Analysis of reaction layer formed on steel strips during hot dip aluminising

Abstract

Aluminised steel sheets are currently used in many applications where the surface properties of aluminium combined with the formability, mechanical strength, and economic advantages of sheet material are obtained. During aluminising by hot dipping, an intermetallic reaction occurs due to diffusion between the aluminium bath and the steel sheet. The phases formed within the intermetallic layer and its thickness are important for determining the behaviour of the coating. In the present work, low alloy, medium carbon, low carbon 9Γ2C (9G2S), and low carbon steel sheets are used for hot dipping in a commercial purity aluminium bath. The temperatures used for hot dipping were 710 and 770°C and the immersion times were 2, 4, and 6 min. The results indicate that the thickness of the intermetallic layer X increases with increasing bath temperature and immersion time t. The variation of X with t followed the relationship X=Ktⁿ, where the constant K and the exponent n were found to depend on the carbon content and on the total alloying element content in the steel strip. With increasing total alloying elements, the thickness decreased. Analysis of the reaction layer indicates the formation of FeAl₂ in most cases in addition to FeAl₃, Fe₂Al₅, and Fe₂Al₃.