Passivity electrochemical reduction charges

Based on the clear interpretation of the electrochemical features of the polarization curve of Figure 5.5 provided by the XPS data, the charges for electrochemical reduction can be reevaluated [67]. The cathodic charge of peak Cl is always larger than that of CII. This means that at Cl two Cu(I) parts are reduced, the fraction which was Cu(U) before being reduced to Cu(l) at Cll and the fraction which was present as Cu(I) before reduction. This result suggests the presence of a duplex film at potentials above AH. Confirmation has been obtained by quantitative ISS depth profiling of a Cu specimen passivated for lOmin... 

Ox and Red are general symbols for oxidation and reduction media respectively, and n and (n-z) indicate their numerical charge (see Section 2.2.2). Where there is no electrochemical redox reaction [Eq. (2-9)], the corrosion rate according to Eq. (2-4) is zero because of Eq. (2-8). This is roughly the case with passive metals whose surface films are electrical insulators (e.g., A1 and Ti). Equation (2-8) does not take into account the possibility of electrons being diverted through a conductor. In this case the equilibrium... 

A good overview on the various passivation and deposition processes can be found in Refs. [267-269]. In Table 1.5 the resulting Dit trap densities for the various possible passivation techniques are shown. Thermal passivation yields the highest interface quality, that is the lowest Dit can be achieved. Quality wise the electrochemical passivation is next. However, electrochemical reactions at a semiconductor surface are only possible in the accumulation mode. Therefore, anodic reactions only take place at p-type doped Si electrodes (accumulation of majority charge carriers, i.e. holes), whereas on n-Si only reduction reactions are possible. Consequently, only p-type doped Si can be anodically passivated. This can be changed by an illumination... 

Fig. 10.12 - Plots showing the dependence of (a) A and (b) Ij on the charge passed during the reduction of passive iron oxides. The passive films were formed for 300 s at (A) 0.75 V and (B) 1.35 V and then reduced at —0.25 V vs NHE. The wavelength was 460 nm and the angle of incidence 68°. Reproduced with permission from C. T. Chen B. D. Cahan, J. Electrochem. Soc., 129, (1982), 17.

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