D.M. Dražić

Adsorption and corrosion inhibitive properties of some organic molecules on iron electrode in sulfuric acid

Adsorption and corrosion inhibitive properties of three different organic molecules: 2-naphthalenesulfonic acid, 2,7-naphthalenedisulfonic acid and 2-naphthol-3,6-disulfonic acid are investigated on Armco-iron electrode cathodically polarized, in 0.5 mol dm−3 H2SO4 solution. The examinations show that the three organic molecules behave as a cathodic corrosion inhibitors. The inhibitive efficiency, changes with the number of functional groups substituted on benzene ring and increases with concentration. The experimentally obtained adsorption isoterms follow the Frumkin equation [θ/n(1−θ)n]exp(−2aθ)=Bc and the best fit is obtained for n=2 (for 2-NSA) and n=3 (for 2,7-NDSA and 2N-3,6-DSA). The calculated values for the parameter a are 2.1–2.9 and Gibbs energies are 17–21 kJ mol−1. The adsorption rate constants are very small, proving that iron surface permanently changes due to the corrosion processes.

Oxygen reduction on iron: Part III. An analysis of the rotating disk-ring electrode measurements in near neutral solutions

Abstract The kinetics of oxygen reduction were studied on iron in borate and bicarbonate buffered solutions, pH 9.8, using the rotating disk-ring method. Under these conditions O2 reduction occurs both on the oxidized surface and on the oxide-free one, depending on potential. On the oxidized surface the series mechanism was found to be operative. On the oxide-free surface the four-electron reduction was observed, but, due to the very small rate of H2O2 formation with respect to the total reduction current, a reliable distinction between the parallel and series mechanism could not be made. All the rate constants of the O2 and H2O2 reactions on the Fe surface were determined. No catalytic decomposition of H2O2 was found. The kinetics of the O2 reduction reaction is slower on more oxidized surfaces.

Kinetics of chloride ion adsorption and the mechanism of AgCl layer formation on the (111), (100) and (110) faces of silver

Abstract The kinetics of chloride anion adsorption on the (111), (100) and (110) faces of silver are investigated by linear sweep voltammetry, potentiostatic pulse and impedance (admittance) measurements. It is shown that the first voltammetric (capacitive) peak corresponds to the adsorption of randomly arranged chloride anions with complete charge transfer between the anions and the substrate ( l ≈ 1) and a relatively slow rate of adsorption: k s = (8−9) × 10 −6 cm s −1 . The second (third) voltammetric (capacitive) peak is found to be very sharp, representing (according to the earlier findings) the formation of an ordered AgCl adlayer formed with the topmost Ag layer. At more positive potentials a three-dimensional (3D) growth of AgCl takes place, characterized by the anodic “crystallization loop” and the j − t transients typical of 3D nucleation. It is shown that after prolonged polarization in the potential region of 3D AgCl formation, irreversible formation of AgCl takes place on the electrode surface.

Oxygen reduction on iron. IV, The reduction of hydrogen peroxide as the intermediate in oxygen reduction reaction in alkaline solutions

Abstract The kinetics of hydrogen peroxide reduction were studied on iron in borate buffered and unbuffered solutions of pH ranging from 8.6 to 13.8, with and without the presence of chloride ions. A rotating disk electrode was employed. Measurements were taken using the linear sweep technique and the potential step steady-state technique. The H 2 O 2 reduction on iron under these conditions was found to occur on the oxidized surfaces. The E-j polarization curves showed a significant hysteresis which is due to oxide film formation. On less oxidized and/or prereduced surfaces the H 2 O 2 reduction was found to be faster than on more oxidized and/or prepassive surfaces. The reaction order with respect to H 2 O 2 was found to be one. The rate determining step most likely involves the transfer of the first electron to the molecular H 2 O 2 . The ionic peroxide form HO − 2 was not observed to be an electroactive species. The H 2 O 2 reduction was found to be pH dependent at pH > 11. The observed pH dependence mostly reflects the acid-base equilibrium properties of H 2 O 2 and partially those of the passive iron oxide film. Chloride ions were found not to have any influence on the reaction kinetics.

Kinetic and mechanistic study of hydroxyl ion electrosorption at the Pt(111) surface in alkaline media

Abstract The adsorption of OH− ions on the Pt(111) plane has been studied by fast cyclic voltammetry in sodium hydroxide solutions (0.03 to 1 M), under quasi-equilibrium and Tafel approximation conditions. It was shown that the OH− ion adsorption is an electrosorption process with one electron exchanged between an OH− ion and the platinum surface. The electrosorption process follows the Frumkin adsorption isotherm with low intensity repulsive interactions of the adsorbed species (f=2–3). The estimated values of the standard electrochemical rate constant (k°=5.6×10−4 cm s−1) and the standard exchange current density (joo=5.45×10−2 A cm−2) indicate a rather fast electrochemical process.

