Microstructure studies of an aluminide coating on 9Cr-1Mo steel during high temperature oxidation

9Cr-1Mo steel coated by hot-dipping into a molten mixture with 7wt.%Si/93wt.%Al was oxidized at 750, 850, and 950 °C in static air. The oxidation kinetics followed a parabolic rate law at all temperatures. Intermetallic iron–aluminum compounds FeAl3, Fe2Al5, and FeAl2 initially formed on the steel substrate at elevated temperature. At 850 °C the FexAly layer thickness increased rapidly during the first 20 min and Fe2Al5 mixed with FeAl2 became the main phases in the aluminide layer. Some cracks propagated through the FexAly brittle FeAl2 and Fe2Al5 layers. After 24 h oxidation at 850 °C, all FeAl2+Fe2Al5 transformed to FeAl, while FeAl2+Fe2Al5 still existed at 750 °C after 56 h exposure. The Kirkendall effect is a plausible mechanism for the voids observed at the interface between the aluminide layer and the steel substrate.