Partial polarization curves

Figure 2. Partial polarization curves for (a) hydrogen evolution and (b) metal dissolution (c) polarization curve due to superposition of the two processes. The limiting diffusion current for curve b is influenced by stirring with hydrogen.

Other research in the field of simultaneous dissolution has focused on the active dissolution of Fe—Cr alloys, which was shown to proceed in the simultaneous mode at quasi-steady state conditions [40]. Applying y-spectroscopic methods, Kolo-tyrkin [41] measured the partial anodic polarization curves of the components Fe and Cr and was able to show that the dissolution rate of Cr from the alloy is more decreased than would have been expected on the basis of its bulk mole fraction (that is, Cr becomes the slow-dissolving component), and the contrary is true for the dissolution of Fe. This implies an enrichment of the Cr in the corroding alloy surface that may promote its subsequent passivation [34]. Also, with increasing Cr concentration of the alloy, the Tafel slope of the partial polarization curves of the components was shown to change from values that are typical for pure Fe to values that are typical for pure Cr [40, 41]. It appears, therefore, that for Fe—Cr alloys, the dissolution of the alloy components occurs in an interdependent... 

Mishra et al. [198] discussed in an exemplary way the dark and photocorrosion behavior of their SnS-electrodeposited polycrystalline films on the basis of Pourbaix diagrams, by performing photoelectrochemical studies in aqueous electrolytes with various redox couples. Polarization curves for the SnS samples in a Fe(CN) redox electrolyte revealed partial rectification for cathodic current flow in the dark, establishing the SnS as p-type. The incomplete rectification was... 

The straight lines for the partial CD t andT in Fig. 63b intersect at the equilibrium potential AE = 0. The value of CD corresponding to the point of intersection is that of the exchange CD f, according to Eq. (6.11). It follows that the exchange CD can be determined when the linear sections of the anodic or cathodic polarization curve, which have been measured experimentally and plotted as log i vs. AE, are extrapolated to the equilibrium potential. Moreover, according to Eq. (6.19) the exchange CD can be determined from the slope of the polarization curve near the equilibrium potential when the curve is plotted as i vs. AE. 

FIGURE 13.2 Polarization curves for the partial current densities of reactions involving the metal and hydrogen, and the polarization curves for the overall current density. 

Figure 1T2 shows anodic d cathodic polarization curves for the partial CD of dissolution 4 and deposition 4 of the metal and for the partial CD of ionization 4 and evolution 4 of hydrogen, as well as curves for the overall reaction current densities involving the metal (4) and the hydrogen (4). The spontaneous dissolution current density 4 evidently is determined by the point of intersection. A, of these combined curves. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

The surface of the base metal is anodically polarized under the effect of local cells. For a graphical analysis of the phenomena, one must construct the polarization curves for the partial currents at the base metal as well as the overall anodic 4 vs. E curve reflecting the effective rate of dissolution of this metal under anodic polarization. The rate of the cathodic process, 4, at the inclusions is described by the corresponding cathodic polarization curve (since the surface areas of anodic and cathodic segments differ substantially, currents rather than current densities must be employed here). At open circuit the two rates are identical. 

As demonstrated in Section 5.2, the electrode potential is determined by the rates of two opposing electrode reactions. The reactant in one of these reactions is always identical with the product of the other. However, the electrode potential can be determined by two electrode reactions that have nothing in common. For example, the dissolution of zinc in a mineral acid involves the evolution of hydrogen on the zinc surface with simultaneous ionization of zinc, where the divalent zinc ions diffuse away from the electrode. The sum of the partial currents corresponding to these two processes must equal zero (if the charging current for a change in the electrode potential is neglected). The potential attained by the metal under these conditions is termed the mixed potential Emix. If the polarization curves for both processes are known, then conditions can be determined such that the absolute values of the cathodic and anodic currents are identical (see Fig. 5.54A). The rate of dissolution of zinc is proportional to the partial anodic current. 

Figure 1 shows a generalized representation of an electroless deposition process obeying MPT [28]. Polarization curves are shown for the two partial reactions (full lines), and the curve expected for the full electroless solution (dashed curve). The polarization curve for anodic and cathodic partial reactions intersect the potential axis at their respective equilibrium potential values, denoted by / j]cd and respectively. At Emp, the anodic and cathodic partial current densities are equal, a... 

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

When there was no electric field on the condenser the light from the particles, on entering the chamber, was partially polarized as would be expected from the observations of Stark. This polarization decreased as we pass along the bundle at a rate that could be easily observed. This behavior is shown in figure 1. The upper curve shows the decrease of the in-... 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

Though processes occurring under photopassivation have not so far been understood in detail, they may be related with certainty (Izidinov, 1979) to the acceleration, under illumination, of one of the two conjugated reactions, which constitute the overall process of electrochemical corrosion. Depending on the initial state of corroding silicon, either the anodic (at the active surface) or the cathodic (at the passive surface) partial reaction is accelerated. This leads to the shift of the potential, and the system jumps over the maximum of the polarization curve from one stable state to the other. 

Corrosion — Corrosion current density — Figure. Polarization curves of a metal/metal ion electrode and the H2/H+ electrode including the anodic and cathodic partial current curves, the Nernst equilibrium electrode potentials E(Me/Mez+) and (H2/H+), their exchange current densities / o,M> o,redox and related overpotentials Me) and 77(H), the rest potential r, the polarization n and the corrosion current density ic at open circuit conditions (E = Er) [i]... 

Equation (7) is valid for the complete active electrode surface. On the other hand, if the inert substrate is partially covered with the same active material, the polarization curve equation is given by (44).7... 

The polarization characteristic of a partially covered inert macroelectrode is easy to determine, but it is very difficult or even impossible to do the same for microelectrodes placed on it. On the other hand,30 the morphology of metal electrodeposits indicates the conditions of deposition. Hence, the type of process control on the microelectrodes can be derived from their morphology and correlated with the polarization curve for the partially covered macroelectrode. 

A completely new approach to the analysis of experimental data is introduced by the use of the complete polarization curve equation and by the method of digital simulation. It was possible in this way to elucidate the polarization behavior of the partially covered inert... 

---