Ionic liquids with superior protection for mild steel in acidic media: Effects of anion, cation, and alkyl chain length

Abstract

The inhibiting effect of seven ionic liquids (ILs) with various anions, cations, and alkyl chain lengths on mild steel corrosion in acidic solution (1 M HCl) was demonstrated using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). ILs with different chain lengths such as 1-ethyl-3-methyl-imidazolium hexafluorophosphate (EMIMPF6), 1-methyl-3-pentyl-imidazolium hexafluorophosphate (PenMIMPF6), and 1-methyl-3-octyl-imidazolium hexafluorophosphate (OctMIMPF6) were examined. In addition, ILs with various anions such as 1-methyl-1-octyl-pyrrolidinium hexafluorophosphate (OctMpyrPF6), 1-methyl-1-octyl-pyrrolidinium thiocyanate (OctMpyrSCN), and 1-methyl-1-octyl-pyrrolidinium dicyanamide (OctMpyrN(CN)2) were considered. Furthermore, ILs with various cations such as 1-octylpyridinium hexafluorophosphate (OctpyPF6), OctMpyrPF6, and OctMIMPF6 were compared. Of all the ILs successfully synthesized in our laboratory, three of them were new: OctMpyrPF6, OctMpyrSCN, and OctMpyrN(CN)2. PDP experiments indicated that the investigated ILs were inhibitors of the mixed type. The adsorption of the studied ILs at the interface of the mild steel/electrolyte obeyed the Langmuir adsorption isotherm, which was also confirmed by SEM and attenuated total reflectance (ATR)-FTIR. The adsorption free energy (ΔG°ads) of these ILs showed negative values, which confirms their spontaneous physical adsorption. In comparing ILs with the same anion and cation, the IL OctMIMPF6, which had the longest alkyl chain, gave the best protection efficiency. The N(CN)2 anion showed better protection for mild steel than SCN and PF6. Imidazolium cation gave better inhibition than pyrrolidinium and pyridinium cations. SEM and ATR-FTIR analyses confirmed the strong adsorption of the studied ILs on the metal surface to form a protective film, which supports the electrochemical and the theoretical results.