Electrodeposition of Coherent Deposits of Refractory Metals: I . Niobium

Abstract

A general method has been found for the electrodeposition of coherent, dense deposits of refractory metals by electrolysis of molten salts. By dissolving the refractory metal fluoride in mixtures of alkali metal fluorides, one may deposit coherent thick coatings of chromium, hafnium, molybdenum, niobium, tantalum, tungsten, vanadium, and zirconium. Not only do the deposits in all cases have the theoretical density, but in most cases are extremely pure and equal to or better in mechanical properties than electron beam melted material available commercially. While differences exist from metal to metal in the details of the compositions of the electrolyte and the valence state of the refractory metal, the general process can be described in terms of one of the first metals studied, niobium. Niobium metal is electrodeposited from a solution about 10 w/o in a mixture of alkali fluorides at a temperature of about 775°C and a current density of about 50 ma/cm2 with commercially pure niobium as anode material. Anode and cathode efficiencies are close to 100% and deposits with hardness of 85–100 DPH are obtained regularly. Deposits with densities of 99.8% of theoretical and higher are obtained and the throwing power of the bath is better than that usually associated with commercial nickel plating. There is no major restriction on substrates, since steel, stainless steel, graphite, copper, Hastelloy alloys, and others have been used. Thicknesses up to 1/4 in. have been electrodeposited with a surprisingly smooth surface. The microstructures of the deposits are the typical columnar grain structure usually observed in electrodeposits without addition agents and their soundness has been confirmed by mechanical testing. Also, successful electrorefining has been accomplished.