Abstract: Corrosion failure was found in the course of acid pickling after welding of a flange with a pipe. The corrosion phenomenon at the central hole of the flange was analyzed by means of macroscopic examination, chemical composition analysis, metallographic examination, SEM analysis, heat treatment simulation. The results show that intergranular sensitization is found on the central hole of the flange after welding, the resistance to intergranular corrosion of the flange is seriously weakened and it finally causes the intergranular corrosion thinning in the course of acid picking.