Anodic dissolution under film-free

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

Anodic Dissolution under Film-free Conditions... 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

The detailed anodic and cathodic reactions under the self-corrosion condition are schematically presented in Fig. 1.13. In general, anodic reactions occur mainly in film-free areas whereas in film-covered areas anodic dissolution is negligible. Cathodic reactions with hydrogen evolution as a main process, can take place in both the film-free and film-covered areas, although in a film-free area much faster than on a film-covered surface. 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

Not only nanometric structures such as tubes or particles are important but also the application of nanoporous materials such as the case of nanoporous gold films [48, 87] (NPG) that could be prepared by selective dissolution of silver form Au/Ag alloys under free corrosion conditions. In this case, stability of the electrode can be increased and biomolecule immobilization could happen inside the mnable pores. Due to their well-defined nanopores, anodized alumina membranes have found many applications, among them, impedimetric immunosensing [63]. 

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