Segregation of alloying elements during the hot-dip coating solidification process

Abstract

Hot-dip galvanized steel is the most frequently used among coated steels. Hot dip coatings are characterized by their appearance. Visually, they show shiny and dull grains with different morphologies. The solidification morphology and the alloying elements segregation affect the final properties of the coating. In this paper, the surface segregation of aluminium and antimony was studied in relation to the coating topography, which is the result of the orientation of the solidification structure with respect to the sheet. Two morphologies can be observed on the galvanized material: one of a feather type appearance and another with a granular appearance. Solidification structures corresponding to these surface morphologies were characterized: in the feather type the dentrites grow parallel to the substrate surface, and in the granular one they grow forming an angle. Microsegregation in the grains with the dendritic solidification of zinc was detected. A surface enrichment of aluminium and antimony was also observed on areas with dendritic growth running at an angle to the substrate surface. This segregation helps to explain the ‘surface darkening’ defect which is observed in aged galvanized materials.