Cathodic polarization curve potential portions

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). 

Using the example of iron corroding in a hydrochloric acid solution, if the iron sample is maintained at the natural corrosion potential of —0.2 V, no current will flow through the auxiliary electrode. The plot of this data point in the study would equate to that of A or C in Figure 1.5. As the potential is raised, the current flow will increase and curve AB will approximate the behavior of the true anodic polarization curve. Alternatively, if the potential were lowered below —0.2 V, current measm-ements would result in the curve CD and approximate the nature of the cathodic polarization curve. By using the straight line portions, or Tafel regions, of these curves, an approximation of the corrosion ciurent can be made. 

The error of the Tafel-plot technique due to the neglect of the double-layer effect was reported to be unpredictable. The doublelayer effect distorts both the anodic and cathodic polarization curves, not necessarily to the same degree this results in possible cancellation or enhancement of the error caused by the doublelayer effect. Furthermore, because the Tafel lines are distorted and may not even have a truly linear portion in any potential range, the results strongly depend on the rather arbitrary drawing of a straight line through the data points. 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Unlike the cathodic portion of the polarization curve, the anodic portion of the curve in Fig. 3(b) does not exhibit clear Tafel-type behavior. The mechanism for Fe dissolution in acids is quite complex. A line can be drawn in the region just above the corrosion potential, giving a Tafel slope of 34 mV decade k Extrapolation of this line intersects the zero-current potential at 7 X 10 A cm , a considerably different value than the extrapolation of the cathodic portion of the curve. This is not uncommon in practice. When this happens, it is usually considered that the anodic portion of the curve is affected by changes on the electrode surface, that is, surface roughening or film formation. The corrosion rate is typically determined from the extrapolated cathodic Tafel region. 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

Molybdenum exhibits unusual polarization behavior. The initial portion of the curve, shown dashed in Fig. 5.20, is very difficult to determine experimentally because it occurs at very low current densities indicating that the passive state is very rapidly established by traces of dissolved oxygen or by very low concentrations of other cathodic reactants. In fact, many of the published curves show only the transpassive range over which the current density rapidly increases. The implication is that as long as the potential is below 200 mV (SHE), the corrosion rate of molybdenum would be very low and this is observed. 

Linear Polarization (LP) as schematically shown in Figure 3.4 covers both anodic and cathodic portions of the potential E versus current daisity curve for determining Rp. 

Figure 31.1 shows a classic electrochemically measured Tafel polarization diagram [33. The Tafel analysis is performed by extrapolating the linear portions of both cathodic and anodic curves on a log (current) versus potential plot to their point of intersection. This intersection point provides both the corrosion potential con and the corrosion current density for the system unperturbed. This is a very simple yet powerful technique for quantitatively characterizing a corrosion process. The Tafel equation can be simplified to provide Eq. (7) by approximation using a power series expansion. 

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