Kinetics of layer formation and corrosion processes of passive iron in acid solutions

The kinetics of the layer formation process and the corrosion process have been determined from the relation between the stationary (i_c,0) and non-stationary (i_c) corrosion rates and the rate of layer formation (i_ι) at passive iron in acid solutions. The rates of both processes depend on the potential difference ε_2,3 at the passive-oxide/electrolyte-solution interface. The fact that i_c and i_c,0 are independent of the electrode potential is explained by a nearly constant composition of the passive oxide in contact with the solution (Fermi level within the band gap).Linear relations, such as log i_ι⁺ = a + (α_ι⁺/α_c⁺)·log i_c, are obtained, which are explained by charge-transfer hindrance. The apparent charge-transfer coefficients are α_ι⁺ = 1·43 and α_ι^? = 0·57 for the layer formation and removal reaction, and α_c⁺ = 0·84 for the corrosion reaction. From the pH dependence (pH = 0.35–2·90) of i_ι and i_c,0 the reaction orders of the hydrogen ions are found to be approximately μn_ι⁺ = ?1 and ν_c = 0. The corrosion cd depends on the sulphate concentration, with a reaction order ν_s = ß = 0·16. From this, the kinetics of the oxide formation H₂O·aq ? OH^? · ox + H⁺·aq (preceding equilibrium), OH^?·ox?O^2?·ox + H⁺·aq (rate-determining) and the kinetics of the corrosion process Fe³⁺·ox + SO₄^2?·aq ? FeSO₄⁺·ad (adsorption equilibrium, Temkin conditions), FeSO₄⁺·ad → FeSO₄⁺·aq (rate-determining), with following dissociation of the complex can be deduced. The apparent charge-transfer coefficients are interpreted by α_ι⁺ = 1 + α_ι, α_ι^? = 1 ? α_ι and α_c⁺ = α + 2ßγ_s (ga_ι, α, true charge-transfer coefficients).     

The rate of the hydrogen discharge reaction on platinum in sulfuric acid solution

The i_o for the H⁺/H exchange on Pt was found to be dependent on H₂SO₄ concentration, giving a linear log—log relation with a slope of 0·5. For any given H₂SO₄ concentration, i_o was virtually independent of potential to an anodic value at which the Pt-associated H concentration dropped to a low value. Comparisons are made with literature results. The influence of the dilute double layer on i_o appeared to be minimal over a wide range of H₂SO₄ concentrations.     

Reduction of cadmium-oxalate-malate complexes at the DME

The reduction of mixed cadmium-oxalate-malate complexes has been studied at the dme. It has been found to be reversible and diffusion-controlled, involving two electrons as indicated by slope of the straight line of E_devs log i/(i_d ? i) and the direct proportionality of the diffusion current to the effective height of the mercury column. The logarithms of stability constants β₁₂, β₁₁ and β₂₁ for complexes Cd(ox) (mal)₂, Cd(ox) (mal) and Cd(ox)₂(mal) are 4·670, 0·989 and 5·234 respectively.     

Anodic oxide films at nickel electrodes in alkaline solutions—II. pH dependence and rate determining step

Room temperature growth kinetics of thin anodic films of the β-Ni(OH)₂ phase is determined in alkaline solutions of different pH's. The kinetics follows the Cabrera—Mott formalism of the high field assisted growth. The potential at which the hydroxide films begin to grow, V_o, depends on pH according to V_o(pH) = 0.377 - 2.3(RT/F)pH [V vs she]. The exchange current density, i_o, also is pH dependent. It is first order with respect to OH^?. For the high field assisted growth it is usually assumed that the rate determining step is either a process at the metal—oxide film interface or a process within the oxide film itself. It is difficult to see how i_a, for either of these processes, could depend on pH. It is suggested that the rate determining step is a process at the oxide film—solution interface. The pH dependence of the rate for this step is discussed in relation to the structure of the double layer.     

Faradaic rectification studies of two electron charge transfer redox couples at the metal (solid) interface

The Faradaic rectification studies of the two electron charge transfer processes, cationic and anionic redox couples, have been carried out. The mechanism of the iodide—iodine reaction through single charge transfer comprises the following steps, of which reaction (b) is the slowest and controls the rate of overall reactions.

