The metal-solution interface

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. 

Electrochemical Impedance Spectroscopy (EIS) and AC Impedance Many direct-current test techniques assess the overall corrosion process occurring at a metal surface, but treat the metal/ solution interface as if it were a pure resistor. Problems of accuracy and reproducibility frequently encountered in the application of direct-current methods have led to increasing use of electrochemical impedance spectroscopy (EIS). 

S. Amokrane, J. P. Badiah. Analysis of the capacitance on the metal-solution interface role of the metal and metal-solvent couphng. In J. O M. Bockris,... 

Consider now the transfer of electrons from electrode II to electrode I by means of an external source of e.m.f. and a variable resistance (Fig.. 20b). Prior to this transfer the electrodes are both at equilibrium, and the equilibrium potentials of the metal/solution interfaces will therefore be the same, i.e. Ey — Ell = E, where E, is the reversible or equilibrium potential. When transfer of electrons at a slow rate is made to take place by means of the external e.m.f., the equilibrium is disturbed and Uie rat of the charge transfer processes become unequal. At electrode I, /ai.i > - ai.i. 3nd there is... 

Before electron transfer can occur the oxygen in the atmosphere must be transported to the metal/solution interface, and this involves the following steps... 

The relation between free phosphoric acid content and total phosphate content in a processing bath, whether based on iron, manganese or zinc, is very important this relation is generally referred to as the acid ratio. An excess of free acid will retard the dissociation of the primary and secondary phosphates and hinder the deposition of the tertiary phosphate coating sometimes excessive loss of metal takes place and the coating is loose and powdery. When the free acid content is too low, dissociation of phosphates (equations 15.2, 15.3 and 15.4) takes place in the solution as well as at the metal/solution interface and leads to precipitation of insoluble phosphates as sludge. The free acid content is usually determined by titrating with sodium... 

Similarly, all points within a metal, which consists of an ordered rigid lattice of metal cations surrounded by a cloud of free electrons, are electrically neutral. Transport of charge through a metal under the influence of a potential difference is due to the flow of free electrons, i.e. to electronic conduction. The simultaneous transport of electrons through a metal, transport of ions through a solution and the transfer of electrons at the metal/solution interfaces constitute an electrochemical reaction, in which the electrode at which positive current flows from the solution to the electrode is the cathode (e.g. M (aq.) + ze M) and the electrode at which positive flows from it to the solution (e.g. M - M (aq.) -)- ze) is the anode. 

If the potential of the metal/solution interface is made more negative than p.z.c. by giving it an excess negative charge the positively charged ion will... 

When a cathodic process occurs at a finite rate the concentration of the electron acceptor (cathode reactant) at the metal/solution interface (x = 0) will become less than that in the bulk solution c, and as the rate increases it will continue to decrease until it becomes zero, i.e. as soon as the electron acceptor arrives at the interface electron transfer occurs. 

The most important quality of the pzc is that it contains information about the structural details of the metal/solution interface. In the absence of surface-active electrolytes, the pzc depends only on the nature of the metal and the solvent.3,4,5 Conversely, the pztc is not exclusively relevant to the structure of the interface this is truer the larger the value of in Eq. (8) (or of At where i is the species to which the electrode is reversible e.g., H+ for the Pt group metals in the H adsorption region). 

Although the pzc contains all the essential structural information about the metal/solution interface, this information is not immediately apparent but must be appropriately decoded. This necessitates a description of (M - A) in microscopic terms that require a minimum of model assumptions.3 Another problem is that (0M - 0s)o is not directly accessible to experimental determination. What is actually measured, usually de-... 

The relevance of the pzc to the structure of the metal/solution interface and its relation to the metal/vacuum situation was first emphasized by Frumkin and Gorodetzkaya in 1928.18 The first compilation of pzc values was prepared by Frumkin in 1933.19... 

A comparison of the adsorption of a given molecule at the air/solution and at the metal/solution interface is a convenient way of obtaining some information on the role of the metal surface.93,94 At the air/solution interface the potential shift is simply... 

An ideally polarized electrode is rigorously defined as the electrode at which no charge transfer across the metal/solution interface can occur, regardless of the potential externally imposed on the electrode. At any fixed potential, such an electrode system attains a true state of equilibrium. 

Badiali, J. P. Analysis of the Capacitance of the Metal-Solution Interface. Role of the Metal and the Metal-Solvent Coupling 22... 

Kripsonsov and quantum mechanical calculations for the metal-solution interface, 174... 

Quantum chemical calculations, 172 Quantum chemical method, calculations of the adsorption of water by, 172 Quantum mechanical calculations for the metal-solution interface (Kripsonsov), 174 and water adsorption, 76 Quartz crystal micro-balance, used for electronically conducting polymer formation, 578... 

When a zinc strip is dipped into the solution, the initial rates of these two processes are different. The different rates of reaction lead to a charge imbalance across the metal-solution interface. If the concentration of zinc ions in solution is low enough, the initial rate of oxidation is more rapid than the initial rate of reduction. Under these conditions, excess electrons accumulate in the metal, and excess cationic charges accumulate in the solution. As excess charge builds, however, the rates of reaction change until the rate of reduction is balanced by the rate of oxidation. When this balance is reached, the system is at dynamic equilibrium. Oxidation and reduction continue, but the net rate of exchange is zero Zn (.S ) Zn (aq) + 2 e (me t a i)... 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

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