Influence of water content on corrosion behavior of repurposed gaseous CO₂ pipeline weld joints and corresponding corrosion rate control criterion

When redundant oil and gas pipelines are repurposed for gaseous CO₂ transport, weld joints become highly susceptible to internal corrosion due to microstructural inhomogeneity, residual stress concentration, and legacy corrosion product films. Establishing the corrosion evolution behavior and control mechanism of weld joints under varying water contents is essential for defining corrosion rate control criteria for these repurposed pipelines. Tests were conducted on newly-built and repurposed X65 pipeline weld joint specimens using the weight loss method and microscopic characterization. Under gaseous CO₂ conditions(50℃, 5 MPa), water mole fractions were set at 0.01%, 0.04%, 0.07%, and 0. 10%, respectively. The uniform corrosion rate, corrosion morphology, and FeCO₃ formation behavior were analyzed, and a risk classification model was established based on the corrosion depth-to-wall thickness failure criterion. Under low water content conditions(up to 0.07%) where a continuous water film was not yet formed, micro-galvanic coupling frequently occurred in repurposed specimens due to pores and cracks in legacy corrosion product films. Consequently, these specimens exhibited higher corrosion rates than newly-built ones. However, at 0. 10% water content, a stable liquid film facilitated rapid interfacial Fe²⁺ supersaturation, promoting FeCO₃ deposition that healed film defects. Under these conditions, the repurposed pipeline corrosion rate fell below that of newly-built specimens. As the corrosion product morphology evolved from dispersed clusters to a continuous layer, 0.07% was identified as the critical water content for the transition from crack-accelerated to deposition-controlled corrosion. A risk classification standard for uniform corrosion rate in repurposed gaseous CO₂ pipeline weld joints was established based on the corrosion depth-to-wall thickness failure criterion. The results indicated that to ensure long-term safe operation, corrosion rates must be maintained below 0. 12 mm/a, requiring CO₂ water content to remain under 0.07%. These findings align with current internal corrosion control standards and provide a technical basis for the life management of repurposed gaseous CO₂ pipelines.