The Influence of Pd on the Structure and Oxidation Performance of β-NiAl Diffusion Coatings

Aluminized coatings on Ni based alloys greatly contribute to achieve process efficiency at higher operating temperatures. The present study characterized Pd-modified and unmodified aluminized coatings and compared with those of Pt-modified coatings regarding the mechanisms of formation and oxidation performance. The results show that Pd reduces the driving force for diffusion of Al during coating formation, increases outward diffusion of Ni and reduces diffusion of alloying elements (Cr and Ti) into the intermetallic layer. In contrast, Pt increases the driving force for diffusion of Al and the mobility of Al in the intermetallic layer of the aluminized coating. These characteristics have a direct impact on oxidation at 1000 degrees C that showed that Pd reduced the rate of theta-Al2O3 -> alpha-Al2O3 transformation, accounting for higher density of voids at the interface beta-(NiAl)/theta-Al2O3, diffusion of oxygen into the coating, spalling and faster degradation of coatings.