A detailed, numerical prediction of the time-domain response to external excitation of a system with macroscopic but unevenly distributed corrosion is presented. Model development for a corrosion macrocell on a steel bar in concrete subjected to a potential-step excitation is used as an illustration of the computational approach. The electrolyte is considered to be purely resistive, while the metal-steel interface is subject to nonlinear polarization boundary conditions, including Butler-Volmer kinetics for the cathodic and anodic reactions. A purely capacitive charge storage mechanism is assumed for the metal-electrolyte interface. The polarization of the cathodic reaction is subject to nonuniform mass transport through the electrolyte. The performance of the model is demonstrated in a simulated polarization experiment. Applications of the model for determining the capabilities of electrochemical test techniques for steel in concrete are examined.