Repaired reinforced concrete structures often suffer from a reinforcement corrosion problem known as the incipient anode (or halo) effect. The diagnosis of this problem is widely reported to be macrocell activity caused by small changes in reinforcement potential in the parent and repair concrete. Cathodic protection is a technique that is used to prevent or arrest such corrosion by inducing small potential changes on the reinforcement. This study looks at the effects potential in both causing and arresting corrosion in concrete. Potential measurements on repaired structures suggest that potential changes on passive steel cannot cause corrosion. Further, the protective effects of a negative steel potential shift induced by the relatively small practical current densities applied in cathodic protection are negligible when oxygen has easy access to the reinforcement. Other factors are suggested to affect both of these processes.