Many systems benefit from the ability to autonomously signal the occurrence of damage. The development of smart polymer coatings on metals can address scientific challenges such as nondestructive detection of early corrosion to avoid further destruction of materials. Here, pH- responsive polymer coatings on metals such as steel, aluminum, magnesium, and copper alloys are reported. The defect areas of coatings can gradually exhibit strong fluorescence as the corrosion starts. Based on the fundamental understanding of electrochemical mechanisms in metal corrosion, the designed pH-responsive polymer coating is dormant before crack occurrence. However, the on-demand release of fluorescent molecules from nano-containersin coatings occurs as corrosion proceeds with increasing pH value by transformation into highly active fluorescence indication from the dormant state at the stage of corrosion commencement. The developed smart polymer coatings can report the corrosion caused by a coating failure which provides a new strategy for nondestructive corrosion detection.