Research On The Corrosion Inhibition Of Imidazolium Based Ionic Liquids For Mild Steel In Acid Solution

Ionic liquids, due to their green feature and designability, and their potentially huge numbers and inhibition performance, comply with times demand of the development of environmental friendly corrosion inhibitors. However, the researches on the ionic liquids as corrosion inhibitors are still scanty, and these studies are not systematic. In the thesis, the infullence of anions, chain length of alkyl, the number and type of substituent and corrosive medium on the inhibition performance of imidazolium based ionic liquids was systematically investigated through the weight loss experiments and electrochemical measurements, and the adsorption behavior of imidazolium based ionic liquids on the surface of mild steel also was deeply discussed, moreover, based on the theoretical calculation, the quantitative structure-activity relationship(QSAR) between the quantum chemical parameters of imidazolium based ionic liquids and their inhibition efficiencies was analyzed and established. In addition, the inhibition performance and mechanism of the several specific structure of imidazolium based ionic liquids for mild steel in H2SO4 solution was also studied and discussed. The main work and innovations of this thesis as follows:① The inhibition performance of six kinds of 1-hexyl-3-methylimidazolium ionic liquids with different anions, namely, chloride ion([HMIM]Cl), bromide ion([HMIM]Br), iodide ion([HMIM]I), hydrogen sulfate ion([HMIM]HSO4), nitrate ion([HMIM]NO3) and dihydrogen phosphate ion([HMIM]H2PO4), for mild steel in H2SO4 and HCl solution was respectively investigated. Specifically, an amino acid ionic liquid, namely 1-octyl-3-methy- limidazolium L-prolinate([OMIM]LPro), its inhibition performance and mechanism for mild steel in H2SO4 solution was also determined and discussed. The research findings are briefed as follows:Anion was an important factor which influence the inhibition performance of the imidazolium based ionic liquid. The influence of anion depended on its adsorptivity and the type of acid. There were two different adsorption ways to the anions of ionic liquid and the anions of acid on the surface of mild steel, namely, competitive adsorption and cooperative adsorption.In 0.5 mol/L H2SO4 solution, [HMIM]NO3 was found to have no inhibition effect on the corrosion of mild steel, and the performance of [HMIM]H2PO4 and [HMIM]HSO4 was also very poor, however, the effect of [HMIM]Cl, [HMIM]Br, [HMIM]I was enhanced in turn. Except [HMIM]I was a mixed-type inhibitor with a predominantly cathodic action, the other four kinds of ionic liquids for mild steel in the H2SO4 solution acted as modest cathodic inhibitors. But, in 1.0 mol/L HCl solution, all the six studied ionic liquids were mixed inhibitors. And, at low concentrations of ionic liquids, [HMIM]HSO4 and [HMIM]NO3 exhibited better corrosion inhibition than [HMIM]Cl, and [HMIM]H2PO4 showed a good inhibition effect, at the same concentration, its inhibition efficiency was equivalent to that of [HMIM]I, and reached 92.7 % when its concentration was of 10 mM. To those imidazolium based ionic liquids, whose anions were halide ions, the inhibition efficiency was increased in the order of Cl-, Br- and I-, however, the inhibition performance of [HMIM]Cl was close to that of [HMIM]Br.In the 0.5 mol/L H2SO4 solution, the Langmuir isotherm and modified Langmuir isotherm by Villamil, El-Awady thermodynamic-kinetic model and Flory-Huggins isotherm were found to have a good relationship with the experimental data, but [HMIM]HSO4 only had a better response to the El-Awady thermodynamic-kinetic model. In 1.0 mol/L HCl solution, the adsorption of ionic liquids on the mild steel surface was still in accordance with the other two kinds of adsorption isotherms except the Flory-Huggins isotherm.