Effect of fluid flow velocity on CO 2 /H 2 S corrosion mechanism in X65 carbon steel pipelines

The corrosion behavior of X65 carbon steel under combined CO 2 /H 2 S conditions was investigated at flow velocities of 0, 1.2 and 2.4 m s -1 over 28 days. Electrochemical tests and surface analyses revealed that the corrosion mechanism is governed by a velocity-dependent transition between flow-accelerated corrosion and scale-controlled corrosion. During the first week, increasing flow velocity significantly enhanced mass transfer and reduced the boundary-layer thickness, leading to higher corrosion current densities. Peak i(corr ) values of 120 ± 10, 180 ± 15, and 210 ± 15 μA cm -2 were observed at 0, 500 and 1000 RPM, respectively. In weeks 2–3, the accelerated transport of Fe 2 + away from the surface and of CO 3 2 -/S 2 - toward the steel promoted supersaturation and precipitation of more compact FeCO 3 and FeS 2 scales. By week 4, corrosion rates for all velocities converged to approximately 100 μA cm -2 , indicating a diffusion-controlled regime dominated by the protective scale rather than flow effects. These findings provide mechanistic insight into sour-service flow conditions and offer quantitative guidance for corrosion management in high-velocity pipelines.