TENSILE STRENGTH OF ELECTROLYTIC COPPER ON A ROTATING

mandrel, rotating vertically, over which agate burnishers rotated, keeping the deposited copper tubes smooth, and of constant thickness throughout...
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TENSILE STRENGTH OF ELECTROLYTIC COPPER ON A ROTATING CATHODE BY C . W. BENNETT

As early as 186.5,’it was noticed that during electrolysis, if the cathode were rotated, a higher current density could be used. Among those actually making use of this was Wilde who patented a process in 1875, using for cathode a vertical iron cylinder which was rotated. The current density used in this process was never more than 2 0 amperes~persquare foot. The next important commercial application of this principle was the Elmore process. In this, the cathode was a .mandrel, rotating vertically, over which agate burnishers rotated, keeping the deposited copper tubes smooth, and of constant thickness throughout. This process is used commercially in Europe a t present in the manufacture of seamless copper tubes. The current density used is not more than 30 amperes per square foot. The copper thus obtained has a tensile strength of from 36,000-50,000 pounds per square inch, depending, it is stated, on the speed of rotation.2 However, a part of this increase in tensile strength, over that of copper precipitated on a stationary cathode, is most likely due to the increase in the rate of burnishing, for we have here a true analogue t o the process of rolling. Due to the increased rate of rotation, the tube passes under the agate burnisher faster, thus giving more and more an approximation to the cold rolled copper, with its correspondingly higher tensile strength. The Dumoulin process substituted a burnisher of sheepskin for the agate of the Elmore process. It was claimed that the animal fat insulated the projections, thus tending to give a more even surface. The current density was run up t o 40 amperes per square foot. No See “Electrochemical and Metallurgical Industry,” 6, 4 1 2 (1908),for review of these processes. “Electrochemical and Metallurgical Industry,” 3, 83 (1905).

Electrolytic Copper o n a Rotating Cathode

295

mention is made of the strength of the deposit. This was tried commercially in England but failed completely. Emerson in 1899 patented a process’ for making a copper wire by plating copper on a rotating mandrel, wound around with a spiral of insulating material. The strip between the insulating material was then pulled off and drawn down. In 1899 the same man patented a processZfor making copper bars. A copper strip was wound spirally around a cathode which was rotated. The strip was thickened to a bar by depositing the copper on it while rotating. A large (sic) cathode was rotated slowly. S. 0. Cowper-Cowles patented3 practically the same process, using a smaller cathode and rotating it more rapidly. The United States Patent called for a process using a cathode moving ‘‘ at such a rate of speed that will cause the hydrogen bubbles to be thrown off from the metal deposited on the cathode, and cause such friction between the metal deposited on the cathode and the electrolyte as t o yield tough and smooth deposits. Using a cathode twelve inches in diameter rotating 1000 R. P. M., and an electrolyte of 12.5 percent copper sulphate and 13 percent sulphuric acid, at about 70’ C, with a current density of 200 (preferably 170) amperes per square foot, copper foil was obtained which was even stronger than the best cold worked copper. The results would have been more conclusive if a thicker deposit had been precipitated and tested. The liability t o error will be apparent when the thickness of an average sample is considered. This is given : Dimensions Inches

1.109X 0.006

I

I

Square inch

I

0.0066

1 I

1 1

Tensile strength per square inch

5 I,000 pounds

U. S. Patent No. 395,773, January 8, 1899. 1899. English Patent No. 26,724, 1898; U. S. Patent No. 644,029, February 20, 1900. “Mineral Industry,” 9, 229 (1900).

‘ U. S. Patent No. 638,917,

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C. W . Bennett

These results, as given, mean absolutely nothing in themselves, as will be seen below in the work on brass and bronze. Five reasons are given for rotating the cathode. ( I ) The electrolyte is stirred and impoverishment is prevented. ( 2 ) The copper is burnished by the friction with the electrolyte . (3) Foreign matter is eliminated, thus preventing “ tree formation.” (4) Air bubbles are brushed away, thus preventing ‘‘ nodule formation.” (5) Thickness of deposit is uniform. There is little doubt that all of these factors enter in and tend to increase the tensile strength, and t o enhance the character of the deposit. However, it seems highly improbable that the factors given above are the only ones entering into the equation. For this reason, it was deemed expedient to make some experiments, to see if other factors could be found, and to find their true relation to the tensile strength of the deposit. Then the principle was t o be applied to the brasses and bronzes, with the hope that alloys of high tensile strength would be obtained. With the copper the various factors were studied by holding others constant and varying one for a series of runs. In this way the effect of speed of rotation, current density, concentration of electrolyte, and temperature were determined. The apparatus consisted of an electrode holder designed to rotate continuously a t any speed up to 6000 R. P. M., and carry 300 amperes. This has been described in a previous paper. For currents up to 150 amperes, the I I O volt circuit, from the university power house, was used. For the higher a current, a motor generator set was used. The test pieces were prepared by the general method outlined in the previous paper referred to above. These pieces ran from 0.040-0.060 inch thick. The actual deposit Jour. Phys. Chem., 16, 287 (1912).

