Electroplating; a treatise for the beginner and for the most experienced electroplater

Front matter

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Introduction

ELECTROPLATING, as commonly understood, is the coating of an inferior with a more valuable metal by means of electricity. It is scientifically defined as "the art or process of covering any electrically conducting material with an adherent and lasting film of metal, in a bath containing a solution of that metal, by means of the electrolytic action induced by a current from a battery or dynamo." This technical definition may seem rather difficult for the average reader to understand, but will be made plain as we proceed step by step with the subject.

It is not necessary that the electroplater, in making a beginning, should have a scientific knowledge of chemistry and electricity, although, unquestionably, the better posted he is in those branches of the sciences that relate to his trade, the more likely he is to succeed. While it is quite possible for a man of average skill to fit himself to be an expert electroplater in certain lines, by a careful study of methods and apparatus, and especially by imitating a practical plater, yet it goes without saying that, the more he can absorb of the real science of the business, the why of the how, the less will be his trouble with batteries and solutions and the greater will be his success.

In this handbook, therefore, we will endeavor to give brief and practical directions calculated to be of benefit to those already engaged, as well as to those about to engage, in the electroplating business, with no more technical detail than is necessary for practical work, and yet with such explicit directions concerning the actual operations as an

10 INTRODUCTION

old hand at the business thinks may be useful to the beginner.

Two things will be urged at the start, and, but for the necessity of being brief, they would be repeated on every page, for without them success cannot be expected. They are:

Use care at every step. Nowhere is carelessness more costly than in the electroplating shop. Constant vigilance is the price of success.

Study the why of things. Know why you do this and that, and you will be more apt to do it right. If you know what you are about at every stage of the process and do things right, the work will come out right, and all will be well. If you blunder along, hit or miss, you will be inquiring the price of junk before you are six months older.

OLD AND NEW METHODS

In electroplating, the apparatus which supplies the electric current, through the aid of which the film of metal is deposited on the article to be plated, is, of course, the essential feature. The ancients used to hammer out a thin leaf of gold or silver and solder it to a surface, often doing really excellent work, but this was a most tedious and expensive process. A kind of plating is also done by immersion in melted metal, usually a soft composition. This is not permanent and could not be employed with precious metals on account of the expense.

Modern plating, or real plating, is electroplating, by which we use a small amount of metal, deposit it evenly, thoroughly, and in a short time, and yet obtain lasting and most satisfactory results. Electric current does the work.

HOW METAL IS DEPOSITED

In simple forms of electroplating apparatus, the bath containing the metallic solution may itself form the battery, and copper is easily deposited in this manner. The

INTRODUCTION more common and practical plan, however, is to obtain current from a source outside the bath, as from a regularly , constructed electric battery or dynamo.

When articles are to be electroplated, they are suspended by wires in the plating tank, which contains a solution of the metal which is to be deposited on them. From another

Fig:. 1. Connections for Electroplating:

wire is hung a piece of the same metal as that in the solution. The articles to be plated are connected by a wire to the negative pole of a battery and the metal plate is connected in the same way to the positive pole of the battery.

Now the metallic solution in the tank will conduct the electricity, and so a complete electric circuit is formed

INTRODUCTION by which the current flows from the positive pole of the battery through the connecting wire to the metal plate hanging in the tank. From the metal plate, it flows through the solution to the article to be plated and thence back through the other wire to the negative pole of the battery — completing the circuit. Fig. i shows the path of the current in an electroplating circuit.

The metal plate attached to the positive pole of the battery is called the "anode" and the objects to be plated in the tank are called "cathodes." The use of these terms can be understood if we think of the battery as being below

Fig. 2. Simple Electroplating Outfit the plating tank, as shown in the illustration. "Anode" is a Greek word meaning "the way up"— from the battery to the solution — and "cathode" means "the way down." By the action of the current, particles of metal are attracted from the solution to the articles to be plated, and at the same time an equal amount of the same metal is thrown off the anodes into the solution. Thus, although the solution is being constantly robbed of metal, it is being fed at the same time, and so its strength is maintained. Of course, the anodes must be plates of the same metal as that contained in the solution; thus, silver anodes in a sil-

INTRODUCTION 13

ver solution, nickel anodes in a nickel solution, etc. The process by which the electric current affects the decomposition of the metal to be deposited on the articles to be plated is called "electrolysis."

To make all this very plain, and to start the beginner at once with a practical working experiment, the diagram, Fig. 2, shows a very simple plating outfit which anyone can make at home. The explanations given will help the beginner to master the first principles of electroplating. A silverplating outfit has been selected for illustration because silver is the metal most easily deposited and therefore is a favorite with experimenters.

