Large Anodic Current

2 Large Anodic Current When r] has large positive values, the second exponential term in Eq. (19) decreases, while the first term (corresponding to the anodic partial reaction) increases. Under these conditions, the smaller term exp[—azFrj/RT] can be neglected, yielding an approximate form which is essentially single exponential Eq. (17). 

there is an exponential relationship between i and high values of //, usually greater than 0.10 V. Taking the logarithm of both sides of Eqs (16 and 17) and expressing t] as a function of the current density, we have 

The experimentally determined cathodic and anodic Tafel lines for the electrodeposition of copper in acid copper sulfate solution are given in Fig. 4. Data presented in Fig. 4 can be used to evaluate the Tafel 

Under these conditions, the exponentials in the Butler-Volmer equation (19) can be approximated using a power series, and taking only the first-order terms in rj, we get 

for small values of rj, less than about 0.01 V, when the electrode potential is near the reversible potential, the current varies linearly with the overpotential. 

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This important expression is known as the Eyring equation. Note that inet depends on both the potential and the surface concentration of each form of the couple. For example, a high concentration of R and a very positive potential combine to result in a large anodic current. 

At E > 1.0 V transpassive dissolution as QxCY2 is obtained in 0.5 M H2SO4, with a strong increase of the current density (Fig. 5). At E > 1.7 V oxygen evolution contributes to the large anodic current. These characteristic electrochemical properties indicate the importance of Cr as an alloying additive to obtain corrosion-resistant alloys, especially at negative potentials and in acidic solutions. 

Imposing an anodic current density on the iron with an external device results in the generation of the anodic branch of the polarization curve. Increasing the applied anodic current decreases the reduction reaction rate as the surface is polarized in the positive direction. At small anodic current densities, the HER current density is still an appreciable fraction of the anodic current density. Under these conditions the applied current density is less than anodic current density. For example, at a potential of -0.225 V(NHE), 4 is 2 X 10 3 A/cm2, 4pP is 6 X 10 3 A/cm2, and 4 is 8 X 10 3 A/cm2. At sufficiently large anodic current densities (e.g., 10 2 A/cm2 in Fig. 26), the cathodic reaction is insignificant rela-... 

Anodic behavior of sihcon can best be characterized by i-V curves. Neglecting the details associated with a silicon substrate such as doping, the current-potential relationship of silicon in aqueous solutions can be considered to be principally determined by the pH and HE concentration as illustrated in Pig. 5.1. In nonalkaline and nonfluoride aqueous solutions, silicon as an electrode is essentially inert, showing a very small current at anodic potential due to the presence of a thin oxide film. In alkaline solutions, silicon is also passivated by an oxide film at anodic potentials but is active below the passivation potential, Vp. In fluoride solutions, the silicon electrode is active in the whole anodic region as shown by the large anodic current. 

Deposition of metals on a silicon surface can be either a conduction band process or a valence band process depending on the redox potential of the metal and solution composition. Deposition of Au on p-Si in alkaline solution occurs only under illumination indicating that it is a conduction band process due to the unfavorable position of the redox couple for hole injection. " On the other hand, deposition of platinum on p-Si can occur in the dark by hole injection into the valence band. For Cu, although the deposition proceeds via the conduction band as shown in Fig. 6.9, it can also proceed via the valence band because a large anodic current of n-Si occurs in the dark in copper-containing HF solution as shown in Fig. 6.10. The reduction of copper under this condition is via hole injection. The holes are consumed by silicon dissolution and the silicon reaction intermediates then inject electrons into the conduction band, resulting in the anodic current on n-Si in the dark. 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

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