Formation of High-Oxidation-Resistant NiAl Surface Layer by Simultaneous Electrodeposition of Al and Zr in Molten Salt

Abstract

A coating layer consisting of Zr-containing NiAl was prepared by the simultaneous electrodeposition of Al and Zr using a molten salt as the medium. In particular, the morphology and chemical composition of the coating layer were investigated based on the influence of the ZrF₄ concentration of the bath and the electrodeposition potential. Furthermore, in the surface layer consisting of the NiAl phase, the Zr concentration was measured by ICP analysis of the solution in which the surface layer was dissolved. For the prepared samples, the cyclic oxidation resistance was evaluated in air at 1423 K. For the Ni sample after the electrodeposition in a NaCl–KCl–3.5 mol%AlF₃ bath without ZrF₄ at − 1.5 V, a surface layer consisting of Ni₂Al₃ was formed. On the other hand, when the simultaneous electrodeposition of Al and Zr was carried out in a NaCl–KCl–3.5 mol%AlF₃ salt containing 0.05 mol%ZrF4 at − 1.3 V and the molten salt containing 0.1 mol%ZrF₄ at − 1.3 ~ − 1.5 V, the formation of a surface layer consisting of NiAl was observed. In particular, by the simultaneous electrodeposition in the molten salt containing 0.1 mol% ZrF₄ at − 1.5 V, a thick surface layer consisting of NiAl (about 40 μm) was formed. Furthermore, the detailed analysis of the Zr concentration in the surface layer revealed that the Zr content in the layer was 0.05 at%. Cyclic oxidation tests revealed that a mass loss was observed for the samples coated with the NiAl layer formed by the electrodeposition of only Al. However, for the sample coated with the NiAl layer containing a small amount of Zr, no decrease in the mass due to exfoliation of the oxide was observed. After the cyclic oxidation tests, a scale consisting of α-Al₂O₃ with good adhesion to the substrate metal was formed on the samples coated with the NiAl layer containing a small amount of Zr. This scale included ZrO₂ particles.