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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.

The goal of this study was to identify a pre-coated steel sheet that will satisfy press formability, seam weldability, spot weldability, brazing property, external corrosion resistance and internal fuel resistance. We established an experimental method for anti-fuel resistance measurement by applying synthetic spoiled gasoline by investigating the composition of spoiled gasoline and corrosion products in the fuel tank, which was collected in North America after 160,000 kilometers' running. Newly developed lead-free pre-coated steel sheet shows excellent anti-fuel corrosion resistance in comparison with that of usual terne coated steel sheet according to this method.