Cathodic polarization of variously corroded iron electrodes in acid perchlorate solutions

The steady-state cathodic reaction taking place on iron electrodes in deaerated, acid perchlorate solutions free of iron salts in the hydrogen evolution reaction. However, reduction of corrosion products formed during preceding anodic polarization will also occur in various potential regions. Cathodic polarization diagrams recorded with rotating disc electrodes (rde) show three reaction regions. In region I, starting below the corrosion potential, the reaction is influenced by pH of the solution and by potential. This is followed, at lower potentials, by region II, where the reaction is limited by diffusion of hydrogen ions to the electrode. The limiting cd is determined by pH of the solution and by the speed of rotation of the rde. At still lower potentials, in region III, there is a further increase of cd due to discharge of water molecules. This reaction is influenced by potential, but not by pH or rde rotation rate.Experiments lasting a few minutes demonstrate that the behaviours in the regions I and II depend on the type of preceding anodic polarization; the alternative effects will be cathodic peak in region I, attributed to the reduction of a previously formed oxidic iron compound approaching the composition of magnetite, or an additional contributon to the cd due to reduction of a different iron compound and in excess of the quantity accounted for the hydrogen evolution.A detailed, rather novel and powerful type of analysis of the reaction recorded in region I indicates a low and constant coverage by adsorbed hydrogen atoms giving rise to a parallel combination of a Volmer—Tafel mechanism (the minor part) and a Volmer—Heyrovsky mechanism (the major part) for the hydrogen evolution reaction.     

Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes

Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na₂PdCl₆ in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode.     

Electrochemical investigations of the autophoretic coating process

Autophoretic coating (AC) is a complex chemical process consisting of various electrochemical partial reactions such as metal corrosion, hydrogen evolution and peroxide and oxygen reduction. While the redox potential of the bath is very positive, about 0.6V, the coating process takes place at about –0.2 V dependent on the bath composition. The low conductivity of the bath is necessary to maintain the dispersion, but it hinders the kinetic investigations. Nevertheless potentiodynamic polarization curves show that the diffusion limited peroxide reduction and the corrosion of iron are the most important reactions. From the corrosion current density a maximum weight loss of 0.1 mg cm^–2 is estimated for a standard film of 20 m thickness. Hydrogen evolution is almost negligible during the first period of dipping. This conclusion can be justified by measurements in the hydrogen permeation cell. The partial current densities are not influenced by various emulgators, but the dispersion catalyses the hydrogen evolution and inhibits the anodic corrosion. The diffusion of peroxide is hindered. Potentiostatic measurements in various solutions show the role of ferrous ions deriving from corrosion in the case of iron or by reduction of ferric ions in the case of gold. Measurements at a disc electrode show that the cathodic reactions are dominated by diffusion and migration.This paper is dedicated to Professor Dr Fritz Beck on the occasion of this 60th birthday.     

Hydrogen evolution reaction on smooth tungsten carbide electrodes

The open-circuit potential in H₂-saturated electrolytes, and the kinetics of hydrogen reactions, in particular of hydrogen evolution reaction, on smooth, rotating disc tungsten carbide WC electrode have been investigated, in 0.5 M sulfate solutions of pH 0.3 and 1.85, in the temperature range 30 to 80°C. Anodically activated electrodes were used too.

It was proved that values of the open-circuit potential in the near vicinity of reversible hydrogen electrode potential can be observed, under condition of high purity, and of tight mounting of the sample in the electrode holder.

Two Tafel regions have been observed for hydrogen evolution reaction, both the Tafel slopes having no systematic variation with temperature. In the lower overpotential region the experimental charge transfer coefficient attained the value of 1, when the temperature was increased up to 80°C. It was suggested that the process approached the barrierless form when the temperature was increased.

The mechanism of hydrogen reactions on WC was discussed. It was suggested that the rate of the process is possibly determined by the rates of its two steps simultaneously. The role of pH on the activation process of WC was emphasized, and implications of the hydrogen evolution reaction parameters on the mechanism of this process were pointed out.     

Flow influenced electrochemical corrosion of nickel aluminium bronze – Part II. Anodic polarisation and derivation of the mixed potential

The mixed charge and mass transfer influenced anodic current response of CA 104 nickel aluminium bronze (NAB) is presented as a function of both laminar (rotating disc electrode) and fully turbulent (rotating cylinder electrode) fluid flow. At low values of positive polarisation, the overall behaviour of the freshly polished material in filtered and artificial seawaters is closely related to that of unalloyed copper. The primary anodic reaction in this case is the selective dissolution of the copper component via a cuprous di-chloride complex anion. At large positive polarisation, the solid solution mole fraction for the production of a discrete film of protective alumina (Al₂O₃) is examined as a function of Reynolds number and discussed in terms of a new mechanism for the passivation NAB in seawater. The polarisation data is used to replicate experimental Reynolds number dependent, corrosion potentials and corrosion current densities over a wide range of electrode angular velocities.     

