ORGANIC-COMPOSITE-COATED STEEL SHEET EXCELLENT IN RESISTANCE TO RUSTING OF ADVENTITIOUS ORIGIN

摘要

An organic-composite-coated steel sheet excellent in the resistance to rusting of adventitious origin, comprising a galvanized steel sheet having on its surface a chromate coating in a given coating weight and further an organic coating formed on the chromate coating, having a thickness of 0.2-2.0 νm and containing the following components (i) to (iii): (i) a base resin comprising an epoxy resin having at least one basic nitrogen atom and at least two primary hydroxyl groups at its ends, (ii) a polyfunctional polyisocyanate compound having at least three, preferably at least four and still preferably at least six isocyanate groups in its molecule, particularly preferably a hexafunctional polyisocyanate compound comprising a polyfunctional derivative of hexamethylene diisocyanate, and (iii) a rust-preventive additive in a compounding ratio (weight ratio of nonvolatile components) as defined by the following equation: (base resin + polyfunctional polyisocyanate compound)/rust-preventive additive = 90/10 - 40/60. Examples of preferred additive include a difficultly soluble chromate and a mixture of silica with a difficultly soluble chromate (35/5-1/39). A remarkably excellent resistance to rusting of adventitious origin can be obtained by the synergistic effect due to the resin composition constituting the organic coating and the rust-preventive additive.