11331 Mixed Potential Theory and Evans Diagrams

The mixed-potential theory was developed to address the problem of several electrochemical reactions proceeding simultaneously at the same metal-solution interface. The rest potential (corrosion potential) of, for example, iron immersed in an acidic solution, is a mixed potential and lies between the equilibrium potentials of the two participating reactions (i.e., anodic dissolution and hydrogen evolution).

The Evans diagram is a kinetic diagram representing electrode potential in volts versus corrosion current in amperes per unit area. Figure 11.3 presents an Evans diagram for iron immersed in an acidic solution. Theoretically, four reactions are possible for this system, that is, iron dissolution to from ferrous ions (Fe2+), the reverse of this process, that is, ferrous ions attaining electrons to form Fe, hydrogen ions in solution forming hydrogen gas or the reverse of this process. Hence four E vs. i lines are presented in Fig. 11.3. For clarity the two thermodynamically feasible reactions are given as solid lines. An extrapolation of the portions of the two solid lines gives rise to an intersection at the corrosion potential (Ecorr) and corrosion current density (icorr) for the given system. Evans diagrams provide kinetic information, that is, the rate of a corrosion reaction, as a function of the applied potential.

The corrosion potential Ecorr is in effect the line (on the Pourbaix diagram) that differentiates the corrosion and immunity regions. That is, if the potential is held below Ecorr in Fig. 11.3, the rate of iron dissolution decreases and the hydrogen generation reaction dominates. On the other hand if the potential is held above Ecorr, the metal dissolution reaction dominates and the rate of corrosion increases. Maintaining the potential below that of Ecorr is adopted in practice and is known as cathodic protection (see Section 11.7.3)

0 0

Post a comment