Fe Solubility in the Zn-Rich Corner of the Zn-Al-Fe System for Use in Continuous Galvanizing and Galvannealing
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INTRODUCTION
THE production of high-quality continuous galvanized (GI) and galvannealed (GA) products requires good bath management practices and, in particular, precise knowledge of the bath dissolved Al concentration.[1–4] Changes in Al concentration, for example, from 0.10 to 0.30 wt pct, have a significant impact on the structure and properties of the resultant coatings by influencing the formation of the so-called ‘‘inhibition layer,’’ a 50- to 250-nm-thick layer of the g-Fe2Al5ZnX intermetallic at the substrate/bath interface.[1,5–7] This layer acts as a transient barrier to further reactions between the liquid Zn overlay and the steel. The effectiveness of Al in this role is complicated by the partitioning of Al between the coating and intermetallic compounds that form within the galvanizing bath.[1–6] At the low bath dissolved Al concentrations commonly employed in the production of GA coatings (i.e., less than 0.14 wt pct at 460 C), the g-Fe2Al5ZnX layer is thin or discontinuous[1,5–7] and inhibition breakdown (the dissolution/destruction of the g-Fe2Al5ZnX layer) occurs rapidly. This facilitates the transformation of the Zn overlay to the desired sequence of Fe-Zn intermetallics during the subsequent galvannealing operation.[1,8] Conversely, the higher bath dissolved Al concentrations (i.e., greater than 0.15 wt pct) used in producing GI coatings promotes the formation of a continuous, relatively thick g-Fe2Al5ZnX layer in which further reaction of the Fe substrate with the Zn overlay is prevented or inhibited, thereby producing a highly J.R. McDERMID, Associate Professor, is with the Department of Mechanical Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. Contact e-mail: [email protected] M.H. KAYE, Research Associate, is with the Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada K7K 7B4. W.T. THOMPSON, Professor, is with the Department of Chemistry and Chemical Engineering, Royal Military College of Canada. Manuscript submitted January 22, 2006. Article published online April 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
formable, essentially pure Zn metallic coating.[1,5,6] The bath dissolved Al concentration plays a central role in the kinetics of the nucleation and growth process of the inhibition layer,[9–11] as well as in the overall Al concentration in the coating.[7,12] In particular, it is believed that a high Al concentration in the coating has a detrimental effect on the longevity of welding electrodes in spot resistance welding, a commonly employed technology in manufacturing processes involving galvanized or GA steel sheet.[13] The intermetallic particles found in continuous galvanizing line (CGL) baths are determined by the Al concentration in the molten Zn: f-FeZn13 at low concentrations; d-FeZn10AlY at intermediate concentrations; and g-Fe2Al5ZnX at higher concentrations,[1–4,14–18] as illustrated in Figure 1. The origin of these intermetallics in the CGL bath is a direct consequence of the galvanizing bat
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