Reinforced concrete has been used in civil infrastructure development for several decades. However, steel bars used in some concrete structures are easily corroded by chlorides. In particular, when an existing concrete structure, after a patch repair, has a large quantity of unremoved chloride, local corrosions are generated in the steel bars. The purpose of this study is to analyze the macrocell corrosion mechanism in a reinforced concrete structure after a patch repair and to determine the influence of corrosion on structural performance. Additionally, a method to continue safely using reinforced concrete after a patch repair is proposed. The results of this study suggest that theoretically, the macrocell corrosion rate is high, based on an intersection point of an anodic and a cathodic polarization curves. It is also found that the macrocell corrosion increases when the chloride ion content exhibits a large difference. Additionally, it can be confirmed that increasing the extent of corrosion decreases the bending fracture load of the beam. Finally, a method is proposed to control macrocell corrosion.