2010 Volume 59 Issue 12 Pages 468-477
In order to characterize the corrosion products near the shear cut edge of the 55 mass% Al-Zn alloy coated steel sheet, galvanized steel sheets with scratch exposing substrate steel and galvanic couple electrodes composed of steel and Al-Zn alloy were subjected to atmospheric corrosion test using artificial seawater simulating the marine atmospheric environment. The corrosion products formed on Al-Zn coating, exposed steel and the coating near the exposed steel were analyzed by EDS, XRD and FT-IR. NaZn4(SO4)(OH)6Cl·6H2O, CaCO3 and Mg(OH)2 were identified on the exposed steel. Precipitation of these corrosion products was rationalized by thermodynamic analysis considering solubility products. Furthermore, it was concluded that the corrosion products exhibit a high electrical resistance to depress electrochemical reactions and also act as a barrier for oxygen diffusion, resulting in suppressed corrosion rate.