Evans diagram zinc corrosion

Figure 1.3 Evans diagram for corrosion of zinc as a function of pH. (Reproduced by permission, Elsevier Ltd., (2).)...

The concept of polarization in a corrosion cell can be explained by considering a simple galvanic cell, such as a Daniel cell, with copper and zinc electrodes. The Evans diagram of a Daniel cell shown in Fig. 3.5 is the basis for understanding the underlying corrosion process kinetics [26,27]. 

Case Study 3.1—Construction of the Evans Diagram for the Corrosion of Zinc... 

Electrochemical corrosion systems can be characterized using the kinetic parameters previously described as Tafel slopes, exchange and limiting current densities. However, the mixed potential theory requires a mixed electrode system. This is shown in Eigure 5.1 for the classical pure zinc (Zn) electrode immersed in hydrochloric (NCl)acid solution [1,8-9]. This type of graphical representation of electrode potential and current density is known as Evans Diagram for representing the electrode kinetics of pure zinc. 

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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