Attention has focused on a Ni aluminide containing a small amount of Y as a high oxidation resistant material, and preparation of a coating consisting of this material was attempted by the molten salt electrodeposition which is a low cost coating method. In the present study, the preparation of the coating consisting of this material was carried out by the electrodeposition of Ni using an aqueous solution as the electrolyte and the electrodeposition of Y and Al using a molten salt electrolyte. Furthermore, in order to change the depth of the region containing Y in the Ni aluminide layer, a three step electrodeposition of Ni, Y and Ni was carried out, and the times of the first and third Ni electrodepositions were changed. As a result, surface layers consisting of Ni aluminide with a different depth of the region containing Y were formed. Furthermore, the cyclic oxidation resistance of these specimens was examined in air at 1423 K. As a result, for the specimen with a shallow Y containing region in the Ni aluminide layer, the specimen mass reduction due to spalling of the scale was observed, whereas for the specimen with a deep Y containing region, no specimen mass reduction was observed and a high cyclic oxidation resistance was obtained. For the specimen with a deep Y containing region, which showed the high cyclic oxidation resistance, the scale formed on the coating after the oxidation test consisted of Al 2 O 3 containing Y 2 O 3 , and this scale entered the metal substrate under the scale in the form of a wedge.

A coating of Ni aluminide/Ni or TiAl 3 bilayer on a TiAl substrate was formed by the electrodeposition of Ni with a different deposition time and the electrodeposition of Al using a molten salt electrolyte. The electrodeposition of Al was carried out using a potentiostatic polarization method in molten NaCl KCl AlF 3 at 1023 K. By this electrodeposition treatment, surface layers mainly consisting of Ni 2 Al 3 and intermediate layers consisting of Ni or TiAl 3 were formed. The cyclic oxidation resistance of these specimens was examined in air at 1323 K. As a result, for the specimens with a thick Ni layer as the intermediate layer, an exfoliation of the coating was observed and the mass reduction due to spalling of an oxide scale was observed. On the other hand, for the specimens with TiAl 3 or thin Ni layer as the intermediate layer, the exfoliation of the coating was not observed and a high cyclic oxidation resistance was observed.

The preparation of a coating consisting of Ni aluminide containing a small amount of Hf was carried out by the electrodeposition of Ni using an aqueous solution and the electrodeposition of Hf and Al using a molten salt. Furthermore, in order to change the depth of the layer containing Hf in the coating, a four-step electrodeposition of Ni, Hf, Ni and Al was carried out, and the times of the first and third Ni electrodepositions were changed. As a result, coatings consisting of Ni aluminide, which contained different depths of the Hf-containig layer, were formed. The cyclic-oxidation resistance of these specimens was evaluated in air at 1423 K. For the specimen with a shallow Hf-containing layer in the coating, the specimen mass reduction due to spallation of the scale was observed, whereas for the specimen with a deep Hfcontaining layer, the specimen mass reduction was only slightly observed and a high-cyclic oxidation resistance was obtained. Among the specimens with a deep Hf-containing layer, the specimen prepared by Hf deposition for 0.6 ks showed the highest cyclic-oxidation resistance. For this specimen, the scale formed on the coating after the oxidation test consisted of Al 2 O 3 . This scale was adhesive, and locally entered the metal substrate.

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 PtAl 2 and Ni 2 Al 3 and an inner layer consisting of Ni 2 Al 3 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 Al 2 O 3 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 Al 2 O 3 scale.

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