Unalloyed steel corrosion in Cl(-) or/and H(2)O-contg. 1-butyl-3-methyl-imidazolium tetrafluoroborate (I) was studied at 500-200 degrees C by electrochem. and gravimetric methods. The addn. of Cl(-) to I resulted in a decrease in its viscosity and an increase in its cond. while the addn. of H(2)O had an opposite effect. Both the addn. of Cl(-). and H(2)O resulted in an increase in pitting corrosion rate. The I decomp. products contributed to the corrosion acceleration.