Current efficiency and kinetics of cobalt electrodeposition in acid chloride solutions. Part I: The influence of current density,pH and temperature

In the electrodeposition of cobalt in chloride electrolytes the evolution of hydrogen is a parasitic reaction. On a rotating platinum disc electrode the current efficiency was calculated as the charge used for anodic dissolution of cobalt at a potential where no other reactions were taking place, divided by the total cathodic charge used for cobalt deposition. The results show that the current efficiency could be measured accurately in this way. In part I the current efficiency and deposition potential are studied as a function of current density and pH. The results show an increase in current efficiency with increasing current density, pH and temperature. The results also indicate a change in the reaction mechanism for electrodeposition when the pH is changed.     

Electrochemical behaviour of copper in neutral aerated chloride solution. I. Steady-state investigation

The electrochemical behaviour of a Cu rotating disc electrode in neutral aerated NaCl solution was investigated in the cathodic and anodic ranges and at the corrosion potential. In the cathodic range, where the reduction of oxygen takes place, reduction peaks allow the identification and quantitative evaluation of insoluble corrosion products (CuCl and Cu₂O). In the anodic range Cu is dissolved, most likely as CuCl ₂ ^– . A new mechanism for the anodic dissolution is proposed after comparing our data with previously published mechanisms. Corrosion currents were found to decrease with time and to be a function of the rotation rate of the electrode. Both the mixed kinetics of the anodic partial reaction and diffusion through a porous layer seem to be relevant in controllingl _corr.     

The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm⁻³ HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10⁻⁹ m s⁻¹. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 2: Cyclic voltammetry and characterisation of the corrosion mechanism

The cyclic voltammetry and mixed potential behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE). Reynolds number dependent corrosion potentials are derived for a wide range of Reynolds numbers and good comparison is made with directly measured values. The films formed on initial exposure of the 90-10 copper-nickel alloy to the seawaters did not confer any additional protection relative to unalloyed copper. Indeed, the 90-10 copper-nickel tended to exhibit relatively high anodic and cathodic currents when directly compared to both unalloyed copper and a nickel aluminium bronze under identical conditions. The corrosion potential of the alloy becomes more negative at higher flow velocity, due to the influence of mass transport on the anodic reaction mechanism at potentials close to the mixed potential. Overall, the results indicate that the character of the freshly polished material/electrolyte interface has a much greater influence over the magnitude of the corrosion rate than the turbulence intensity of the fluid.     

Dissolution anodique de l'etain en milieu basique

A non uniform dissolution was observed on a tin rotating electrode in an alkaline medium. Anodic dissolution takes place only at the inner disc when the electrode is polarised in the overhang of the polarisation curve which is Z shaped. The study of the impedance diagrams has shown the importance of a diffusion impedance of a reaction product. The diffusion coefficient of this species is measured by impedance method.     

Correlation of electrochemical and ellipsometric data in relation to the kinetics and mechanism of Cu2O electroformation in alkaline solutions

The kinetics of copper anodization were studied in the potential range of Cu₂O electroformation in different alkaline solutions by using voltammetric, rotating disc electrode, rotating ring-disc electrode and ellipsometric techniques. Thin layer and massive copper electrodes were employed. Results indicate the formation of soluble Cu(I) species at the early stage of copper anodization. Other soluble Cu(II) species are produced after the Cu₂O layer electroformation. Data are discussed on the basis of a reaction mechanism involving different reaction products and the influence of the copper surface on the initial electrochemical reaction.     

Rotating nanoparticle array electrode for the kinetic study of reactions under mixed control

A method for the determination of the intrinsic electrocatalytic activity of metal nanoparticles for reactions under mixed diffusion-activation control is proposed. The working electrode consists of a nanoparticle array supported on a rotating disc of an inert conducting substrate. This electrode configuration reduces significantly the contribution of the reactant diffusion to the experimental current-potential curves and consequently, the kinetic parameters of the reaction can be evaluated more accurately. A model was developed for the diffusion process in the proposed configuration, which includes a parameter related to the degree of dispersion of the nanoparticles on the electrode surface (active area factor). It allows one to evaluate changes in the current-potential plot under conditions of constant particle diameter, which is essential for the appropriate analysis of the electrocatalytic activity of nanoparticles. On this basis the dependences of the current and current density for the hydrogen oxidation reaction on overpotential, rotation rate, particle size and active area factor were derived. The validity of these expressions was verified through the analysis of experimental results.     

