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.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

In situ gravimetry of nickel thin film during potentiodynamic polarization in acidic and alkaline sulfate solutions

The passivation process of nickel thin films during potentiodynamic polarization in acidic and alkaline sulfate solutions was analyzed by an electrochemical quartz crystal microbalance (EQCM) to separate the partial current density of nickel dissolution through the passive film, i_Ni²⁺, and the partial current density of film growth, i_O^2-. The values of i_Ni²⁺ and i_O^2- during potentiodynamic polarization (20 mV s⁻¹) in the passive potential region could be separated by comparing the mass change rate and net polarization current as a function of electrode potential. It is found that i_Ni²⁺ is larger than i_O^2- in pH 3.0, 0.5 M sulfate solution, while the situation reverses in pH 12, 0.5 M sulfate solution. The sulfate concentration dependences of i_Ni²⁺ and i_O^2- in pH 3.0, sulfate solution were significant as compared to those in pH 12, sulfate solution.     

Electrogeneration and electrochemical properties of hybrid materials: polypyrrole doped with polyoxometalates PW12−xMoxO403− (x=0,3,6,12)

Hybrid materials such as polypyrrole doped with polyoxometalates PW_12−xMo_xO₄₀³⁻ (x=0,3,6,12) were electrogenerated from organic solution. The effects of the Mo-substitution on the electrogeneration and electrochemical properties of the hybrid materials were studied by potential step and cyclic voltammetry. It was found that Mo-substituted polyoxometalates could stimulate the oxidation–polymerization of pyrrole. The oxidation–polymerization potential of pyrrole decreases as the Mo content increases, correlating the redox properties of the polyoxometalates. During the initial cathodic sweep, a great cathodic peak (altogether a blue cloud of material release) with a shoulder is shown by all the studied materials. A substantial loss of weight was always observed during this reduction process. Polyoxometalates and polypyrrole oligomers determined by FT-IR [J. Chem. Phys. B 104 (2000) 10528] and UV–VIS contributed to the weight loss giving rise to a partial electrochemically stimulated dissolution of the material. After this initial cycle, the consecutive voltammograms present two redox processes. Charges and current densities involved in those two cathodic processes show a linear variation, in opposite directions, at almost the same rate upon cycling. Those shifts of the cathodic processes are related to a continuous change of the counter-ion involved in the redox process from Li⁺ to ClO₄⁻, altogether the subsequent release of polyoxometalate (that required the Li⁺ counter-ion) from the hybrid.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more negative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more regative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E_h = α + β log t. The rate of oxide film thickening in the respective solutions was in the order Na₃PO₄ > Na₂SO₄ > NaCl > NaNO₃. The equilibrium potential was found to be independent of anion concentration in the case of Cl^- and NO^-₃ and was slightly dependent on the PO^3-₄ and SO^2-₄ concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.     

Electrochemical behaviour of nickel anode in H2SO4 solutions and the effect of halide ions

The electrochemical behaviour of pure nickel in H₂SO₄ solutions has been potentiodynamically investigated. The effects of the following factors on the anodic dissolution and passivation of the metal are discussed: potential scan rate, successive cyclic voltammetry and progressive additions of Cl^?, Br^? and I^? ions. Increasing the potential scan rate increases the critical current density i_cc, denoting that the active dissolution of nickel in H₂SO₄ is a diffusion controlled process. Cyclic voltammetry shows that the reverse excursion does not restore the anode to its active state. On successive cycling, the height of i_cc decreases; this could be attributed to the decrease in the reduction efficiency of passivating oxide film during the cathodic half cycles. The presence of the halogen ions below a certain concentration specific to each anion inhibits the anodic dissolution both in the active and passive states. The inhibitive action of these additives decreases in the order I^?, Br^?, Cl^?. Beyond the specific concentrations, the halogen ions accelerate the anodic dissolution and shift the active passive transition to more positive values. The aggressiveness of these anions decreases in the sequence Cl^?, Br^?, I^?, Further increase in the halogen ion concentrations can lead to breakdown of the passive film and initiate pitting. The susceptibility of nickel to pitting attack enhances with increasing H₂SO₄ concentration.     

