Corrosion mechanism of X65 steel exposed to H2S/CO2 brine and H2S/CO2 vapor corrosion environments

Wet gas gathering and transportation in natural gas production has good economic benefits, but it also brings many risks. Due to the synergistic effect of corrosive gas and multi-phase flow in the wet gas pipeline, there are two corrosive environments, which leads to frequent accidents of pipeline corrosion failure. In this paper, the corrosion experiments of X65 steel in two environments (H₂S/CO₂ vapor; H₂S/CO₂-dissolved brine) were completed by a high-temperature and high-pressure reactor. Combined with SEM, EDS and XRD instruments, the morphology, elements and compounds of corrosion products were analyzed. The corrosion impact of temperature, flow rate, CO₂ and H₂S in both environments was determined. Finally, corrosion mechanism in two corrosion environments were established. When CO₂ and H₂S coexisted, both in two corrosive environments, the two gases were involved in the corrosion of X65 steel, and the corrosion products formed were FeCO₃ and FeS in the liquid phase. The difference was that the corrosion product film in the gas phase was denser than that in the liquid phase and the corrosion rate in the gas phase was smaller than that in the liquid. There was a large amount of Cl⁻ and high shear force brought by the flowing, the corrosion product film fell off and formed local corrosion. In the gas phase, due to the H₂S and CO₂ higher concentration, a dense corrosion product film rapidly formed in the droplets. In the two environments, the order of corrosion factors is $P_{H_{2}S}$ ≫$P_{C{O}_2}$ Velocity > Temperature. But in the gas phase environment, H₂S dominates in the gas phase more than in the liquid phase because it is more soluble in droplets.