Formation of Ni-aluminide Coating Containing Hf by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

Abstract

The formation of a coating layer consisting of Ni-aluminide containing Hf on a Ni–10at.%Cr–8at.%Al alloy substrate was attempted by the electrodeposition of Hf, Ni, and Al. The cyclic-oxidation resistance for the alloy covered with this coating was then evaluated in air at 1,423 K. Ni was deposited by aqueous solution electrolysis. Hf and Al were deposited by molten-salt electrolysis. For the sample first treated with the Hf-deposition, subsequently treated with the Ni-deposition, followed by Al-deposition, a coating consisting of Ni₂Al₃ of about 40-μm thickness was uniformly formed on the alloy. At the center region of this coating, a Hf-concentration layer was formed. The cyclic-oxidation test showed that, for the untreated sample and the sample with only Ni and Al depositions, a mass reduction due to spallation of a scale took place during the initial oxidation period. On the contrary, for the sample with Hf, Ni, and Al depositions, a mass reduction due to spallation of a scale scarcely took place. The cross-sectional observation using SEM showed that, for the sample with Hf, Ni, and Al depositions after the oxidation test, an adhesive scale having a spiked shape was formed. This scale mainly consisted of α-Al₂O₃, and contained HfO₂ particles. It was postulated that, for this sample, the Hf in the Hf-concentration layer diffused into the surface region of the Ni-aluminide layer, contributing to improvement in the exfoliation resistance of the scale.