EPS-contact-leaching mechanism of chalcopyrite concentrates by A. ferrooxidans

The effect of extracelluar polymeric substances（EPS） on the bioleaching chalcopyrite concentrates in the presence of ironand sulphur-oxidizing bacteria （A. ferrooxidans） was studied. The bacterial number, pH, redox potential, and the concentrations of Fe^2＋ and Cu^2＋ ions were investigated. The leached residues were analyzed by the X-ray diffraction and FT-IR. The results indicate that the EPS makes the bacteria adhere to the chalcopyrite surface easily and it is helpful for bacteria in disadvantageous environment At the same time, EPS film layer with Fe^3＋ deposits on the surface of chalcopyrite and becomes a barrier of oxygen transfer to chalcopyrite to passivate chalcopyrite, and creates the high redox potential space through concentrating Fe^3＋ ions to accelerate bioleaching pyrite in chalcopyrite concentrates. The results suggest that EPS formation promotes bioleaching pyrite and inhibits bioleaching chalcopyrite, especially under high potential condition.     

Electrochemical assessment of chromium in passivation films on tinplate

Abstract

When tinplate, passivated in chromate solutions, is anodically polarised either with current measurement at steadily increasing potential or with potential measurement at constant current, an indication of the oxidation of Cr^III to Cr^VI in the surface film is obtained. The indication can be made quantitative for the estimation of Cr^III. The constant-current method is preferred as permitting an easy measure of the charge passed during the oxidation which is indicated as an arrest on the potential/time curve. The preferred conditions are a chloride-free neutral phosphate solution and a current density of 25 μA/cm².

For most commercial tinplates the estimated Cr^III is similar to the total chromium but the few exceptions and others giving unusual potential/time curves may help in relating treatment td performance in laboratory cathodic-anodic passivation treatments, the chromium was wholly oxidised to Cr^VI by passage of a sufficient charge in the anodic period.     

The corrosion behaviour in organic acid solutions—II. A steel electrode

The corrosion behaviour of steel in stagnant de-aerated citric and oxalic acid solutions (10^?5–10^?1M) in the pH range 2–6 was studied. It was found that the corrosion behaviour of steel in both acids was generally the same but it depended on the acid concentration and the pH value.In 10^?4–10^?1M, the corrosion rate (W) increased with the increase of acid concentration and decreased with the increase of pH value as follows:

$\text{log}\text{W = a+b}\text{log}\text{C, where b=0.485 and}\text{d}\text{log}\text{W}\text{d}\text{pH}\text{=?0.15?0.25.}$

At high concentration (10^?1 and 10^?2) and in the pH range 2–5, the steady state potential varied by 64 and 58 mV for a unit change of pH in oxalic and citric, respectively. The corrosion process was found to be cathodically controlled.In dilute solutions (10^?5–10^?3M) in the pH range of 2–5 and in all acid concentrations at pH 6, the steady state corrosion potential shifted in the negative direction with increase of acid concentration accompanied by an increase in the corrosion rate, indicating that the corrosion process became anodically controlled.     

Corrosion behaviour in organic acid solutions—I. Tin electrode

The corrosion behaviour of tin in oxalic, citric and tartaric acids in the pH range 2–6 and at concentrations 10^?1–10^?5 M was investigated. The results indicate that increasing the concentration of the organic acid and the pH of the medium up to 4 shifts the corrosion potential to a more negative value and increases the rate of corrosion. At pH values 5 and 6, a shift in the corrosion potential in a noble direction is accompanied by a decrease in the corrosion rate. A linear relationship is observed between the steady state potential (E) and the logarithmic concentration of the organic acid (log C) for the three acids studied:

$\text{E = a ? b}\text{log}\text{C}$

. It was concluded here that the corrosion mechanism in the pH range 2–4 is anodically controlled by the complexing of Sn²⁺ ions with organic acid anions and that the order of decreasing aggressiveness is oxalic > citric > tartaric acids. Corrosion was inhibited at pH values 5 and 6 and this was attributed to film formation on the surface of tin which may result from the hydrolysis of tin complex species.     

