Dynamic reactive wetting and its role in hot dip coating of steel sheet with an Al-Zn-Si alloy
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INTRODUCTION
THE importance of dynamic wetting on processing conditions and product quality is widely recognized in the liquid film coating industry.[1] However, its relevance, or how it contributes to metallic hot dip coating processes, remains unclear or not well understood. In such processes, the continuously moving substrate is part of a system capable of forming intermetallic compounds. Hence, it may be anticipated that dynamic physical-reactive wetting[2] inherently assumes a key role in the establishment of a stable and uniform contacting meniscus at the coating bath entry (Figure 1) an essential prerequisite to the development of a continuous and uniform alloy layer. A nonuniform alloy layer thickness can affect the formability of the coated strip product, while a discontinuous alloy layer can lead to surface defects such as uncoated areas and pinholes, which may ultimately impair the corrosion performance of the final product. Steady-state wetting studies provide only a limited understanding of the influence of dynamic physical-reactive wetting on the kinetics of the alloy layer formation during the initial stages of solid-liquid contact.[3,4] Dynamic wetting studies related to hot dip coating have mainly been performed using the “meniscograph” technique.[5] Contact angles are determined indirectly by this technique, which in effect measures the adhesive force between a melt and a NICHOLAS EBRILL, Research Engineer, YVONNE DURANDET, Senior Research Scientist, and LES STREZOV, Principal Research Engineer, are with the Centre for Metallurgy and Resource Processing, BHP Research and Technology Development, Shortland NSW 2307, Australia. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
substrate as the latter is withdrawn from the molten bath. Because the substrate is typically immersed in the melt for 5 to 10 seconds, meniscograph measurements give only an indication of the wetting behavior between the melt and the alloy layer already formed on the substrate. To simulate the transient stages of the initial solid-liquid contact, a few wetting studies of hot dip coating melts on iron or steel substrates have been performed at milliseconds resolution using the sessile drop deposition technique.[4,6,7] In these studies, it was shown that under isothermal conditions, a “quasi-equilibrium” contact angle was established within 20 ms of the liquid droplet being deposited onto the solid substrate. It was believed that alloy layer growth had started within this short time period. In further isothermal studies,[8] where the substrate and droplet were heated well above the droplets melting point, wetting was improved by an increase in temperature. In practice, however, the initial substrate temperature is often different and much lower than the coating bath temperature. The work described in this article was undertaken to investigate, in millisecond r
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