溶融Zn-6%Al-3%Mg合金めっき鋼板の大気暴露腐食生成物

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Atmospheric corrosion of hot-dip Zn-6%Al-3%Mg alloy coated steel sheet after 5 years of exposure in a severe marine and a rural sites have been investigated compared with Zn-0.2%Al and Zn-4.5%Al-0.1 %Mg alloy coated steel sheets. The corrosion resistance of Zn-6%Al-3%Mg alloy coated steel sheet is more superior than that of Zn-4.5%Al-0.1%Mg alloy coated steel sheet in both exposure sites. An extensive study using XRD and TEM revealed that the corrosion products consisting of amorphous, fine-crystal Zn_5(OH)_8Cl_2-・H_2O and Zn_6Al_2(OH)_<16>CO_3・4H_2O are formed on Zn-6%Al-3%Mg alloy coated steel sheet after 1 year of exposure in a severe marine site. Even after 5 years of exposure the formation of zinc oxide, which have little protective action for coatings, is suppressed on Zn-6%Al-3%Mg alloy coated steel sheet. It is concluded that the surface of Zn-6%Al-3%Mg alloy coated steel sheet is covered with the protective corrosion products in the early stage of exposure, resulting in the remarkably improvement of atmospheric corrosion resistance.
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