The values of the transfer coefficient and rate constant are obtained for the second step, which is rate determining in both the directions. The values of the parameters obtained when the concentration of the electrochemical species (I₂) is much less in comparison to that of iodide, are α_c = α_a = 0·5 and k^O_a of the order of 5 × 10^?5 cm/s. The parameters for the iodide—iodine reaction, using 1·0 M KNO₃ as supporting electrolyte, are α_c = 0·49, α_a = 0·51 and k^O_a of the order of 5 × 10^?3 cm/s. For the stannous-stannic reaction either of the two steps appears to be rate determining in both directions and the values of the parameters obtained are α_c = 0·48, α_a = 0·52 and k⁰_a = 2·1 × 10^?3 cm/s.     

Iron and hydrogen electrodes in liquid ammonia

Equilibrium potentials of the iron(II)—iron electrode vs TlTlCl; 0.1 M NH₄Cl in liquid ammonia are determined from intersections of Tafel lines for iron deposition and dissolution. The hydrogen electrode is shown to behave reversibly at palladium hydride, but not at platinum with respect to the dependence of hydrogen pressure. The standard potential of iron at 293 K is estimated at E° = ?0,356 (±0,010)V in 0.1 M NH₄NO₃. Standard potentials of other electrodes are reevaluated and shown to be consistent with earlier measurements except for the standard potential of lead. Steady state and transient polarization curves indicate in simple mechanism of the iron electrode involving transfer of iron(II) in one step. The temperature-independent anodic transfer coefficient is α₊ = 0.41 (±0.02), the cathodic transfer coefficient α_? = 0.61 (±0.03). The standard exchange current density at E° is j°_° = 7.10^?7 Acm^?2 for 293 K. Exchange current densities of the hydrogen electrode are j_° = 1.6·10^?9 Acm^?2 at platinized platinum in 0.1 M NH₄Cl both for 293 K.     

Linear sweep voltammetry of manganate(VII), manganate(VI), and manganate(V) in alkaline media

Electrochemical reactions were assigned to the voltammetric waves obtained in alkaline KMnO₄ and K₂MnO₄ solutions. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that at slow potential scan rates in MnO^?₄ solutions ranging from 0·07 to 0·19 F in NaOH the first step in MnO^?₄ reduction is a l-electron reversible charge transfer with no coupled chemical reaction; at fast scan rates (? V s^?1 the process becomes quasi-reversible. The standard rate constant and the activation energy for the MnO^?₄/MnO^2?₄ charge transfer step were estimated. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that in 4·O F KOH and at potential scan rates between 0·005 and 2·V s^?1 the reduction of MNO^2?₄ to MnO^3?₄ is a reversible charge transfer with no coupled chemical reaction. The diffusion coefficients of MnO^?₄ and MnO^2?₄ in alkaline solutions are reported.     

Determination du courant limite d'electrodialyse par conductivite pour les faibles nombres de reynolds. Cas des solutions de tartrate acide de potassium

In the case of an electrodialysis operation, it is possible to obtain the limiting current by measuring the conductivity of the solution undergoing dilution. Starting from the equation of the constant flux of matter, when the concentration at the membrane falls to zero, (i/σ_d)_crit = δ relates the current density i and the conductivity of the diluate σ_d. The critical value of (i/σ_d) is easily obtained from the drawing of σ _dvs the potential applied to the apparatus. Experiments carried on aqueous solutions of KHC₄H₄O₆ at 8 g/l show that (i/σ_d)_crit) is well related to the Reynold's number by the relation αRe^β. It is then possible to calculate the thickness δ of the diffusion layer as a function of the Reynold's number.     

The influence of temperature on the growth and fatty-acid composition of Skeletonema costatum in a batch photobioreactor

The influence of temperature on the growth and fatty-acid composition of the microalga Skeletonema costatum (CCAP 1077/b) has been studied. The maximum specific rates of growth (μ_m) and productivity (P_B) were reached between 293 and 298 K. The experimental values of μ_m–T were adjusted to the function: μ = μ′_o · T · exp(-E_a/RT) ? μ_d · exp(-E_d/RT). The total content of the saturated acids reached a minimum at 293 K, wheras the maximum amounts of polyunsaturated acids and Σn ? 3 HUFA (highly unsaturated n ? 3 fatty acids) occurred at 293 and 295·5 K, respectively.     

