INVESTIGATION OF CORROSION RESISTANCE OF PRE-PAINTED Zn-11Al-3Mg-0.2Si ALLOY COATED STEEL SHEET THROUGH OUTDOOR EXPOSURE TEST IN OKINAWA(54)

摘要

The Zn-11%Al-3%Mg-0.2%Si alloy coated steel sheet was well known as a coated steel exhibitinghigh corrosion resistance. In order to develop the enhance the cut edge corrosion resistance ofchromate –free pre-painted 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 Okinawawas evaluated and investigated in this study. As a result, the corrosion resistance of pre-painted SDwas better than that of pre-painted Zn-0.2%Al coated steel sheet. The analysis of corrosion productsof the corroded pre-painted SD by EPMA and XRD were carried out. The simonkolleite;Zn5(OH)8Cl2?H_2O, the gordaite;;NaZn4(SO4)Cl(OH)6?6H_2O and the zinc aluminium carbonatehydroxide, Zn6Al2(OH)16CO3?4H_2O were found in the corrosion product under the paint film andthe corrosion product of Mg-rich was observed on the steel cut edge of pre-painted SD. From theseresult, the mechanism of the cut edge corrosion resistance of pre-painted SD was discussed in thestudy.