Metal-electrolyte interface current density

If the density of the electrical current i corresponds to the diffusive flow on the metal-electrolyte interface j, then the electrochemical balance equation can be presented as follows ... 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

When using this definition, one must pay attention to the fact that the current density is a local quantity. Consequently, when writing the sum one should only take into account the species actually present at the point in question. In an electrochemical system, the nature of the charge carriers may vary from one point to another within the device. This frequently occurs in large-sized cells with different species at the anode and the cathode. Yet this is always the case in the area surrounding a metal electrolyte interface, as shown in the example below. 

According to (6.24), the ionic current density in the film varies exponentially with the electric field. Even though this relation has been derived here from a rather simple model, it holds true quite generally. We therefore can also look at equation (6.24) as an empirical equation that describes the relation between the ionic current, the potential and the thickness of solid oxide films, in a similar way as the Butler-Volmer equation describes the relation between current and potential for a metal-electrolyte interface. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

Ehiring corrosion (oxidation) process, both anodic and cathodic reaction rates are coupled together on the electrode surface at a specific current density known ds icorv This is an electrochemical phenomenon which dictates that both reactions must occur on different sites on the metal/electrolyte interface. For a uniform process under steady state conditions, the current densities at equilibrium are related as o = — c = ieorr Ecorr- Assume that corrosion is uniform and there is no oxide film deposited on the metal electrode surface otherwise, complications would arise making matters very complex. The objective at this point is to determine both Ecorr and icorr either using the Tafel Extrapolation or Linear Polarization techniques. It is important to point out that icorr cannot be measured at Ecorr since ia = —ic and current wfll not flow through an external current-measuring device [3]. 

In the case of a painted Al-Zn alloy coating, two parallel scribes (distance about 2 mm) were prepared. The SVET measured two different current density distributions for the two scribes. To recalculate the current density at the metal/electrolyte interface from the values measured at a certain distance, mathematical modeling was done in which the surface and its scratches were considered to be composed of elemental strips 30 pm wide. Each strip was then replaced by a line, which was the source of current. In the presence of many line sources, the current at a particular point in solu-... 

Electrolytic plating rates ate controUed by the current density at the metal—solution interface. The current distribution on a complex part is never uniform, and this can lead to large differences in plating rate and deposit thickness over the part surface. Uniform plating of blind holes, re-entrant cavities, and long projections is especiaUy difficult. 

Again the extent to which such parallel reactions contribute to the measured current is not very easy to quantify. However, fortunately, such a quantification is not necessary for the description of NEMCA. What is needed is only a measure of the overall electrocatalytic activity of the metal-solid electrolyte interface or, equivalently, of the tpb, and this can be obtained by determining the value of a single electrochemical parameter, the exchange current I0, which is related to the exchange current density i0 via ... 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Now, to go to the experimental situation, what happens as we insert a metal electrode into an electrolyte solution without connecting it to an external electron source As we have discussed before (p. 22), an El is built up and hence a certain potential is established across the interface region. At this potential, charge transfer between electrode and electroactive species takes place, but, since no net current flows, the rates of electronation and de-electronation are identical. The system has reached the equilibrium potential at which the current density z for electronation is equal to the current density of de-electronation i. This current density is designated i0, the equilibrium exchange current density (cf. Table 6), given by the expression ... 

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