| abstract |
The present invention provides a method for producing a highly corrosion resistant organic composite-plated steel sheet which is inhibited from liberation of chromium, which comprises forming a chromate film comprising a composition of the following (A) as a first layer at a coating amount of 5-150 mg/m2 as a solid film on the surface of a zinc or zinc alloy plated or zinc composite alloy plated steel sheet and then, coating on the first layer an organic solvent coating composition of the following (B) as a second layer at a thickness of 0.3-5 (my)m as a solid film. (A) coating type chromate bath composition +E,crc/1/ Cr6+: 1-30 g/l, +E,crc/2/ Cr3+: 1-30 g/l, +E,crc/3/ Cr6+/Cr3+weight ratio: 0.2-2.0, +E,crc/4/ HF+H3PO4: 1-60 g/l, +E,crc/5/ HF+H3PO4/Cr3+weight ratio: 0.5-3.5, and +E,crc/6/ silane coupling agent/Cr6+molar ratio: 0.05-0.3 (B) organic solvent coating composition (a) a bisphenol epoxy resin having a number-average molecular weight of 300-100,000 in an amount of 30% by weight or more based on the solid content of the coating composition; (b) at least one curing agent selected from the group consisting of polyisocyanate compounds and blocked polyisocyanate compounds at 1/10-20/10 in a weight ratio to solid content of the epoxy resin; (c) fumed silica of 0.1-100 m(my) in an average particle size in an amount of 5-50% by weight based on the solid content of the coating composition; (d) a ketone type organic solvent in an amount of 40% by weight or more based on the weight of the coating composition; and the solid content of the coating composition being in the range of 10-50% by weight. |