PURPOSE: To obtain an effective and cost effective corrosion inhibitor for steels to an aq. compsn. of the halide of alkaline metals by constituting the corrosion inhibitor of a mixture composed of strong bases, molybdates and phosphates.

CONSTITUTION: The corrosion inhibitor for steels which is effective even under severe conditions of about 220°C by the synergistic effect of the ternary mixture contg. at least one kind of the strong bases, at least one kind of the molybdates and at least one kind of the phosphates and comes into contact with the aq. compsn. of the halide of the alkaline metals is obtd. The strong bases described above are preferably LiOH. The phosphates and the molybdate are preferably the salts of Li, Na, K, Ca, Mg, Si, etc. The corrosion inhibitor described above is effective to the aq. compsn. of the working fluid of a heat pump contg. 40 to 70wt.% LiBr as the halide of the alkaline metals. At this time, the aq. compsn. contg. 0.01 to 0.2mol/l strong bases, 0./001 to 0.2mol/l molybdates and 0.001 to 0.2mol/l phosphates is preferably used as the effective amts. of the ternary mixture described above.