zinc concentration by ICP analysis further confirmed the plausible complexing mechanisms. While the formation of water‐soluble products was occasioned by the electrostatic attraction mechanism, the water‐insoluble products were predominantly formed by chelation of Zn²⁺ with O,N‐carboxymethyl chitosan. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2246–2253, 2000     

Electrochemical study on the inhibitory effect of the underpotential deposition of zinc on Zn-Co alloy electrodeposition

Zn-Co alloy electrodeposition from chloride baths containing different Zn²⁺/Co²⁺ ratios was investigated by cyclic voltammetry and anodic linear sweep voltammetry using a Pt electrode. The peaks were attributed by means of EDX analysis, SEM and TEM observations performed on some alloys potentiostatically deposited. In the range of potential where zinc deposits underpotential, cyclic voltammetry showed a complex cathodic peak with one maximum and two shoulders, correlated with the deposition of different cobalt rich alloys. Up to four anodic peaks, two correlated with zinc oxidation from η and γ phases and two correlated with oxidation of solid solutions of zinc in cobalt, were observed. ALSV and TEM indicated that the remarkable increase in Zn content of the alloy, which occurs with a strong inhibition of the process at potentials more negative than that of the cathodic peak and more positive than the bulk deposition potential of zinc, is due to the deposition of γ phase. No inhibition of the alloy deposition process was observed with very low concentrations of zinc (<0.015 M) in the bath containing 0.19 M Co²⁺.     

Characterisation of titanium oxide film grown in 0.9% NaCl at different sweep rates

The nature of the anodic oxide film that forms on titanium on titanium in 0.9% NaCl has been investigated using a wide range of techniques. A linear relationship was found between the critical current density required for passivation of titanium in 0.9% NaCl and the sweep rate. Anodic oxide films formed on titanium in 0.9% NaCl appear to consist of two layers, an inner compact layer, the growth of which continues to follow a high field growth law, and a porous less protective outer porous layer. XPS and XRD indicated that passive films on titanium consist mainly of TiO₂. However, hydroxides and lower oxides are also present, especially in rapidly grown films. XRD data indicated that in 0.9% NaCl the anodic oxide film is formed through the preferential removal atoms in the plane of (0 0 2) in the course of electrochemical reaction. A model based analysis XPS spectra was proposed to explain the growth rate dependence of the degree of protection offered by anodic oxide films on titanium. XPS clearly demonstrated the present of Ti(III) and Ti(II) cations in the passive film. This is strong evidence that cation migration more likely dominates over anion migration in the growth mechanism of anodic oxide film. XPS data also revealed that the concentrations of Ti(III) and Ti(II) species within the oxide films increased as the oxide/metal interface was approached.     

Influence de l'antimoine et du cuivre sur la cementation du cobalt par le zinc

Zinc sulphate solution (160g/l Zn²⁺) used for zinc electrowinning is purified for cobalt by cementation with metallic zinc powder. Industrial practice shows that considerable acceleration of this operation is achieved by the presence of trace impurities in solution. Eventually, CuSO₄ and Sb₂O₃ are deliberately added to the solution. The aim of this work is to elucidate the mechanism of action of those impurities.Zn²⁺ ions present in the solution are responsible for the slow speed of cobalt deposition (160g/l Zn²⁺; 10 mg/l Co²⁺).The cathodic part of the cementation reaction was simulated on a flat electrode through potentiostatic deposits at ?730 mV/ENH (potential at zero current of metallic zinc in the solution). The deposits were studied by anodic dissolution, radioactive tracers, X-ray diffraction and fluorescence, atomic absorption spectrophotometry, optical and electronic microscopy (transmission and scanning). The results were compared with cementation on zinc plate and zinc powder.Antimony and copper are deposited together with cobalt and form alloys with reduced cobalt activity. Total voltage available to overcome the inhibitor effect of zinc ions is thus increased and the reduction of cobalt ions accelerated. Antimony, copper and cobalt triple alloys resist particularly well to corrosion with hydrogen evolution. Copper has a higher accelerating effect on cobalt cementation than antimony, but the latter stabilizes the deposit very effectively.The electrochemical methods used and the knowledge of the mechanism of action of the impurities open new trends for industrial practice.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH^–]9 M), the dissolution process appeared to follow zero-order kinetics.     

Comparison of poly(ethylene-co-acrylic acid) loaded Zn2+–montmorillonite nanocomposites and poly(ethylene-co-acrylic acid) zinc salt

Novel nanocomposite films based on poly(ethylene-co-acrylic acid) (PEAA) and zinc montmorillonite (Zn²⁺–MMT) were fabricated using a solution casting method with water as the solvent. Transmission electron microscopy indicated that Zn²⁺–MMT was distributed finely in the PEAA matrix. X-ray diffraction indicated that an ion exchange process occurs between Zn²⁺–MMT and PEAA. The nanocomposites filled with a low Zn²⁺–MMT loading increased the tensile strength and elongation at break. The significant improvements in these mechanical properties were attributed to the fine dispersion of Zn²⁺–MMT in the polymer and the covalent interaction between the polymer chains and Zn²⁺ cations. Thermogravimetric analysis and differential thermal calorimetry confirmed that PEAA formed a network through the presence of Zn²⁺ cations. A poly(ethylene-co-acrylic acid) zinc salt (PEAAZn) film by hot pressing was introduced for comparison. Zn²⁺–MMT improved the mechanical properties of the PEAA significantly compared to that of PEAAZn.