Dissolution Current and Pitting Potential of Zinc in KOH Solutions in Relation to the Concentration of Aggressive Ions

Abstract

Potentiodynamic anodic polarisation curves for zinc were obtained in dilute (10^?3 M) solutions of KOH that favoured metal dissolution with only a slight tendency to passivate the zinc, and in more concentrated (10^?2 M) solutions that favoured passivation of the zinc. Addition of aggressive halide ions (Cl^?, Br^?, I^?) to the 10^?3 M solution caused a shift of the dissolution potential to more negative values and a marked increase in corrosion rate, suggesting adsorption of the anions on to the bare metal surface. The difference between the dissolution current density, i_o, in 10^?3 M KOH alone, and in the presence of halide ions, i_agg, was given by: log (i_agg?i_o) = a + b log C_agg, where a and b are constants.

In 10^?2 M KOH, small additions (2 × 10^?4 M) of aggressive ions did not affect the dissolution kinetics. At higher concentrations the c.d. increased sharply at a well defined potential, or pitting potential, E_p, given by the relation: E_p = α ? β log C_agg, α and β being constants. It is assumed that in 10^?2 M KOH the aggressive anion acts by penetrating the oxide, or by becoming incorporated in it as an impurity, altering its physical and electrical properties.     

Investigation on temperature dependence of photoluminescence in Sr1.7Eu0.3MxCeO4.15+x/2 (M = Li, Na, K, x = 0, 0.3) red phosphors

The temperature-dependent luminescence of Sr_1.7Eu_0.3M_xCeO_4.15+x/2 (M = Li⁺, Na⁺, K⁺, x = 0, 0.3) samples was investigated and discussed in the temperature range from 303 to 573 K. It is found that the thermal quenching temperature of samples decreases with Li⁺-/Na⁺-doping but increases with the incorporation of K⁺. We suggest that these observations are resulted from two factors. One is that the incorporation of Li⁺/Na⁺/K⁺ ions reduces the strength of potential field at the O²⁻ sites, and then results in a red-shift of the Eu-O charge transfer band. The other is that Δr expands with Li⁺-/Na⁺-doping but shrinks with K⁺-doping. We consider that it is a feasible way to adjust the temperature-dependent luminescence properties of materials by introducing appropriate impurities.     

Effects of applied potential and solution temperature on the pitting corrosion of pure aluminium in sulphate ion-containing chloride solution

Effects of applied potential and solution temperatureT _s on the pitting corrosion of pure aluminium (Al) were investigated in 0.01 M NaCl solutions containing various sulphate (SO₄ ^2-) ion concentrations using a potentiodynamic polarisation experiment, the potentiostatic current transient technique, ac impedance spectroscopy and atomic force microscopy (AFM). The potentiodynamic polarisation curves showed a rise in the pitting potentialE _pir values and a simultaneous increase in anodic current density at potentials much higher than theE _pit value as the SO42~ ion concentration increases. This implies that (SO₄ ^2-) ions impede pit initiation at potentials belowE _pit but enhance pit growth aboveE _pit. This was confirmed from the larger pit growth rate parameterb values of pure Al exposed to (SO₄ ^2-) ion-containing chloride solutions during the abrading action than those exposed to (SO₄ ^2-) ion-free chloride solution. Furthermore, at 7_s=25°C, the charge densityQ values for the Al metal dissolution in the presence of (SO₄ ^2-) ions were smaller than the value in its absence. By contrast, as validated by the capacitance values and the AFM images of the re-anodized specimens, an enhanced metal dissolution was observed in (SO₄ ^2-) ion-containing chloride solutions at 7_s=60° and 80°C. From the experimental findings, it is suggested that (SO₄ ^2-) ions act as inhibitors of pitting corrosion on pure Al belowE _pit and at 7_s=25°C, whereas they act as promoters at 7_s=60 ° and 80°C. This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive Cl ion attack, which occurs at potentials aboveE _pit     

