Reaction between Solid Iron and Molten Zinc Containing Aluminium

Ti interstitial free steel specimens with grain size of 50 μm in diameter polished with buffs were immersed in molten Zn containing 0.1-1.0 mass% Al for 4 to 3600 s at 733K. Formation and growth kinetics of Fe-Zn and Fe-Al alloy layers was examined metallographically. A fragmentary δ1(f.δ1) layer composed of small discrete δ1 phase particles embedded in a matrix of η phase was formed on the surface of the specimen in molten Zn containing 0.1 mass% Al and its thickness increased with immersion time according to a linear rate law. In molten Zn containing more than 0.3 mass% Al, a very thin layer composed of small Fe2Al5 crystals was formed on the specimen surface, and then another type of small Fe2Al5 particles in the shape of lens were formed between the Fe2Al5 crystal layer and the specimen. The Fe2Al5 crystal layer disappeared, as the lens-like particles of Fe2Al5 phase developed to a layer, the thickness of which increased with immersion time according to a parabolic rate law that means the layer growth is controlled by diffusion. In molten Zn containing 0.15-0.25 mass% Al, f. δ1 phase was formed between the crystal or diffusion layer of Fe2Al5 phase and the specimen. It appears that the formation of f. δ1 phase is caused by penetration of the molten Zn through cracks of the Fe2Al5 layer.