HOW TO MAKE A SMALL SILVERPLATING OUTFIT.

For a tank take an ordinary glass fruit jar or any other receptacle of glass, not metal, which will hold I qt. of liquid. Fill it nearly full with rain or distilled water and then add J4 oz. of silver chloride and iy 2 oz. of chemically pure potassium cyanide. Let this dissolve and incorporate well with the water before using. This is the bath.

Take an ordinary wet battery and fasten two copper wires to the terminals, as in Fig. I. Fasten the other ends of the wires to two rods of heavy copper wire or J^-in. brass pipe, which are to be laid across the top of the "tank." The wires must be well soldered to the rods to make a good connection.

When the solution is ready, which is when the crystals are entirely dissolved, the outfit is ready for work. Procure a small piece of silver, a silver button, ring, chain, or anything made entirely of sterling silver, fasten a small copper wire to this, and hang it by this wire to the rod connected with the carbon of the battery. This forms the anode. The article to be plated is to be suspended in a similar manner from the other rod and will form the cathode.

Be sure that the article to be plated is chemically clean.

14 INTRODUCTION

It may be cleaned by scrubbing with pumice and a brush saturated in water. When cleaning an article, there should always be a copper wire attached to it. It should never be touched by the hand after you have once started to clean it, because the touch of the fingers will deposit enough grease to cause the silver plate to peel ofif when finished. When well scoured, run clear, cold water over the article and, if it appears at all greasy, place it in hot water. When well cleaned, place it in the plating bath and carefully watch the results.

If small bubbles appear on the surface of the bath, you will know that you have too much of the anode immersed and you must draw out the piece until you can see no more bubbles. The surface of the anode should be about the same as the surface of the article to be plated. If the anode is too large, bubbles will appear, as stated; if too small, the metal contained in the solution will be exhausted.

Leave the piece to be plated in the solution for about half an hour, then take it out, clean off* the yellowish scum with a toothbrush and some pumice, rinse in clean water and dry in sawdust. When it is thoroughly dry, take a cotton flannel rag and some polishing powder and polish the article. It must have a fine polish before plating if it is desired to have a finely polished surface after the plate is put on.

To see if your battery is working, take a small copper wire and touch one end to the anode rod and the other to the cathode rod. When these two ends are touched and separated, there should be a small spark.

This description applies only to silverplating. Articles of lead, pewter, tin or any other soft metal cannot be silverplated unless they have first been copperplated. This, as stated, is merely an experiment to interest the beginner and to form a sort of object lesson which will enable him to grasp more easily the details which follow and the reasons why.

Chapter II

ELECTRIC current used for electroplating is derived either from voltaic cells or from dynamo-electric generators, usually called "dynamos. " If power is available, a dynamo is of course in the long run the most economical means of producing the electric current required for electroplating. But if a dynamo is out of the question, either for want of power or on account of the initial expense, its place can be very satisfactorily filled by batteries coupled together in sufficient number to produce the required current.

ELECTRIC BATTERIES

An electric battery is composed of several cells, although a cell is sometimes called a battery. The cells are of various kinds, wet and dry, and the wet cells may be of single or double liquid. The types described below refer to kinds or patterns of cells, and not to any special patents. There are as many special kinds of electric cells, or batteries, as there are manufacturing concerns, and even more. As a general rule, the higher-priced cells are likely to be worth all the money asked, and the cheaper ones really cost more in the end. This rule will apply pretty generally to chemicals and to all supplies. The beginner is advised to get as few articles as possible, and get the best ; then to use them carefully, so as to get the greatest value out of them.

With regard to the different kinds of cells, the Smee and Wollaston are perhaps the most common types of singleliquid cells, and the Daniell, Fuller, Bunsen, and gravity cells may be taken as representative patterns of doubleliquid cells. The principle consists in the exciting of an electric current by the solution (by an acid or an acid salt) of the readily attacked (or "negative") metal, in conjunction with a metal or other element (as carbon) not so readily attacked (this being called the "positive" metal or element), the effect being to produce what is called a "difference of potential" which sets up an "electromotive force" that tends to drive a current of electricity around the circuit when a passage is completed for it outside the cell.

