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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.

A passenger vehicle in the subtropical marine environment of Okinawa Island for eight years was submitted for corrosion investigation.The precoated sheet steels such as Zn-Ni and Zn-Fe showed excellent corrosion resistance in the door hem flange and a scribed outer panel, compared to cold rolled steel. Cosmetic corrosion of a North America vehicle was more severe than that of a Okinawa vehicle, but a similar corrosion rate for perforation was shown in door hem flange. Basic zinc chloride and β-FeOOH were observed by using micro X-ray diffraction method in the initial stage of corrosion, and then these changed to ZnO and α-FeOOH, Fe304.