Electrolytic Copper ow a Rotating Cathode

397

before turning down was much thicker. The measurements of the cross section were taken with micrometer calipers, reading directly to 0.001 inch. These pieces were then broken in an Olsen testing machine. Five or more tests were made, and their average taken as the true tensile strength. During the runs it was found necessary to burnish the deposit once or twice. A mechanical burnisher was not desirable, for it would be open to the objection that the copper was being “rolled” as in the Elmore process. Therefore it was deemed best to stop the run once or twice, depending on the relative rate of stirring and on the current density, and burnish with emery paper. This was done by holding the paper against the rotating tube. The surface was then washed, treated with a strong solution of potassium cyanide to remove grease, and then with I : I nitric acid solution to slightly roughen the surface and ensure the adherence of the next layer of copper deposited. In general, if a solution be stirred during crystallization, the crystals resulting are smaller than those from the same solution without stirring, because more nuclei are formed. In depositing a metal, then, if it be precipitated directly in the crystalline state, we shall expect to get smaller crystals if the solution be stirred vigorously. However, the precipitation of the metal in the crystalline state directly, is not a t all probable. It is likely that the metal comes down in a condition analogous to a “melt,” and then crystallizes from this. By rotating the cathode the uncrystallized material is agitated and smaller crystals result. The force of the rotation tends to move the material, forcing it to develop new crystal centers, and in this way prevents the growth of large crystals. Hence in precipitating copper, smaller crystals were expected from the run where the rotation was rapid. It is also known that the tensile strength of steel, copper, etc., is increased by rolling. Rolling, it is generally admitted, does nothing more than break down crystal aggregates, giving a more finely crystalline mass. Hence, with the

C. W . Bennett

298

precipitated copper an increase of tensile strength was expected with a decrease of crystal size, as the rotation was increased. When this was tried the results showed that the theory was correct. A solution containing 2 0 percent CuS0,.5H,O, and 12 percent H,SO, was used. The temperature a t starting was 35’ C. This was desirable, for trial showed that this was the temperature maintained throughout a run a t the current density used, i. e . , 500 amperes per square foot. The deposits were treated alike, and every precaution was used t o keep all conditions, save speed of rotation, constant. Results for four runs are tabulated below :

Revolutions per minute

1

Tensile strength in vertical direction Pounds per sq, in.

1

37,000

~

Voltage

I750

3.2 3.8

2 500

49,000

1 direction Tensile strength in of rotation Pounds per sq. in.

I

-

I,

41,000

5 1,000

3 500

R.P M

IS

20

25

30

5 1,000

I” Hundreds 35

Fig.

40

I

45

50

55

Electrolytic Coppel. o n a Rotating Cathode

299

A gradual increase in the ’tensile strength, with increased rotation of the cathode, is seen from the curves. There has been therefore a gradual decrease in the size of the crystals due to the increased rate of agitation of the uncrystallized matrix. The observations given in the last column were taken to show that the theory, assigning the increase in tensile strength to the mechanical deformation of the crystals or particles of copper, by the friction against the solution, is untenable. This theory states that the particles are drawn out in the direction of rotation, and a fibrous interlacing mass is obtained. According to this, the tensile strength would be greatest in the direction of the lamination. The test pieces in the form of rings were cut off the bottom of the tube, while it was in the lathe. These were broken by applying pressure in the direction of a tangent. Fig. z gives a

sketch with detail of the apparatus used for these tests. This was prepared in the following way: A disk of steel I ” diameter and 9/16” thick was sawed through with a hack

