PURPOSE: To eliminate the corrosion of a metallic member contg. a joint between iron or steel and a zinc surface-treated steel sheet by using a soln. obtained by dissolving a specified amt. of a nitrogen-contg. heterocyclic compd. in a degreasing soln. or in the washing water generated from washing in succession to the treatment with the degreasing soln.

CONSTITUTION: A soln. obtained by adding 0.01W5% additive of ≥1 kind of nitrogen-contg. heterocyclic compd. consisting of a pyrazole derivative (A), a benzopyrazole derivative (B), a triazole derivative (C) and a benzotriazole derivative (D) which are respectively shown by formulas IWIV into a degreasing soln. is used. When the additive is applied to the washing water generated from washing in succession to the treatment with the degreasing soln., the effectiveness is unchanged. The metallic member is degreased by such a soln. In the formulas, R is a hydrogen atom or a 1W5 C acyl group and X, Y and Z show respectively a hydrogen atom, a hydroxyl group, a lower alkyl group, a carboxyl group, an amino group or a nitrogroup.