Butler-Volmer equation corrosion rates

The introduction of 0 in the equations for current density need by no means refer only to the adsorbed intermediates in the electrode reaction. What of other entities that may he adsorbed on the surface For example, suppose one adds to the solution an oiganic substance (e.g., aniline) and this becomes adsorbed on the electrode surface. Then, the 0 for the adsorbed organic substance must also be allowed for in the electrode kinetic equations. So, in Eq. (7.149), the value of 0 would really have to become a 0, where the summation is over all the entities that remain upon the surface and block off sites for the discharging entities. Many practical aspects of electrodics arise from this aspect of the Butler-Volmer equation. For example, the action of organic corrosion inhibitors partly arises in this way (adsorption and blocking of the surface of the electrode and hence reduction of the rate of the corrosion reaction per apparent unit area).67... 

The concept of the corrosion potential can also be illustrated in a Knear i -E plot. Figure 7 shows two curves representing the Butler-Volmer equation for the metal and hydrogen reactions. The pointat which the rate of metal dissolution equals the rate of hydrogen evolution is the potential at which the metal curve is as high above the potential axis as the hydrogen curve is below the axis. That distance is the corrosion rate. 

Here again, the concentration of the species might enter additionally into the rate equation which catalyze the cation transfer. This situation is often encountered for metal corrosion as, e.g., in the OH catalysis of active iron dissolution. In the following part, the Butler-Volmer equation will be developed for a simple redox process. For metal dissolution and deposition the rate equation is similar. The concentrations [Red] and... 

Equation (1) has a form analogous to the Butler-Volmer equation of electrode kinetics therefore, it is not surprising that the techniques of determinations are analogous in many ways to those for the determination of exchange current density in electrode kinetics (see, for example. Ref. 15 and 16). The corrosion rate measurement techniques can be classified in two ways. First, one can consider the different ways that Eq. (1), or its equivalent, is used to calculate the corrosion current density from measured current-density-polarization data. Second, one can consider the different ways that the current-density-polarization data are measured experimentally. The first classification is used in this chapter. The second classification is discussed briefly in Section II.7. 

For many reactions, the charge transfer is only one elementary step in a sequence of many others. Some substances break chemical bonds and form new ones. Oxygen reduction is a relatively complicated process with several intermediate species corresponding to a sequence of reaction steps. Nevertheless, a corrosion reaction is often ruled by the Butler-Volmer equation, although reaction steps other than the charge transfer may be rate determining as well. 

Friedrich and Filers [113] have proposed a corrosion model of development and derived electron transfer equations based on the Butler-Volmer expression which can be simplified into three cases. Case 1 is when both the forward and reverse processes of developer oxidation are important. Case 2 is when the net rate is limited by the forward rate of developer oxidation. Case 3 corresponds to a rate which is limited by the kinetics of both developer oxidation and silver halide reduction. 

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