Corrosion Mechanism of Zn-Mg-AI Hot-dip Coated Steel Sheet

摘要

Recently, we have developed the hop-dip alloyed steel sheet having the coating composition of Zn-3.0%Mg-2.5%AI. It has a higher corrosion resistance than galvanized steel sheet, Gl, because of having both the sacrificial ability of Zn and the passive property of Al. We have investigated the corrosion properties of Zn-Mg-AI alloy coated steel sheet by using Salt Spray Test with varying corrosion time. And corrosion products were analyzed by XPS, EDS, FE-SEM, EPMA, GDOES and XRD. We have found that there were several different corrosion behaviours with compared to Gl. Initially, Mg contents were diffused toward top surface of the coating and assisted the formation of densely packed corrosion products with Zn and forming Zn related oxide. However, Al contents were attributed to lock up the aggressive Cl component inside coating materials with increasing corrosion time. Therefore, the excellent corrosion resistance of Zn-Mg-AI coating was shown to exhibit with two types of effects, such as dense formation of corrosion product and Cl contents confinement by passivation of Al inside a coating.