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Attempts to prepare tantalum coating on tungsten have been performed in 55 mol%LiF-35 mol%NaF-10 mol%CaF 2 melt containing K 2 TaF 7 . Electrolytic deposition of tantalum was carried out by galvanostatic polarization. A conproportionation reaction also occurred that interfered with the electrodeposition and resulted in decrease in current efficiency. However, the product of this reaction was soluble, which diffused away from the electrode without contaminating the deposit. Hence, a compact electrodeposited tantalum coating was obtained on tungsten substrate. An excellent interface was possible when coating was performed in the melt containing 2 mol% K 2 TaF 7 .

Electrochemical study has been carried out on the electro-deposition of tantalum in 55 mol%LiF-35 mol%NaF-10 mol%CaF 2 melt containing 1-2 mol%K 2 TaF 7 at 700 C. This has been done for determining the mechanistic features for preparing electrolytic coating of tantalum on nickel and tungsten substrates. Electro-deposition of metallic tantalum occurs primarily by electro-reduction of Ta(V), i.e., TaF 5 also undergoes reduction to form metallic tantalum. The oxide ions generated by reduction of TaOF 2À 5 are removed from the surface by an oxide ion getter in the LiF-NaF-CaF 2 melt. Pure metallic tantalum without any entrapped salt is successfully deposited on tungsten by galvanostatic polarization at reasonably low current densities. An additional feature on nickel is the formation of an intermetallic compound at potential $0:25 V nobler than that of pure tantalum as a result of underpotential deposition of tantalum. This intermetallic compound covers the surface within a short time followed by deposition of pure tantalum, although intermetallic compound keeps growing at the interface of pure tantalum deposit and the substrate as a result of diffusion. Other secondary reactions are (i) a low activity reduction of Ta(V) to Ta(II) at À0:25 V, (ii) a conproportionation reaction at the surface of electrode leading to partial dissolution of tantalum and, (iii) a corresponding disproportionation reaction that forms powdery metallic tantalum at the salt surface.

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