Double anodic metal dissolution

Let us consider as an example a metallic iron electrode in an aqueous solution. The anodic metal dissolution is a process in which iron ions in the metallic bonding state in the metal phase transfer across the interfacial compact double layer (Helmholtz layer) into the hydrated state of the ions in the solution ... 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

If a positive potential is applied to the metal, as shown in Fig. 10.3, the ionization of the surface atoms will be promoted, and thus more metal ions will be produced at the surface. In the solution, water molecules, positive ions (cations), and negative ions (anions) drift around. The adsorbed layer of positive metal ions attracts nearby water dipoles in a preferential direction. The negative ions in the solution near the anode surface are also attracted toward the surface. The adsorbed fixed layer and the negative ion layer (Fig. 10.3) together are the so-called electrical double layer. Details about the double layer are available elsewhere [3]. Electrochemical reactions and mass transport for further electrochemical dissolution occur and pass through this double layer. 

The first equation represents the equilibrium between hydrated Ag+ ions and Ag atoms in a single-crystal configuration. Alternatively, we may say that there is a heterogeneous thermodynamic equilibrium between Ag+ ions in the solid phase (where they are stabilized by the gas of free electrons) and Ag+ ions in the liquid phase (stabilized by interaction with water molecules). The forward reaction step corresponds to the anodic dissolution of a silver crystal. On an atomic level, one may say that a Ag" " core ion is transferred from the metallic phase to the liquid water phase. In an electrochemical cell, an electron flows from the Ag electrode (the working electrode) to the counter electrode each time that one Ag+ ion is transferred from the solid to the liquid phase across the electrochemical double layer. Although the electron flow is measured in the external circuit between the working... 

For heavily doped materials, either notp type, the surface is degenerated and the material behaves like a metal electrode, meaning that the charge transfer reaction in the Helmholtz double layer is the rate-determining step. This is supported by the lack of an impedance loop associated with the space charge for the heavily doped materials. Also, for heavily doped n-Si large current in the dark is due to electron injection, which is not characterized by a slope of 60 mV/decade. For p-Si, electron injection into the conduction band may also occur during the anodic dissolution. 

The electrochemical reduction of benzenediazonium chloride was also studied in the presence of unsaturated compounds like styrene, using Pt as cathode and Cu, Fe or Ti as anode. The main processes observed were addition of a phenyl and a chloro group to the double bond [PhCH(Cl)CH2Ph (I)] and additive dimerization [PhCH2CH(Ph)CH(Ph)-CH2Ph (II)]. These results are due to the catalytic effect of the cations formed by the anodic dissolution of the metal on the reaction between the diazonium salts and the unsaturated compound. The absence of products (I) and (II) when both electrodes were Pt confirms this redox catalysis. 

Sites where dissolution is occurring are assumed to be generally anodic to the equilibrium potential and sites where oxygen reduction or hydrogen evolution is occurring are assumed to be cathodic. At the anodic sites, because of local interactions between the metal atoms and the electrolyte, the electron cloud is assumed to be oriented toward the electrode. The capacitance of the double layer is thereby decreased and makes the metal atom cores more susceptible to ionization and dissolution. 

Actually, the model introduces a strong coupling between charge transfer and double-layer stmcture, a feature generally discarded in spite of the high interfacial concentration of cations released by the dissolution. In some way it may be regarded as a -potential approach to the problem. The topic of tiie kinetics content of the inductive impedance of anodically dissolving metals is still an open question. Obviously, it deserves further theoretical and experimental efforts. 

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