§ 138 Coppering Aluminium.
A good adherent deposit of copper on aluminium used to be considered a desideratum in the days when it afforded the only means of soldering the latter. Many receipts have been published from time to time, and I have tried, I think, most of them. On no occasion, however, till this year (1896), have I succeeded in obtaining a deposit which would not strip after it was tinned and soldered, though it is not difficult to get apparently adherent deposits so long as they are not operated upon by the soldering iron. The best of the many solutions which have been proposed in years gone by is very dilute cupric nitrate with about 5 per cent of free nitric acid.
The problem of electroplating aluminium which I have indicated as awaiting a solution has at last found one. In the Archives des Sciences physiques et naturelles de Genève for December 1895 (vol. xxxiv. p. 563) there is a paper by M. Margot on the subject, which discloses a perfectly successful method of plating aluminium with copper. The paper itself deals in an interesting way with the theory of the matter — however, the result is as follows.
(1) The aluminium articles are boiled for a few minutes in a strong solution of ordinary washing soda. The aluminium surface is thus corroded somewhat, and rendered favourable to the deposit of an adherent film of copper. After removal from the soda solution the aluminium is well washed and brushed in running water.
(2) The articles are dipped for thirty seconds or so in a hot 5 per cent solution of pure hydrochloric acid.
(3) After dipping in the hydrochloric acid, the work is instantly plunged into clean water for about one second, so as to remove nearly, but not quite, all of the aluminium chloride.
(4) The work is transferred to a cold dilute (say 5 per cent) solution of cupric sulphate slightly acidulated with sulphuric acid. The degree of acidulation does not appear to be very important, but about one-tenth per cent of strong acid does well.
If the preliminary processes have been properly carried out the aluminium will become coated with copper, and the process is accompanied by the disengagement of gas. It appears to be a rule that if gas is not given off, the film of copper deposited is non-adherent. The work must be left in the copper sulphate solution till it has received a uniform coating of copper.
(5) When this is the case the work is removed — well washed so as to get rid of the rest of the aluminium chloride, and then electroplated by the battery in the ordinary copper sulphate bath.
If the operation (4) does not appear to give a uniform coat, or if gas is not evolved from every part of the aluminium surface, I find that operations (2) and (3) may be repeated without danger, provided that the dip in the hydrochloric acid is shortened to two or three seconds.
The copper layer obtained by Margot’s method is perfectly adherent — even when used as a base for ordinary solder — though in this case it can be stripped if sufficient force is applied.
Since the solder recommended by M. Margot for aluminium contains zinc, it does not run well when used to unite aluminium to copper, brass, iron, etc. In this case, therefore, I have found the most advantageous method of soldering to be by way of a preliminary copper-plating.
The success of M. Margot’s method depends in my experience on obtaining just the proper amount of aluminium chloride in contact with the aluminium when the latter is immersed in the copper sulphate solution.
§ 139. The process of copper-plating from sulphate or nitrate may, according to Mr. Swan (Journal of the Royal Institution, 1892, p. 630), be considerably accelerated by the addition of a trace of gelatine to the solution. As success appears to depend upon hitting the exact percentage amount of the gelatine, which must in any case be but a fraction of one per cent, and as Mr. Swan refrains from stating what the amount is, I am unable to give more precise instructions. A few experiments made on the subject failed, doubtless through the gelatine content not having been rightly adjusted. Mr. Swan claims to be able to get a hard deposit of copper with a current density of 1000 amperes per square foot, but seems to recommend about one-tenth of that amount for general use.
The solution employed is a mixture of nitrate of copper and ammonium chloride — proportions not stated. Electrolytic copper, as generally prepared, is very pure, but this is a mere accident depending on the impurities which, as a rule, have to be got rid of. Electrolysis seems to have no effect in purifying from arsenic, for instance.
Roughly speaking, about 11 grms. of copper are deposited per ampere hour from cupric salt solutions. When the current density is too high the anode suffers by oxidation, and this introduces a large and very variable resistance into the circuit.