Evans diagrams processes

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

Fig. 1.28 Evans diagram illustrating a corrosion process (e.g. a bimetallic couple) in which the area of the cathode is not equal to that of the anode, (o) so that and (b) > S(,...

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

An alternative method of presenting the current-potential curves for electroless metal deposition is the Evans diagram. In this method, the sign of the current density is suppressed. Figure 22 shows a general Evans diagram with current-potential functions i = f(E) for the individual electrode processes, Eqs (43 and 44). According to this presentation of the mixed-potential theory, the current-potential curves for individual processes, ic = iu = f(E) and ia = = f(E), intersect. The... 

The study of corrosion processes in recent years has been almost entirely based on the so called Evans diagram. Considering how often this diagram has been misused or misrepresented suggests that a brief discussion may again be in order at the risk of being repetive or trivial. 

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]. 

The Evans diagram is also very useful in estimating the current required in the external circuit to stop the process of corrosion. If an external current is appHed cathodicaUy (negative current), the potential on the cathodic polarization line crosses the equihbrium potential of the anode and the anodic reaction is not thermodynamically feasible. Thus, the corrosion process stops. This process is the basis of cathodic protection and is discussed in Chapter 15. 

The i-E curves used in this chapter are related to the Evans diagram, beloved by corrosion chemists, but follow the conventions used throughout the rest of this book. While they are greatly simplified polarization diagrams, their usefulness lies in their ability to predict and rationalize the kinetics of corrosion processes. The figures show, in a diagnostic fashion, the following features ... 

In order to achieve either an anodic or cathodic process, it is necessary to apply an overvoltage ri). If AG (activation energy) for the reaction is small, the over-voltage required is small and if it is large, the over-voltage required is also small. In the Evans diagram, the relationship between log I and p is observed to be linear for activation polarization ... 

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)-... 

The predictions of the CM model are exactly the same. In line with a simple Bell-Evans-Polanyi diagram (e.g. Fig. 18), stabilization of the product configuration leads to an earlier transition state, while stabilization of an intermediate configuration leads it increasingly to mix into the transition-state wave-function. For example, stabilization of the carbocationic configuration [36] results in the transition state acquiring more of that character so that an E2 process becomes more El-like (Fig. 266). 

Fig. 11. Universal Minerals and Metals Inc., simplified flow diagram for production of copper powder from scrap, ammonium carbonate process. Evans (ElO).

---