Nucleation current density

In seawater, lead anodes with 1 or 2% silver may be used for cathodic protection of ships " at current densities of up to 120Am Lead with 6Vo antimony and 1 Vo silver has also been recommended. It is thought that silver might provide small stable nucleation sites for PbOj formation " in a manner similar to the Pb/Pt bi-electrode " (see Section 11.3), which is serviceable at 250 A m . A lead. Wo Ag, 0.5% Bi or 0.5% Te alloy with a platinum micro-electrode will perform well at 500 A m. ... 

Figure 14. Charge-discharge curves for the upper plateau in the LirSi system inside a matrix of the Li2 f)Sn phase at 415 °C. The upper panel shows the effect of current density, whereas the lower panel shows that the potential overshoot related to the nucleation of the second phase is mostly eliminated if the electrode is not cycled to the ends of the plateau (441.

To study the nucleation and growth of Au nanoclusters in silica within the above theoretical frame, we implanted fused silica slides with 190keV-energy Au ions, at room temperature and current densities lower than 2 pA/cm, to reduce sample heating [49,50]. The implantation conditions were chosen to have, after annealing, a subsurface buried layer of Au nanoparticle precipitation of about... 

The capacitance determined from the initial slopes of the charging curve is about 10/a F/cm2. Taking the dielectric permittivity as 9.0, one could calculate that initially (at the OCP) an oxide layer of the barrier type existed, which was about 0.6 nm thick. A Tafelian dependence of the extrapolated initial potential on current density, with slopes of the order of 700-1000 mV/decade, indicates transport control in the oxide film. The subsequent rise of potential resembles that of barrier-layer formation. Indeed, the inverse field, calculated as the ratio between the change of oxide film thickness (calculated from Faraday s law) and the change of potential, was found to be about 1.3 nm/V, which is in the usual range. The maximum and the subsequent decay to a steady state resemble the behavior associated with pore nucleation and growth. Hence, one could conclude that the same inhomogeneity which leads to pore formation results in the localized attack in halide solutions. 

The incorporation of discreet nucleation events into models for the current density has been reviewed by Scharifker et al. [111]. The current density is found by integrating the current over a large number of nucleation sites whose distribution and growth rates depend on the electrochemical potential field and the substrate properties. The process is non-local because the presence of one nucleus affects the controlling field and influences production or growth of other nuclei. It is deterministic because microscopic variables such as the density of nuclei and their rate of formation are incorporated as parameters rather than stochastic variables. Various approaches have been taken to determine the macroscopic current density to overlapping diffusion fields of distributed nuclei under potentiostatic control. 

Both Eqs. (10.28) and (10.31) predict a current density which first rises as the perimeters of the clusters grow, and then decreases rapidly as the clusters begin to overlap. They can be cast into a convenient dimensionless form by introducing the maximum current density jmax and the time fmax at which it is attained. A straightforward calculation gives for instantaneous nucleation and progressive nucleation, respectivly,... 

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

Raeissi et al. [236, 237] showed that temperature, pH, and current density affected the morphology and texture, as well as the nucleation mechanism of the zinc deposits on carbon steel electrode. 

The cadmium electrodeposition on the solid cadmium electrode from the sulfate medium was investigated [217]. The following kinetic parameters were obtained cathodic transfer coefficient a = 0.65, exchange current density Iq = 3.41 mA cm , and standard rate constant kg = 8.98 X 10 cm s . The electrochemical deposition of cadmium is a complex process due to the coexistence of the adsorption and nucleation process involving Cd(II) species in the adsorbed state. 

In interfacial electrochemical reaction rates given by the Butler—Volmcr equation (7.24), the current density, or rate of reaction per unit area, is zero at zero oveipotential (equilibrium), but significant net currents are observed if the potential of the working electrode is displaced from the reversible potential by only 1 mV. In the case of rate-controlling nucleation, however, there is no detectable current until the oveipotential exceeds a few millivolts, after which (at, say, 7 mV), the reaction rate suddenly undergoes an explosive increase. 

---