Formation and oxidation performance of low-temperature pack aluminide coatings on ferritic–martensitic steels

A pack cementation process was developed to coat commercial 9% Cr ferritic–martensitic steel T91 at temperatures below its normal tempering temperature to avoid any potential detrimental effect on the mechanical properties of the coated alloy. In order to prevent the formation of Fe2Al5 coatings, the Al activity in the pack cementation process was reduced by substituting the pure Al masteralloy with binary Cr–Al masteralloys containing either 15 or 25 wt.% Al. When the Cr–25Al masteralloy was used, a duplex coating was formed at 700 °C, consisting of a thin Fe2Al5 outer layer and an inner layer of FeAl. With the Cr–15Al masteralloy, an FeAl coating of ~ 12 μm thick was achieved at 700 °C. The pack aluminide coatings fabricated at 700 °C are being evaluated in air + 10 vol.% H2O at 650 °C and 700 °C to determine their long-term oxidation performance.