Nucleation and growth of anodic oxide films on bismuth—I. Cyclic voltammetry

The early stages in the formation of a continuous anodic layer of bismuth oxide on a solid bismuth electrode, in the pH range 5–14, were studied. The oxide covered the surface by the simultaneous thickening and spreading of patches. The metal surface was classified into two different areas with different overvoltage for oxide nucleation. The ratio of the two areas varied according to the history of the surface.The thickening of the newly formed layer (final thickness ～20 nm) followed the high-field growth law i = $\text{A}$exp(B$\text{E}$) where $\text{E}$ is the field in the oxide layer, with B = (2.0 ± 0.5) × 10^?6 V^?1 cm. This value of B gives an activation distance for high-field ion transport of 0.2 nm, comparable to the radius of a lattice site and much smaller than that obtained previously, for much thicker films.Dissolution of the film, giving breakdown of the oxide layer and pitting of the metal, occurred for pH<8. The thickness of the anodic film was thus limited to only 4 nm at pH 5.Cathodic reduction of the anodic oxide resulted in a porous metal surface. The current—voltage curve for the reduction often had a complex shape, which was related to the morphology of the original anodic layer.     

A Study of the Apparent Desorption Energy for OH in the Water Formation Reaction on a Palladium Catalyst

In the H₂/O₂ reaction, the desorption of OH from a polycrystalline palladium foil has been studied with Laser-Induced Fluorescence (LIF) as a function of the hydrogen mixing ratio, α_H2, and the temperature. The water production as a function of α_H2 was also monitored with microcalorimetry. The apparent desorption energy, E_OH ^a, for OH has been measured from α_H2 = 0% to α_H2 = 74%. At α_H2 = 0%, E_OH ^a was measured to be 143 ± 7 kJ/mol; the apparent desorption energy then increases linearly with α_H2 to a maximum value of 223 ± 6 kJ/mol at α_H2 = 40%. At higher α_H2, the apparent desorption energy was measured to be constant at about 200 kJ/mol. The maximum in apparent desorption energy at α_H2=40% occurs at the same α_H2 as the maximum in water production. This phenomenon has also been reported for the platinum metal, although at an α_H2 value of 20%. The relative coverage of intermediates and products on the palladium catalysts was obtained using Chemkin.     

Diffusion and equilibrium properties of hydrogen in palladium measured by the stripping potentiostatic method

Theory and experiments on the (non-equilibrium) stripping potentiostatic method for evaluation of the diffusion coefficient and desorption isotherms of hydrogen in α-Pd are discussed. Diffusion coefficient of hydrogen D can be evaluated either under transient conditions, from the anodic stripping current (I_a) or under steady-state condition, from the anodic charge corresponding to the stripping of hydrogen held initially in Pd membrane (Q_a). Following data are reported: D = (3.60±0.06) × 10⁻⁷ cm² s⁻¹ (transient) and D = 3.41 × 10⁻⁷ cm² s⁻ (steady-state) at 20°C in 0.1 N H₂SO₄. Equilibrium concentration and equivalent pressure of hydrogen at the entrance side of Pd membrane can be calculated from the steady-state permeation current (I^∞_a or the anodic stripping charge (Q_a) and its open-circuit potential just after stopping hydrogen generation, E⁰_c. Reliable Sieverts' constants (K_s) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: ΔH^o = −(2.6±0.1) kcal (mol H)⁻¹ and ΔS^o = −(14.5±0.4) cal K⁻¹ (mol H)⁻¹. These data agree well with those obtained in gas phase measurements.     

The triplet lifetime of poly(phenyl vinyl ketone). A laser flash photolysis study

The photochemistry of poly(phenyl vinyl ketone) in solution has been examined over a temperature range using nanosecond laser flash photolysis techniques. Experiments were performed at low temperatures to eliminate the interference from biradical signals and allow for the separation in time of the behaviour of quencher-free and quencher-containing polymers. For the former the kinetics of triplet decay in toluene-d₈ lead to log $\text{(}\text{A}\text{s}{}^{\mathrm{?1}}\text{) = 9.90 ± 0.50}$ and E_a = (3.56 ± 0.30) kcal mol^?1 where A is the preexponential factor and E_a the activation energy. The extrapolated triplet lifetime at 20°C is 57 ns.     

