Theoretical Investigation On Corrosion Inhibition Mechanism Of Imidazoline Inhibitors

Corrosion is one of the prominent problems in petroleum industry. Among the numerous corrosion-prevention approaches, inhibitor has been widely applied and played a significant role in petroleum and petrochemical fields, with its characteristic of economy, high-performance, and adoptability. And recently, with the recognition of environment protection, the development of new efficient and environment-friendly green inhibitors has gained more attention. Traditionally, design and evaluation of inhibitors are based on trial-and-error experiments, and bear the shortcomings of high-cost, long-cycle, and blindness to some extent. Hence, the theoretical guidance of the R&D for the new inhibitors is in urgent need. In this paper, 8 kinds of 1-(2-aminoethyl)-2-alkylimidazolines with various alkyl chain lengths ((NH2)C2H4-C3H4N2-CH2(CH2)nCH3,n=5, 7, 9, 11, 13, 15, 17, and 19) were chosen to explore their corrosion inhibition mechanism for CO2 corrosion of carbon steel with multi-dimensional simulation by quantum chemistry calculation, molecular mechanics and molecular dynamics modeling. The reactivity of these molecules was analyzed, the influence of alkyl chain length on corrosion performance was investigated, the membrane forming mechanism of these molecules in the presence of solvent was proposed, and the diffusion dynamics of corrosion particles inhibition by inhibitor membrane was studied. The research results are as follows.The reaction active areas of these 8 inhibitor molecules are homogeneously distributed on the imidazoline ring and its polar functional groups. The 3 nitrogen atoms on imidazoline ring and polar functional groups are electrophilic reactive centers, which can donate electrons to metal surface to form coordinate bond. And carbon and nitrogen atoms on both ends of the C=N double bonds are nucleophilic centers, which can accept electrons from metal surface to from back-donating bond. Alkyl chain length generally shows no significant influence on reactivity of the whole molecule and active area distribution. When the adsorption process occurs, the head of imidazoline molecule is preferentially adsorbed on metal surface because of its strong electron transferring effect, and the non-polar alkyl chain deviates from the surface with certain inclination angle and self-assemblies into a compactly-arranged hydrophobic membrane. With the alkyl chain length increases, the stability of the inhibitor membrane and the binding strength between the membrane and the metal substrate increase gradually, which means long alkyl chain is beneficial to inhibition efficiency of the inhibitors. For two kinds of surfaces of Fe and FeCO3, when the alkyl chain length respectively exceeds 13 and 15, the inhibitor can form a high-coverage and compact membrane on metal surface to prevent the corrosion media from diffusion to metal surface and reduce both anode and cathode reactions on Fe surface. But according to this simple model under vacuum conditions, the theoretical evaluation results of inhibited efficiency of the 8 inhibitors are not completely in correspondence with experimental results in literatures.In solutions, when the range of alkyl chain length is from 7 to 15, the binding energy between inhibitor molecules and metal surface increases with the alkyl chain length, and reaches maximum when chain length is 15. With further extension of the alkyl chain from 17 to 21, the adsorption stability decreases gradually, which is mainly because of twist of inhibitor molecules ascribed to the solvent effect of water molecules and its further influence on interaction mode and strength. The formation of self-assembly membrane in solution indicates that imidazoline with alkyl chain length of 15 has good clustering effect and best inhibition performance in that the long chains interweave to form compact hydrophobic membrane, which can cover the metal surface well. When the alkyl chain is shorter or longer than 15, the alkyl chains are mostly in discrete state and the membranes are loose and porous, which results in reduced inhibition performance. Identical inhibitor is adsorbed more stable on FeCO3 surface than on Fe surface. The calculation model considering solvent effect can describe the adsorption and membrane-forming behavior of inhibitors more accurately. And the theoretical evaluation results match well with experimental results in literatures.Free volume, interaction between particles and membrane, and self-diffusion performance of membrane are the 3 critical factors influencing the performance for prevention of corrosion particles diffusion by inhibitor membrane. Identical inhibitor membrane has more prevention ability for cations and anions (H3O+, Cl-, and HCO3-) than that for H2O molecules. Molecules with shorter chain have larger rigidity, which reduces the interweaving and interactions of molecule chains. Therefore, the inhibitor membrane has larger self-diffusion capacity and weaker interaction with corrosion particles with comparatively larger free volume. So the corrosion particles have stronger diffusion ability in short chain molecule membranes. As the alkyl chain extends, the molecular flexibility increases, which makes the alkyl chains easily twist and interweave, to reduce the membrane self-diffusion ability and free volume, and enhance the particles coating by membrane and interaction between particles and membrane, which prevents the corrosion particles from diffusing in membrane. Thus, compared to short chain molecules, long chain molecular membrane has better diffusion-reducing ability, which stops enlargement when alkyl chain length is longer than 15. Theoretical evaluation results were in good accordance with experimental results in literatures when the alkyl chain length is between 7 and 15.