Hot-Dip Aluminizing of Low Carbon Steel Using Al-7Si-2Cu Alloy Baths

Hot-dip aluminizing of low carbon steel was done in molten Al-7Si-2Cu bath at 690°C for dipping time ranging from 300 to 2400 seconds. Characterization of the intermetallics layer was done by using scanning electron microscope with energy dispersive spectroscopy. Four intermetallic phases, -Al7Fe2Si, -FeAl3, -Fe2Al5, and -Al2Fe3Si3, were identified in the reaction layer. - Al7Fe2Si phase was observed adjacent to aluminum-silicon topcoat, -FeAl3 between and -Fe2Al5, -Fe2Al5 adjacent to base material, and -Al2Fe3Si3 precipitates within Fe2Al5 layer. The average thickness of Fe2Al5 layer increased linearly with square root of dipping time, while for the rest of the layers such relationship was not observed. The tongue-like morphology of Fe2Al5 layer was more pronounced at higher dipping time. Overall intermetallic layer thickness was following parabolic relationship with dipping time. 1. Introduction Hot-dip aluminizing is an effective and inexpensive coating process to protect steels from oxidation [1, 2]. The quality of coating depends on the properties of the intermetallics layer forming at the interface. A brittle intermetallic layer may peel off from surface during forming operations [3], which generally follows aluminizing treatment. Therefore it becomes necessary to study the formation of intermetallics layer under different conditions. Gebhardt and Obrowski [4] observed that when steel comes in contact with the molten aluminum, the major intermetallic layer formed is Fe2Al5. Bouché et al. [5] reported the formation of two intermetallic layers, namely, Fe2Al5 and FeAl3, when solid iron is dipped in liquid aluminum over the temperature range 700°C to 900°C. They reported that the growth behaviour is initially nonparabolic which is followed by parabolic. Kinetic studies done by Bouayad et al. [6] for medium dipping times (<45？min) showed that the growth of Fe2Al5 layer is diffusion controlled and FeAl3 layer growth is linear with time. Many researchers tried to explain the observed tongue-like morphology of the intermetallic layers [5–7] and are of the opinion that the anisotropic diffusion is responsible for this growth. Springer et al. [8] investigated interdiffusion between low carbon steel and pure Al (99.99%) and Al alloy (Al-5%Si) between temperatures 600°C and 675°C and showed that growth rate of -layer (Fe2Al5) is diffusion controlled and it governs overall intermetallic layer growth. Cheng and Wang [9] observed that as the silicon content in the molten bath increases, the thickness of intermetallic layer decreases as well as the interface