The electric current has several properties, among which we may consider force and quantity, both of which are governed or influenced by the resistance of the bodies through which the current passes. This has often been compared to water in a pipe, which may be under a varying

pressure according to the level, and may have a flow according to the opening at the faucet. The difference of potential may be likened to the water level, the electromotive force to the pressure, and the current to the flow or volume of water. For some purposes, a very high voltage* is required, and for others, as in electroplating, a low voltage and a large volume of current are needed. We will now consider the construction of the cells, and then the methods of connecting them»to get the desired results.

THE SMEE CELL

The smee cell, Fig. 3, is a square glass jar having two zinc plates held together by a clamp, and between them, well insulated, a platinum plate, or a silver plate covered with "plat-

Fig. 3. Smee Cell unit

The "volt" is the practical unit of electromotive force. The "ohm " is the of resistance. The "ampere" is the unit of current.

Fig. 4. Daniell Cell inum black," which is a preparation of finely divided platinum, black in color, obtained by the action of potassium hydrate on platinum chloride.

Platinum black has the property of absorbing or occluding gases, and is employed in the Smee cell for this purpose. The plates must have sufficient room so as not to come in contact with the zinc sulphate that forms by the dissolving of the plates. The vessel is filled with a solution of sulphuric acid (by measure, i part of the acid to 7 parts of water) and is then ready for use. Connections are made by a small copper wire, No. 16 gauge, with the zinc element to the cathode rod, and from the platinum element to the anode rod. Several cells may be placed together in a frame and connected to form a battery. The Smee cell has the advantage of being able to give a fairly steady current for a long time, as the platinum black prevents 'the accumulation on the positive element of bubbles of gas which would result in a decrease and final stoppage of the current, an efifect known as "polarization."

THE DANIELL CELL

The Daniell cell, Fig. 4, is composed of a glass or stone jar containing a cylindrical plate of sheet copper and an inner porous pot or jar of unglazed earthenware containing a zinc rod. The porous pot is filled with dilute sulphuric acid, while in the outer part the copper plate is immersed in a saturated solution of copper sulphate. The electromotive force of the Daniell cell is about 1 volt. The internal resistance is about 3 ohms.

The Daniell cell is recharged thus: Thoroughly clean all the parts and re-amalgamate the zincs. Re-amalgamation is for the purpose of freeing the surface from impurities and is thus accomplished: Take a flat earthenware dish a little larger than the zinc plate, and put in just enough water to cover it. Then slowly add sulphuric acid, one part to the ten of water — that is, if you have 10 oz. of water, add i oz. of sulphuric acid. Immerse the zinc in the diluted acid and pour over it a small quantity of mercury which is to be painted onto the zinc with a little brush or mop made up of tow and a few brass wires. See that the zinc is well covered. The acid may be afterward used with the charging solution, and the mercury that is left may be poured back into the bottle.

After the zincs are thus re-amalgamated, charge the porous pot with a solution of i part of sulphuric acid to 12 of water, and the outer jar with a saturated solution of copper sulphate. On a sieve suspended in the liquid, place crystals of copper sulphate, to keep the solution saturated. Do not throw a handful of the sulphate into the jar to settle at the bottom. Care should be taken to keep the zinc and the zinc sediments from touching the porous pot, as this would attract particles of copper from the copper solution to form on the outer wall of the partition and cause a higher internal resistance and short circuit.

If properly understood and kept in working order, the Daniell battery is very constant, and is well suited to silverplating.

THE BUNSEN CELL

The Bunsen cell, Fig. 5, has several forms, of which the English and the French are the best known. The English Bunsen is similar in appearance to the Daniell, with a cylinder of amalgamated zinc in place of copper, and a square of carbon in place of the zinc rod. The outer cell is charged with dilute sulphuric acid, 1 part of acid to 10 parts of water, and the inner cell with strong commercial nitric acid. Run the nitric acid into the porous cell until about three-quarters full, and fill the space between the porus cell and the outer jar with the dilute sulphuric acid. The French Bunsen has sulphuric acid in the porous cell with the carbon, producing a constant generator with a low voltage, or electromotive force. This cell gives 1.8 volts at starting, falling to 1.5 volts when the circuit is closed. This form is less troublesome to keep in working order than the English Bunsen and is free from noxious fumes.

An English authority says : "The type of cell known as the Daniell is best for depositing copper from its sulphate solution and silver from the usual plating solution, but the Bunsen may be used for this purpose if the exciting liquid is sufficiently weakened. One or two large cells in series will be enough for all ordinary purposes" (presumably, for small work, such as the country repair shop).