Anodic Dissolution of Nickel from the Inherent and NiZn Intermetallic Phases in Acidic Sulfate Solutions. I. Nickel

For comparative investigation of the anodic dissolution kinetics of nickel from its own phase and NiZn intermetallic compound, the literature data are generalized and supplemented with new results concerning the anodic behavior of individual nickel in acidic sulfate environments. During the anodic dissolution of nickel at all the stages either in its passive or active state, the main role is shown to be played by the adsorption of water. HSO^– ₄, Cl^–, and I^– anions decelerate the active dissolution of nickel in a potential range from 0.0 to 0.2 V and accelerate it in a range from 0.2 to 0.3 V.     

The mechanism of dezincification and the effect of arsenic. II

Abstract

Dezincification occurs by dissolution of both zinc and copper, followed by redeposition of copper from the oxygen-free cuprous chloride/zinc chloride anolyte. Deposition of copper from such a solution by reduction of Cu⁺ requires a potentIal below ?0·41 V but Cu²⁺, formed by disproportionation of cuprous chloride, can be reduced to copper at potentials up to ?0·16 V. As the potential of film-free beta-brass iS ?0·56 V, reduction of copper at the advancing front of dezincification in beta-brass can occur by direct cathodic re~uction of cuprous ions. Alpha-brass, with a potential of ?0·38V, can only reduce cupricions; dezincification of alpha-brass therefore depends upon formation of Cu²⁺ by disproportionation of cuprous chlonde at the advancing front.

Under the conditions existing at the advancing front, disproportionation of cuprous chloride can be suppressed by a small amount of arsenic, acting through reactions in which it cycles between the As³⁺ and AS_(solid)states. Arsenic consequently inhibits dezincification of alpha-brass but the dezincification of beta-brass, which does not require the formation of cupric ions, is unaffected.     

Determination of the diffusivity of cation vacancy in a passive film of Ni using Mott-Schottky analysis and in-situ ellipsometry

The diffusivity of cation vacancies in a passive film formed on Ni in a pH 8.5 buffer solution was estimated based on the Point Defect Model with high field migration equations. The parameters required to calculate the diffusivity of the cation vacancies include the flux of cation vacancies, the acceptor density, and the thickness of the passive film as functions of the film formation potential and they were measured from potentiostatic polarization, a Mott-Schottky analysis, and in-situ ellipsometry, respectively. The passive current density, proportional to the flux of the point defects, linearly increased with an increase of the passivation potential, while the acceptor density was reduced with the potential. The thickness of the passive film appeared to increase very rapidly and the growth became stable in thousands of seconds, and increased linearly with an increase of the potential. The dependence of all parameters investigated in this study on the passivation potential corresponded well to the theory. The diffusivity of the cation vacancies in the passive film of Ni was determined to be 2.7×10^?18 cm²s^?1.     

Current and potential measurements along simulated cracks

Measurements of the potential gradients along real, and simulated cracks have been made for an HY80 steel in natural seawater and on simulated cracks for a C-Mn steel in a CO^2-₃/HCO^-₃ solution. The simulated crack cell also allowed measurement of the current flow in the crack enclave. The results show that large potential drops only occur along cracks if attempts are made to pass relatively large currents along the crack enclave, so that if the applied potential is not too far removed from the free corrosion potential or the environment is one that promotes passivation of the steel surface then any initial potential drop will decay relatively quickly.     

The dissolution of iron under cathodic polarization

It has been shown that the dissolution rate for iron at cathodic potential is higher than expected on the grounds of the Wagner-Traud mixed potential theory. This observation cannot be explained by a parallel chemical process as suggested by Kolotyrkin et al. The dissolution appeared to be unaffected by potential, pH, cations (Li⁺, Na⁺, K⁺), anions (SO^2?₄, Cl^?, ClO^?₄) and solvent (water-dioxan mixtures). However, the dissolution rate was decreased by an order of magnitude when the electrode was connected to a strong magnet, suggesting that the mechanical degradation of the surface under the influence of the cathodically evolved hydrogen is the main cause of the measured effects.     

The dissolution of iron under cathodic polarization

It has been shown that the dissolution rate for iron at cathodic potential is higher than expected on the grounds of the Wagner-Traud mixed potential theory. This observation cannot be explained by a parallel chemical process as suggested by Kolotyrkin et al. The dissolution appeared to be unaffected by potential, pH, cations (Li⁺, Na⁺, K⁺), anions (SO^2-₄, Cl^?, ClO^-₄) and solvent (water-dioxan mixtures). However, the dissolution rate was decreased by an order of magnitude when the electrode was connected to a strong magnet, suggesting that the mechanical degradation of the surface under the influence of the cathodically evolved hydrogen is the main cause of the measured effects.     

Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K₂Cr₂O₇. Raman spectra acquired in and near corrosion pits allowed identification of two products of Cr^VI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an Al^III-Cr^VI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al³⁺ and CrO₄²⁻. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a Cr^III-Cr^VI mixed oxide, formed by partial reduction of Cr^VI to Cr^III by exposed Al metal or by H₂. At the pitting potential, a mixture of Al^III and Cr^III mixed oxides was observed, with the Al^III species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by Cr^VI are considered. In particular, the concentration of Cr^VI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.     

Kinetics of phase transformations in the surface layer of a binary alloy at the selective dissolution: III. Cu-Au | Cu<Superscript>2+</Superscript> system

Conditions of the phase transformation of gold on the surface of Cu-Au alloys selectively dissolved in an acidic sulfate solution at the overcritical anodic polarization are experimentally found. Kinetic regularities of the formation of an individual Au⁰ gold phase on the alloy surface depending on the alloy composition, anodic potential, and the presence of organic surfactants in the solutions are determined.     

Vorwort

Investigation into the pitting corrosion of passive austenitic CrNi steels in neutral chloride solutions Stainless steels of the 18/8 CrNi-Type suffer pitting corrosion by halogen ions. Potentiokinetic, galvanostatic and potentiostatic tests as well as the ferro-ferricyanide-tests showed that pitting susceptibility increases with Cl^? content, temperature and oxygen content of the electrolyte, with decreasing homogeneity and purity of the material. Cold-working is without significant influence on the pitting potential. Mn up to 11,2% increases pitting potential by 50 mV, Ni up to 25% increases the potential by 200 mV, Cr up to 30 and Mo up to 4,6% increase the potential by max. 900 mV in 3% NaCl of p_H 7,5 at 22° C. The four methods employed gave the same pitting potentials. Before arriving at the potential of stationary pitting all steels showed a region where formation and repassivation of single pits occur. Cathodic protection to suppress pitting causes H₂-absorption. The amount of absorbed H₂ increases as the potential becomes more negative. Hydrogen embrittlement was not observed. The absorbed H₂ impairs pitting resistance. The study of Cl^?-adsorption as a pitting releasing process by help of the potentiostatic method, working with a reference source of triangular alternating voltage gave no indication of a preferential Cl^?-adsorption or an Cl^?-adsorption-potential near the pitting potential.     

Korrosionsmonitoring mit elektrochemischen Sensoren unter Einsatz von Evolutionsstrategien

Corrosion monitoring with electrochemical sensors by using evolution-inspired optimization techniques A complex array of sensors for measurements of corrosion influencing parameters was investigated at the example of pitting corrosion of the alloy X 5 CrNi 18 10. Potentiodynamic measurements of pitting potential were performed in a flow circuit by varying chloride ion concentration (10^?3 … 1 mol 1^?1), pH-value (2 … 8), content of dissolved oxygen (0 … 8 mg 1^?1), temperature (10 … 60°C) and flow velocity (0 … 2 m s^?1). The influence of these parameters on the pitting potential was evaluated by using evolution-inspired optimization technique with an equation derived from literature data. This technique is indicated by stable convergence in the present application and results in an empirical equation facilitating the forecast of pitting potential in the investigated parameter field.     

The application of noble metals in light-water reactors

Corrosion potential is a primary determinant of the stress-corrosion cracking susceptibility of structural materials in high-temperature water. Efforts to minimize stress-corrosion cracking in light-water reactors include adding hydrogen. In someplants’ out-of-core regions, the hydrogen required to achieve the desired corrosion potential is relatively high. In-core, more hydrogen is needed for an equivalent reduction in corrosion potential. Additionally, sIDe effects of high hydrogen-addition rates, including increased ¹⁶N turbine shine and ⁶⁰CO deposition, have also been observed in some cases. An approach involving noble-metal coatings on and alloying additions to engineering materials dramatically improves the efficiency with which the corrosion potential is decreased as a function of hydrogen addition, such that very low potentials are obtained once a stoichiometric concentration of hydrogen (versus oxygen) is achieved.     

