The Effect Of Electrolysis Parameters On The Coating Composition And Properties During Electrodeposition Of Tungsten Carbides And Zirconium Diborides

Abstract:

The potential ranges of the deposition of tungsten, carbon, and tungsten carbide dissolved in a NaCI-LiCl (3:1 mass ratio) melt were determined. The reduction of tungstate ions to tungsten proceeds at potentials by 100-150 mV more negative than the reduction of carbon to carbonate ions. At the concentrations corresponding to the composition of the melt used in the synthesis, the electroreduction of tungstate and carbonate ions is accomplished simultaneously in a sufficiently narrow potential range.Experiments at 1173 K have shown that tungsten carbide coatings can be deposited from a melt containing 5 wt % Na2WO4. When the Na2CO3 content does not exceed 0.2 wt %, continuous deposits of W-W2C alloys (the composition of which also depends on the carbonate concentration) are deposited on the cathode. Continuous W2C deposits appear at a concentration of 0.2-0.5 wt %; continuous W2C-WC deposits appear at 0.5-1.0 wt %; poorly adhered deposits W2C-WC are formed at still higher concentrations. To study the effect of the other electrolysis parameters on the properties and structure of W2C coatings, we have chosen the optimal melt NaCl-LiC1: 0.5 wt %, Na2WO4-0.4 wt % Na2CO3. Continuous deposits were obtained at 1073-1173 K. At lower temperatures, tungsten oxides are deposited together with metal and carbide. Well adhered, uniform, and nonporous coatings were obtained at a cathodic current density of 2-15 A/dm2 and a deposition rate from 2 to 20 μm/h. Chronovoltammetric studies have shown that stable discharge of combined complexes of zirconium and boron can be achieved by maintaining the molar ratio at a level [Zr(IV) + B(III)] : [F-] > 1 : 4 (at a ratio [Zr(IV) : [B(III)] = 1 : 2) in the KC1-NaC1 melt. Under these conditions, the voltammograms demonstrate a wave of electroreduction of chloride-fluoride heterocenter complexes of zirconium and boron in place of the waves of partial electroreduction of zirconium and boron. In this case, the uniform phase of zirconium diboride is formed in a wide current density range. The absolute magnitude of the overall concentration of Zr(IV) and B(III) in the electrolyte (in a range of 0.6-30 wt %) has no significant effect on the product composition, as well as technological and economic parameters of the process. The temperature threshold of synthesis in the melt studied lies in the interval of 923-943 K. The temperature interval of 1073-1173 K is optimum at a cathodic current density of 5-20 A/dm2.

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