9. Macrocell Corrosion Mechanism

When a macrocell is formed in a corrosion process, an electrical field is established in the environment because a net current flows from the anode to the cathode, which are physically separated. This situation occurs in galvanic corrosion, differential aeration, localized attacks such as pitting and crevice, and cathodic protection. Potential and current distributions are extremely important because they determine the corrosion rate. Analytical solutions of electric fields exist only for very simple geometry and simplified conditions. In the last two decades, the use of numerical calculations based on Finite Element Methods (FEM) has overcome these difficulties. This Chapter gives an overview of macrocell electrical field and current distribution, giving analytical solutions for both quantities in simple geometries, such as inside and outside a pipe. In some of these geometries the throwing power is also evaluated.