(a) Nickel deposited by the wet way is white, with a slightly yellow tinge, having a dull pearl-grey dead lustre; it is obtained by dissolving nickel nitrate in its own weight of ammonia, and diluting the whole with 20-30 times its volume of liquid soda bisulphite, marking about 24° B. This application is found useful when articles require to be protected against the oxidising action of damp or salt air, sulphurous gases, and weak acids. Nickel electrotypes stand the wear and tear caused by ink and press much better than the ordinary copper ones.
(b) Another bath is a solution of nickel nitrate, without excess of acid, precipitated by potassium cyanide, and the precipitate redissolved by more cyanide. An acid solution of nickel may be precipitated by alkalies, such as potash, soda, or ammonia; after washing the precipitate, dissolve in potassium cyanide. A moderate battery power and nickel anodes are employed.
(c) Without A Battery
Into the plating vessel, which may be of porcelain or copper, place a concentrated solution of zinc chloride, dilute it with 1-2 volumes of water, and heat to boiling. If any precipitate separates, it is to be redissolved by adding a few drops of hydrochloric acid. As much powdered zinc as can be taken on the point of a knife is thrown in, which covers the vessel internally with a coating of zinc. The nickel salt, for which purpose either the chloride or sulphate may be used, is added until the liquid is distinctly green; then put in the articles to be plated, previously thoroughly cleaned, together with some zinc fragments. Continue the boiling for 15 minutes, when the coating of nickel is completed. Well wash the articles with water, and clean with chalk. If a thicker coating is desired, the operation may be repeated. Wrought and cast iron, steel, copper, brass, zinc, and lead have been successfully coated by this process. It is necessary that the objects should be entirely covered by the plating liquid, and that their surfaces should be thoroughly cleaned.
(d) The plant necessary to commence nickel plating consists of a battery, preferably of the Smee type, with carbon negative; a well-bolted oblong wooden tank, of a size to suit the articles to be plated, coated on the inside with good asphalt, and nearly filled with the nickel solution; nickel plates for anodes, and brass rods to suspend the plates and work in the bath; suitable vessels for an alkali, an acid, and soft water for cleaning the work before placing it in the nickel bath; polishing and buffing lathes, rouge, crocus, etc. The bath may be composed either of nickel chloride and ammonia or the corresponding sulphate, dissolved in pure water. If the latter is used, the solution must be kept neutral and up to about 6°. It is prepared by dissolving 3/4 lb. of the salt in each gallon of water. This salt is generally considered the best for nickel plating. The chloride bath requires about 4 oz. of the fait per gallon, and works better with a slight acid reaction, the tendency in working being towards alkalinity, even with great exposure of anode. The intensity of battery* current must be proportioned to the bath, and remain constant.
Large baths offer less resistance to the electric current than those of smaller dimensions, and can therefore be worked with a current of somewhat less tension. For a bath of 10 gal. or less, the tension of the current should be equal to that of 2-3 Smee cells (carbon and zinc) in series. The exposed surface of the nickel anodes should in no case be less than the surface to be coated, but may with advantage be greater. The amount of battery power for a given amount of work should be in zinc.surface equal to the surface to be coated, with care to preserve the normal tension of the current. If the current is too intense the coating will present a dull white or frosted appearance. The anodes must be in connection with the negative plate (carbon) of the battery. Damage is not iinfrequently done to the bath and work by misconnection. The work should be scrupulously clean when entered to the bath, and should be carefully moved about after entering to free it from any adhering air-bubbles. If the finished work is to have a smooth polishing surface it must present such a surface before entering the bath. Nickel is hard and cannot well be burnished. Traces of oil and grease are removed by a hot soda solution. After dipping in clean water the surface is freed from films of oxide by an acid bath.
