Corrosion kinetics Evans diagram

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

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... 

The less well known (so-called) Evans diagrams involve kinetics and allow one to make a rough first cut at the order of magnitude of a corrosion rate (for a pure surface without blocking oxide films). 

It is well known that Pourbaix diagrams give the thermodynamic limits of corrosion. However, it is possible that corrosion in a system may be limited by kinetics to rates so low that corrosion that is thermodynamically possible can be neglected under practical circumstances. In this light, (a) construct an Evans diagram, i.e., a plot of the actual relevant electrode potentials against log i for... 

Figure 25 Evans diagram for Fe in acid showing use of conservation of charge to determine Eom and corrosion rate (7corr), given complete knowledge of the kinetic parameters involved.

Figure 11 Schematic Evans diagram illustrating the effect of a change in the cathodic reaction kinetics on the corrosion conditions. Case 1 would be representative of Fig. 5. Case 3 would lead to the polarization behavior described in Fig. 6. Case 2 would lead to the polarization behavior shown in Fig. 8. (After Ref. 71.)...

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

Fig. 3 Electrochemical framework for intergranular corrosion described by an Evans diagram depicting the anodic half-cell reaction kinetics for the grain boundary zone and the grain matrix. In this case, enhanced active dissolution occurs in both the grain boundary region and in the matrix. At a fixed potential, given by Egpp, the anodic dissolution rate is accelerated along the grain boundary compared to the matrix.

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 required quantities can be obtained from an Evans diagram for the corrosion of iron in hydrogen-saturated, oxygen-free solution. Assume that charge-transfer kinetics controls the reaction rates and that the high-field approximation applies. In this example, iron corrodes by the electrochemical reaction producing iron ions at the anode ... 

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 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 ... 

The reason for the success of Evans diagrams in corrosion is that they combine thermodynamic Victors ( values) with kinetics factors (i values). The usefulness of corrosion kinetics in the study of corrosion rates is, therefore, obvious. The exchange current densities have been included in the polarization diagram by Stem, and such diagrams are called Stem diagrams. Evans diagrams do not include exchange current densities. 

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

Figure 18.4 Evans diagram for the corrosion of iron in the presence of two simultaneous cathodic reactions. The dotted line represents the sum of the two cathodic currents for oxygen reduction and hydrogen evolution. pH =4.Jl — 0-63 mA cm". Other kinetic parameters are as in Figure 18.1.

---