Anode partial current density

In this type of corrosion, metal ions arising as a result of the process in Eq. (2-21) migrate into the medium. Solid corrosion products formed in subsequent reactions have little effect on the corrosion rate. The anodic partial current-density-potential curve is a constant straight line (see Fig. 2.4). 

For large negative or positive overpotentials, i.e., for r)t P RT/nF, either the cathodic or the anodic partial current density predominates, so that according to eqn. 3.19... 

Potential Difference A< Departs from Equilibrium Butler-Volmer Equation, When the interphase is not in equihbrium, a net current density i flows through the electrode (the double layer). It is given by the difference between the anodic partial current density i (a positive quantity) and the cathodic partial current density i (a negative quantity) ... 

Anodic partial current (density) — For a single - electrode reaction, the total current is composed of an anodic and a cathodic - partial currents. 

When the rate is controlled by the - charge transfer step according to the - Butler-Volmer equation the anodic partial current density ( a) can be expressed as follows ... 

Figure 23. Schematic representation of the cathodic and anodic partial current densities at a metal electrode in contact with a simple redox system as a function of the over potential U - The bars on the vertical axis indicate one decade of current, the bars on the horizontal axis indicate 0.1 V (or 0.1 eV for the electrochemical potential scale).

If we keep these reflections in mind, interpretation of the morphology of the GaP surface etched in alkaline K3Fe(CN)g solutions is straightforward. At the (lll)-fac the overall etch rate is kinetically controlled. The i-Fcurves for the p-type (111) GaP-face are schematically shown in Fig. 13 a, which demonstrates that the situation at the rest potential corresponds to the rising part of the anodic partial current density. Etch pits are formed due to local differences in hole capture reactivity, analogously to the anodic etching of p-GaP in the rising part of the i- V curve (Fig. 1... 

The difference between the anodic external current and the independently determined anodic partial current (dissolved Fe) is the cathodic partial current density. The results as obtained by Hoar and Holiday are shown in Fig.6. The dashed curve represents the external polarization behavior in the absence of inhibitor and the black lines are the Tafel slopes for the anodic partial current density (the metal dissolution) for different inhibitor concentrations. The cathodic partial current density (hydrogen eveolution) is found for all values of the inhibitor concentrations in the shaded area. Therefore, it is obvious that the inhibitor in this case acts exclusively by reducing the anodic reaction rate but not the cathodic one. 

In Eq. (18), the first term represents the anodic partial current density I a (taken as positive) and the second term is the cathodic partial current density ic (taken as negative). The net current is the sum of the two terms. It is positive when the electrode is polarized anodically and negative when the electrode is polarized cathodically. 

Charge transfer coefficient, anodic partial current density... 

The corrosion current density is equal to the anodic partial current density at the corrosion potential. Its value, and therefore the rate of corrosion, depends on the kinetic parameters of both electrode reactions involved in the corrosion process. 

The current density measured during a polarization experiment is the sum of all anodic and cathodic partial current densities (Chapter 4). Figure 6.3 schematically shows the variation of the anodic partial current density of a passivating metal as a function of the potential. It allows us to define a number of quantities that describe the polarization behavior of passivating metals. 

Figure 6.7 illustrates why a less noble passivation potential facilitates spontaneous passivation. It shows anodic partial current densities for two metals, whose only difference is the value of their passivation potential. The cathodic partial current densities are identical for the two metals. We observe that the corrosion potential of metal a having a lower passivation potential is located in the passive region, while that for metal b is in the active region. Metal a, although less noble, is therefore expected to better resist corrosion. 

Figure 6.16 Anodic partial current density for the dissolution of iron measured in 0.15 M phosphate solutions of different pH. Dashed lines show the sum of the dissolution current densities and the formation of oxygen by the oxidation of OH ions and H2O [11].

Figure 6.28 Cathodic partial current density for the reduction of oxygen, of Fe ions, and of Ce ions on an austenitic stainless steel of type AISI304 in 5% H2SO4. Concentration of the oxidizing agents (3/n) mM 1 (n= charge number). Dashed hne anodic partial current density [30].

For a n-type semiconductor the anodic partial current density, i a,cond> corresponds to ... 

According to Faraday s law the anodic partial current density is proportional to the rate of corrosion. Thus, if the currents /cj and Zaji are negligible, we get from equation (7.5) for the corrosion rate Vj of the anode ... 

When the surface area of electrode (I) is small compared to that of electrode (II) and/or the oxygen concentration in compartment (I) is much lower than in compartment (II), the cathodic partial current of electrode (I) can be neglected I4j l l/cjil- By setting 4,11 = 4,n we obtain thus for the anodic partial current density of electrode (I) ... 

In the subcritical region, the anodic partial current density varies little with potential. Its value, generally small, depends on the applied potential sweep rate and on the alloy... 

Crack propagation by anodic dissolution, implies that a corrosion cell develops between the active crack tip and the remainder of the surface, which remains passive. The crack grows by anodic dissolution at the crack tip, whereas a passive film protects the walls of the crack. The rate of crack growth therefore is proportional to the anodic partial current at the crack tip. Because the anodic surface is much smaller than the cathodic surface (in fact, one does not know exactly what is the surface area of a crack tip), the anodic partial current density at the crack tip can reach very high values on the order of several A cm . Such high current densities correspond to substantial rates of crack growth. 

The polarization curves for iron, measured in 6M HCl solutions containing different amounts of trimethylene diamine (Figure 12.28), illustrates the described behavior [19]. This compound, whose formula can be found in Figure 12.26, reduces the cathodic partial current density more than the anodic partial current density and its presence lowers the corrosion potential. On the other hand, the slope of the Tafel lines is not affected by the inhibitor, indicating that the mechanism of the electrode reaction remains unchanged. The reduction of the partial currents can be explained by postulating that the inhibitor adsorbs on the metal and thus reduces the surface area available for the corrosion reactions. 

Figure 12.33 shows the principle of cathodic protection for a system obeying Butler-Volmer kinetics. Because the anodic partial current density is negligible at the protection potential, iprot is equal to the cathodic partial current density at the potential E — ffprot- The Butler-Volmer equation thus yields for the protection current density ... 

The rate of corrosion of a common steel in de-aerated acidic solution is 30 /im year Calculate the anodic partial current density in /xA cm . ... 

Cr"" ions. The anodic partial current density is 2 mA cm Calculate the weight loss in mg dm day. ... 

Note that the current density, /, for oxidation has a positive sign whereas /, the current for the reduction process has a negative sign. / is referred to as the anodic partial current density and I as the cathodic partial current density. The measured current density, 7, is therefore made up from contributions of opposite sign from the anodic and cathodic processes as shown in Fig. 3.1. 

Fig U Illustration of the way the experimental current density I varies with potential. Remember that cathodic and anodic partial current densities have opposite signs by convention, anodic currents are taken as positive. 

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