Effect of Process Variables on the Grain Size and Crystallographic Texture of Hot-Dip Galvanized Coatings
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to their relatively low corrosion resistance, sheet steels are often coated with a protective metallic zinc (Zn) layer. Among the various metallic coating processes, continuous hot-dip galvanizing is considered to be a cost-effective means of coating large quantities of sheet steel products.[1] In this process, the steel strip is immersed in a molten zinc alloy bath for a defined time followed by gas jet wiping and natural cooling at rates ranging from 3 to 10 K/s, depending on the substrate thickness. The final quality of the zinc coating is based on its corrosion resistance and mechanical properties, which can in turn depend on the size and crystallographic orientation of the solidified zinc grains.[1–3] It has been observed that coatings comprising small zinc grains with a preferred basal crystallographic texture exhibit optimal corrosion resistance and cracking SHIRIN KABOLI, formerly M.A.Sc. Student with the Steel Research Centre, McMaster University, Hamilton, ON, Canada, is now Ph.D. Student with the McGill University, Montreal, QC, Canada. JOSEPH R. MCDERMID, Professor, is with the Steel Research Centre, McMaster University. Contact e-mail: [email protected] Manuscript submitted October 15, 2010. Article published online May 29, 2014 3938—VOLUME 45A, AUGUST 2014
behavior during forming operations.[4–7] It has been deduced by previous workers that the chemical composition of the liquid zinc bath, steel surface condition, and coating cooling rate are the most important process variables governing the nucleation and growth of zinc grains during coating solidification and, subsequently, control the final size and crystallographic orientation of the zinc coating.[8,9] In general, bath composition is known to strongly affect the formation of large zinc grains or spangles with a preferred crystallographic orientation.[10] Small concentrations of various alloying elements are usually introduced to the galvanizing zinc bath for a variety of purposes. Small aluminum (Al) additions (0.16 to 0.25 wt pct) are commonly employed in continuous galvanizing baths to form a thin g-Fe2Al5ZnX intermetallic layer at the bath-substrate interface, commonly referred to as the inhibition layer, to temporarily retard or inhibit the formation of brittle Fe-Zn intermetallics at the steel-coating interface and allow the coating to remain ductile. In addition, small quantities of antimony (Sb) are sometimes added to the galvanizing bath to reduce bath viscosity and ease the gas jet-wiping operation. However, coatings made using baths containing more than 0.04 wt pct Sb typically consist of very large METALLURGICAL AND MATERIALS TRANSACTIONS A
Faderl[3] showed that Zn coatings containing 0.05 wt pct Pb displayed a variety of orientations varying from having the basal planes parallel to the substrate surface to have the basal plane oriented at approximately 90 deg to the substrate surface. However, it should be pointed out that these orientations were selected by the authors to study the effect of orientation on the angle between dendrite arms
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