Influence of the slurry thickness and heat treatment parameters on the formation of aluminium diffusion coating

Formation of the FeAl3 or Fe2Al5 phase during the aluminization of iron-based alloys causes detrimental behaviour of the material due to the brittleness of these phases and the different coefficient of thermal expansion between the base material and the resulting diffusion coating. In order to control the microstructure of the produced diffusion zone and its evolution, two different slurry thicknesses (30-50 and 100-150 µm) using high-purity aluminium spherical particles and three different heat treatments times (5, 10 and 20 h) were tested over ferritic-martensitic P92 steel. After the heat treatment, iron-aluminide phases rich in aluminium content were formed. After 1350 hours of exposure in air at 650 °C, it was found that for the coatings in the range of 30-50 µm neither FeAl3 nor Fe2Al5 phase remained in the diffusion coating, while Fe2Al5 did remain in those coatings in the range of 100-150 µm.