The Bunsen cell is best for depositing copper and brass from their alkaline solutions, and also for the deposition of nickel, because its electromotive force is high, enabling it to pass through high resistances. It is not suitable for the work of silverplating, gilding, and electrotyping because its e.m.f. (electromotive force) causes the metal to go on too fast, and in a granular condition. In all these operations the Daniell cell will be found to be the best, because its e.m.f. is lower than that of the Bunsen, and its current equally constant in volume. The Smee cell is eminently useful for gilding small pieces of jewelry. Batteries with a high e.m.f. cause gold to go on too fast, and to give the deposits a brown color. So much has been said, though

Fig. 5. Bunsen Cell very briefly, descriptive of the leading types of batteries in use. The beginner will find it to his advantage to study the battery which he has adopted, and to become thoroughly familiar with its operation.

HOW A BATTERY WORKS

The cells forming the battery are connected up in various ways to obtain different results. We will explain, as briefly and simply as possible, the working of the cells in the battery, and how the electric current is controlled, since it is important in plating operations to get the proper pressure and current, depending upon a known resistance. Scientific terms will be avoided, except when necessary to make the statement exact.

In considering the flow of current in an electric circuit, we can compare it nicely with the flow of water in a water pipe. Suppose we had a pail of water with a pipe in the bottom. We know that the higher the water stands in the pail, the more rapidly the water will flow through the pipe, and that it makes no difference how big the pail, providing the water level is the same. Now we can think of a battery cell as a pail of water and an electric current as the flow through a pipe leading from the pail. To correspond to the pressure of the water on the bottom of the pail, we have the electromotive force which, in fact, is usually called the "pressure" or "voltage," and which we will call hereafter the e.m.f. The rate of flow of the electric current corresponds to the rate of flow of water, and it depends upon the pressure (e.m.f.) of the battery, just as the rate of the water in the pipe depends upon the pressure of the water in the same way. No matter how big the battery, the current will be no greater in the circuit if the pressure remains the same. We also know that in a water pipe, other things being the same, the larger the pipe, the more rapidly will the water flow (in gal. per min.). In the electric circuit, the larger the wire, the greater the current will be, for the larger wire offers less resistance to the flow of electricity. Some substances offer more resistance to the flow of electricity than do others ; that is why copper wire is used so much, as copper offers less resistance to the current than most other substances. Now if we put a valve in our water pipe, we can change the flow of the water (the current) by opening or closing the valve, that is, we put more resistance in the path of the water. In the same way, we can put a valve in the electric circuit, by which we can vary the current from the battery by introducing more or less resistance. Such a valve is called a "resistance box" or "rheostat."

The pressure of water is measured in lb. per sq. in. ; electric pressure is measured in volts. The rate of flow of water is measured in gal. per min. ; the rate of flow of electric current is measured in amperes ; and the resistance offered to this flow is measured in ohms. There are two resistances to be considered, that of the cell or battery itself, called "internal resistance," and that of the circuit (the wires, solution, etc.) called "external resistance."

There are many ways of grouping the cells in a battery, but they are comprehended in the systems known as "series" and "parallel" and combinations of the two, called "series-parallel."

In series, the cells are connected with the positive element of one joined to the negative of the next and so on, and the free elements connected to the main circuit, as shown at A, Fig. 6. In this arrangement, the current generated in the first cell has to flow through the second, and the e.m.f. of one is added to that of the other. This method corresponds to setting one pail of water on another, in which case we double the pressure.

The parallel grouping of the cells has a different effect. In this, all the negative elements are joined together on one wire, and all the positive elements to the other wire, as shown at B. This is called "joining for quantity." The zincs are practically all one large zinc, and the coppers (or carbons) are one big positive element. The force is no greater, but the internal resistance is decreased in propor-

B

Fig. 6- Grouping of Cells— A f Series— B, Parallel— C, Series-Parallel

tion to the number of cells, as there are more paths through which the current may flow. The current is therefore increased in proportion to the number of cells thus connected. The e.m.f. is no more altered than would be the total water pressure, produced by placing two pails of water side by side on a level floor, in place of one ; for both cells are giving the same pressure, and coupling them in parallel is only equivalent to increasing the size of a single cell, which has no influence on the e.m.f.

The series-parallel grouping, shown at C, unites two or more sets of cells which were joined in series in a parallel arrangement.

The practical results of these combinations may be figured out from a knowledge of the capacity of the cells and the resistance, by a formula known to electricians as Ohm's law, which declares that the current is equal to the e.m.f. divided by the resistance. This law can be expressed by a

E

formula, which for any cell is I = where I equals

r + R

the current in amperes, E = the e.m.f. in volts, R = the external resistances of the circuit in ohms, and r the internal resistance of the cell in ohms.

