Anodic process dissolution efficiency

The above-mentioned factors are not so important in case of conventional ECM due to comparatively large lEG and flowing nature of electrolyte. However, all the factors play a vital role during anodic dissolution in microscopic domain especially in the case of micro-ECM. Targeted research attempts have been made throughout the world by scientists to solve the effects of all these factors. Some amount of success has been achieved however, it needs further efforts to make the process more efficient. Attempts have been made to put forward more information regarding aU these in the context of micro-ECM in the later chapters for better understanding of all these factors. 

In terms of the developed ideas, the possible complications are caused by the fact that the conditions for successful cathode and appropriate anode processes are mutually incompatible hi iraiic craiductivity is required for the efficient cathode deposition, whereas high electrcMiic cOTiductivity is necessary for easy anode dissolution. ... 

When n = 2, Ti is equivalent to the theoretical value, meaning that Mg dissolution is utilized 100% for current generation. If n is smaller than 2, for example, n = 1.2, then T) will be as low as 60%, implying that only some of the dissolution of Mg has contributed to the Faraday current generation and the remaining consumed in other processes involved in the dissolution. Table 1.5 lists the anodic dissolution efficiency of Mg in some electrolytes. The apparent valence appears to be a function of the electrolyte solutions. 

The strange anodic polarization behaviors (negative difference effect, lower apparent valence, low anodic dissolution efficiency, low anodic polarization resistance and poor passivity ) are closely associated with the AHE process which is further related to the onset of localized corrosion or pitting . A comprehensive anodic dissolution model can be employed to understand these. 

The question arises as to what effects are responsible for the generation of the very first tiny etch pits on the surface. However, this question is misleading in some respects. A flat n-type surface anodized in HF is in an unstable condition. Tiny inhomogeneities of flatness in the electrode surface, inevitably produced by the dissolution process, are amplified, because the concave areas will focus the electric field and thereby become more efficient in collection holes than flat areas. An increase in anodization bias accelerates pore initiation at flat electrodes. 

The principles of dissolution have been reviewed by Bloom and Nater (1991), Blesa et al. (1994) and Casey (1995). The driving force for dissolution is the extent of undersaturation with respect to the oxide. Undersaturation is thus a necessity for dissolution as is supersaturation for precipitation. Other factors being equal, the rate of reaction will increase as degree of undersaturation rises. The extent of undersaturation varies from one system to the next. Dissolution of anodic Fe oxide films often takes place in nearly saturated solutions, whereas extraction of iron from its ores requires markedly undersaturated solutions in order to be efficient. In most natural systems (soils and waters) the aqueous phase is fairly dose to saturation with respect to the iron oxides and dissolution may, therefore, be extremely slow. The dissolution process can be accelerated by the presence of higher levels of electrons or chelating ligands. 

It should be noted that in practical batteries such as coin cell (parallel plate configuration) or AA, C, and D (jelly-roll configuration), there is a stack pressure on the electrodes (the Li anodes are pressed by the separator), and the ratio between the solution volume and the electrode s area is usually much lower than in laboratory testing. Both factors may considerably increase the Li cycling efficiency obtained in practical cells, compared with values measured for the same electrolyte solutions in the Li half-cell testing described above. It has already been proven that stack pressure suppresses Li dendrite formation and thus improves the uniformity of Li deposition-dissolution processes [107], The low ratio between the solution volume and the electrode area in practical batteries decreases the detrimental effects of contaminants such as Lewis acids, water, etc., on Li passivation. 

Anodic polarization of Ca electrodes in TC leads to current passage and dissolution of the active metal at high efficiency. As expected for SEI electrodes, a Tafel-like behavior connects the current and the overpotential applied [see Eqs. (5)—(11) in Section V.C.3], It is assumed that upon anodic polarization the anions (Cl-) migrate from the surface film s solution interface to the surface film s metal interface. Two processes can thus occur ... 

The water temperature will also influence the electrocoagulation process. A1 anode dissolution was investigated in the water temperature range from 2 to 90°C. The A1 current efficiency increase rapidly when the water temperature increase from 2 to 30°C. The temperature increase will speed up the destructive reaction of oxide membrane and increase the current efficiency. However, when the temperature was over 60°C, the current efficiency began to decrease. In this case, the volume of colloid Al(OH)3 will decrease and pores produced on the A1 anode will be closed. The above factors will be responsible for the decreased current efficiency. 

Anodic dissolution of hard alloys has been enhanced by the application of ultrasound, apparently because of the increase in cavitation and the hydrodynamic pressure resulting in an increase in current density. The cyclic nature of the hydrodynamic pressure helps to remove passivating oxide films from the surface of the workpiece, thereby raising the process efficiency. This increase in current density resulting from the application of ultrasonic vibrations was most evident in hard alloys containing appreciable quantities of Ti and Ta carbides [117]. 

The electrolyte and other conditions must be selected so that both the anodic dissolution and the deposition of the metal occur with high efficiency while none of the impurity metals can transfer from the anode to the cathode. Certainly there must be no passivation of the anode (see Chapter 9) and the objective is to obtain a good-quality, often highly crystalline, deposit at the cathode. Where necessary, additives are added to the electrolyte to enforce the correct behaviour at both electrodes. Chloride ion is a common addition to enhance the dissolution process and, where essential, organic additives are used to modify the cathode deposit. Since, however, organic compounds can be occluded to some extent and reduce the purity of the metal, their use is avoided when possible. 

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