Sum cathodic curves

The individual curves (M, H, and W), the sum cathodic curve (SC), and the net curves (N) are shown in Fig. 5.9. The net curves only are shown in Fig. 5.10. The net curves pass to very low values and become zero at Ecorr- being net cathodic below this potential and net anodic above Ecorr- It is evident from the net curves (Fig. 5.10) that Ecorr is easily determined but that iCOrr would be estimated by extrapolation of the Tafel region of the cathodic curve to Ecorr- Also, the portion of the cathodic polarization curve above ECOrr and the portion of the anodic curve below ECorr must be estimated by extrapolation of the experimentally determined portions (Fig. 5.9 and 5.10). 

Again, emphasis is placed on the fact that experimental poten-tiodynamic scans measure only the net current densities (i.e., the difference between the sum anodic and sum cathodic curves). The net curves... 

Fig. 5.13 Relative positions of anodic metal polarization curve, M, and sum cathodic curve, SC, for cathodic oxygen and hydrogen-ion...

Fig. 5.16 Effect of reducing dissolved oxygen concentration such that the sum cathodic curve, SC, intersects theanodic polarization curve for the metal, M, at three positions. pH = 1. P0 = 0.05 atm...

Interpretation of an experimentally determined polarization curve, including an understanding of the information derivable therefrom, is based on the form of the polarization curve, which results from the polarization curves for the individual anodic and cathodic half-cell reactions occurring on the metal surface. These individual polarization curves, assuming Tafel behavior in all cases, are shown in Fig. 6.2 (dashed curves) with Ecorr and the corrosion current, Icorr, identified. It is assumed that over the potential range of concern, the Iox x and Ired M contributions to the sum-anodic and sum-cathodic curves are negligible consequently, Uox = Iox M and Ured = Ired x. At any potential of the... 

In this section, the relative positions of several schematic anodic and cathodic curves are presented. The sum anodic (X iox) and sum cathodic (X ired) curves are shown relative to the individual curves, and then the net curves are shown as representative of what would be observed experimentally. Figure 5.8 shows an anodic polarization curve (M) repre-... 

Figure 1.32 shows the E-i curves for a metal corroding in an acid in which both dissolved oxygen and HjO ions act as cathode reactants (note that in order to illustrate the summation of the partial currents to give the total current i, = i hi + 02. I rather than log i has been used). The total cathodic curve ABCD is the sum of the partial cathodic currents, and it can be seen that the corrosion potential due to the total cathodic current is more positive than either co.t..hj or However, whereas in the case of oxygen... 

When a metal, M, corrodes in a solution, there must be at least one oxidation and one reduction process. What is measured is the sum total of all partial cathodic processes and partial anodic processes occurring during corrosion of a metal. An anodic curve represents the sum total of all partial oxidation processes and a cathodic curve, the sum total of all partial reduction processes. The point of intersection of anodic and cathodic polarization curves in an Evans diagram gives the mixed potential Ecorr (corrosion potential), also called the compromise potential, or mixed potential, or free corrosion potential, and the corrosion current (icorr)-... 

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. 

The solid curve is the sum of the cathodic and anodic components, which are represented by the respective dashed lines. 

As discussed in detail in Chapter 2, the corrosion potential is determined by the intersection of the sum of the anodic Evans lines and the sum of the cathodic Evans lines. For active-passive materials, the only new wrinkle is the increased complexity of the anodic line. Since the anodic line is not single-valued with respect to current density, three distinct cases can be considered. In all cases, the condition E /a = X Ic determines the position of the corrosion potential, and the condition im = z a - ic determines the appearance of the polarization curve... 

Fig. 5.8 Schematic representation of relative positions of anodic metal, cathodic hydrogen, and cathodic water polarization curves, pH = 1. Curve M, anodic polarization for metal (e.g., Fe-18% Cr) curve H, cathodic polarization for H+ curve W, cathodic polarization for H20 curve SC, sum of H+ and H20 polarization...

Fig. 5.10 Schematic representation of the net anodic and cathodic polarization curves, N, for the anodic metal, M, and for the cathodic hydrogen, H, polarization curves. Note that the net curves deviate from curves M and H only near Ecorr. SC is the sum of cathodic polarization for H+and H20. pH = 1...

Fig. 5.12 Sum (SC) of cathodic oxygen, hydrogen, and water polarization curves of Fig. 5.11. Oxygen curve dominates above -300 mV (SHE) and hydrogen curve below-300 mV (SHE). Water reduction makes negligible contribution to the current density. pH = 1.Po2 =0.2 atm...

The corrosion current density and the potential are in this case most easily determined with a simple graphical analysis as shown in Figure 4.8. The thick curve shows the sum of cathodic current densities in the potential range where both reactions contribute. 

The actual corrosion situation is defined by the crossing of the anodic reaction curve (Equation 19.1) and the sum curve for the cathodic reactions (Equations 19.2 and 19.3). This corresponds to a corrosion potential of E orr and a corrosion current density /<.o 0 = /cor/Area). The corrosion rate is proportional to the current density... 

The net current is the algebraic sum of the cathodic and anodic currents (see Section 4), corresponding to the reduction and oxidation reactions. However, in polarography the solution usually contains only one electroactive species and the contribution from the reverse reaction can usually be neglected in the case of many organic reactions which behave irreversibly. It can be seen that as the applied potential is increased the cathodic current increases exponentially until it becomes diffusion limited, and this is shown in the current-potential curve, or polarogram (Figure 5). 

The basic operation of the LPR instrument uses two electrodes made of the material of interest, i.e., carbon steel. Admiralty, 90 10 copper nickel, etc. One of the electrodes acts as an anode, the other as a cathode. During the operation of the instrument, a very small voltage of 20 mV is applied across the electrodes. In this region, the polarization curves approach linearity, hence the term LPR. The applied potential divided by the measured current i is a resistance term called Polarization Resistance, or Rp. The resistance to current flow between the two electrodes of the LPR probe is the sum of polarization resistance values at each electrode and the resistance of the solution between the electrodes (Rj). 

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