Corrosion Behavior of Organic Composite Coated Steel Sheet

Organic composite coated steel sheets retain their excellent corrosion resistance during cyclic corrosion tests (CCT). To clarify the corrosion behavior of these sheets during CCT, variations in corrosion products and coating components were examined. Moreover, the contribution of the corrosion products, organic composite coating, and chromate film to corrosion resistance was examined by AC impedance measurements.

Formation of crystalline ZnCl₂·4Zn(OH)₂ and amorphous zinc carbonate were detected by X-ray diffraction (XRD) and Fourier transformation infrared spectroscopy (FT-IR). Crystalline ZnCl₂·4Zn(OH)₂ is formed during CCT on and under the organic composite coating. The corrosion products formed on the coating contain silicates from the silica in the organic composite coating.

Consequently, the contents of zinc and silica in the coating decrease, while nickel and chromium in the chromate film and carbon in the coating remain constant during CCT. AC impedance measurements suggest that the chromate film and ZnCl₂·4Zn(OH)₂ formed under the organic composite coating are the source of the excellent corrosion resistance of this type of steel sheet.