Formation mechanism of high activity aluminide coating on Ni-CeO2 coated Rene 80 alloy

High temperature oxidation behavior of protective oxide scales can be improved by surface doping with reactive elements such as cerium. In this investigation, an intermediate Ni-CeO2 layer was electrodeposited on the surface of Rene 80 nickel-based superalloy before pack cementation aluminizing treatment. Electrodeposition of the Ni-CeO2 layer was carried out using direct current and Watts nickel electroplating bath containing 30 g/lit nano ceria particles. Electrodeposited specimens were aluminized in a powder mixture of aluminum, ammonium chloride, and alumina by a two-step treatment at 760 and 1040 °C to form an inward growth-type coating. Cross-sections of the coated specimens were studied by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. The results showed that after aluminizing the composite intermediate layer converts to an Al-rich NiAl layer with a porous and CeO2-free surface zone below which a CeO2-containing layer exists. Three-zone microstructure of two-step aluminide coatings on nickel-based superalloys was observed below the CeO2-containing layer. The mechanism of coating formation was discussed. The oxidation resistance of the CeO2 modified coating in 10 five-hour cycles at 1050 °C was considerably better than the unmodified coating.