Investigation of corrosion resistance of pre-painted Zn-11%Al-3%Mg-0.2%Si alloy coated steel sheet through outdoor exposure test in Okinawa

摘要

Zn-11%Al-3%Mg-0.2%Si alloy coated steel sheet was well known as a coated steel exhibiting high corrosion resistance. In order to develop and enhance the cut edge corrosion resistance of chromate –free prepainted steel sheet, the corrosion resistance of the pre-painted Zn-11%Al-3%Mg-0.2%Si alloy coated steel sheet (pre-painted SD) after 9.5 years of exposure test in Okinawa was evaluated and investigated in this study. As a result, the corrosion resistance of pre-painted SD was better than that of pre-painted Zn-0.2%Al coated steel sheet. The analysis of corrosion products of the corroded pre-painted SD by EPMA and XRD were carried out. Simonkolleite; Zn5(OH)8Cl2.H2O, gordaite; NaZn4(SO4)Cl(OH)6.6H2O and zinc aluminium carbonate hydroxide, Zn6Al2(OH)16CO3.4H2O were found in the corrosion product under the paintfilm and the corrosion product of Mg-rich was observed on the steel cut edge of pre-painted SD. From these result, the mechanism of the cut edge corrosion resistance of pre-painted SD was discussed in the study.