Abstract
Pack cementation process has been applied for depositing aluminide and platinum-aluminide coatings on a nickel-base superalloy IN-738LC. In order to investigate hot corrosion behaviors of the coatings, specimens covered with a Na2SO4 salt containing 25%NaCl have been heated in the air in the temperatures ranging from 750 to 980°C.
As aluminide coating after pack cementation at 750°C mainly consists of brittle Ni2Al3, diffusion treatment is necessary to convert Ni2Al3 to more ductile and stable NiAl. Aluminide coating after pack cementation at 1120°C mainly consists of NiAl, and chromium precipitates contained in the coating are relatively small in quantity. The hot corrosion resistance of the coating made at 1120°C is inferior to that of the coating diffusion-treated after pack cementation at 750°C.
Platinum-aluminide coating has been made by electroplating of platinum plus pack cementation. When the pack cementation is carried out at 750°C, PtAl2 is formed as a thin layer at the surface of coating after diffusion-treatment. While the mixture of PtAl2 and NiAl in the outer layer is obtainable after pack cementation at 1120°C. Hot corrosion resistance of the platinum-aluminide coating is excellent at 980°C, but inferior at 850°C or less to that of the aluminide coating diffusion-treated after pack cementation at 750°C.
