Original Article

Effect of fluid flow velocity on CO₂/H₂S corrosion mechanism in X65 carbon steel pipelines

Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran

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Received: 13 August 2025
Accepted: 18 November 2025

Abstract

The corrosion behavior of X65 carbon steel under combined CO₂/H₂S 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 Fe²+ away from the surface and of CO₃²-/S²- toward the steel promoted supersaturation and precipitation of more compact FeCO₃ and FeS₂ 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.