Preparation of Ni-Al-Pt Coating on Ni-6 at%Cr Alloy by Electrodeposition Method and Cyclic-Oxidation Resistance

The formation of a coating layer consisting of Ni, Al and Pt on a Ni-6 at% Cr alloy substrate was attempted by the electrodeposition of Pt and Al. The Pt and Al were deposited by electrolysis in an aqueous solution and molten salt, respectively. The cyclic oxidation resistance of the alloy with this coating was then evaluated in air at 1423 K. Particular attention has been paid to the effects of Pt in the coating and 6 at% Cr in the alloy substrate on the cyclic oxidation resistance. For the sample treated by Pt deposition, followed by the Al deposition, a coating layer comprised of an outer layer consisting of PtAl2 and Ni2Al3 and an inner layer consisting of Ni2Al3 was formed. This coating layer contained about 2 at% Cr. The cyclic oxidation test showed that for the sample with a coating consisting of Ni aluminide without Pt, a mass reduction due to spallation of an oxide scale took place during the oxidation test, whereas for the sample with a coating consisting of Ni aluminide containing Pt, no mass reduction took place. For the sample with a coating consisting of Ni aluminide containing Pt after the oxidation test, the formation of an adhesive Al2O3 scale was observed. The cyclic oxidation test also showed that for the sample with a coating consisting of Ni aluminide containing Pt formed on a pure Ni substrate, a large mass increase took place during the oxidation test, while for the sample with the coating formed on the Ni-6 at% Cr alloy substrate, such a mass increase did not take place. Such effects of Pt in the coating and 6 at% Cr in the alloy substrate on the cyclic oxidation resistance are discussed from the viewpoint of the formation of a protective Al2O3 scale.