An acid inhibitor concentrate is provided which contains water, at least one polyamino-aldehyde resin such as a quaternized polyethylenepolyamine-glyoxal resin, and at least one compound selected from the group consisting of acetylenic alcohols, ethoxylated fatty amines, ethoxylated fatty amine salts, and aldehyde-releasing compounds (such as hexamethylenetetramine). Such concentrates form useful metal cleaning and pickling solutions when combined with aqueous acid, wherein such solutions, when contacted with a metal surface, are effective in removing scale, smut and other deposits from the metal surface but exhibit a reduced tendency for the aqueous acid to attack or etch the metal itself.