Formation of Al₂O₃/FeAl coatings on a 9Cr-1Mo steel, and corrosion evaluation in flowing Pb-17Li loop

Iron aluminide coating layers were formed on a ferritic martensitic grade 9Cr-1Mo (P 91) steel using pack aluminizing process. The formation of different aluminide compositions such as orthorhombic-Fe₂Al₅, B2-FeAl and A2-Fe(Al) on the pack chemistry and heat treatment conditions have been established. About 4–6 μm thick Al₂O₃ scale was formed on the FeAl phase by controlled heat treatment. The corrosion tests were conducted using both the FeAl and Al₂O₃/FeAl coated specimens in an electro-magnetic pump driven Pb-17Li Loop at 500 °C for 5000 h maintaining a flow velocity of 1.5 m/s. The detailed characterization studies using scanning electron microscopy, back-scattered electron imaging and energy dispersive spectrometry revealed no deterioration of the coating layers after the corrosion tests. Self-healing oxides were formed at the cracks generated in the aluminide layers during thermal cycling and protected the base alloy (steel) from any kind of elemental dissolution or microstructural degradation. - Highlights: •Al₂O₃/FeAl coating produced on P91 steel by pack aluminizing and heat treatment. •Corrosion tests of coated steel conducted in flowing Pb-17Li loop at 500 °C for 5000 h. •Coating was protective against molten metal corrosion during prolonged exposure. •Self-healing protective oxides formed in the cracks generated in aluminide layers.