S. Fletcher

A computer simulation of the voltammogram describing the active-passive transition of the Pb(Hg)/PbCl2 interface

Abstract A computer simulation of the voltammogram corresponding to a three-dimensional nucleation and growth mechanism is described. Theory is compared to experiment for the Pb(Hg)/PbCl 2 interface. Good agreement is found.

The anodic behaviour of lead amalgam electrodes in HCl solution

Abstract The anodic behaviour of the lead amalgam electrode has been investigated in aqueous hydrochloric acid solutions. Both voltammetric and potential pulse results are described. The mechanism of passivation is shown to be the nucleation and growth of three-dimensional nuclei of PbCl2 which progressively block the electrode. The nuclei are considered to be right circular cones, distributed at random on the electrode surface. As in the case of solid lead electrodes, a simultaneous dissolution of PbCln2−n complexes is observed, which diffuse away from the electrode under mass transport control. Evidence is also presented that the first stage in the growth of the anodic film is the two-dimensional nucleation and growth of a monolayer of PhCl2. Unfortunately, this process is partially obscured by the dissolution reaction. A reaction scheme is proposed.

Nucleation of 2-d films — I. Some phenomenological considerations

Abstract Some of the assumptions inherent in electrochemical nucleation-growth theory are briefly reviewed. Particular attention is paid to the point-process approximation, which is shown to conflict with established results in heterogeneous nucleation theory. A simple phenomenological formalism is introduced in order to circumvent the difficulties.

Studies in electrocrystallization: Part III. The effect of ohmic and finite risetime distortions on data acquisition in 2D nucleation and growth

Abstract The effect of uncompensated iR-drop on two-dimensional nucleation and growth is described. Both instantaneous and progressive nucleation laws are considered. Also the effect of the finite risetime of potential square-waves on the experimental data acquisition is presented. Data are examined in terms of current-time transients and also in terms of the transient maximum parameters im and tm. It is demonstrated that neglect of ohmic and finite risetime factors causes complex distortions of rate data, particularly at short times. Prospects of minimising these distortions are briefly considered.

A computer simulation of the voltammogram corresponding to the two-dimensional progressive nucleation and growth of a passivating film

Abstract A computer simulation of the voltammogram corresponding to the two dimensional progessive nucleation and growth of a passivating film is presented. The effect of varying parameters is discussed in terms of the peak potential and peak current. A description of the influence of ohmic overpotential is also included.

Some statistical aspects of the growth of crystals in passivation, metal deposition and anodic film formation

Abstract The formation of solid crystalline phases is a common feature of many electrochemical reactions including corrosion, metal deposition, and anodic and cathodic film formations. Unfortunately the problem of modelling the crystal growth processes in an accurate and verifiable way has raised a number of serious theoretical difficulties which as yet have not been resolved. Among the many difficulties we address ourselves in the present paper to three important questions: o (a) What is the spatial distribution of growth centres, and how may this be statistically described? (b) How may the growth geometry of individual crystals be recovered from experimental data? (c) How do crystals of the emergent phase interact with each other and with the boundaries of the electrode? The statistics of a large number of potentiostatic experiments are theoretically described, and we indicate how the geometries and the spatial distribution of crystals may be investigated via an analysis of the moments of the experimental ( i−t ) responses.

Temperature effects in the electrochemical behaviour of cycled lead electrodes in HCl solutions

The electrochemical behaviour of cycled lead electrodes has been investigated in HCl solutions in the temperature range −6° to 30°C. Because of the low ionic strength of solution below ~0.8 M, and the associated ir drop problem, solutions stronger than 1.0 M were principally considered. In weaker solution (0.1–0.8 M) only derived current functions were analysed, whilst all potential-dependent parameters were rejected. Results are reported for both stationary and rotatiang disc electrodes. Because of the chemical dissolution of the surface layer of PbCl2 formed on anodic scan, measurements were confined to time-scales of < 1 s. By applying these strict criteria, the overall reaction scheme was deduced. In particular, the kinetics of passivation were found to involve the three-dimensional growth of isolated PbCl2 nuclei, which slowly dissolve into solution. At extreme anodic potentials, or at low temperature, passivation occurs by progressive, two-dimensional growth under diffusion control. A simultaneous electrochemical dissolution of Pb2+ at somewhat more negative potentials is noted, for which the rate-determining step is the diffusion of lead ion complexes away from the electrode surface. The nature of the reactivation of the electrode at extreme anodic potentials is also discussed.

The electrochemical reduction of maleimide: The synthesis of succinimide

Abstract The electrochemical reduction of maleimide has been investigated in the pH range 0–14 at a variety of electrode materials. The reaction scheme was found to fall into three distinct sections, depending upon the pH at which the reduction was performed. At low pH the reduction product was found to be succinimide, and the mechanism of its formation is described. At higher pH values, the hydrolysis of starting materials complicates the interpretation of the electrochemical measurements, but nevertheless the overalls reduction mechanism could be deduced. Polarographic data complement information obtained from rotating disc experiments, and comparison is made with the electrochemical reduction of phthalimide.

Expander action at the Pb(Hg)/PbCl2 interface

Abstract The influence of ammonium lignosulphonate on the kinetics of formation of thick films of PbCl2 is described. It is shown that geometric factors need to be considered as well as the influence of the additive on the rates of lattice growth (k1, k2) and the nucleation rate constant (A). The results indicate that expanders may be of use in sea-water activated batteries.