Abstract
A mathematical model was developed to predict the movement of ions resulting from the flow of direct current through concrete. The model is based on applying a differential mass balance and incorporating flux equations based on dilute solution theory. Electrochemical reactions establish flux equations which determine the boundary conditions at the electrode surfaces. The model was used to predict the effects of different variables on the development of chloride and hydroxide concentration profiles. Variables examined include chloride concentration, chloride distribution, pH, temperature, current density, and chloride and hydroxide diffusion coefficients. The chloride/hydroxide ratio at the surface of the steel was also determined as a function of time. At cathodic protection current densities, the chloride/hydroxide ratio was found to drop significantly over the first few thousand hours of operation. Results of the model were combined with corrosion rate experiments to predict the proper cathodic protection current at various conditions. This current was found to be related to chloride concentration to a good approximation. A constant current criterion for cathodic protection is suggested based on this relationship.
