Objective To study the internal AC corrosion of old strain clamp. Methods The strain clamp which has been run for 36 years was cut and dissected. Scanning electron microscope(SEM), X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to observe and analyze the microstructure, elements and corrosion products in order to explore the evolution of internal corrosion damage of strain clamps. Results Lots of white corrosion product and black covering occurred on the inner surface of strain clamp and surface of aluminum conductor. White corrosion product presented two forms, powder and slice. The product consisted of Al2O3, AlO(OH) and AlOOH. The black covering reduced with increasing depth. The main elements were Al, O and C, and the phases were Al2 O3 , Carbon (graphite) and organic compounds. The aluminum conductor was corroded more seriously near the steel core where the Zn protective layer was damaged. The corrosion region was black. There were some iron-rich particles embedded in other corrosion products. The main elements were Al, C, O, Fe and Zn in the corrosion region, and the existing form of Fe element was Fe2 O3 . The local corrosion of the surface of the steel core was serious. It exhibited a planar pitting morphology. The main elements were Zn, Al, C and O, and the corrosion product of Zn was hexagonal wurtzite ZnO. Conclusion Liquid entering into strain clamp resulted in serious internal AC corrosion and produced a lot of heat, which decomposed the seal material.