Abstract: The zirconium composite layer in CPE reactor cracked at the weld. The failure cause was analyzed by optical microscopy, EDS and XRD. The results show that the main corrosion forms of the internal zirconium composite layer include pitting, intergranular corrosion and corrosion due to the second phase. The main reason for the occurrence of pitting is the formation of an activation-passivation corrosion cell between the metal substrate and the oxide film, so that the corrosion continuously extended from the surface to the inside, forming punctiform pores. The main cause of local severe corrosion is insufficient gas protection during welding, poor thermal conductivity during welding or excessive welding parameters, resulting in the formation of zirconium dioxide at the weld and then resulting in the electrochemical inhomogeneity between grain boundaries and grains. The inhomogeneity produced intergranular corrosion.