| 摘要 |
It is an object of this invention to provide an organic composite coated steel sheet having an excellent corrosion resistance in a rust-contaminated environment. The coated steel sheet of this invention carries a chromate film formed on the surface of a zinc or zinc alloy plated steel sheet and having an appropriate coating weight, and an organic resin film formed on the chromate film, having a thickness of 0.2 to 2.0 microns, and comprising: (i) a base resin obtained by adding at least one basic nitrogen atom and at least two primary hydroxyl groups to the ends of molecules of an epoxy resin; (ii) a polyfunctional polyisocyanate compound having at least three, preferably at least four, and more preferably at least six, isocyanate groups per molecule (and still more preferably, a polyfunctional hexamethylene diisocyanate); and (iii) a rust-preventing additive, the total amount of the base resin and the polyisocyanate compound and the amount of the additive being in a ratio of from 90/10 to 40/60 by weight of nonvolatile matter. The rust-preventing additive may be a sparingly soluble chromate, or a mixture of silica and a sparingly soluble chromate having a silica/chromate ratio of from 35/5 to 1/39. The combination of the resin composition forming the organic resin film and the rust-preventing additive produces an outstandingly high corrosion resistance in a rust-contaminated environment.
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