ORGANIC COMPOSITE COATED STEEL SHEET HAVING A HIGH CORROSION RESISTANCE IN A RUST-CONTAMINATED ENVIRONMENT

摘要

It is an object of this invention to provide an organiccomposite coated steel sheet having an excellent corrosionresistance in a rust-contaminated environment. The coatedsteel sheet of this invention carries a chromate film formedon the surface of a zinc or zinc alloy plated steel sheet andhaving an appropriate coating weight, and an organic resinfilm formed on the chromate film, having a thickness of 0.2to 2.0 microns, and comprising:(i) a base resin obtained by adding at least one basicnitrogen atom and at least two primary hydroxylgroups to the ends of molecules of an epoxy resin;(ii) a polyfunctional polyisocyanate compound having atleast three, preferably at least four, and morepreferably at least six, isocyanate groups permolecule (and still more preferably, apolyfunctional hexamethylene diisocyanate); and(iii) a rust-preventing additive, the total amountof the base resin and the polyisocyanatecompound and the amount of the additive beingin a ratio of from 90/10 to 40/60 by weightof nonvolatile matter.The rust-preventing additive may be a sparingly solublechromate, or a mixture of silica and a sparingly solublechromate having a silica/chromate ratio of from 35/5 to 1/39.The combination of the resin composition forming the organicresin film and the rust-preventing additive produces anoutstandingly high corrosion resistance in arust-contaminated environment.