- Image via Wikipedia
At the meeting of the American Electrochemical Society, Professor Haher demonstrated experimentally that when an aqueous solution of a salt of sodium or potassium is electrolyzed, using a cathode of tin or lead, hydrogen is formed, and a dark cloud, consisting of the finely-divided heavy metal, envelops the cathode.
With a lesser current density, the cloud does not form, the surface of the heavy metal lining simply roughened.
It was pointed out by Professor llaber that here the alkali metal actually is deposited on the cathode, forming an alloy with the latter, which is then decomposed by water, yielding hydrogen secondarily and the finely divided metal.
This experiment is of great value in settling the mooted question of whether the hydrogen liberated at the cathode during the process of electrolysis of an aqueous solution of a salt of an alkali metal is of primary or secondary origin.
While, as Professor llaber staled in the conclusion of his paper, the experiment is perhaps of more theoretical than practical interest, nevertheless it is simply a striking illustration of a general fundamental principle that underlies all electrolytic deposition of metals.
When one metal is deposited electrolytically upon another, the process of alloying always goes on to a greater or less extent, which is determined primarily by the nature of the metals, and secondarily by the conditions of experiment.
Among the latter are of special importance the original condition of the surface of the cathode, nature and concentration of the electrolyte, temperature, potential and current density.
These all have been well recognized and thoroughly discussed in text-books as important factors in securing desirable, dense, well-adhering deposits that will admit of being polished and burnished.
The important fact that the alloying power of the electrolytic deposit with the coated metal underneath is a factor determining not only the strength with which the deposit adheres, but also the length of time the plated article will wear and resist corrosion, has not been sufficiently recognized.
The alloying power of two metals consists of a specific attraction of the metals for each other, a tendency to dissolve in each other, or, as it is put by others, a tendency that the metals have to bring about a mutual interpenetration of their masses.
This force is no doubt chemical in character, slight though the affinity may be in sonic cases.
The alloys of mercury, the amalgams, especially those of the alkali metals, have become of considerable importance in the electrolytic preparation of caustic alkalies and of alkali metals.
Here the alloying of the alkali metal with the mercury constituting the cathode is well recognized. But when gold is plated on lead, for example, an alloy of gold and lead is just as truly formed at the point of contact between the two metals.
A thin plating of gold on lead will gradually soak into the lead entirely as the alloying process continues. This goes on at room temperatures.
The experiments that Roberts-Austen made in 1900 demonstrating how gold diffuses into lead at ordinary temperatures are of special interest in this connection.
A thin plating of gold on zinc is absorbed in a few days. The “tarnish” becomes visible very soon. After the deposit has been absorbed, the gold may be exposed again by carefully etching away the zinc from the surface by use of a dilute acid.
The experiment of Gore, illustrating how copper plated on platinum penetrates into the latter, is well known to electroplater’s.
This affinity between metals asserts itself the moment one metal is deposited on the other. The stronger the affinity of the metals for each other is, the better the deposit is apt to adhere. On the other hand, the plating is also more apt to be absorbed by the base metal underneath the greater the tendency of the metals to alloy is.
A higher temperature generally aids this alloying process, and so it is that frequently much better adhering deposits are obtained by working at temperatures above the ordinary.
A clean, slightly roughened (etched or scratch-brushed) surface of the cathode clearly facilitates the alloying of the deposit with the metal of the cathode, for thus a more intimate contact is secured. When two metals are to be alloyed under ordinary conditions, the affinity between them must overcome their cohesion’s. This frequently requires a high temperature.
In the process of plating the affinity between the metals clearly does not initially have to overcome the cohesion of the deposited metal, since the latter is gradually built up, starting with an exceedingly thin film. Thus it is that alloys of metals may readily be formed at room temperatures by electrolysis, whereas under ordinary conditions the alloys would not form at all, or would form but very slowly.
The fact that platinum will not amalgamate with mercury under ordinary conditions, and that platinum amalgam may readily be prepared electrolytically, is a good illustration of this.
In the case of any electroplated article, the deposit and the metal underneath continue to act upon each other, or mutually diffuse into each other.
This action is great between zinc and gold or lead and gold, as has been stated above.
Between copper and gold and brass and gold it is less noticeable. And yet who has worked with brass, gold or platinum-plated weights and has not noticed that the plating is, after all, but a relatively poor protection against corrosion?
Especially as the weights grow older they are all the more easily corroded by moisture and traces of salts in the atmosphere of the laboratory. If the plating is heavy, this corrosion will, of course, not take place so early as when the coating is thin, which is so frequently the case.
The mechanical character of the alloy formed by the deposited and the base metal is also of practical consequence. If the alloy has a strong tendency to become crystalline, or is brittle, or has a considerably different co-efficient of expansion from that of the pure metals, the deposit is more apt to chip off than if the alloy is pliable and does not exhibit tendency to crystallize.
- Image by Michael Buck via Flickr
Nickel may be successfully plated on copper and its alloys— brass, bronze, etc.—for with these nickel alloys readily. On the other hand, when an object of lead is to be nickeled, it is first copper plated, and then the nickel is deposited on the copper. Nickel Plating Kits.
In this operation use is made of the fact that copper alloys readily with nickel, and copper alloys better with lead than docs nickel; and so copper serves here as a cement, as it were. Many other similar illustrations from practice might be cited.
- Image by bragadocchio via Flickr
In general, then, metals that will alloy readily with each other may, other things being equal, successfully be plated on each other.
Whether the plating will be absorbed faster or slower will depend on the character of the metals and the temperature at which the object is kept, as noted above.
If the deposit consists of a metal of the same color as the base metal underneath, as, for instance, gold over brass or copper, the effect of this diffusion of the metals into each other will not be so apparent as when the metals are of different hue— gold on lead or zinc.
And yet metals of very different color may at times be plated on each other, and the effect of this diffusion remains unnoticed because the alloy formed has the color of the plating even up to very high concentrations of the alloy. Nickel 011 copper is a good illustration of this.
While it thus appears that a strong alloying power is desirable between base metal and electrolytic deposit, as it tends to secure good adhesion of the latter, it is also evident that this tendency to form an alloy frequently militates against the very purpose for which the electrolytic deposition was made, namely, to secure a better appearance or to protect from corrosion.
It is evident that in the long run all plated objects must slowly deteriorate, even when not in use and when carefully protected from the atmosphere by a lacquer of some kind. The electroplater must study the subject carefully so as to plate those metals or alloys upon each other that possess sufficient affinity to coalesce well, and yet will, at the temperatures at which the plated objects are to be kept, diffuse but very slowly into each other.
He will also choose the color of the metals or alloys so that the final alloy formed may have the same, or nearly the same, shade as the electrolytic deposit and will itself be corroded less readily than the base metal alone.
The connection between soldering and electroplating of metals is evident from what has been stated. So, for instance, the difficulty of plating on aluminum and of soldering the metal go together, though, of course, the fact that this metal is readily oxidized, and that the oxide coating interferes with both processes, must not be lost sight of.
It is not so easy to solder iron with the usual half-and-half solder, as it is to solder zinc, though the latter metal is much more readily oxidized than iron. The explanation is clear: iron does not alloy as readily with the tin and lead solder as does zinc.
A careful, systematic study of the electrolytic deposition of the metals from the standpoint of the alloys formed will no doubt yield results of further practical value. Laboratory Of Physical Chemistry,
By Prof. Louis Kahlenbkkc, Ph.d. relies
University of Wisconsin, Madison.
1903
