The mechanism of tungsten alloy plating was investigated. Absorption spectra indicate the possibility of cobalt tungstate complexes in the citrate cobalt‐tungsten plating bath. Polarographic evidence is given for the existence of complexes of tungstate and citrate. Calculations of energies of activation of iron‐tungsten alloy deposition show that the tungstate reaches the cathode by diffusion rather than by migration. The mechanism postulated for the plating of tungsten alloys includes (a) the deposition of a film of partly reduced tungstate on the cathode and (b) the catalytic reduction of this film by hydrogen in the presence of freshly deposited iron, cobalt, or nickel.
An aqueous bath suitable for the electrodeposition of cobalt‐tungsten alloys, containing approximately 50 per cent tungsten, is described. The bath contains cobalt sulfate, sodium tungstate, and citric acid in the approximate mole ratio of 1:1:1.5. A bath pH of about 7 is most satisfactory and is obtained by the addition of ammonium hydroxide. The bath temperature should be 70°C. or above. A cathode current density of 15 amp./dm.2 is suitable, although lower current densities decrease the tungsten content of the deposit only slightly and result in higher current efficiencies. Bright cathode deposits are obtained over a wide range of current densities. Anodes of cobalt or of tungsten, of both of these metals, or of some inert material may be used.
MECHANISM OF ANODE THERMAL REACTION AI~O~ -4-3C ~ 2AI -4-3C~O is --1.45 v at 971~ with the AI in the pure molten state, the carbon in the pure solid state, the Al~O~ dissolved in cryolite to give a 5 % solution by weight, and the compound CsO attached to the carbon anode surface. The free energy change for the reaction as written above is then 6 X 96,500 >< 1.45 AG~ = --n FE = = -4-200,609 cal 4.185 CO2 AG~ = --151,500 cal/g mole CO.~ CO AGf = --76,570 cal/g mole CO CsO AGf = --67,150 cal/g mole C~OThus it appears that the compound C~O is thermodynamically less stable than either CO or CO2, and its formation during electrolysis must be postulated to account for the observed reversible voltage and its decay with time on open circuit.
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