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C. W . Bennett

saw, in the middle, giving two half discs. Holes were drilled in these, as shown in the figure, a little to the right and left of the centers of gravity of the sections. These were then fastened in two forks or brackets by pins, as shown. The brackets could then be held in the testing machine, the ring of copper slipped over the disc sections, the brackets pulled apart and the ring broken. The breaking load was then divided by two, thus assuming that it was evenly divided. In the last run, at 5500 R. P. M., the difference is a bit greater than the experimental error and needs a little comment. At this rapid rate of rotation the solution is very violently agitated, and very fine bubbles of air are carried down into the solution. These are to some extent, most likely, entrapped and occluded in the copper. Now the bottom end of the cathode would receive the least amount of this air, and here the most compact deposit would be found, and consequently the tensile strength would be highest. This is the most probable explanation, for, on examination, the texture of the deposit a t the end proved to be finer. Anyway, if the theory 'were true, all the measurements should be higher than those on the vertical strip, by an amount depending on the speed of rotation. Since they do not show this general tendency it may be concluded that the deformation of the crystals is not a factor. I n order to check up the inverse ratio of crystal size and tensile strength this series was polished, etched with ammonium hydroxide ( I : 3), and the crystal sizes and shapes noted under the microscope. It was hoped that the measurements of the actual size of the crystals could be obtained, but they were so small that the results would have meant nothing had they been taken. However, in passing along the series rapidly a decrease in the crystal size could bc detected. When they were mixed, one could arrange thc series by noting the crystal size under the microscope. Nc difference in the shape of the crystals could be seen, as woulc be expected according to the deformation theory. Agair

Electrolytic Copper on a Rotating Cathode

301

the structure of the piece, from planes at right angles to each other, appeared t o be the same. With the higher rates of rotation the deposit was smoother and better, showing that there was more efficient stirring and hence less impoverishment. In Fig. I , a curve is given, showing the voltage drop across the cell at varying speeds of rotation. An astonishingly large increase in voltage with increase in rotation is noted. The increase was more than was anticipated. It may be due to an increased resistance of the cell, or a back electromotive force. Increased resistance of the cell may be caused by an increase in the resistance of the brush contact, an air film on the cathode, or a tendency for the electrolyte t o concentrate in the outer portion of the cell by being subjected to centrifugal force. These tendencies would increase with increased rotation. The back electromotive force may be due to frictional electricity, from the friction of the cathode against the liquid, or a more rapid solubility of the cathode than the anode. An increase in relative solubility of the . cathode may be caused by the fact that the crystals at this electrode are much smaller than those of cast copper, or increased stirring, tending to equalize the concentration of the liquid about the cathode, and aiding diffusion, would allow the cathode to dissolve more rapidly than the anode, and thus set up a back electromotive force. Lastly the increased relative solubility of the cathode and anode may be due to a combination of the last reason with another. There may be an impoverishment with respect to copper ions in the film about the cathode. This then would give the cell,

cu 1 x -y cu

'

x CU'

'

I cu,

or a concentration cell, manifesting its electromotive force in a direction opposite the charging voltage. When now, the cathode is rotated, the increase in solubility may be greater than the reverse tendency, caused by stirring the solution faster, and consequently the final effect would be a total ncrease in the solubility of the cathode, and hence a higher

302

,

C. W . Bennett

back electromotive force. Which and how many of these factors enter in, determining the increase in voltage, cannot be said a t present. A detailed study of this, however, has been begun, and it is hoped that results will be forthcoming. This seems to be an important point in explaining some of the results gotten with revolving cathodes. These will be dealt with in detail in a future paper. The variation of current density was then studied. If. is generally known that rapid crystallization from a solution or “melt” gives smaller crystals than that which takes place more slowly. If now in depositing a metal the current density be increased, the amount of metal t o crystallize in unit time is increased, and hence the crystals should be smaller. With the smaller crystals goes more even loading stress and hence higher tensile strength. Therefore, for any given set of conditions the tensile strength ought to increase as the current density is increased. The upper limit will depend on the rate of stirring, other things being equal. If this be true, the maximum tensile strength would be found a t a higher current density, if the stirring were more vigorous. To show this, two series of observations were made, one a t 2500 R. P. M., another a t 5500 R. P. M. These were not, however, run under the same conditions. Due to the large current used, the temperature rise was found to be different in the series of the first run. To decrease this difference a higher initial temperature was used in the second series. The effect of this increase had been anticipated, and will be discussed under temperature, below. The desired point, however, is illustrated, that is, that the maximum tensile strength is obtained a t a higher current density where the rate of rotation and hence rate of stirring is higher. The solution was the same as was used before. I n the first series, the initial temperature was that of the room, in the last it was about 50’ C. Results are tabulated below.

