Corrosion Products near the Shear Cut Edge of 55 mass% Al-Zn Alloy Coated Steel Sheets under Simulated Marine Atmospheric Environment

Abstract

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.