What Is Overpotential

Thermodynamic principles can explain a corrosion situation in terms of the stability of chemical species and reactions associated with corrosion processes. However, thermodynamic principles cannot be used to predict corrosion currents or corrosion rates. In reality, polarization effects control the cathodic and anodic currents that are integral components of corrosion processes. 

When two or more complementary processes such as those illustrated in Chap. 3 occur over a single metallic surface, the corrosion potential that results from such situations is a compromise between the various equilibrium potentials of all the anodic and cathodic reactions involved. The difference between the resultant potential (E) and each individual reaction equilibrium potential (E ) is called polarization and is quantified in terms of overpotential described in Eq. (5.1)  

The polarization is said to be anodic when the anodic processes on the electrode are accelerated by moving the potential in the positive (noble) direction or cathodic when the cathodic processes are accelerated by moving the potential in the negative (active) direction. There are three distinct types of polarization and these are additive, as expressed in Eq. (5.2)  

Copyright 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use. 

---