Electrolytic Co@per o n a Rotating Cathode _ _ _ ~ _ _ _ 2500

-~

1 1 I

revolutions per minute

I

Amueres 1 Tensile square Voltage foot I 1 per sq. in.

+

300

400 510 I100

1700 -

:::t~

I

'I I

I '

2.5

60,000

3.3 3.9

68,000 40,000 35,000 14,000

7.2 12.2

~

I

5500

revolutions per minute

Amueres

I

1

Tensile

foot

1

i

per sq. in.

~

340 500

-

-

303

34,000 j0,ooo 4 I ,000 32,000 28,000 13,000

2.7 3.8 7.I

I000 I 600 2 400

11.1 20.3

4000

27.5

These results are shown graphically by curves in Fig. 3.

I

300

I

I

400

,

Current Deneity in Ampa. per eq.ft. I

,

I

,

l

,

,

,

I

,

I

IO00

500

,

I

I

I

1100

Fig. 3

The first three points were checked, that there might be no doubt as to the form of the curves. The tensile strength reaches a maximum and then drops off. This is due, no doubt, t o the inefficient stirring, at the higher current densities. The current density was run up to a limit where the deposit was noticeably bad, due to the separation, probably, of cuprous oxide. The sample a t 2400 amperes per square foot was very ductile, could be bent and worked nicely. No trace of oxide or other impurity was noted. The sample at 4000 amperes per square foot was brittle, but

C. W . Bennett

304

worked nicely in the athe giving a perfect copper sur ace. The fracture, however, showed a redd.ish brown color. Upon examination under the microscope a gradual decrease in the size of the crystals with increase in current density could be noticed. A few runs were made to show that variation of the concentration of the electrolyte makes little difference in the tensile strength. The following experiments were made, varying first the concentration of copper sulphate and then that of the sulphuric acid. CURRENT

DENSITY500 AMPERESPER

SQUARE

FOOT. -

Percent CuSO,. 5 H I o

Percent H,SO,

Tensile strength Pounds per square inch 60,000 58,000 551000 60,000

57,000

The last one of each series is a bit low, but this is due t o the slightly elevated initial temperature required to hold the copper sulphate in solution. The current efficiency was not so good where a large percentage of acid was used. Now as to temperature variation, which has been anticipated above. If copper wire be drawn while cold through a die, the effect is a decrease in ductility and an increase in tensile strength. This is caused by the breaking down of the large crystals giving smaller ones embedded more or less in amorphous material. The distance of shear through the crystal is relatively short, and hence the elongation is short, or in other words the ductility is slight. The tensile strength goes up, for the material is worked more or less toward the amorphous end, i. e . , the crystals become smaller and under stress show a more even loading. If the hard drawn copper wire be annealed, the tensile strength goes down to about 30,000 pounds per square inch as a limit, and the ductility is increased. The crystals become larger, the distance of

Electrolytic Copper on a Rotating Cathode

305

shear be x e they break is longer and hence the ductility is increased. By depositing copper on a rotating cathode, from a solution a t room temperature, it is possible to duplicate the cold worked copper, getting a hard compact sample with a very slight ductility, and a high tensile strength. Now by raising the temperature of the solution and hence that of the uncrystallized copper deposited, it ought to be possible to anneal the metal, while it is being precipitated. Or looked at from the other point of view, crystallization from a hotter solution should give larger crystals, other things being equal. The following illustrates the principle admirably. _ ~_ _

CURRENT DENSITY500

_ _ _ _ ________ _ _ _ ~ _ _

Temperature _

AMPERESPER SQUARE FOOT ___ _

~~_~

I

_

_

_~~ _ _ _ _ _

Tensile strength

_ I_ _ -

___

I

C

I

50° C

I

25O

75O

c

-

63,000 pounds per square inch 49,000 pounds per square inch 30,000 pounds per square inch

'

This is graphically illustrated by a curve, Fig. 4.

e

Temperature I

I

260

I

'

I

of Elecirolyte I

60-

J

I

J

'

'

75'

Fig. 4

The last sample, especially, was very ductile and soft, and very similar in properties to the annealed copper wire. In this work a t the current densities above 500 amperes per square foot, no attempts were made to cool the solution during the progress of the run. The large rise in temperature explains why the tensile strength a t the higher current densities

