Composition and Formation Mechanism of the Alloy Layer in Aluminum-Coated Steel

Abstract

The composition and the structure of the alloy layer formed on steel, which was hot-dipped in molten aluminum and Al-3.3%Si alloy, were determined by some metallurgical methods including metallography, micro-hardness test, etching pits, X-ray diffraction and electron probe X-ray micro-analysis. The change in the thickness of alloy layer against immersion time at 700°∼850°C was also measured. The results obtained were as follows: (1) In the pure aluminum coating the alloy layer was composed of FeAl3 and Fe2Al5 but in the Al-3.3%Si alloy coating it was sheer FeAl3. (2) The FeAl3 layer was constituted of fine grains and the Fe2Al5 layer of relatively large columnar ones almost perpendicular to the steel surface. (3) The thin layer in the Al-Si coating is due to the accelerated diffusion of iron into the molten Al-Si alloy. (4) The mechanism of the formation of the alloy layer was also discussed.