Corrosion performance of Zinc and Zinc alloy coated steel sheets used for field vehicle

摘要

Corrosion performance of zinc and zinc alloy coated steel sheets in field vehicle was investigated with some microscopic and analytical methods after 10 year's field running in subtropical marine environment of Okinawa island in Japan. Zinc alloy coated steel sheets such as Galvannealed steel sheet and Zn-Ni electroplated steel sheet showed excellent corrosion resistance in the paint scribed outer panel and the door hem flange, compared to cold rolled steel sheet in such a real environment. The coating weight of these Zn alloy coated steel sheets and also Zn coated steel sheet strongly affected on the corrosion resistance of car body. The width of scab blister of painted Zn-Ni and galvanealed steel sheet was very small and decreased with the increase of Zn coating weight. The corrosion depth in hem flange also decreased with increasing coating weight. Corrosion mechanism was also studied by comparing the corrosion behavior in such a marine environment of Okinawa with that in snow-belt area of North America which is more severe environment. In the cosmetic corrosion of outer panel, corrosion products grew under blister at cross scribe and zinc chloride were observed to the tip of blister. This cosmetic corrosion behavior was relative similar to that of North America The difference of corrosion structure under blister depended on the coating type such as Zn, Zn-Fe, Zn-Ni and coating weight of coating layer. In perforation corrosion of the hem flange, Cl in the corrosion products was not partially clearly observed. This result was some different from the corrosion in snow-belt areas in North America. It suggested that another type of perforation corrosion such as small amount of Cl type corrosion also occurred at car body in Okinawa. From these results in the fieled viecle, the previous presumption of cosmetic and perforation corrosion mechanism based on cyclic corrosion tests was proved.