Effect of sulphate ion on the electrochemical polymerization of pyrrole and N-methylpyrrole

Pyrrole and N-methylpyrrole were electrochemically polymerized in an identical manner in aqueous electrolytes containing (Et₄N)BF₄, LiClO₄ or Na₂SO₄ to give films of the corresponding p-doped polymers. The polymer obtained from the aqueous Na₂SO₄ electrolyte, [(poly N-methylpyrrole)^+y (SO₄)_y/2^2-]_x, differed from the other polymers in that it exhibited a very low conductivity (≈ 10^?7 S/cm), contained a high concentration of > C=O groups and had an EPR linewidth, ΔH_pp, more than an order of magnitude greater than that found in the corresponding polymers obtained from the (Et₄N)BF₄ and LiClO₄ electrolytes. A sample of [(polyN-methypyrrole)^+y(BF₄)_y^?]_x electrochemically synthesized in CH₃CN and subsequently converted electrochemically to the corresponding sulphate in the Na₂SO₄ aqueous electrolyte exhibited normal behaviour. A mechanism is proposed for the introduction of > C=O groups into the poly(N-methylpyrrole) polymer during its electrochemical polymerization in the Na₂SO₄ aqueous electrolyte.     

Influence De Certains Cations Alcalins Sur La Cinetique De Dissolution Anodique Du Rhenium

The influence of the alkaline ions on the kinetics of anodic dissolution of rhenium has been investigated in neutral and basic solutions. In these media, the K⁺, Rb⁺ and Cs⁺ ions have an inhibiting effect on the rhenium corrosion rate, corresponding to some peak shaped polarization i—U curves. The observed passivation is due to the formation of an alkaline perrhenate layer on the electrode. The position of the observed passivation peak on the potential scale depends on the pH of the electrolyte. The higher is the pH, the better is the passivation.     

The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry

The dissolution of Mg has been investigated with atomic emission spectroelectrochemistry at open circuit and during potentiostatic control in chloride (NaCl) and sulfate (Na₂SO₄) electrolytes. A dissolution stoichiometry of approximately n = 2 is observed under all the conditions of these experiments with no evidence for the commonly invoked Mg⁺ intermediate. Nevertheless, in chloride electrolyte it is shown that anodic polarization results in a cathodic activation of the surface with increased hydrogen production during the polarization and an increased corrosion rate following the pulse. The formation of insoluble Mg oxides/hydroxides was also investigated in the sulfate electrolyte.     

Surface charge and interaction with sulphate ions at the boehmitized aluminum

Measurements were made of the streaming potential and the contact angle as functions of pH, for aluminum samples with oxidized boehmitized surfaces, i.e. after treatment in boiling water. The experiments were done in solutions that contained 10^-4 M KCl and up to 5 × 10^-4 M K₂SO₄. Also investigated was a system in which the boehmitized sample was first treated by a solution of KCl and K₂SO₄ and then washed with water before the streaming potential and the contact angle were estimated. The results obtained for the streaming potential were used to calculate the zeta potential and the surface charge density. The results show with certainty that, in addition to the main interaction of the solid phase (i.e. boehmite and H⁺ and OH^- ions) there is also interaction with the sulphate ions from the solution. The effects measured by both experimental methods show this essential correlation. Some specific possible interactions are discussed and the results point to sulphate ions bonding in two ways: strong interaction with the solid substrate, or a looser bonding. Both experimental methods appear very sensitive for the study undertaken.     

