Analysis of coating layer formedon steel strips during aluminising by hot dipping in Al-Si baths

Abstract

Aluminising of low carbon (0.19 wt-%C) steel was carried out using AI-0, 4, 8, and 12 wt-%Si melts. Different dipping times and melt superheat were used. In all cases, a coating layer was formed which is composed of an intermetallic layer and an aluminium top coat layer. The thickness of the intermetallic layer increased with bath temperature, especially in pure aluminium baths, and decreased with increasing silicon content. Addition of more than 8 wt-%Si to the bath had no detectable effect on the thickness. This thickness X increased with time following the parabolic relationship X =KTⁿ, where the growth rate constant K decreased with silicon content. Energy dispersive X-ray analysis revealed that the intermetallic layer is composed of a thick layer of AI₅Fe₂ followed by a much thinner one of Al₃Fe on the aluminium side. In the case of a silicon containing bath, different Al_xFe_ySi_z compounds were identified. The kinetics of the reaction between solid steel and liquid aluminium were studied. In AI-Si baths, the growth rate of the intermetallic layer was lower and its dissolution rate higher compared with a pure aluminium bath. Separation of parts of the layer was also found. Iron loss from the steel strip, especially in the case of AI-Si baths, was partly used in the formation of both the measured and the dissolved layer and partly dissolved in the melt. The growth rate of the layerwas evaluated and the activation energy was found to be 138.46 and 106.65 kJ mol^-1, for pure aluminium and Al-8Si baths respectively.