Effects Of O₂ And H₂S Impurities On The Corrosion Behavior Of X70 Steel In Water-Containing Supercritical CO₂ Transport System

Transmission of CO₂ from capture area to sequestration place in pipelines is a vital project in the process of carbon capture and storage.According to the difference from capture sources to technologies,it is inevitable containing a certain amount of impurities in captured CO₂-riched atmosphere,which would become a potential threat to the corrosion of pipeline under supercritical CO₂ conditions.In recent years,in-depth studies about the corrosion behavior of pipelines usually conducted in water-saturated supercritical CO₂ system.And mainly focus on O₂,SO₂ and NO₂impurities.Nevertheless,the effect of H₂S on the corrosion behavior of steels in supercritical CO₂ system was still unclear.There still existed major divergences in the typical corrosion characteritics.X70 steel was used in this study.And the effect of water content,H₂S content and corrosion time on the corrosion behavior of X70 steel were systematically investigated by means of simulate test of corrosion,kinetic analysis and morden analysis methods.The main conclusions are as follows:?1?In water-unsaturated?2500 ppmv?supercritical CO₂-O₂-H₂S system,the corrosion rate of X70 steel increased with the rising of H₂S content.O₂ impurity could destroy the integrity of FeCO₃ scales and induce the forming of Fe₂O₃ and FeOOH.H₂S impurity could suppress the supersaturated precipitation process of FeCO₃ and make the corrosion scale mainly composed of FeS.?2?The formation of corrosion pits and spherical corrosion products on X70 steel surface were related to the non-uniform deposition of water droplets on initial corrosion scale.At the initial stage,the droplets condensed on the steel surface with a smaller contact angle?⁸8°?and spread out to form the initial water layer gradually.Subsequently,many droplets locally deposited on initial corrosion scale with a higher contact angle?¹30°?and induced the generation of corrosion pits and spherical corrosion products underneath the droplets.?3?The corrosion rate of X70 steel increased with the rising of water content.The increase of H₂S content reduces the critical water content.Thus,controlling water content is more effective than limiting H₂S content.?4?The particle species mainly contains H₂CO_3?aq?,HCO^-_3?aq?,H⁺_?aq?and H₂S_?aq?in the condensed water phase.And FeS corrosion products formed via solid state reaction between H₂S_?aq?and Fe.While FeCO₃ corrosion products formed according to supersaturated precipitation process.?5?The corrosion scale of X70 steel had a double-layer structure.The outer layer of FeS initially formed,while the inner layer of FeCO₃ gradually formed on the interface of initial corrosion scale/steel substrate.And the corrosion type changed in the sequence of uniform corrosion-localized corrosion-uniform corrosion with prolonging of corrosion time.?6?In supercritical CO₂-saturated water phase,OCP and LPR increased sharply at the initial stage and reached a steady state in 3 h.The rising of OCP was related to the adsorption and accumulation of FeHS_a^-_ds,which was formed of multi-step reaction process between H₂S_?aq?and Fe,on the steel surface.?7?In supercritical CO₂-saturated water phase,the Nyquist diagrams presented a capacitive reactance in the high frequency region and an inductive reactance in the low frequency region when the corrosion time was less than 24 h.However,as the corrosion time exceeded 72 h,the Nyquist diagrams only contained an incomplete capacitive reactance in the high frequency region.