If the work is of iron the acid may be hydrochloric diluted with 3-4 volumes of water; if of copper or brass, of nitric acid diluted with about 20 parts water. Brighten the work in the acid dip, then immerse momentarily in water; go over it with a clean stiff brush and very fine sand; again dip in the acid, then quickly in soft water, and place immediately in circuit. The hand must not come in contact with the surface of the work after removal from the alkali, as the slightest touch may spoil all. On removal of the work from the plating bath it should be immediately dipped in cold water and transferred to hot water, which will cause it, when taken out, to dry quickly and perfectly. The bath should be covered when not in use, to keep out dust and prevent as much as possible its evaporation.
By a little practice and proper attention to these simple rules the nickel bath may be worked continuously, month after month, and the metal deposited smoothly and with certainty. Magleto-electric machines, such as those of Gramm and Weston, are now gradually replacing galvanic batteries in large electro-plating establishments.
(e) Complaint is often heard against white nickeling not succeeding. This may be due to the current, that is too weak or too strong, or to the composition of the bath, but frequentlythe cause is to be sought in the nickel film being too thin, so that the metal beneath, which is generally brass, shows through. In the case of iron this is not so striking, owing to the similarity in the colour of iron and nickel. But here there is another disadvantage of thin nickeling, that the iron rusts. There is always danger of rusting, even when well plated, if the iron has been cleansed in acid. This evil may be entirely overcome by using the sand blast instead of acid pickle. Another advantage gained is that the surface is roughened and the nickel adheres to it better, while subsequent polishing is unnecessary.
Experience has shown that scythes cannot be put in pickle before nickeling, as they soon become checked or cracked in the bath. Some scythes were polished with sand blast, and then nickel plated with entire success. This would seem to solve the problem of how to best protect scythes from rust, for the innumerable experiments and attempts to protect them with varnishes have always given negative results.
One difficulty often met with in nickel plating brass and zinc should not go unmentioned. These and other metals which are flexible, yet only slightly elastic, do not quite return to their original shape after the bending force has been removed, while nickel is so elastic that it endeavours to return to its former position.
This is frequently the cause of nickel plate getting loose when deposited on these metals. A thin layer of nickel sticks better, but, as already mentioned, does not prevent the other metal from showing through, while it offers little or no protection against oxidation.
(/) By the process used at Mons, in Belgium, a thick plating of nickel may be deposited upon any metal by a feeble electric current in a very short space of time. The composition of the bath is as follows: Nickel sulphate, 1 kilo.; neutral ammonia tartrate, '725 kilo.; tannic acid, '005 kilo.; water,20 litres. The neutral ammonia tartrate is obtained by saturating a solution of tartaric acid by ammonia. In the same manner the nickel sulphate must be exactly neutralised. Three or four litres of water are at first aided, and the solution is made to boil about 1/4 hour. The rest of the water is then added, and the liquid is filtered or decanted. This bath may be renewed indefinitely by adding the same materials and in the same proportions. The deposit obtained is brilliantly white, soft, and homogeneous. Even when obtained of great thickness there are no irregularities on the surface, and it has no tendency to scale. Some very thick deposits of nickel upon both rough and polished cast-iron goods have been obtained by this process at a cost scarcely exceeding that of copper.
(g) For the baths, Gaiffe gives the following formula:
Nickel and ammonium, double sulphate...... 1 kilo.
Distilled water...... 10 litres.
Roseleur prefers to take:
Double sulphate...... 400 grm.
Ammonium carbonate .. 300 grm.
Distilled water...... 10 litres.
Each of the two salts is dissolved separately in a part of the water. The solution of ammonium carbonate is gradually poured into that of nickel, taking care not to pass the point of neutrality. The quantity of 300 grm. ammonium carbonate indicated above is not obligatory, but may be varied according to the quality of the salt of nickel.
(A) Adams proposes the two following mixtures:
Take 135 grm. pure nickel, and dissolve in hydrochloric acid, avoiding excess, and heating gently. When all is dissolved, add 2.25 litres coll water, and aid gradually ammonia until the liquid is neutral to litmus paper. Dissolve separately 70 grm. sal ammoniac in water, and mix with the former solution, and make up to 10 litres with cold water.