Room temperature ionic liquids were found to be a kind of versatile lubricant with excellent friction reduction, antiwear performance. However, our knowledge is insufficient for understanding their corrosive properties. In the present work, the effects of a series of 1-n-alkyl-3-methylimidazolium hexafluorophosphates on the corrosion properties of AISI 52100 carbon steels were studied using potentiodynamic polarisation, electrochemical impedance spectroscopy and electrochemical noise. The results show that the corrosion activity decreases with increasing the chain length of alkyl substituent. The corrosion ability of 1-decyl-3-methylimidazolium hexafluorophosphate was the lowest among four kinds of 1-n-alkyl-3-methylimidazolium hexafluorophosphates absolute alcohol solution.