Abstract: Dozens of kilometers of petroleum pipeline painted with pitch was buried underground for almost 8 years.A section of this pipe underground of a pond pored a hole in it.Great economic losses have been caused by the failure of the pipeline.It is urgently necessary to investigate the cause of this failure.Optical metallography,scanning electronic microscope,energy spectrum analysis and X-ray diffraction were used in the investigation of the fracture,microstructure,chemical composition and phases in the alloy.The possibility of arbitrary penetration of the steel pipeline was excluded by microstructure analysis and phases detective technique.The hole in the pipeline was made by localized corrosion.The major mechanism in this failure case is occluded cavity corrosion.In the corrosion process,the corrosion is accelerated by a high cathode and low anode.