Influence of substrate oxidation on dynamic wetting in hot dip Al-Zn-Si coatings

In metallic hot dip coating processes, the formation of a thin continuous and uniform alloy layer is critical in providing the vital metallurgical bond at the substrate-coating melt interface. When the alloy layer isn’t uniform, bare spots or uncoated areas are formed which greatly affect the corrosion resistance and surface appearance of the coated product. Direct and indirect dynamic measurements of mass transfer during metallic coating are difficult, if not impossible. However, the concept of interfacial resistance to mass transfer is analogous to that of interfacial resistance to heat transfer. Using this concept, measurements based on dynamic reactive wetting and thermal interfacial resistance were performed at millisecond resolution to study the effects of low carbon substrate oxidation on the formation of 55Al-Zn-1.6Si hot dip coatings [1, 2]. The effect of substrate oxidation on the dynamic wetting of 55Al-Zn-1.6Si hot dip coating melts on low carbon steel substrates was investigated using an experimental apparatus based on the sessile drop technique. The initial wetting and spreading of a molten metal droplet on a substrate was recorded using a high speed digital camera operated at 1000 frames per second. The interaction between Zn-55Al-1.6Si droplets and low C steel substrates changed from wetting to non-wetting when the substrate was oxidised. Contact angles of θ ~ 20° were measured on clean substrates and increased to θ > 90° for oxidised substrates. The effect of substrate oxidation on thermal interfacial resistance was investigated using a dip testing apparatus. Immersion experiments using a thermocouple instrumented substrate dipped into a coating bath were performed in gas atmospheres of varying oxygen content. The interfacial heat flux and “apparent” thermal interface resistance were calculated from the substrate temperature response for various substrate oxidation conditions. Calculated “apparent” thermal interface resistances were consistent with observations of droplet wetting behaviour, as the thermal response of the substrate embodied the coupled effects of heat and mass transfer phenomena during the initial stages of solid-liquid contact. The interfacial resistance increased by an order of magnitude when a clean substrate was oxidised. Poor wetting of Zn-55Al-1.6Si melts on oxides was responsible for the increased interfacial resistance which resulted in the mass transfer of Fe-(Al,Zn) at the interface being hindered. These effects accounted for the increase in the occurrence of bare spots on the coated substrate surface.