The high-temperature oxidation behavior of hot-dipping Al–Si coating on low carbon steel

Low carbon steel was coated by hot-dipping into a molten bath containing Al–10 wt.% Si. The high-temperature oxidation behavior of the specimen was tested at 750, 850 and 950 °C for 72 h in air using a thermobalance. The element distribution, phase composition, and morphology of the aluminide layer and the oxide scale were characterized by OM, XRD and SEM/EDX. After hot-dip treatment, the coating layers consisted of three phases, where Al, FeAl3, and Fe2Al5 were detected from external topcoat to the aluminide/steel substrate. The result of high-temperature oxidation test showed oxidation kinetics basically followed the parabolic rate law at all temperatures. The Fe2Al5 formed during the immersion process completely transformed to FeAl2, FeAl and α-Fe(Al) phases because of the composition gradient and the chemical diffusion by oxidation. In the present study, Kirkendall voids were found to form at the coating/substrate interface due to the rapid inter-diffusion of iron and aluminum during oxidation and, therefore, the adherence of the coatings should be compromised. Afterwards, loss of protective behavior of coating layer occurred only by iron oxide nodules formed on the coating specimen exposed at 850 °C for 24 h.