Solidification of Hot-Dip Galvanized Coatings

Abstract

<p>Continuous hot-dip galvanizing is a common industrial process in which a steel sheet is immersed in a molten zinc alloy bath. After solidification, a thin zinc-rich layer is formed on the steel surface which protects the substrate in corrosive environments. Small amounts of antimony are sometimes added to the galvanizing bath to reduce zinc viscosity and ease the gas jet wiping operation. One of the side effects of antimony addition to the zinc alloy bath is the formation of very large zinc grains. One of the problems associated with coatings containing large zinc grains are relatively poor paint adhesion and detrimental mechanical properties. In this study, a galvanizing simulator was used to investigate the influence of important process variables such as bath composition, steel surface roughness and cooling conditions on the solidification of zinc coatings. The coating surface and cross-sectional microstructures were characterized via optical microscopy and Scanning Electron Microscopy (SEM). In addition, the zinc grain orientation distribution was investigated using Electron Backscattered Diffraction (EBSD). Furthermore, Scanning Auger Microscopy (SAM) was carried out on the coating surface to study the distribution of alloying elements and bath impurities in intermetallic phases. The results showed that the presence of small amounts of antimony in the zinc alloy bath enhanced the grain growth in preferred crystallographic orientations on both substrates. It was also found that the substrate surface roughness had a strong influence on the coating crystallographic texture such that zinc grains had a strong basal preferred orientation on smooth substrates while exhibiting prismatic or pyramidal orientations onrough substrates. Finally, zinc crystals were smaller for the slow-cooled coatings while zinc grains had almost the same diameter for the intermediate and fast-cooled coatings on both the smooth and rough substrates. Factors affecting the solidified microstructure, crystallographic orientation of zinc grains and phase assemblage will be discussed.</p>