Anodic metal oxide films

Impressed current anodes of the previously described substrate materials always have a much higher consumption rate, even at moderately low anode current densities. If long life at high anode current densities is to be achieved, one must resort to anodes whose surfaces consist of anodically stable noble metals, mostly platinum, more seldom iridium or metal oxide films (see Table 7-3). 

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

A variety of nanomaterials have been synthesized by many researchers using anodic aluminum oxide film as either a template or a host material e.g., magnetic recording media (13,14), optical devices (15-18), metal nanohole arrays (19), and nanotubes or nanofibers of polymer, metal and metal oxide (20-24). No one, however, had tried to use anodic aluminum oxide film to produce carbon nanotubes before Kyotani et al. (9,12), Parthasarathy et al. (10) and Che et al. (25) prepared carbon tubes by either the pyrolytic carbon deposition on the film or the carbonization of organic polymer in the pore of the film. The following section describes the details of the template method for carbon nanotube production. 

Figure 13 Reaction pathway for the electrochemical incineration of p-benzoqui-none at a Pt anode covered with a quaternary metal oxide film. (From Ref. 54.)...

A corrosion inhibitor may (a) decrease the rate of the cathodic or anodic process per apparent unit area by simply blocking active redox sites on the metal surface, (b) remove electrons from the metal thereby shifting the potential of the metal surface into a positive range of passivation, in which a metal oxide film is spontaneously formed, or (c) contribute to the formation of a thin protective coat on the surface, which stifles corrosion. 

Historically, this is the material which really sparked interest in the solar photoelectrolysis of water. Early papers on TiCh mainly stemmed from the applicability of TiCh in the paint/pigment industry255 although fundamental aspects such as current rectification in the dark (in the reverse bias regime) shown by anodically formed valve metal oxide film/ electrolyte interfaces was also of interest (e.g., Ref. 52). Another driver was possible applications of UV-irradiated semiconductor/electrolyte interfaces for environmental remediation (e.g., Refs. 256, 257). 

Finally, the electrochemistry of porous metal oxides prepared as films from anodic treatment of metal electrodes will also be discussed. Porous metal oxide films on electrodes have applications in a variety of fields, from corrosion protection to batteries and catalysis. 

In these equations, A, , represents the exposed area of the specimen, p,. is the Him resistivity, e is the vacuum permittivity, and Eq/ is the relative permittivity of the metal oxide. Consistently, with such equations, the reciprocal capacitance of anodic aluminum oxide films increases on increasing the formation potential in both alkaline (Figure 6.15) and acidic media (Figure 6.14). This behavior reflects the increase in the film thickness on increasing the formation potential. 

There are a lot of investigations of the breakdown of anodic valve metal oxide films in the electrolyte or in a metal-oxide-metal contact. Because of the enormous differences in all of these parameters, there is no general model that can explain all the experimental findings. In fact, each model has only a limited range of vahdity and should not be stressed beyond this range. 

Tungsten (together with Al, Ti, Zr, Bi, Ta, and Nb) belongs to the group of the so-called valve metals, which passivate and show a very high corrosion resistance in most common aqueous media. The composition of naturally or anodically produced oxide films is essentially identical to WO3. 

Figure 4.4.29. Interfacial defect generation/annihilation reactions that occur in the growth of anodic barrier oxide films according to the Point Defect Model (D. Macdonald [1999]). m = metal atom, = cation vacancy on the metal sublattice of the barrier layer, MP = interstitial cation, Mu = metal cation on the metal sublattice of the barrier layer, Vo = oxygen vacancy on the oxygen sublattice of the barrier layer, Oo = oxygen anion on the oxygen sublattice of the barrier layer, = metal cation in solution.

Mixed metal oxide coated anodes, also called dimensionally stable anodes (DSA), are based on electrode technology developed in the early 1960s for the production of chlorine and caustic soda. The mixed metal oxide films are thermally applied to a noble metal such as titanium, niobium, and tantalum as substrate materials and are available in a variety of sizes and shapes. These oxide coatings have excellent conductivity, are resistant to acidic environments, are chemically stable, and have relatively low consumption rates. Groundbed installation in soils usually specifies that the anode be prepackaged in a canister with carbonaceous backfill material. 

Pitting is defined as localized corrosion attack occurring on exposed metal surfaces in the absence of any apparent crevices. This pitting occvirs when the potential of the metal exceeds the anodic breakdown potential of the metal oxide film in a given environment. When the anodic breakdown (pitting) potential of the metal is equal to or less than the corrosion potential imder a given set of conditions, spontaneous pitting can be expected. 

Naturally occurring oxide films on most metals do not usually provide optimum corrosion protection, and this may be modified or replaced to provide a further means of corrosion control. Common examples are the anodizing of aluminium alloys or the chromating of aluminium, zinc, cadmium or magnesium. With anodizing, the natural oxide film on the aluminium is thickened electrolytieally by up to 5 p.m. Chromating, described in detail later in the chapter, replaces the existing metal oxide film with a mixed chromium/metal oxide film of better corrosion resistance. 

Because the oxidative potential of polypyrrole and hydrogen are very close, the electrochemical p-doping and dedoping processes can be simultaneously accompanied by the reduction/oxidation of hydrogen in aqueous environments. Which process dominates can depend on many factors such as pH, nature of the electrolyte, additives, and dopants. Also to be considered when the polymer is coupled with a passivating metal are two possible anodic reactions oxidation of the polymer backbone and polarization of the metal surface to produce a metal oxide film. Another possibility is an irreversible anodic overoxidation of the polymer that can occur at potentials more positive than that of the reversible doping-dedoping process. 

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