Mechanism of Corrosion Resistance in Organic Composite Coated Steel Sheet during Cyclic Corrosion Test

Organic composite coated steel sheets (OCC) show excellent corrosion resistance during cyclic corrosion tests (CCT). To clarify the mechanism behind this corrosion resistance, we studied changes in corrosion products and elements of plating, chromate, and organic resin layers of OCC with different organic resin layer weights. Larger amounts of corrosion products were detected on OCC with a smaller weight organic resin layer. Larger amounts of basic zinc chloride [ZnCl2·4Zn(OH)2] forming under the organic resin layer were detected by X-ray diffraction and electron probe microanalysis (EPMA) on OCC with a larger weight organic resin layer. Chromium remained in the chromate layer longer in OCC having the larger weight organic resin layer. The contribution of the corrosion products on the organic resin layer, the organic resin layer, the basic zinc chloride under the organic resin layer and the chromate layer to resistance was estimated by AC impedance measurements. That of the basic zinc chloride under the organic resin layer and the chromate layer was largest, followed by that of the organic resin layer. The excellent corrosion resistance of OCC therefore depends on the high barrier of the corrosion products on the organic resin layer and the ability to maintain the chromate layer and the basic zinc chloride under the organic resin layer in a corrosive environment.