Intermetallic formation of Al-Fe and Al-Ni phases by ultrafast slurry aluminization (flash aluminizing)

Aluminizing of Fe and Ni-based substrates is a conventional process often made by gas phase related techniques (CVD, pack cementation and above the pack), whereby the coating thickness is controlled by solid-state diffusion. Similar coatings can be achieved by slurry aluminizing where self-propagating high temperature synthesis and solid-state diffusion mechanisms are involved. The present work investigates ultrafast (5 min annealing) aluminization to reduce coating time and compares the results on pure iron and nickel substrates with a conventional slurry aluminizing treatment (2 and 5 h). The use of fast heating ramps (25 and 100 degrees C/min) led to outwardly diffused coatings even with very high Al slurry activities. On nickel, the coatings were homogeneous while on iron, Kirkendall porosity occurred in the outermost layers due to the great dissolution of Fe in the Al melt and its subsequent outward diffusion towards to the Al source.