The slurry aluminizing process is widely employed to enhance the oxidation and corrosion resistance of nickel-based superalloys used in high-temperature environments such as gas turbines and aerospace engines. This study investigates the effects of the concentration of Al vapors in the reactor chamber and the initial slurry layer thickness on the microstructure, chemical composition, and phase composition of aluminide coatings. Coatings were manufactured on Ni-based superalloy substrates using CrAl powders as an aluminum source and chloride- and fluoride-based activator salts. The effect of the initial thickness of the slurry layer was studied by varying the amount of deposited slurry in terms of mg_slurry/cm²_sample (with constant mg_slurry/cm³_chamber). The microstructure and phase composition of the produced aluminide coatings were evaluated by SEM, EDS, and XRD analysis. Slurry thickness can affect concentration gradients during diffusion, and the best results were obtained with an initial slurry amount of 100 mg_slurry/cm²_sample. The effect of the Al vapor phase in the reaction chamber was then investigated by varying the mg_slurry/cm³_chamber ratio while keeping the slurry layer thickness constant at 100 mg_slurry/cm²_sample. This parameter influences the amount of Al at the substrate surface before the onset of solid-state diffusion, and the best results were obtained for a 6.50 mg_slurry/cm³_chamber ratio with the formation of 80 µm coatings (excluding the interdiffusion zone) with a β-NiAl phase throughout the thickness. To validate process flexibility, the same parameters were successfully applied to produce platinum-modified aluminides with a bi-phasic ζ-PtAl2 and β-(Ni,Pt)Al microstructure.

Nickel-based superalloys require protective low-activity aluminide coatings to withstand high-temperature oxidation and corrosion in turbine applications. As opposed to conventional gas processes, this study investigates the mechanisms of formation of alternative low-activity nickel aluminide coatings on the René N5 superalloy through electroless nickel pre-deposition followed by slurry aluminizing. Different thicknesses of electroless nickel layers (5, 10, 25 μm) were deposited and subsequently aluminized with varying slurry amounts (5–16 mg/cm²) under controlled heat treatments at 700–1080 °C with heating rates of 5 and 20 °C/min. Without electroless pre-deposition, high-activity coatings with refractory element precipitates formed. With electroless nickel, a precipitate-free low-activity coating developed, with thickness increasing linearly from 15 to 40 μm proportional to the initial electroless layer. An increasing slurry amount raised the overall coating thickness from 27 to 67 μm. Kirkendall porosity formed exclusively during the δ-Ni₂Al₃ to β-NiAl phase transformation at elevated temperature. Reducing the heating rate from 20 to 5 °C/min significantly decreased void formation by promoting more balanced Ni-Al interdiffusion. This work demonstrates that combining electroless nickel with slurry aluminizing provides an efficient route for producing low-activity coatings with controlled microstructure and minimal porosity.