ANALYSIS OF COATING LAYER FORMED ON STEEL STRIPS DURING ALUMINIZING BY HOT DIPPING IN AL-SI BATHS

Aluminising of low carbon (0.19 wt-%C) steel was carried out using Al- 0, 4, 8, and 12 wt-%Si melts. Different dipping times and melt superhe at were used. In all cases, a coating layer was formed which is compos ed of an intermetallic layer and an aluminium top coat layer. The thic kness of the intermetallic layer increased with bath temperature, espe cially 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 followin g the parabolic relationship X = KTn, where the growth rate constant K decreased with silicon content, Energy dispersive X-ray analysis reve aled that the intermetallic layer is composed of a thick layer of Al5F e2 followed by a much thinner one of Al3Fe on the aluminium side. In t he case of a silicon containing bath, different AlxFeySix compounds we re identified. The kinetics of the reaction between solid steel and li quid aluminium were studied. In Al-Si baths, the growth rate of the in termetallic layer was lower and its dissolution rate higher compared w ith a pure aluminium bath. Separation of parts of the layer was also f ound. Iron loss from the steel strip, especially in the case of Al-Si baths, was partly used in the formation of both the measured and the d issolved layer and partly dissolved in the melt. The growth rate of th e layer was evaluated and the activation energy was found to be 138.46 and 106.65 kJ mol(-1), for pure aluminium and Al-8Si baths, respectiv ely. (C) 1997 The Institute of Materials.