Formation of the β-NiAl Containing Hf by the Simultaneous Electrodeposition of Al and Hf using a Molten-Salt and the Cyclic Oxidation Behavior

Abstract

The formation of a coating layer consisting of a Ni aluminide containing Hf on a Ni substrate was attempted by the electrodeposition of Al and Hf. Especially, the effects of the HfF₄ concentration in the electrolytic bath, electrodeposition temperature and potential on the type of Ni aluminide phase and the morphology of the Hf-rich precipitates in the Ni aluminide were investigated. The cyclic oxidation resistance of the Ni covered with the coating was then evaluated. Al and Hf were deposited by molten salt electrolysis. For the sample treated with the Al and Hf deposition at 1023 K, a Ni₂Al₃ layer was formed, and no Hf in the layer was observed. On the other hand, for the sample treated with the Al and Hf deposition at 1173 K and −1.25 to −1.8 V, a NiAl layer containing Hf was formed. It was found that a thick NiAl layer was formed at 1173 K and −1.5 V. The cyclic oxidation test result showed that the oxidation rate of the untreated sample, the sample coated with a Ni₂Al₃ layer without Hf and the sample coated with a thin NiAl layer, were high. However, for the sample coated with a NiAl layer containing a small amount of Hf, the mass gain rate was low. On the sample after the cyclic oxidation test, a scale consisting of α-Al₂O₃ and HfO₂ adhering to the substrate was formed.