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 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 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; Zn_5(OH)_8Cl_2-H_2O, gordaite; NaZn_4(SO_4)Cl(OH)_6-6H_2O and zinc aluminium carbonate hydroxide, Zn_6Al_2(OH)_(16)CO_3-4H_2O were found in the corrosion product under the paint film 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.