A water-based surface-treating agent for
metallic materials which gives a film highly satisfactory in
corrosion resistance, blackening resistance, wet secondary adhesion to topcoatings, low-
pollution characteristics (amount of fixed
chromium),
chemical resistance (especially
acid resistance and alkali resistance), etc., while retaining the intact practical liquid stability of conventional chromate-containing resinous
coating agents. The water-based surface-treating agent comprises a
synthetic resin emulsion and
hexavalent chromium ions and has a pH of 5 or lower, the
synthetic resin emulsion being one obtained by
emulsion-polymerizing the following ingredients using a nonionic polymerizable emulsifier and a nonionic nonpolymerizable emulsifier: (1) an ethylenic
carboxylic acid, (2) a functional acrylic
monomer having at least one of N-((un)substituted methylol)carbamoyl,
phosphonate, alkoxy, cyano, and carbamoyl groups, and (3) a third
monomer which is different from the ingredients (1) and (2) and forms the skeleton of the
copolymer to be obtained.