[Omim]Lpro was a mixed-type inhibitor with a predominantly cathodic action for mild steel in 0.5 mol/L H2SO4 solution, and its inhibition e?ciency increased with the increase in the concentrations of [Omim]Lpro, but reduced with rising temperatures and was strongly influenced by temperature. The cation of OMIM played a more major role in the corrosion inhibition of [Omim]Lpro than prolinate anion, at low concentrations, there is a synergistic effect between the OMIM cation and prolinate anion, but at high concentrations, due to the competitive adsorption, which showed an antagonistic effect. Moreover, the adsorption of [OMIM]LPro on the mild steel surface was found to obey the El-Awady thermodynamic-kinetic model and Flory-Huggins isotherm equation.② The inhibition performance of six kinds of 1-alkyl-3-methylimidazolium bromide with different chain length of alkyl for mild steel in H2SO4 solution was investigated, and the effects of chain length of alkyl on the perfomance of imidazolium based ionic liquids and the QSAR between the quantum chemical parameters and their inhibition efficiencies was analyzed. In particular, the inhibition performance and mechanism of an novel ionic liquid, namely 1-butyl-3-methyl benzimidazolium iodide(BMBIMI), for mild steel in H2SO4 solution was also investigated. The research findings are briefed as follows:The inhibition performance of 1-alkyl-3-methylimidazolium bromide for mild steel in H2SO4 solution generally increased with the increase of the chain length of alkyl, and ionic liquids exhibited a good corrosion inhibition when the chain length of alkyl reached up to 12. The six studied ionic liquids were modest cathodic inhibitors, and their adsorption on the surface of mild steel was found to obey the Langmuir isotherm and modified Langmuir isotherm by Villamil, El-Awady thermodynamic-kinetic model and Flory-Huggins isotherm. Theoretical calculation showed that there was a good agreement between the quantum chemical parameters of 1-alkyl-3-methylimidazolium cations and their inhibition efficiencies, and the QSAR equations were obtained.It was found that BMBIMI was an effective mixed-type corrosion inhibitor for mild steel in 0.5 mol/L H2SO4 solution, its inhibition ef?ciency was up to 97 % at the concentration of 5.0 mM, and BMBIMI showed a good property of temperature resistance, because its inhibition ef?ciency increased with increasing temperature at higher BMBIMI concentrations. Moreover, the BMBIMI cation adsorbed on the mild steel surface by benzimidazole ring using ?at mode, and the adsorption obeyed Langmuir isotherm.③ The inhibition performance of five kinds of imidazolium based ionic liquids with different substituents, including N-octylimidazolium bromide([OIM]Br), 1-octyl-3- methylimidazolium bromide([OMIM]Br), 1-octyl-2,3-dimethylimidazolium bromide([ODMIM]Br), 1-allyl-3-octylimidazolium bromide([AOIM]Br) and 1-benzyl-3-methylimidazolium bromide([BzMIM]Br) for mild steel in H2SO4 solution was investigated. The research findings are briefed as follows:There was a significant effect of the substituent on the corrosion inhibition of the imidazolium based ionic liquids. Because of the steric effect, the inhibition efficiencies of the ionic liquids decreased with the increase in the number of the substituent, and were also affected by the electron donating ability of the substituent and the spatial configuration of the alkyl chain. The order of inhibition performance was as follows: [OIM]Br > [AOIM]Br > [OMIM]Br > [ODMIM]Br > [BzMIM]Br. Except for [OIM]Br was a mixed inhibitor for mild steel in H2SO4 solution, the others were modest cathodic inhibitors. Their adsorption on the mild steel surface was in accordance with the Langmuir isotherm and modified Langmuir isotherm by Villamil, and the El-Awady thermodynamic-kinetic model.It was found that there was no a fixed relationship between the quantum chemical and molecular structure parameters of studied ionic liquids and their inhibition efficiencies, even the relationship was completely opposite. Therefore, only the main factor was found out which caused the difference in molecular properties, the effect of the difference on the inhibition performance could be analyzed.