Characterization of Niobium Platings Obtained from NaCl-KCl Melts

NaCl-KCl-Nb(X) and LiCl-KCl-Nb(X) melts have been investigated as possible media for deposition of niobium. Constant current and different pulse programs were applied. The average current densities employed were in the range 10 to 220 mA/cm2. Niobium ions were generated in the melts by chemical oxidation of metallic niobium with NiCl2.

The initial valency of niobium was found to 3 when a NaCl-KCl melt was applied as solvent. The deposits were characterised by X-ray analy-sis and Scanning Electron Microscopy (SEM) combined with EDX analysis. From both NaCl-KCl and LiCl-KCl melts dense and coherent deposits with an uniform thickness (>50µm) could be obtained at temperatures in the vicinity of 750°C.

In general the temperature had a great impact on the material distribution and crystallisation. Thus deposits obtained from LiCl-KCl melts at temperatures lower than 650°C were porous and non-uniform in thickness. At temperatures below 550°C no niobium metal could be identified in the product.When metallic nickel was used as substrate intermetallic compounds (e.g.

NbNi3) were formed in the interface between the substrate and the deposit. This resulted in poor adherence of the deposit. In the case of AISI316 stainless steel substrates no intermetallic phases were observed. Further an excellent adhesion of the deposited niobium layer was obtained.Deposits (on nickel substrates) from NaCl-KCl melts at 750°C with oxide added (molar ratio O/Nb greater than 0.5) were thin and consisted of niobium-nickel alloys contaminated with NbO2.