Theoretical Study On The Inhibition Mechanism Of Ionic Liquid Corrosion Inhibitors

Metal corrosion is a ubiquitous phenomenon in the national economy,national defense construction,science and technology and other relative fields.It has tremendous negative effects and needs being treated.Corrosion inhibitor as a simple and effective additive can effectively prevent the corrosion of metals.Inoic liquids with heterocyclic structures,containing heteroatoms such as N,O,S and multiple bonds,have been used as a potential inhibitor for corrosion protection.In recent years much research progress has been achieved in corrosion inhibition with ionic liquids.However the work is mainly focused on effect and efficency of corrosion inhibition,leaving the microscopic mechanism of corrosion inhibition less studied.In this dissertation,three types of ionic liquids(1-alkyl-3-methylimidazole chloride[XMIM][Cl],1-alkyl-3-methylimidazolium acetate[XMIM][Ac],1-octyl-3-methylimidazole salt[OMIM][Y],X = ethyl,butyl,hexyl,octyl,and Y = Cl,BF4,HSO4,Ac,TfO)were used as corrosion inhibiton medium for corrosion protection of carbon steel.Electronic structures and reactivity of these ionic liquids,surface energies and electronic structures of the iron surface were systematically analyzed by quantum chemical calculations.The adsorption behavior of ionic liquids on the iron surface was studied by molecular dynamics simulations and the microscopic mechanism of corrosion inhibition was revealed.The research results are given as follows.With an increase in the alkyl chain length of the[XMIM][Cl]and[XMIM][Ac]systems,the highest occupied molecular orbital energy(EHOMO),the lowest unoccupied molecular orbital energy(ELUMO),the softness(S),and polarizability(a)increase gradually,whereas energy gap(?E),dipole moment(?),electronegativity(?),hardness(?),and electrophilic index(?)gradually decrease.For the[OMIM][Y]system,the quantum chemical parameters of ionic liquids are quite different,and only the polarizability(a)gradually decreases with an increase in the alkyl chain length.The order of the surface energies from calculation for Fe(100),Fe(110)and Fe(111)surfaces follows as>?Fe(111)>?Fe(100)>?Fe(110),which indicates that Fe(110)surface is dense and stable.The 3d orbital of iron surface is the most active.The Fe(110)surface not only easily to interact with oxygen-containing anions of ionic liquids,but also can interact with the imidazole ring of cations.Therefore,Fe(110)surface was chosen as the adsorption surface of ionic liquids.The results show that the inhibition is mainly[XMIM]+ cations of the[XMIM][Cl]system,and the order of inhibition efficiency follows as[OMIM][Cl]>[HMIM][Cl]>[BMIM][Cl]>[EMIM][C1].Both[XMIM]+ cations and the Ac-anion have inhibition effect for the[XMIM][Ac]system,and the order of inhibition efficiency is[OMIM][Ac]>[HMIM][Ac]>[BMIM][Ac]>[EMIM][Ac].For the[OMIM][Y]system,[XMIM]+ cations and anions(BF4-,HSO4-,Ac-,TfO-)have inhibition effect,and the order of inhibition efficiency is[OMIM][TfO]>[OMIM][Ac]>[OMIM][HSO4]>[OMIM][BF4]>[OMIM][Cl].No matter what ionic liquids are used,cations or cations and anions of ionic liquids are adsorbed to cover on the Fe(110)surface as a protective film,which hinders the contact between iron surface and corrosion medium(H2O,H3O+,Cl-).In summary,the best corrosion inhibitor in the eleven studied ionic liquids is[OMIM][TfO].