Abstract: The cooler of a converter transfomer located in an extra-high voltage substation leaked oil. After inspecting the cooler, it was found that a large number of leakage points distributed in 3003 aluminum alloy oil pipes. Stereomicroscopy, optical microscopy, spectrography, SEM, EDS and other techniques were adopted to analyze the reasons causing leakage of the oil pipe. Results indicate that there were several corrosion pits on the inner surface of the aluminum alloy pipes, and the corrosion products contain high concentration of S element and Cl element. The corrosion process made the thickness of aluminum alloy pipes thin. Eventually, the aluminum alloy pipes were totally corroded in the direction of thickness, which resulted in the oil leakage.