Abstract: Leakage incidents occurred at girth welds of surface pipelines in Northwestern China's Sulige Gas Field due to localized corrosion and perforation, with distinct erosion-corrosion features observed on the external surface adjacent to perforation sites. Comprehensive failure analysis was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and micro-hardness testing to characterize the morphology, chemical composition, phase structure, and mechanical properties of failed pipe samples. The investigation revealed the corrosion mechanism and perforation evolution process. The results show that welding defects in the root pass created pits on the pipe's inner wall where condensate of moisture in the pipe preferentially accumulated. A small amount of CO₂ gas in the transported medium dissolved into the liquid accumulation, forming a corrosive electrolyte that initiated localized corrosion at the weld. Additionally, porosity defects within the weld zone facilitated progressive corrosion penetration, causing gradual wall thinning. Under the pressure differential between the internal and external environments, perforation occurred, resulting in localized leakage. After the perforation of the pipe wall, the fluid inside the pipe formed high-speed leakage outward, and the scouring effect of high-speed leakage caused the perforation to continuously expand, leaving distinct erosion-corrosion marks on the adjacent external surface.