Suppose we have four cells of a voltage of 1.5, an external resistance in tanks, wires, etc., of 2 ohms, an internal resistance of .05 ohm, and wish to know the current given by the three combinations. Here E = 1.5, r = .05, R = 2. The various methods of grouping are as follows :

A. — SERIES GROUPING.

Electromotive force of one cell = 1.5; e.m.f. of 4 cells in series = 4 X 1.5 = 6 volts.

Internal resistance of one cell = 0.05 ohm ; internal resistance of 4 cells = 4 X 0.05 = 0.2 ohm. External resistance = 2 ohms. Total resistance = internal resistance of 4 cells -f- external resistance = 0.2 -f- 2 = 2.2 ohms.

electromotive force

Current (I) =

total resistance

Current = = 2.73 amperes, nearly.

2.2

B. PARALLEL GROUPING.

Electromotive force of cells in parallel is the same as that of a single cell, if the cells are identical. That is, the e.m.f. = 1.5 volts.

The internal resistance of 4 cells in parallel is *4 °f that of one cell. That is, the internal resistance is 0.05 -f- 4 = 0.0125 ohm. The external resistance is 2 ohms. Therefore, the total resistance = 0.0125 -f 2 = 2.0125 ohms.

electromotive force

Current (I) = •

total resistance

= = 0.74 ampere

2.0125

C. SERIES-PARALLEL GROUPING.

Electromotive force is double that of one cell; that is, the e.m.f. = 1.5X2 = 3 volts.

The internal resistance of each set of two cells in parallel is one-half that of each cell, = 0.05 -=- 2 = 0.025 ohm. The total internal resistance of the two groups in series is twice that of either group singly, that is, 0.025 X 2 = 0.05 ohm. External resistance = 2 ohms. Therefore, total resistance = 0.05 + 2 = 2.05 ohms.

electromotive force

Current (I) =

total resistance

3 = = 1.46 amperes

2.05

Suppose, however, that the external resistance is not so great as assumed in the above examples, in which it is purposely made rather high. On a "short circuit" (very little

3o amperes.

.2

120 amperes.

—

•05^-4

6o amperes.

external resistance), R = o and we find the current from these three combinations as follows: -

(A) I

.05 .05

So that, while with a heavy external resistance we get the greatest volume of current from the series coupling, with a low resistance we get the greatest volume of current from the parallel, which at the same time gives the weakest voltage. From the above it is evident that the voltmeters and rheostats which will be treated later, must be sharply watched if the plater would know what he is doing.

THE DYNAMO

After learning the rudiments of the trade with such small electric batteries as are found most practical, the electroplater may feel warranted in using a dynamo. If one has some electrical training and mechanical ability he may be able to build a very serviceable dynamo for himself. This, however, is a separate trade, and the electroplater is apt to have his hands full in his own particular line. As a rule, it will pay him better to buy a good dynamo and use all his time in his regular business. The advantages of a dynamo in a shop where a good custom trade is established are that it yields a current in every respect more suitable to the work. of depositing metals than from the best batteries, it is more cleanly in working, eliminates noxious gases, and is more easily managed.

The best form of dynamo for the use of the electroplater is some sort of shunt-wound or compound-wound machine, giving a large volume of current at low pressure, the current being delivered constantly and in one direction. In the shunt-wound dynamo, there is automatically a continuous balancing of the current as the load varies in the external circuit. The compound-wound machine, however, can also be designed to give a constant current, and has, in addition, self -regulating features not possessed by the shunt-wound machine. It is the best type of machine to use for electroplating. Such a generator is shown in Fig. 7. Machines for electric lighting work are designed to give a moderate volume of current at high pressure, and while a plating dynamo will generate current at, say, 10 volts, the lighting current may have a pressure of no or 220 volts, or higher. The unfitness of the lighting current for the purposes of the electroplater is therefore manifest.

Fig. 7 8 Compound- Wound Dynamo

In purchasing a dynamo, avoid all offers of second-hand machines, although warranted "as good as new." Get a small machine of a responsible dealer, at a cost of $50 or upwards, and then master its construction and working so as to get the best results. It may be set down as a law in electrical engineering, though probably never before printed, that a dynamo will deteriorate and go to scrap with a velocity equal to the square of the ignorance of the operator.

REGULATING AND MEASURING INSTRUMENTS

A rheostat, to enable the plater to control the current, is necessary where a dynamo or large battery is used. One style of rheostat is show T n in Fig. 8. Throwing aside technical terms, the rheostat is similar to a tap or valve in a