Evans diagram iron system

The data discussed in Figs. 20 through 24 represent the type of data in a polarization curve combinations of potential and applied current density. Figure 26 shows a complete polarization curve for the iron in acid systems for which the Evans diagram is shown in Fig. 25. The Evans lines are included as dotted lines in the figure. The difference between the Evans diagram and the polarization curve is that the polarization curve data display applied current densities, whereas the Evans diagram displays the reaction rates in terms of current densities. 

A preliminary knowledge of which reaction steps could be key in determining the overall corrosion rate can be assessed by measurements of Corr as a function of important system parameters, e.g., oxidant concentration, solution composition, temperature. The proximity of ACOrr to either eM/Mn+ or /Red can indicate which of the two half-reactions may be rate determining. This is illustrated in Fig. 3A, which shows an Evans diagram for the combination of a fast anodic reaction coupled to a slow cathodic one. The corrosion of iron or carbon steel in aerated neutral solution would be an example of such a combination. The anodic reaction requires only a small overpotential (1) = /Mn+ - Ecorr) to sustain the corrosion current, /COrr, compared to the much larger overpotential required to sustain the cathodic reaction at this current. The anodic reaction would... 

Graphical and analytical determination of the corrosion current is explained in detail in Chapter 3. The Evans diagram for an iron system in an oxygen-saturated solution with a pH of 7 is shown in Fig. 1.2. 

Fig. 1.2 Evans diagram for iron system in oxygen-saturated solution.

Solving these two equations for a particular value of angular velocity, we obtain the corrosion potential and the corrosion current. Figure 3.16 shows the Evans diagram plotted for various angular velocities. As noted from the plot, below 600 rpm the anodic and the cathodic slopes intersect closer to the equilibrium potential of iron, while beyond 600 rpm, no matter how much the rotation speed is increased, the corrosion potential is always equal to the corrosion potential of the system when there are no diffusion limitations (shown in the cathodic dark line). 

The Evans diagram shows the electrode potential in volts in the ordinate and the reaction rate (ampere) in the abscissa (Fig. 3.8). Consider a base metal, such as iron or zinc, placed in acidic solution the metal will dissolve at the same rate as hydrogen is evolved and the two reactions mutually polarize each other. This is shown in a very simple form by Evans diagram. In Evans diagram, either current or current density can be plotted against potential. If the ratio of anodic to cathodic areas is taken as unity, current density (i) may be used rather than current. A negligible resistance is assumed between the anode and cathode. As the change in the anodic and cathodic polarization has the same effect on corrosion current, the system is considered to be under mixed control. 

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