Electrochemical behavior of Li+, Mg2+, Na+ and K+ in LiFePO4/ FePO4 structures

Besides Li⁺ and Mg²⁺, the electrochemical behavior of Na⁺ and K⁺ in LiFePO₄/ FePO₄ structures was studied since they naturally coexist with Li⁺ and Mg²⁺ in brine. The cyclic voltammogram (CV) results indicated that Na⁺ exhibits some reversibility in LiFePO₄/FePO₄ structures. Its reduction peak appears at ?0.511 V, more negative than that of Li⁺ (?0.197 V), meaning that a relatively positive potential is beneficial for decreasing Na⁺ insertion. The reduction peak of K⁺ could not be found clearly, indicating that K⁺ is difficult to insert into the FePO₄ structure. Furthermore, technical experiments using real brine with a super high Mg/Li ratio (493) at a cell voltage of 0.7V showed that the final extracted capacity of Li⁺, Mg²⁺ and Na⁺ that can be attained in 1 g LiFePO₄ is 24.1 mg, 7.32 mg and 4.61 mg, respectively. The Mg/Li ratio can be reduced to 0.30 from 493, and the Na/Li ratio to 0.19 from 16.7, which proves that, even in super high Mg/Li ratio brine, if a cell voltage is appropriately controlled, it is possible to separate Li⁺ and other impurities effectively.     

Thermodynamic analysis of Na-S-Fe-H2O system for Bayer process

Thermodynamic diagrams of Na-S-Fe-H₂O system were constructed to analyze the behavior of sulfur and iron in the Bayer process. After digestion, iron mainly exists as Fe₃O₄ and Fe₂O₃ in red mud, and partial iron transfers into solution as Fe(OH)₃^-, HFeO₂^-, Fe(OH)₄^- and Fe(OH)₄^2-. The dominant species of sulfur is S^2-, followed by SO₄^2-, and then SO₃^2- and S₂O₃^2-. The thermodynamic analysis is consistent with the iron and sulfur species distribution in the solution obtained by experiments. When the temperature decreases, sulfur and iron can combine and precipitate. Controlling low potential and reducing temperature are beneficial to removing them from the solution. XRD patterns show that NaFeS₂·2H₂O, FeS and FeS₂ widely appear in red mud and precipitates of pyrite and high-sulfur bauxite digestion solution. Thermodynamic analysis can be utilized to guide the simultaneous removal of sulfur and iron in the Bayer process.     

Anodic behaviour of tin in maleic acid solution and the effect of some inorganic inhibitors

The anodic behaviour of a tin electrode in maleic acid solutions was investigated by potentiodynamic and chronopotentiometric methods. Measurements were conducted under different experimental conditions. The results demonstrated that the polarization curves exhibit active/passive transition. In active regions, tin dissolves as Sn²⁺ which is subsequently oxidized to Sn⁴⁺ and the dissolution process is controlled partly by diffusion of the solution species. The passivity is due to the presence of thin film of SnO₂ on the anode surface formed by dehydration of precipitated Sn(OH)₄. The active dissolution of tin increases with increasing acid concentration, temperature and scan rate. The potential transients showed that the passivation time decreases with increasing applied current density. The effect of adding increasing concentrations of CrO₄²⁻, MoO₄²⁻ and NO₂⁻ ions on the anodic behaviour of tin in maleic acid was studied. These ions inhibit the active dissolution of tin and promote the attainment of passivity. The extent of these changes depends upon the type and concentration of the inhibitor.     

Study of the dissolution of lead in nitric acid by the thermometric technique

The thermometric technique was used to study the dissolution of lead in HNO₃. It was found that ΔT and the reaction number increase almost linearly with an increase in the acid concentration to a certain limit, whereafter the reaction is inhibited by a further increase owing to the formation of a protective layer of PbO₂.The mechanism of dissolution is confirmed by the effects of some additives, e.g. NO₂^_ ion, NO₃^_ ion as KNO₃ and NH₄NO₃, Cl^_ ion as HCl and NaCl, as well as urea. These additives were found to increase or decrease the dissolution process depending on whether they or their reaction products were involved in the dissolution reaction.