The role of Si and Ti additions on the formation of the alloy layer at the interface of hot-dip Al-Zn coatings on steel strips

The reaction between solid steel and liquid Al-Zn-Si alloy leads to formation of a solid intermetallic interfacial layer. In the case of industrial Al-43.5Zn-1.5Si coated steel strips the thickness of the intermetallic layer is on average 1.35 mu m and the kinetics of the reaction is controlled by the effect of Si on Al and Fe diffusivities through the solid intermetallic interface. In this paper it is shown that increasing the Si content in the liquid alloy to values near 1.45% causes a reduction of the average thickness of the intermetallic interfacial layer to values near 1 mu m. This effect is further enhanced by the addition of small quantities of Ti to the liquid alloy. In this case, average interfacial layers less than 1 mu m thick were observed. EDS microanalysis at the interface of industrial coated steel strips showed that the interfacial intermetallic compounds are chemically similar to those formed in the bulk of Al-43.5Zn-1.5Si liquid baths in continuous coating lines. Differential acid dissolution of the coatings revealed that the intermetallic layer is not planar at the interface with coating overlay and is formed by crystals of different size and chemical composition. (c) 2005 Elsevier B.V. All rights reserved.