Effect of silicon and manganese on the kinetics and morphology of the intermetallic layer growth during hot-dip aluminizing

Hot dip aluminizing (HDA) of low carbon steel in aluminum bath containing various amounts of silicon and manganese was studied in this work. The effect of bath alloying elements on the morphology and growth kinetics of intermetallic layer during the reaction of steel substrate and aluminum bath was investigated by scanning electron microscope equipped with EDS. Results showed that Fe2Al5 (eta) and FeAl3 (theta) layers are detectable when pure aluminum bath is used. In contrast, when the bath contains 6 and 12 wt% silicon, new intermetallic layer Fe2Al7Si (tau(5)) has appeared adjacent to the topcoat aluminum layer. Moreover, dispersed Fe2Al7Si particles were observed in the topcoat. Presence of silicon in the compound layer had a significant effect on the growth kinetics and intermetallic layer thickness. The inhibiting effect of silicon on the growth of compound layer is possibly due to silicon placement inside the compound layer which retards the diffusion. However, in the bath with 3 wt% manganese, no changes were observed in the composition of the intermetallic layers. Phases of Fe2Al5 (eta) and FeAl3 (theta) were the dominant intermetallic layers with no traces of manganese. Although, dispersed intermetallic particles of the topcoat revealed amounts of manganese. Existence of manganese in the bath indicated no significant effect on the growth rate of the layer and its morphology. Furthermore, a method for evaluating roughness intensity of the compound layer has been developed in this work. Investigations on the tongue-like morphology and roughness of the intermetallic layer revealed that the interface roughness increases linearly with the thickness of the layer. Presence of 3 wt% manganese in the aluminum bath indicated a negligible effect on the thickness and roughness of the layer. However, silicon eliminated the tongue-like morphology of the interface by retarding the rate of the growth.