Large Cathodic Current

At first sight the mechanism of crevice corrosion appears to be simply the formation of a differential aeration cell in which the freely exposed metal outside the crevice is predominantly cathodic whilst the metal within the crevice is predominantly or solely anodic the large cathode current acts on the small anodic area thus resulting in intense attack. However, although differential aeration plays an important role in the mechanism, the situation in reality is far more complex, owing to the formation of acid within the crevice. 

Large Cathodic Current We have seen from Figure 6.7 that for the large negative values of overpotential r], the partial cathodic current density i approaches i, i i. For these conditions the Butler-Volmer equation (6.45) can be simplified. Analysis of Eq. (6.45) shows that when rj becomes more negative, the first exponential term in the equation (corresponding to the anodic partial current) decreases, whereas the second exponential term (corresponding to the cathodic partial reaction) increases. Thus, under these conditions. 

We will consider this relationship for the large cathodic and anodic values of rj. For large cathodic current densities, we start with Eq. (6.47), omitting the minus sign and writing the absolute value of i ... 

Large Cathodic Current. We have seen from Figure 6.7 that for the large negative values of overpotential 17, the partial cathodic current density i approaches 2, i = i. 

An example of the use of the technique to obtain information during a cyclic voltammagram is shown in Fig. 113 for n-ZnO [192], As the crystal was cycled to very negative potentials, a large cathodic current was found due both to hydrogen evolution and Zn-metal deposition from the reductive decomposition of the ZnO. Re-oxidation of the Zn metal is seen as an anodic peak in the CV, but it is very difficult, in the presence of both cathodic and anodic currents, to form a clear impression from the CV of the amount of Zn... 

For large cathodic current densities smaller than the limiting current density, the deposition overpotential increases with increasing current density and vice versa, according to the Bulter-Volmer equation ... 

In Chapter 8 crystallisation and scale formation are discussed and the effect of pH was demonstrated as being a factor in deposit formation. Furthermore the equilibrium of ions in solution is likely to be affected by the presence of electric currents and the current density. As a consequence, a characteristic feature of cathodically protected metallic surfaces in contact with solutions containing mineral ions, notably sea water, is the formation of deposits or cathodic protection scale. The use of controlled scale formation is mentioned as a method of corrosion control in Chapter 14. Cox [1940] proposed the deliberate formation of calcium deposits on steel by the imposition of large cathodic currents to act as anticorrosive self healing layer. 

Figure 19 represents a DC voltammogram of superoxide anions obtained from a porcine neutrophil in contact with an electrode opsonized with IgG. The large cathodic current at around —0.3V is ascribed to the reduction of dissolved oxygen in the buffer solution, and... 

Figure 3 shows experimentally observed current-potential relations of the (a) p- [22] and (b) n-GaAs [23] electrodes in 10 mM HCl in dark. As described above, while a large cathodic current and a negligible anodic current were observed at n-GaAs electrode, a large anodic... 

Panel (a) in Figure 8 shows a cyclic voltammogram at a slow scan rate of 0.5 mV s of HOPG basal plane in 1 M LiClO/EC + DEC." In the first cycle, three major cathodic peaks appeared at about 1.0, 0.8 and 0.5 V. These cathodic peaks disappeared in the second cycle, and hence are attributed to irreversible decomposition reactions of the electrolyte solution that are closely related to SEI formation as mentioned in the previous section. A large cathodic current rise observed at potentials close to 0.0 V could be assigned to lithium intercalation because of the presence of an anodic lithium deintercalation peak at about 1.0 V. However, the charge consumed for the current rise at around 0 V was much greater than that for the anodic peak, and hence a substantial fraction of the cathodic current at around 0 V was consumed by irreversible processes such as solvent decomposition. 

On titanium alloys, however, the metal still has an active-passive transition in the crevice environment and the surface potential must be low enough for the active corrosion to occur. Thus, IR drop is required to stabilize the active dissolution [9,74], particularly when large cathodic currents are available. 

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