Surface-treated steel sheets of good white rust resistance, and method for producing them

A surface-treated steel sheet of good white rust resistance, which is fabricated by applying a surface-treating composition that contains:

• (a1) an aqueous epoxy resin dispersion of a modified epoxy resin dispersed in water, in which the modified epoxy resin is prepared by reacting an epoxy group-containing resin (A), an active hydrogen-containing compound (B) a part or all of which is an active hydrogen-containing hydrazine derivative, and a hydrophilic organic component (C),
• (b) a silane coupling agent,
• (c) a phosphoric acid and/or a hexafluoro-metal acid, and
• (d) an aqueous dispersion of an urethane resin, wherein the hydrazine derivative is at least one member selected from hydrazide compounds, pyrazole compounds, triazole compounds, tetrazole compounds, thiadiazole compounds and pyridadine compounds, the amount of the silane coupling agent is from 15 to 50 parts by mass relative to 100 parts by mass of the solid content of the organic resin, and the amount of the phosphoric acid and/or the hexafluoro-metal acid is from 0.1 to 80 parts by mass relative to 100 parts by mass of the organic resin, to a zinc-plated or aluminum-plated steel sheet followed by drying it to form thereon a surface treatment film having a thickness of from 0.02 to 5 μm.

A method for producing a surface-treated steel sheet of good white rust resistance, which comprises applying a surface-treating composition that contains:

• (a1) an aqueous epoxy resin dispersion of a modified epoxy resin dispersed in water, in which the modified epoxy resin is prepared by reacting an epoxy group-containing resin (A), an active hydrogen-containing compound (B) a part or all of which is an active hydrogen-containing hydrazine derivative, and a hydrophilic organic component (C),
• (b) a silane coupling agent, and
• (c) a phosphoric acid and/or a hexafluoro-metal acid, wherein the hydrazine derivative is at least one member selected from the group consisting of hydrazide compounds, pyrazole compounds, triazole compounds, tetrazole compounds, thiadiazole compounds and pyridadine compounds, the amount of the silane coupling agent is from 15 to 50 parts by mass relative to 100 parts by mass of the solid content of the organic resin, and the amount of the phosphoric acid and/or the hexafluoro-metal acid is from 5 to 50 parts by mass relative to 100 parts by mass of the organic resin, and has a controlled pH of from 0.5 to 6, onto the surface of a zinc-plated or aluminium-plated steel sheet, and not washing it with water, drying it under heat at an ultimate sheet temperature of from 50° C. to 300° C. to thereby form thereon a surface treatment film having a thickness of from 0.02 to 5 μm.

A method for producing a surface-treated steel sheet of good white rust resistance, which comprises applying a surface-treating composition that contains:

• (a1) an aqueous epoxy resin dispersion of a modified epoxy resin dispersed in water, in which the modified epoxy resin is prepared by reacting an epoxy group-containing resin (A), an active hydrogen-containing compound (B) a part or all of which is an active hydrogen-containing hydrazine derivative, and a hydrophilic organic component (C),
• (b) a silane coupling agent,
• (c) a phosphoric acid and/or a hexafluoro-metal acid, and
• (d) an aqueous dispersion of an urethane resin, wherein the hydrazine derivative is at least one member selected from the group consisting of hydrazide compounds, pyrazole compounds, triazole compounds, tetrazole compounds, thiadiazole compounds and pyridadine compounds, the amount of the silane coupling agent is from 15 to 50 parts by mass relative to 100 parts by mass of the solid content of the organic resin, and the amount of the phosphoric acid and/or the hexafluoro-metal acid is from 5 to 50 parts by mass relative to 100 parts by mass of the organic resin, and has a controlled pH of from 0.5 to 6, onto the surface of a zinc-plated or aluminium-plated steel sheet, and not washing it with water, drying it under heat at an ultimate sheet temperature of from 50° C. to 300° C. to thereby form thereon a surface treatment film having a thickness of from 0.02 to 5 μm.