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Electro-deposition of Cobalt

Although nickel has been very largely used for commercial electroplating purposes, there would appear to be a useful field for a cobalt electroplate industry. The earlier literature on the subject is full of contradictions,2 but this is readily understood when it is remembered what an important influence upon the nature of deposited metals is exerted by different factors such as temperature, current density, chemical composition of the solution employed, etc.

A solution of a cobalt salt, or a mixture of salts containing cobalt, is prepared and a cobalt anode inserted. The carefully cleaned article to be plated is introduced as cathode, and a suitable current passed through the system.

It is most important that the anode shall consist of cobalt in a high state of purity. In view of the improvements in the metallurgy of that metal, this condition is not now an insuperable barrier to success as a very pure metal may now be obtained on a commercial scale. The anodes may be either cast or rolled sheet according to whichever is the more convenient or more easily obtainable. The difference between the two is not so important as in the case of nickel, since cobalt dissolves rather more readily; the anodes, however, should be annealed and thoroughly cleansed before use.

The problem of a suitable solution has long been a matter of discussion. Sylvanus Thompson in 1887 recommended and patented the following mixtures:

Cobalt ammonium sulphate1 lb. or 500 grams
Magnesium sulphate½ lb. or 250 grams
Ammonium sulphate½ lb. or 250 grams
Citric acid1 oz. or 31.2 grams
Water1 imperial gallon or 1½ U.S. Gallons or 5 litres

Cobalt sulphate½ lb. or 250 grams
Magnesium sulphate¼ lb. or 125.grams
Ammonium sulphate¼ lb. or 125.grams
Water1 imperial gallon or 1½ U.S. gallons or 5 litres

These solutions yield their optimum results at about 35° С. A simple solution, rapidly made up, consists of:

Cobalt potassium sulphate - 1 lb.

Water - 1 imperial gallon

and its efficiency is increased by the addition of approximately 1 oz. of sodium hypophosphite.

For these solutions a cathode current density of 1½ amperes per square foot is recommended, although for the first few seconds of immersion a little higher current may be applied, but should be quickly reduced. The voltage may advantageously lie in the neighbourhood of 2 volts.

Kalmus and his co-workers recommend the following compositions:
  1. A solution of 200 grams of cobalt ammonium sulphate, (NH4)2SO4.CoSO4.6H2O, equivalent to 145 grams of the anhydrous salt, in one litre of water. The density of the solution is 1.053 at 15° C.

    This solution is useful at all current densities up to 4 amperes per sq. dcm. or 37.2 amperes per sq. foot.
  2. A solution in one litre of water of the following:

    Cobalt sulphate, CoSO4 - 312 grams

    Sodium chloride, NaCl - 19.6 grams

    Boric acid, H3BO3 - nearly to saturation

    The density of the above solution is 1.25 at 15° C.

    This solution plates most satisfactorily with a current density ranging from 3.5 to 26.4 amperes per sq. dcm., the latter figure being equivalent to over 240 amperes per sq. foot; and even at this speed the limit of the solution is not reached. The solution does not change appreciably either in cobalt content or in acidity over long periods of time, and is thus particularly useful for commercial practice.
The following conclusions are arrived at by Kalmus:
  1. The electro-deposition of cobalt from either of the foregoing solutions on to brass, iron, steel, copper, tin, lead, Britannia metal, and German silver may be effected under conditions similar to those employed for nickel. The deposit is firm, adherent, hard, and uniform. It is readily buffed to a finished surface having a beautiful lustre, possessing a slightly bluish cast, although beautifully white.
  2. The deposit is much harder than nickel, so that less weight (about one-fourth) is needed for equal protection. The plating satisfactorily withstands the various hammering, bending, and burnishing tests usually applied.
  3. The electric-conductivity of the cobalt solutions is considerably higher than that of commercial nickel solutions. Other things being equal, therefore, the cobalt solutions may be worked at a lower voltage for a given speed of plating; or solution (1) may be worked four times, and solution (2) some fifteen times as fast as the most rapid nickel solution.
Other workers, who have had occasion to repeat the work of Kalmus report considerable difficulty in obtaining thick deposits of cobalt consistent with a high quality of adhesion. The solutions recommended by Kalmus yield quite satisfactory results tor very thin deposits, but the case is otherwise where deposits are required ranging in thickness from 3 to 5 thousandths of an inch and upwards. In these cases the deposited metal is found to separate very readily from its base. Furthermore, in view of more recent research on the rapid deposition of nickel, it would appear that the possibilities of improvement in this direction have by no means been exhausted. Consequently the third conclusion of Kalmus relating to the relative speeds of deposition of cobalt and nickel may shortly require revision.

From the foregoing it is evident that the whole subject is worthy of further careful commercial investigation.

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