Conducting polymers from anodic coupling of dithienylacetylenes. Electrochemistry and potential-driven conductive and magnetic properties

Anodic coupling of dithienylacetylene and dithienylacetylenes substituted with alkyl and alkoxy groups has been investigated in acetonitrile. The resulting polymers have been characterized by cyclic voltammetry, UV-Vis spectroscopy, electrochemical quartz crystal microbalance, in situ ESR and in situ conductivity. The degree of conjugation of the polymers is increased as the monomer oxidation potential is lowered, which occurs with alkyl-substitution and even more with alkoxy-substitution. Alkoxy-substitution in position 3 of the thiophene ring produces the dimer, whereas 4-substitution yields an extensively crosslinked polymer. A regularly coupled polymer is obtained by 3,4-disubstitution with the ethylenedioxy group. The low conductivity (1 × 10⁻³ S cm⁻¹) and the high unpaired spin density in the oxidized state of this polymer are explained by localization of charge and spin operated in the dithienyl moieties by the triple bonds.     

Prevention of passive film breakdown on iron in a borate buffer solution containing chloride ion by coverage with a self-assembled monolayer of hexadecanoate ion

Breakdown of a passive film on iron in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻ was suppressed by coverage of the passive film surface with a self-assembled monolayer (SAM) of hexadecanoate ion C₁₅H₃₁CO₂⁻ (C₁₆A⁻). The pitting potential of an iron electrode previously passivated in the borate buffer at 0.50 V/SCE increased by treatment in an aqueous solution of sodium hexadecanoate for many hours, indicating protection of the passive film from breakdown caused by an attack on defects of the film with Cl⁻. No breakdown occurred over the potential range of the passive region by coverage with the SAM of C₁₆A⁻ in some cases. Structures of the passive film and the monolayer were characterized by X-ray photoelectron and Fourier transform infrared reflection spectroscopies and contact angle measurement with a drop of water.     

A conformational study of ethyl-substituted bithiophenes. Semi-empirical versus ab initio methods

Semi-empirical (AM1, PM3) and ab initio calculations (STO-3G, 3-21G¹) are employed to obtain the equilibrium optimized geometries and the torsional potential surfaces of 2,2′-bithiophene as well as its 3,4′- and 3,3′-ethyl-substituted derivatives. For the unsubstituted molecule, ab initio calculations have also been performed at the HF/6-31G¹ level. The geometries were completely optimized along the torsional potential curves to account for the molecular relaxation, yielding a physically meaningful picture of the nonrigid rotation. The results given by each theoretical method are compared and discussed. It is found that ethyl substitution causes rather small changes in the thiophene ring structure. Contrary to these results, ethylation dramatically influences the overall shape of the torsional potentials, leading to a large tilt from planarity. The barrier against planarity is found much higher for the 3,3′-ethyl derivative. It is also observed that the steric hindrance created by ethyl groups is much higher than that induced by methyl substituents.     

The role of repassivation kinetics in the measurement of the pitting potential of AISI 304 stainless steel by the scratch method

The rate of repassivation of AISI 304 stainless steel in Cl^? solutions of pH 2 has been examined as a function of potential. The rate of repassivation was found to decrease in the vicinity of the pitting potential. This decrease could not be used as an accurate measure of the pitting potential because at lower potentials, where repassivation rate was rapid, pits nucleated, provided that the potential was maintained for long periods of time.     

Kinetic study of the electrochemical degradation of polyaniline

The kinetics of the electrochemical degradation of polyaniline (PANI) layers, deposited by electropolymerization and chemical polymerization onto platinum electrode, was investigated in an acid aqueous solution. The degradation rate was shown to depend greatly on the electrode potential applied. First-order rate constants of degradation, obtained from the kinetic data, were shown to vary between 2.87×10⁻⁵ and 3.11×10⁻³ s⁻¹ for thick PANI films, having the electrochemical charge density of 14 mC/cm², and between 2.0×10⁻⁵ and 3.60×10⁻³ s⁻¹ for thin PANI films, having the charge density of 1.5 mC/cm², within the electrode potential range of 0.3–0.9 V versus Ag/AgCl. Two linear regions were found to present on the dependencies of logarithm of the first-order degradation rate constant on electrode potential, one of them having a slope of 0.44 and 1.34 V⁻¹ within electrode potential limits of 0.3–0.6 V, and another one having a slope of 6.37 and 6.39 V⁻¹ within potential limits of 0.6–0.9 V, for thick and thin polymer films, respectively. The results obtained show that the electrochemical degradation of PANI films proceed at a remarkable rate even at low electrode potential values.