Abstract: The etching agent is an aqueous solution including at least one acid selected from the group consisting of inorganic acids other than nitric acid, and organic acids; and an organic nitrogen compound having a molecular structure containing N—OH or N—O?. The acid concentration of the etching agent is 0.05 to 3% by weight, and the organic nitrogen compound concentration of the etching agent is 0.005 to 5% by weight. By bringing the etching agent into contact with a surface of a magnesium component, fine irregularities can be formed on the surface of the magnesium component even when etching depth is large.

Abstract: Disclosed is an etching agent for steel. The etching agent is an acidic aqueous solution including ferrous ions, ferric ions, and an acetylene group-containing water-soluble compound. The concentration of ferrous ion A % by weight and the concentration of ferric ion B % by weight in the etching agent is preferably from 0.1 to 2.5. Also disclosed is a replenishing liquid that is added to the etching agent when the etching agent is continuously or repeatedly used. The replenishing liquid is an aqueous solution including an acetylene group-containing water-soluble compound.