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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.

Modifications in the properties of galvannealed steel sheet have been made to meet the needs of the automotive industry. The press formability (powdering and flaking) of the galvannealed steel sheet is very sensitive to the phase composition and coating weight of the coating. The allowable coatings for a satisfactory anti-powdering property may fail to provide the anti-flaking property, showing a high coefficient of friction due to the existence of a soft phase, ζ, at the surface. In this study, the effects of an inorganic film on the surface of the galvannealed coating were studied improving press formability. It was found that a thin borax film was effective to minimize surface friction and to improve unti-flaking properties. Short electrode life is another problem, as it limits continuous spot welding of the galvannealed steel sheet. During welding, electrode tips (Cu) are consumed by an interaction with the coating and the steel substrate.

The corrosion mechanism of zinc coated steel sheets in automotive bodies was studied in field vehicle tests and several types of accelerated tests. Perforation corrosion starts in unpainted areas of lapped parts, and proceeds in the following steps: i) galvanic protection by the Zn coating, ii) protection by corrosion products, and iii) corrosion of the steel substrate and perforation. Although the corrosion processes were the same in all the cases tested, the corrosion rate depended significantly on the environment, such as atmospheric exposure conditions and the part of the automotive body. In accelerated corrosion environments, Zn coating is largely ineffective against perforation corrosion because galvanic protection and protection by corrosion products cannot be maintained over the long term.

Al2O3 -dispersed Zn-Co-Cr alloy electroplated has been developed for automobile use. Optimum coating composition ranges from 0.5 to 2.0wt% for Co, 0.3 to 1.0wt% for Cr, and 0.1 to 2.0wt% for Al2O3. The coating showed excellent corrosion resistance with and without paint. Superior anti-corrosive property of the coating results from the formation of the mechanically and chemically stable corrosion products, ZnCl2 −4Zn (OH)2, and the passivation in anodic polarization. The coating also showed good stone chipping resistance as well as paintability, formability, and weldability. Accordingly Al2O3 -dispersed Zn-Co-Cr alloy electroplated is suitable for application to both unexposed and exposure parts, including interior and exterior surfaces.