Current-potential curves partial reactions

In general, according to Eq. (2-10), two electrochemical reactions take place in electrolytic corrosion. In the experimental arrangement in Fig. 2-3, it is therefore not the I(U) curve for one reaction that is being determined, but the total current-potential curve of the mixed electrode, E,. Thus, according to Eq. (2-10), the total potential curve involves the superposition of both partial current-potential curves ... 

Fig. 1. Current-potential curves for a generalized electroless deposition reaction. The dashed line indicates the curve for the complete electroless solution. The partial anodic and cathodic currents are represented by ia and ic, respectively. Adapted from ref. 28. 

Wagner-Traud Diagram, According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential i-E) curves, as shown schematically in Eigure 8.2. First, there are two current-potential curves for the partial reactions (solid curves) (1) ic =f(E), the current-potential curve for the reduction of ions, recorded from the rest potential E eq M the absence of the reducing agent Red (when the activity of is equal to 1, eq,M E m) and (2) = f(E), the current-potential... 

Electroless Deposition of Copper. The basic ideas of the mixed-potential theory were tested by Paunovic (10) for the case of electroless copper deposition from a cupric sulfate solution containing ethylenediaminetetraacetic acid (EDTA) as a complexing agent and formaldehyde (HCHO) as the reducing agent (Red). The test involved a comparison between direct experimental values for and the rate of deposition with those derived theoretically from the current-potential curves for partial reactions on the basis of the mixed-potential theory. 

Electroless Deposition in the Presence of Interfering Reactions. According to the mixed-potential theory, the total current density, is a result of simple addition of current densities of the two partial reactions, 4 and However, in the presence of interfering (or side) reactions, 4 and/or may be composed of two or more components themselves, and verification of the mixed-potential theory in this case would involve superposition of current-potential curves for the electroless process investigated with those of the interfering reactions in order to correctly interpret the total i-E curve. Two important examples are discussed here. 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

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) ... 

As mentioned in Sec. 2.1, simple superposition of the respective current-potential curves for the two partial reactions does not always yield the curve obtained with a complete electroless bath. This is illustrated in Fig. 21 [126], in which the current-potential curve obtained with a complete electroless copper bath containing EDTA (curve 1) is compared with the curve obtained in solution in the absence of formaldehyde (curve 2) and with that obtained in solution in the absence of Cu(II) (curve 3). All three curves were recorded at room temperature with a copper disk electrode rotating at 2100 rpm, while scanning the potential in the positive direction... 

In Fig. 9 we consider two simultaneous electrode reactions (O/R and O /R ) with transfer of n and n electrons, respectively and formal equilibrium potentials Ef and E. At open circuit, the mixed potential, Em, adopts intermediate values < Em < closer to the equilibrium potential of the faster partial reaction. The overall current-potential curve is indicated by the broken line. Notice that Em is not determined by the thermodynamic values of E and Ef but by the kinetics of the respective reactions, that is, by the respective anodic and cathodic component curves in Fig. 9 with the condition of Eq. (104). These curves may be altered by mass transport, surface area and specific... 

Fig. 2-4 Current-density-potential curves for an electrochemical partial reaction as in Eq. (2-35).

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

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). 

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... 

This section introduces you to the factors that determine the overall rate of an electrode reaction in a system which is not stirred. This allows predictions of the shape of the resulting current/WE potential curves for a system which is under diffusion control. The work of llkovic in 1934 in deriving the current/analyte concentration relationship for the DME is covered and the llkovic equation is stated and partially derived. The Heyrovsky-Ilkovic equation (1935) is then derived this provides an explanation of the shape of the current WE potential curve. This curve now becomes a polarogram and the half-wave potential is defined and related to the polarogram. Finally the question of the reversibility of the electrode reaction is discussed and tests for reversibility are given. 

FIGURE 20.10 Superposition of the anodic and cathodie partial reactions to form a current density-potential curve. 

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