Reduction of no on smooth,porous platinum and platinum-carbon electrodes in acidic solutions

Rate determining step of one-electron reduction of NO to N₂O and the rate constant of this reaction k° = 1.7 · 10^?4 on smooth platinum electrode were determined. It was shown that PtC electrode activity increases with increasing platinum content of the catalyst and depends largely on the kind of carbon carrier. The most active electrodes prepared of powder platinum black reduce NO to NH⁺₄ even at E ? 0.4 V.     

A study of the solid salt film on nickel and stainless steel

During steady state diffusion controlled dissolution of one-dimensional pit electrodes of stainless steel SS 302 and nickel, impedance and transient measurements were carried out to determine the ohmic potential drop across the salt film Δε_ΩF, the electric conductivity of the film σ_F and the film thickness d_F as a function of potential and solution composition. It was found that Δε_ΩF increases with decreasing diffusion limited current density, i_p, and with potential. For i_p → 0 it approaches a value which is independent on the bulk concentration. The critical potential for salt film formation is −0.9 and −0.08 V vs sce for SS 302 and nickel, respectively. The specific electric conductivity of the salt films is approximately 1.4 × 10⁻⁵ and 4.4 × 10⁻⁵ S cm⁻¹ for SS 302 and nickel, respectively. The salt film thickness linearly increases with potential and bulk concentration and is very thin compared to the pit depth (less than 1.5% at potentials < 1.2 V vs sce and chloride concentrations < 6 moll⁻¹).     

On the fluid dynamics of shaken bioreactors— flow characterization and transition

Phase‐resolved PIV measurements were carried out to provide a thorough characterization of the flow and mixing dynamics occurring in a cylindrical shaken bioreactor when operating conditions such as medium height h, shaking rotational speed N, orbital shaking diameter d_o, and cylinder inner diameter d_i, are varied. A scaling law based on the aspect ratio h/d_i, on the orbital to cylinder diameter ratio d_o/d_i, and on the Froude number Fr = 2(πN)²d_o/g, is derived to predict the incipience of flow transition occurring when the free surface orientation starts to exhibit a phase delay to the shaker table position along its orbit; depending on the combination of Fr, d_o/d_i and h/d_i the transport phenomena in the bioreactor are controlled by a horizontal toroidal vortex, or by a vertical one precessing around the cylinder axis. The free surface interfacial area was directly measured by image analysis to assess oxygen transfer potential and compared to an analytical solution valid for low Fr. © 2012 American Institute of Chemical Engineers AIChE J, 59: 334–344, 2013     

Physico-chemical studies on the interaction of transition metal ions with Schiff bases: polarographic study of copper(II) complex of salicylaldehyde-serine

Copper(II) complex of Schiff base derived from salicylaldehyde-serine has been studied by polarographic method of analysis at 30° and at constant ionic strength in aqueous medium. The plots of i_dvs √h and i_dvsC were linear and passed through the origin, showing that the reduction process was diffusion controlled. The plots of log (i/i_d?ivsE_de were linear in all concentrations of Schiff base but the slope showed that the reduction process was irreversible. The kinetic parameters were determined by Koutecky's method. The composition and stability constant were determined by DeFord and Hume's method.     

Mass transfer at horizontal gas-evolving electrodes

The effect of H₂ and O₂ evolution on the mass transfer coefficient of the reduction of K₃Fe(CN)₆ and the oxidation of K₄Fe(CN)₆ at nickel electrodes was studied up to 105 mA/cm². The relation between the rate gas evolution and the mass transfer coefficient was found to be: log K = a + 0·25 log V for a H₂-evolving horizontal electrode and log K = a + 0·4 log V for an O₂-evolving horizontal electrode.Comparison was made between mass transfer coefficients at horizontal and vertical gas-evolving electrodes. Mass transfer coefficients at horizontal electrodes are much higher than at vertical electrodes.     

Synthesis of a New Modification of Aluminum Oxide, Gahnite, and Zincite in Detonation

Results of studying the products of explosion of aluminum and octogen charges enclosed by a brass shell under conditions of complete burning out in air are described. Gahnite ZnAl₂O₄, a new (metastable) modification of aluminum oxide, and small quantities of zinc oxide were identified in the condensed explosion products. The x-ray pattern of the aluminum oxide was indexed in a face-centered cubic lattice with the parameters a = 7.854(1) Å, V = 484.4(2) ų. The possible Fedorov groups are Fm3m, F432, and F43m.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.