Synthesis Of Quaternary Phosphonium Salt Ionic Liquids And Study On The Inhibition Of Magnesium Alloy

Magnesium and its alloys are regarded as ideal engineering materials because of their superior properties such as high strength to mass ratio,large thermal conductivity,excellent electromagnetic shielding performance,easy recycling,and relatively low environmental pollution.They are widely applied in the automotive industry,aerospace,infrastructure and other fields.However,,magnesium and its alloys are sensitive to be corroded due to their high reactivity.The oxide film is porous after corrosion,which could not protect the magnesium alloy efficiently.As a result,the applications of Mg alloys are greatly suppressed.Therefore,many researchers are devoted to developing the new methods,novel materials and technologies to suppress the corrosion of magnesium alloy.Among all kinds of anticorrosion methods,utilization of corrosion inhibitor has the advantages,such as,simple operation,low cost,and no requirement of other equipment.Since 1950 s,the development of corrosion inhibitor has attracted much attentions,especially for recent years,the central goal is to improve the corrosion inhibition efficiency.In addition,there are two important problems to be considered for magnesium alloy: one is the number of corrosion inhibitor is still not abundant;the other is to develop "green" corrosion inhibitor with less environmental pollution.To solve above two problems,four quaternary phosphonium salt have been synthesized by combination of theory and experiment.They present the excellent corrosion inhibition effect on magnesium alloy in Na Cl solution.Specific research contents are as follows:1.A benzyl quaternary phosphonium salt,[BPP][NTf₂],was synthesized for the first time by a new pathway,i.e.,theoretical prediction?experimental synthesizee?corrosion inhibition.After that,its corrosion inhibition effect on magnesium alloy in 0.05 wt.% Na Cl solution was studied.Firstly,the geometries of [BPP][NTf₂] and [P6,6,6,14][NTF2] were optimized at the B3PW91/6-31G?d,p?level of theory.As compared with [P6,6,6,14][NTF2],the inhibition effect of [BPP][NTf₂] should be better than that of [P6,6,6,14][NTF2],which is a potential inhibitor.Then,[BPP][NTf₂] was synthesized and the corresponding structure was characterized.Then,the electrochemical impedance and potentiodynamic polarization curves of magnesium alloy electrode were measured after adding different amount of [BPP][NTf₂] in 0.05 wt.% Na Cl.The results show that the inhibition efficiency was enhanced with the increase of [BPP][NTf₂] concentration.When 0.30 m M [BPP][NTf₂] was added,the highest inhibition efficiency was 91.4%.The adsorption of inhibitor on the surface of magnesium alloy follows the Langmuir adsorption,which is dominated by chemical adsorption.The first principles and the computational results of molecular dynamics verify the above experimental conclusions.The interaction between [BPP][NTf₂] and magnesium alloy not only includes weak interactions such as van der Waals interaction,but also includes strong interactions such as electrostatic interaction and covalent bond.The results of scanning electron microscopy?SEM?showed that [BPP][NTf₂] can significantly slow down the corrosion of magnesium alloy.Finally,based on the information of corrosion products tested by Fourier transform infrared spectroscopy?FTIR?,a possible corrosion inhibition mechanism was proposed.[BPP][NTf₂] is a magnesium alloy inhibitor with simple synthesis method,low cost and good corrosion inhibition effect.2.On the basis of previous work,three new quaternary phosphonium salts [DTP][NTf₂],[TTP][NTf₂] and [OTP][NTf₂]] were synthesized for the first time.The electrochemical test results presented that all of them could effectively delay the corrosion of AZ31 B magnesium alloy in 0.05 wt.% Na Cl solution.With the increase of the alkyl chain length,the inhibition efficiency is gradually improved,that is,[OTP][NTf₂] > [TTP][NTf₂] > [DTP][NTf₂].For the same corrosion inhibitor,the corrosion inhibition efficiency increases with the enhance of the concentration.The highest inhibition efficiency reaches 91.9% when 0.30 m M [OTP][NTf₂] is included in 0.05 wt.% Na Cl solution.Next,the effect of time on corrosion inhibition was investigated.After 43 days,[OTP][NTf₂] still have a good corrosion inhibition effect on magnesium alloy.There was a coating film on the surface of the magnesium alloy according to the SEM result.According to the test analysis,the composition of the coating film is very complex including corrosion inhibitor,corrosion products of the magnesium alloy,products of the reaction between corrosion inhibitor and magnesium alloy,products of the reaction between corrosion products and corrosion inhibitor,and other components.Based on the results of X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?along with the density functional theory?DFT?,the possible corrosion inhibition mechanism was proposed.3.In order to further compare the corrosion inhibition effect of [OTP][NTf₂] and [BPP][NTf₂],their inhibitions were investigated in different concentrations of Na Cl solution along with different immersion time.The corrosion inhibition effect of [OTP][NTf₂] and [BPP][NTf₂] on AZ31 B magnesium alloy are investigated in 0.05 wt.% Na Cl,0.5 wt.% Na Cl and 3.5 wt.% Na Cl solutions,respectively.The results of electrochemical impedance spectroscopy and potentiodynamic polarization curves indicated that [OTP][NTf₂] and [BPP][NTf₂] have corrosion inhibition effect on magnesium alloy in 0.5 wt.% Na Cl solution.When the concentrations of [OTP][NTf₂] and [BPP][NTf₂] are 0.8 m M and 1.0 m M,the inhibition efficiency is the highest with the value of 87.1% and 90.4%,respectively.When the 0.8 m M [OTP][NTf₂] and 1.0 m M [BPP][NTf₂] were included in 3.5 wt.% Na Cl solution and 0.05 wt.% Na Cl solution respectively,the inhibition efficiency was also determined by the same method.To investigate the corrosion inhibition effect of [OTP][NTf₂] and [BPP][NTf₂] after long-term immersion,magnesium alloy electrodes were immersed in 0.5 wt.% Na Cl solution without or with 0.8 m M [OTP][NTf₂] and 1.0 m M [BPP][NTf₂] for 15 h and 96 h,respectively,to study the effect of immersion time on corrosion inhibition.With the elongation of immersion time,the inhibition efficiency of both inhibitors have a slightly increase.And the inhibition efficiency of [OTP][NTf₂] would be larger than that of [BPP][NTf₂].After immersion for 96 h,the inhibition efficiency of [OTP][NTf₂] can reach 92.3%.On the basis of SEM and EDS results,there was a protective film on the surface of magnesium alloy.The long alkyl chain is included in [OTP][NTf₂],which may take a long time to reach the surface of magnesium alloy.Therefore,the inhibition effect of [OTP][NTf₂] is enhanced after the long time immersion and exceeds [BPP] [NTf₂],which provides a valuable clue to explore a corrosion inhibitor with long-term protection effect on magnesium alloy.