Study On Inhibition Performance Of Imidazoline Derivative Inhibitor Against Co₂ Corrosion In Oil And Gas Field

With the rapid development of oil and gas industry, there are more and more natural gas wells and gas transmission pipelines of oilfield existing CO₂ corrosion in gas and liquid two phases, which is a research hot spot in recent years. Using chemical inhibitors has been acknowledged as a very practical,less influencing and most economical method of combating CO₂ corrosion. Imidazoline and its derivatives have been used successfully as inhibitors in protection of oil field pipe instrument against CO₂ corrosion because of their advantages such as high inhibition efficiency, no special irritative smell, good heat stability and low toxicity. With the boosting up of environmental protection consciousness and rising up of sustained development strategy, it is especially important to search for new, superior and environmentally friendly imidazoline derivative inhibitors and understand their mechanism of inhibition.(1) In consideration of designing inhibitor molecule, our laboratory has developed a kind of newly imidazoline phosphate inhibitor (IPI) which has multi-adsorption centers, good water solubility. This paper aims at investigating the effectiveness and inhibition mechanism of the new inhibitor for Q235 steel against CO₂ corrosion deeply and systematically for the first time.Weight-loss method, polarization test, electrochemical impedance spectrum (EIS) and scanning electron microscopy(SEM) were used to study the corrosion inhibition performance and action mechanism of the IPI inhibitor in an aqueous solution of 2﹪NaCl saturated with CO₂. The results showed that the IPI inhibitor can inhibit Q235 steel from CO₂ corrosion effectively in liquid phase. The inhibition efficiency increasesd with increase in concentration of inhibitor, and which was up to 93.3% and 85.0% at the concentration of 1000 mg·L^-1 at 293 K and 333 K, respectively. The inhibition efficiency decreased slowly with soaking time, so it had very good aftereffect. Corrosion rate increased and inhibition efficiency deceased with increase in temperature at the same concentration. The IPI inhibitor inhibited both the anodic and cathodic reaction, especially anodic reaction, so it belonged to an anodic type inhibitor and its inhibition mechanism was"negative catalysis effect".In addition, some surface analysis techniques such as FT-IR and XPS were used to characterize the adsorption film formed on the Q235 steel. The results proved that the IPI molecules were really adsorbed on the iron surface to form adsorption film and the main component of the film was long chain fatty amides (LFA) which was the hydrolysis product of IPI.Quantum chemistry PM6 semi-empirical method was used to validate the law between the molecular structure and the inhibition performance and study the adsorption models of LFA on the Q235 steel. The optimized geometry of LFA showed that the heteroatoms N,O,P adsorption centers performed a approximate plan surface which made for the multicenter adsorption of LFA molecule on the surface of Q235 steel. The chemical adsorption of the LFA occurred by the formation of coordination bonds between the heteroatom atoms of acyl, amine group and phosphate group and Fe on the metallic surface.(2) A kind of newly thioureido imidazoline inhibitor (TAI) was synthesized in our laboratory. The TAI inhibitor performed excellently as a gas-liquid two-phase inhibitor for CO₂ corrosion. The aim of this work was to study the inhibition performance of TAI inhibitor for Q235 steel against CO₂ corrosion both in gas and liquid two phases.The corrosion inhibition performance and the growth and decay law of film formed on Q235 steel of TAI at different temperatures and concentrations both in gas and liquid two phases were researched by weight-loss method, polarization test and electrochemical impedance spectrum (EIS). The results showed that the TAI inhibitor can protect Q235 steel from CO₂ corrosion efficiently both in gas and liquid two phases. The values of inhibition efficiency showed peak-value-phenomenon at concentration of 50 mg·L^-1 owing to the change of adsorption mode. The inhibition efficiency in liquid phase decreased with immersion time, however, fluctuation of the inhibition efficiency values was shown in gas phase due to the synergistic effect of TAI and corrosion product. Corrosion rate increased and inhibition efficiency deceased with increase in temperature both in gas and liquid two phases. The TAI inhibitor inhibited both the anodic and cathodic reaction, especially anodic reaction, so it belonged to an anodic type inhibitor. At natural corrosion potential, the surface of Q235 steel had always extra negative charges in the solution with and without TAI inhibitor. The addition of TAI in the solution made the potentials of zero charge (PZC) shift to positive direction.The results of FT-IR, AFM and XPS surface analysis techniques proved that the TAI inhibitor and its acid hydrolysis product?amides (AMI) formed adsorption films on sample surfaces in liquid phase and in gas phase, respectively. The surface analysis by AFM displayed that the damage on the metallic surface was considerably reduced in the presence of the TAI inhibitor and bigger adhesive force between AFM probe and steel surface was detected owing to hydrophobic interaction from the inhibitors in the two phases and the long range-repulsive force between the AFM probe and the steel surface increased in gas phase but decreased in liquid phase by screening effect of surface charges.Quantum chemical calculation results showed that the TAI and AMI inhibitors can form multicenter adsorption and adsorb on the Fe surface though the imidazoline ring, thioureido and acyl. The alkyl chain of TAI and AMI were approximately parallel and perpendicular to the Fe surface, respectively.