Surface Characteristics Analysis and Adherence Evaluation of Hot-Dip Galvanized Low Alloy Steel

Abstract

The present study discusses the surface characteristics analysis and coating - substrate adherence evaluation of samples from hot-dip galvanized steel sheets produced by Arcelor Mittal Company. Optical microscopy has revealed that the coating is formed principally of zinc oxide (ZnO), pure zinc (ɳ) layer, intermetallic phase zeta (ζ), the existence of Delta (δ) phase, and Γ-inhibition layer. The Scanning Electron microscopy with EDX studies on substrate-interface and coating showed the presence of oxygen in large quantities localized in the interface and which is the formation of iron oxides that prevent the diffusion of zinc and therefore reduces the adhesion of the coating to the substrate. The low adhesion is also related to the presence of intermetallic phases which are inherently fragile. The interfacial indentation test allows determining the indentation fracture toughness of Fe-Zn coatings. During the test, a crack is initiated and propagated along the interface. It was demonstrated that cracks Palmqvist type appear from 0.5N and the KlC coefficient decreases according to the increase of the normal load. The contaminations of samples coated before and after ion sputtering were analyzed using X-ray Photoelectron Spectroscopy (XPS).Both electrochemical corrosion tests in a 3.5% NaCl solution showed that all Fe-Zn coatings have a much higher corrosion resistance than that for steel uncoated.

Keywords: Low alloy steel - Continuous hot-dip galvanizing - Coatings - Interfacial Indentation - tenacity.