Mixed-potential theory Evans diagram

The rate of deposition and the mixed potential are determined on the basis of the mixed-potential theory using the Evans diagram. First, the current-potential curve... 

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... 

It is tempting to say that the farther apart two metals are in the galvanic series, the more corrosion will occur on the more negative material, or anode. Unfortunately, this is not the case. In order to predict the rate of galvanic corrosion, an Evans Diagram of both metcds involved must be considered. This approach is called Mixed Potential Theory. 

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. 

---