Oxygen reduction on a duplex stainless steel

O2 reduction on duplex stainless steel was investigated in neutral and alkaline solutions with and without the presence of chloride ions. All polarization curves showed hysteresis with higher reaction rate in positive going sweep than in the negative one. Tafel slopes of about −90 mV dec−1 in neutral and alkaline solutions without chloride ions and about −150 mV dec−1 in NaCl solution were obtained. Reaction rate is almost independent on pH in solutions of pH < 12 while in more alkaline solutions reaction order with respect to OH− ions of about −1 was obtained. Reaction rate and kinetic parameters for O2 reduction on duplex stainless steel were compared to data for O2 reduction on pure Fe and great similarity was found.

Influence of chloride ion adsorption on hydrogen evolution reaction on iron

Abstract In order to investigate the effect of chloride ions on the hydrogen evolution reaction (HER) on iron, electrochemical polarization measurements have been carried out in aqueous sulphuric acid with the addition of different concentrations of NaCl. Data for the surface coverage have been calculated from the observed shift of the cathodic polarization curve. The Temkin isotherm has been found to fit the experimental results. The adsorption kinetic analysis shows that adsorption is quite slow, fitting a linear relationship between surface coverage and logarithm of time. Simultaneous a.c. impedance measurements in the HER potential region do not show any sign of pseudocapacitance, indicating that chloride ions adsorb without partial charge transfer.

The kinetics of inhibitor adsorption on iron

Abstract The kinetics of adsorption of Cl − and Br − ions and 2-naphthol-3,6-disulphonic acid (NDSA) on Armco iron electrodes cathodically polarized in 0.5 M H 2 SO 4 was carefully analyzed. Contrary to the initial expectation that the adsorption of these species should be faster on such cathodically protected surfaces as compared to the rates of adsorption during anodic polarization when the metal surface changes permanently, the obtained adsorption rate constants were about 10 −2 mol −1 dm 3 s −1 both for halogen ions and NDSA with a small dependence on electrode potential for halogen ions. The kinetics were analyzed by using a computerized curve fitting procedure based on a Frumkin-type isotherm for the quasi-equilibrium state and Roginskii—Zeldovich-type equations for the adsorption and desorption processes. A computer analysis of the experimental θ vs. time data provided good estimations of k a and a , while the estimation of k d was less reliable, depending on the precision of the experimental data. For halide ion adsorption it was found that n = 1 and a n = 3 and a > 0 (attractive forces). The slow adsorption rates during cathodic polarizations are explained by the “anomalous” dissolution of iron during cathodic polarization by a mechanism similar to that for the anodic polarizations.

Hydrogen peroxide oxidation on passive iron in alkaline solutions

Abstract The kinetics of hydrogen peroxide oxidation were studied on passive iron in alkaline solutions of pH ranging from 9.8 to 13.8, using the rotating disk electrode. Measurements were taken using the linear sweep technique. The kinetic parameters of H 2 O 2 oxidation were determined: the Tafel slope is 110 mV dec −1 and the reaction order with respect to H 2 O 2 is 0.5 and with respect to the OH − ion 0.7. It is assumed that the rate determining step is the transfer of the first electron to the HO − 2 ion adsorbed under Temkin conditions. The pH dependence of the reaction rate is attributed to the pH dependent properties of the passive iron surface. There are no indications for the semiconducting behaviour of the passive film. The rate of H 2 O 2 chemical decomposition was estimated. It was found that the passive film on iron was a poor catalyst for H 2 O 2 chemical decomposition.

Anomalous temperature dependence of the hydrogen evolution reaction on iron

Abstract Cathodic polarizations for the hydrogen evolution reaction on Armco iron in perchloric acid measured in the temperature range 278 −348 K and pH range 0.5–2.0 follow the Tafel relationship but with anomalous temperature dependence. Instead of varying widely with temperature, the measured values of the slope were between 115 and 140 mV dec −1 , i.e., ±10%. This is ascribed to an anomalous change of the apparent transfer coefficient α c with temperature. Various possible reasons for this behaviour, such as adsorption and absorption of hydrogen, mechanism change and impurity effects, are ruled out on the grounds of experimental findings, leading to changes in the symmetry factor, β, as the probable reason. Following the Conway approach, numerical values of the enthalpic and entropic part of the symmetry factor are evaluated.