For this study, three corrosion inhibitors with different anionic carbon chain lengths, 1-vinyl-3-methylimidazolium iodide ([VMIM]I), 1-vinyl-3-proplylimidazolium iodide ([VPIM]I) and 1-vinyl-3-butylimidazolium iodide ([VBIM]I) were studied on the corrosion of X70 steel in 0.5 M H2SO4 solution at 298 K by electrochemical measurements, weight loss methods, scanning electron microscope (SEM), FTIR spectra, atomic force microscopy (AFM) and theoretical calculation. Three inhibitors with synergistic effect of iodide ions are mixed-type inhibitors with excellent inhibition efficiency. The order of corrosion inhibition efficiency is: [VMIM]I < [VPIM]I < [VBIM]I according to theoretical and experimental study, indicating the inhibition performance was improved with the increase of alkyl chain length attached to imidazolium ring, owing to its electron-donating effect of alkyl groups. The adsorption of three inhibitors including physisorption and chemisorption obeyed Langmuir adsorption isotherm. (C) 2018 Elsevier B.V. All rights reserved.