On the Electrodeposition of Tungsten-Cobalt Alloys From Aqueous Solutions

SynopsisSound coherent deposits of tungsten-cobalt alloys with up to 60–66 wt. % of tungsten can be produced at cathode current efficiencies of up to 21% from solutions containing 40–70 g./l. of tungsten as sodium tungstate, 4–12 g./l. of cobalt as cobalt sulphate, 70–150 g./l. of citric acid and 50 g./l. of ammonium chloride, adjusted to room-temperature pH of 7·5–8·5 with sodium hydroxide, and operated at 50–70° C. with interrupted d.c. pulses of 0·05–2 sees, and equal or rather greater “off” periods, the cathode current density during the pulses being 5–20 amps./dm.2. Moderate agitation of the solution is desirable.Hydrogen overpotential for hydrogen deposition from citrate buffers is much less on tungsten-cobalt cathodes than on tungsten or cobalt cathodes. Evidently the electronic structure of the alloy cathodes is much more favourable than that of tungsten for hydrogen deposition: this suggests that it is also more favourable for tungsten deposition, which may help to explain why tungsten-cobalt alloys are so much more easily deposited than pure tungsten.A number of amphipathic organic molecules, among them thiourea, benzenesulphonamide and pyridine, increase the hydrogen overpotential on tungsten-cobalt alloys: but they do not increase the cathode current efficiency of alloy deposition, evidently because they increase the metal-deposition overpotential about equally.