Three 1-alkyl-3-methylimidazolium tetrafluoroborates (I, alkyl = Bu, C(6)H(13), C(8)H(17)) were synthesized in reaction of methylimidazole with resp. alkyl chlorides and used as medium for studying the corrosion of a nonalloyed constructional steel at 25-200 degrees C by dc. electrochem. or gravimetric methods. The corrosion rate decreased with increasing alkyl chain length in I, but increased with increasing temp. A passivation of the steel in I (alkyl = Bu) at 150-200 degrees C was obsd. The traces of Cl ions contributed to pitting corrosion.