Field-assisted dissolution

In the initial stages of the anodization process field-assisted dissolution dominates chemical dissolution due to the relatively... 

Fig. 5.19 Schematic diagram of the evolution of straight nanotubes at constant anodization voltage (a) Oxide layer formation, (b) pit formation on the oxide layer, (c) growth of the pit into scallop shaped pores, (d) metallic part between the pores undergoes oxidation and field assisted dissolution, (e) fully developed nanotubes with a corresponding top view.

What has been described is what is called stress-corrosion cracking. Some common examples of systems that tend to undergo this type of corrosion are given in Table 12.5. But perhaps one should call it yield-assisted corrosion (an electrochemi-cal-plus-mechanical phenomenon) in contrast to normal field-assisted dissolution (an electrochemical phenomenon). 

Figure 12. Schematic diagram of the evolution of an anodic Ti02 nanotube array (a) Formation of a compact oxide layer, (b) Formation of pits due to the dissolution and breakdown of the barrier oxide film, (c) The barrier layer at the bottom of the pits is relatively thin and this leads to the enhanced electric field assisted dissolution of Ti02, which results in further pore growth, (d) Voids formed in the inter-pores region, (e) Fully developed nanotube array with a corresponding top view [51].

In agreement with the foregoing set of considerations, the film resistance increases linearly with film formation potential in alkaline solution (Figure 6.15). However, in acidic media, the resistance of the oxide film decreases on increasing the film formation potential, as can be seen in Figure 6.14. This can be attributed to an increase in the vacancy concentration within the oxide film, which could be explained by a field-assisted dissolution of anodic oxide film at the oxide/solution interface generating aluminum vacancies ... 

Fig. 17 Schematic diagram showing the nanotube on anodized titanium foil (a) oxide layer formation (b) pit formation (c) growth of pit into pores (d) regions between the pores undergo oxidation and field assisted dissolution (e) developed nanotube array. From [34]...

This mechanism is consistent with the hypothesis that in the second stage dissolution kinetics is dependent on diffusion within the concentration boundary layer. It is conceivable that in the first stage field assisted dissolution may be the controlling step. In this stage formation of Ti(OH)4 or of hydroxy-cations, e.g. Ti(OH)3, has different effects on titanium transport. While Ti(OH)4 does not react with organic molecules, Ti(OH)3 can form organometallic complexes which may be transported systemically. 

Three processes are believed to compete during the anodisation process field-assisted oxidation of Ti to TiOi, field assisted dissolution of Ti metal ions in the electrolyte and the chemical dissolution of Ti and TiOi due to etching by fluoride ions, enhanced by the presence of (Equations 3.27-3.29). ° The chemical steps are not thought to... 

The thickness of the tubular structure stops increasing when the chemical dissolution rate of the oxide at the mouth of the tube (top surface) becomes equal to the rate of inward movement of the metal/oxide boundary at the base of the tube. Higher anodisation voltages promote the oxidation and field-assisted dissolution and increase the nanotube layer thickness before equilibrium is reached. A slightly different mechanism occurs in aqueous solvents compared with organic solvents in aqueous solvents the process is subtractive (the nanotube array will be less than the... 

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