The Synergistic Inhibition Effect Between Imidazoline-Based Dissymmetric Bis-Quaternary Ammonium Salts And Thiourea In CO₂Corrosion Process

With the increased of domestic oil demanded and oil production intensification,CO2corrosion problems were more and more prominent in oil production, it wasseriously endangering the safety of the petrochemical industry production, resulting inthe country suffered immeasurable loss.Filling inhibitor in the exploitation of oil and gas wells and gathering pipelineshas many advantages in those measures of preventing CO2corrosion, it has low cost,effective and easy to implement features. Two or more inhibitors are compound usedin order to achieve a better inhibition effect currently, and inhibitors compound usedmechanism is still relatively little. In order to study the synergistic mechanismbetween imidazoline-based dissymmetric bis-quaternary ammonium salts (DBA)and Thiourea (TU), it is using weight loss method (under normal pressure and highpressure), polarization curves, electrochemical impedance spectroscopy, scanningelectron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-rayspectroscopy (EDX) and other methods for the research of corrosion inhibitionmechanism and synergistic mechanism.The main findings are as follows:(1) DBA corrosion inhibition could play a certain role in the25℃,40℃,60℃and80℃. Corrosion inhibition efficiency increased with increasing concentration ofDBA. The inhibition efficiency was up to72%when DBA inhibitor dosage was up to400mg·L-1in25℃. Polarization curves and electrochemical impedance resultsshowed that: DBA inhibitor could reduce both the anode and cathode Tafel slope; itcould be classified as mixed type inhibitors. DBA could form protective adsorbed filmon the Q235steel surface. The film could reach its best protection state at the time of24h, after that the adsorption film began to fall, a good protective effect of adsorbed film can not be formed.(2) The best ratio of the mass fraction was1:1when DBA and TU compoundused, the same rule was obvious in three temperatures. When used in accordance withthe best ratio of the mass fraction, corrosion rate of Q235steel decreased with theincreased of inhibitor concentration and the inhibition efficiency increased with theincreased of DBA-TU concentration. When the concentration of DBA-TU was up to150mg·L-1, the inhibition efficiency was all above90%at four temperatures,followed by inhibition efficiency had little change with increasing the concentrationof DBA-TU. Inhibitor make corrosion potential of steel moving positive, it couldreduced Tafel slope of both anodic and cathodic, it could be classified as an anodicinhibitor. DBA-TU inhibitor could form tough adsorption film on the Q235steelsurface, the dense of adsorbed film increases with increasing concentration ofinhibitor. The adsorbed film on the steel surface formed quickly and it had betteraftereffect which could approve the apparent synergistic inhibition effect betweenDBA and TU.(3) The corrosion rate of Q235steel in the blank solution increased withincreased concentration of HAc in90℃. The corrosion rate of Q235steel was lessthan0.076mm·a-1when the concentration of DBA-TU reached120mg·L-1in thesolution without HAc; The corrosion rate of Q235steel fell to0.076mm·a-1when theconcentration of DBA-TU reached150mg·L-1in the solution with1mL·L-1or2mL·L-1, the inhibition efficiency was above90%. DBA-TU could be a good inhibitorin inhibiting corrosion from CO2and HAc.(4) Sample morphology obtained from SEM suggesting that DBA alone can notbe a good inhibitor in protecting Q235steel from CO2corrosion. The surface addingDBA-TU was more smoothly than the blank which indicated DBA-TU can be a goodCO2corrosion inhibition. Sample morphology after weight-loss under high pressurecould approve that DBA-TU can protected Q235steel against CO2and HAc corrosion.EDX analysis had found S, N elements on well protected sample in150mg·L-1inhibitor solution, these evidence effectively proved adsorption film formed on thesurface of the Q235steel. The crystal of XRD analysis was formed on the surface of Q235steel in the blanksolution at80℃. The result showed that it would form a protective crystal FeCO3atthe temperature more than60℃in CO2corrosion system. It mitigated the corrosionrate of Q235steel to some extent.