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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.