Formation of Aluminide Coatings on Ferritic–Martensitic Steels by a Low-Temperature Pack Cementation Process

A pack cementation process was developed to coat commercial 9% Cr ferritic–martensitic steel T91 at temperatures below its tempering temperature (∼ 750 °C) to avoid any potential detrimental effect on the mechanical properties. Two Cr–Al binary masteralloys with different Al levels were used to explore possibilities of forming FeAl-based coatings without the Al-rich intermetallic phases, such as Fe2Al5. In contrast to the pure Al masteralloy, which led to the formation of Fe2Al5 coatings at 650 °C, a coating consisting of a thin Fe2Al5 outer layer and an FeAl inner layer was formed at 700 °C by utilizing a Cr–25Al masteralloy. When the Cr-15Al masteralloy was employed, thin FeAl coatings (∼ 12 μm) containing < 50 at.% Al were achieved at 700 °C. The effect of the amount of masteralloys on coating growth was investigated using a series of packs containing 2NH4Cl–x(Cr–15Al)–(98–x)Al2O3, where x ranged from 10 to 70 wt.%. Both coating thickness and surface Al content increased, when the amount of the masteralloy was increased from 10 to 40 wt.%. However, no further increase was observed once the masteralloy amount was greater than 40 wt.%.