Effet du pH en milieu basique sur la cinetique d''oxydation de l''hydrogene a une electrode de platine

The polarization curves of hydrogen oxidation in several alkalines media are determined by the rotating disc method. The electrode are made of smooth platinum or platinum covered by a thin layer of black platinum. For the total range of pH we show that the limiting process is always the diffusion of dissolved hydrogen to the electrode.     

Zinc deposition and passivated hydrogen evolution in highly acidic sulphate electrolytes: Depassivation by nickel impurities

In highly acidic sulphate electrolytes used for zinc electrowinning, the steady-state behaviour and the impedance of zinc electrodes have been analyzed under potentiostatic control. It is shown that zinc deposition takes place under conditions where hydrogen evolution remains passivated on the deposit surface. A depassivation of hydrogen evolution leading to the deposit corrosion can be provoked by an electrode excursion near the corrosion potential. The presence of nickel impurities facilitates this depassivation process even though the electrode potential remains cathodic. The depassivation becomes easier with the existence of a scratch on the substrate surface, because of bubbles retained on this defect.     

Kinetics and mechanism of the electrochemical oxidation of the NO−2 ion on platinum in AgNO2-acetonitrile solution

The anodic oxidation of NO^?₂ ion dissolved as AgNO₂ in ACN, in the range ? 12° to 68°C, is investigated using platinum electrodes. Single and multiple linear potential sweep and rotating disc electrode techniques are used. The electrochemical reaction is interpreted with the following sequence of reactions:

Step (1) behaves as an irreversible step which at high potentials is diffusion controlled, while step (4) corresponds to a reversible charge transfer step. The chemical reactions are fast processes. The postulated reaction mechanism for the anodic discharge of the NO^?₂ ion implies the preferential reaction of the initial product with a NO^?₂ ion instead of a solvent molecule as it occurred with solutions employing other aprotic solvents.     

Electrochemical behaviour of copper in neutral aerated chloride solution. II. Impedance investigation

A.c. and EHD impedance measurements were performed on a Cu rotating disc electrode immersed in neutral aerated NaCl and the results were compared with simulated curves according to the kinetic model developed in Part I. The agreement is good over the whole anodic range and at the corrosion potential. A slowing down of the mass transport rate is explained by diffusion through a surface layer (Cu₂O) at the corrosion potential and for the close anodic range. It is confirmed that, on the cathodic plateau, the interface may be considered as uniformly accessible for the cathodic partial reaction of oxygen reduction. However, at less cathodic potential a surface layer effect due to CuCl must also be taken into account. Diffusion coefficients for both O₂ and CuCl ₂ ^– were determined by EHD impedance.     

Cementation and corrosion at a RDE: Changes in flow and transfer phenomena induced by surface roughness

Cementation and corrosion were investigated in a batch cell to highlight the improvement of the kinetics observed after a certain lapse of time. Experiments on cementation with the Cd(II)/Zn and Ag(I)/Cu systems, and on corrosion with the Ce(IV)/Zn and Cr(VI)/Zn couples, were carried out on a rotating disc electrode immersed in relatively concentrated solutions. Time variations of the concentration of the reacting species and SEM observations showed that the change in reaction kinetics was due to the roughness of the changing surface of the electrode, depending on the chemical system considered. For corrosion, the average roughness was shown to exceed the thickness of the Nernst diffusion layer, and the rougher surface created allows local flow disruption and mass transfer enhancement. Besides, corrosion by Cr(VI) species results in greater roughness of the metal surface with more significant flow disruption than with Ce(IV) at the same concentration.     

Lead electrodeposition from very alkaline media

The electrodeposition of lead from very alkaline media has been studied by cyclic voltammetry, chronoamperometry under stationary and convective conditions. Experimental parameters like lead concentration and temperature have been varied. From NaOH 6 M the metal deposition takes place at about −0.90 V versus SCE far from the hydrogen evolution reaction (HER) which is seen at −1.30 V, but both processes are favoured by the lead content increase and the NaOH concentration decrease. The analyses of the chronoamperometric responses support the view of a 3D growth and suggest a substantial influence of lead concentration on the type of nucleation. Progressive nucleation is observed for the deposition from solutions with low content in lead but as this concentration increases a tendency towards instantaneous nucleation is revealed. The voltammetry with the rotating platinum disc electrode has confirmed that the lead electrodeposition is a mass transfer controlled process, and also allowed the estimation of diffusion coefficients.