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

Abstract

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 FeAl 3, Fe 2Al 5, and FeAl 2 initially formed on the steel substrate at elevated temperature. At 850 °C the Fe x Al y layer thickness increased rapidly during the first 20 min and Fe 2Al 5 mixed with FeAl 2 became the main phases in the aluminide layer. Some cracks propagated through the Fe x Al y brittle FeAl 2 and Fe 2Al 5 layers. After 24 h oxidation at 850 °C, all FeAl 2+Fe 2Al 5 transformed to FeAl, while FeAl 2+Fe 2Al 5 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.