12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Due to the increasing demand for material and cost reduction, there has been a global trend to reduce the coating thickness of continuously galvanized sheet and coil products. As the lifetime of a metallic coating depends mainly on its coating weight, reducing the coating thickness could compromise the durability and performance of the coating. Comparatively, Zn-Al alloy coatings, such as GF (Zn-5%Al) and GL (Zn-55%Al-1.5%Si), which are usually two to four times more corrosion resistant than a galvanized (GI) coating, are more likely to provide equivalent corrosion protection to steel with a lighter coating weight. However, there is a lack of published studies on the corrosion performance of thin Zn-Al alloy coatings in corrosive environments. In the present study, samples of GF and GL, both of which had a coating weight of 30-40 g/m^2/side, were exposed at Belmont Beach, Australia, in August 2015. This is a severe marine site falling into the atmosphere corrosivity category of C5 (ISO 9223). For comparison, exposed samples also included GI and GA (galvannealed) coatings with normal coating weight (70 g/m^2/side). During the exposure test, a number of samples were retrieved from the site at periods ranging from 311 to 1373 days, and then examined using stereoscope, X-ray Diffractometer (XRD) and Scanning Electron Microscope (SEM). In addition to evaluating the cut-edge protection, the corrosion behavior of each coating was studied based on the corrosion product formation and the corrosion of coating microstructure. The polarization and electrochemical impedance of the collected samples were analyzed to evaluate the remaining protection of the corroded coatings.