Diffusion aluminide coating modified via electroless nickel plating for Ni-based superalloy protection

Diffusion aluminide coatings are widely employed to increase oxidation resistance of superalloy components. This work presents an innovative modified diffusion coating on a Ni-based superalloy obtained by the deposition of an electroless nickel layer (ENL) before the pack cementation aluminizing process. ENL was also employed as a vehicle to embed α-Al2O3 nanoparticles within the aluminide coating, to promote a faster formation of a stable protective oxide. Plain and modified coatings were compared by means of XRD, SEM and EBSD. Differences in oxidation kinetics were evaluated with isothermal oxidation tests at 1050 °C: modified coatings present higher protective capabilities, characterized by a lower parabolic oxidation constant than standard aluminides. SEM and EDS analysis after 1000 h of test demonstrated a continuous and well-adherent thermally grown oxide (TGO) scale that was still protective, in contrast to the degradation evidenced in plain coatings. Nanoparticles mobility was not promoted by the diffusion treatments, thus preventing any beneficial effect on TGO growth and high temperature oxidation resistance. The proposed procedure based on deposition of an external electroless nickel layer, therefore, demonstrates a remarkable increase of protective performances of the aluminide coatings.