C. W . BelzrLett

306

was always low. The heat developed by the resistance of the solution keeps the temperature up, and anneals the deposit while forming. Under these conditions the maximum tensile strength is that of soft drawn copper, 30,000-35,ooo pounds per square inch. The samples above 500 amperes per square foot, Fig. 3 , were all ductile except the last ones of the two series, given a t 2500 and 5500 R. P. M., respectively. Here the deposit was noticeably bad. It was thought advisable to get an approximation of the current efficiency with the rotating cathode a t a high current density. There being no evolution of gas a t either electrode this was gotten by taking the ratio of the anode loss and cathode gain. Consequently, a run was made, using 1000 amperes per square foot, while the cathode was rotated 5500 R. P. M. The results follow: Weight of anodes before Weight of anodes after Anodes loss Weight of cathode after Weight of cathode before Cathode gain

. . . 52.8 - = 99.6 current efficiency.

.

2582 grams 2529 grams 53.0 grams

869 grams 816.2 grams

--

52.8 grams

53

’

The efficiency therefore is practically IOO percent. Some runs were made with varying amounts of gelatine (1/2-5 grams per liter of solution) to see if the character of the deposit would be improved. Good conditions otherwise, that is, 400 amperes per square foot, a t 2500 R. P. M., were used. The deposits, however, were very hard, and so brittle they could be crushed between the fingers. The brittleness was no doubt due to the mechanical deposition of traces of gelatine, with the copper. The gelatine is undoubtedly present here as a colloid. It is possible that the particles of copper become covered with a surface film of the gelatine, and are thus prevented from adhering to each other. Thus a “brick and mortar” structure would result, which could

Electrolytic Copper o n a Rotating Cathode

307

easily be broken down. This is of importance in accounting for the results gotten with bronze, for here, too, there is certainly colloidal tin oxide present in the solution. A run was made with copper nitrate and nitric acid, the concentrations of which were equivalent t o 15 percent CuS0,.5H20 and 1 2 percent H,SO,. A current density of 500 amperes per square foot, at 2500 R. P. M., was used. The deposit, however, was dark, hard and brittle. In an attempt t o apply the principle of rapid stirring and high current density, to the alloys of copper, bronze was studied. The first question was that of a solution from which t o deposit the copper and tin. The solution recommended by Curry' was first tried. The solution contained 15 grams of CuS0,.5H20, 28 grams of SnC,O,, 5 grams of H,C,O,, and 5 5 grams of (NH,),C,O, per liter of water. The anodes were go percent copper and IO percent tin. The current density varied from 20-200 amperes per square foot, but no satisfactory deposit could be obtained. At first bronze would be deposited, then the deposit became copper-rich until finally only copper was precipitated. CuC1,.2H,O was substituted for CuS0,.5H,O with only greater complications, for here we introduced the possibility of cuprous chloride. The conductivity of the solution was increased by adding ammonium chloride, but the character of the deposit was not enhanced. There are two possibilities here. The tin may go in as stannous tin and be oxidized to the stannic condition, from which it would not be precipitated, or the tin may dissolve and become colloidal. If the latter be true, a copper-rich deposit should be obtained from the beginning, if the solution were boiled before the deposition was started. Here the tin in solution would be changed entirely to the colloidal state, and only copper would deposit. When this was tried the results indicated that the tin went bad, due to the formation of the colloid. A solution was made up as given above. One portion was electrolyzed, cold for a minute, and another, after being boiled, was run under identical conditions. A ~.

Jour. Phys. Chem.,

IO, 515

(1906).

308

C. W . Bennett

bronze was obtained in the firs: case, while in the latter practically a pure deposit of copper resulted. On account of the low conductivity of this solution it was deemed wiser to study the alkaline tartrate, for use in this precipitation. When the solution recommended’ was tried, no deposit a t all could be obtained. The sodium hydroxide concentration was high enough so that presumably it alone was decomposed. Some preliminary runs were made then, on a small scale, t o determine the proper concentration of the various substances. Solutions containing copper sulphate and sodium stannate in the proportion of 90 parts of copper to I O parts of tin, with enough sodium potassium tartrate to diSsolve the whole easily, with varying amounts of sodium hydroxide, were electrolyzed with an anode of 90 percent copper, and I O percent tin. These were run a t a moderate current density using a rotating perforated platinum electrode. After running for a t least 24 hours and after a deposit of constant composition was obtained, the deposits were compared, the solution giving the best bronze was analyzed, and this concentration used. This solution contained 2 2 grams of CuS0,.5H20, 5.5 grams of Na,SnO,, 150 grams of NaKC,H,O,, and 5 grams of NaOH per liter. The anode analyzed 89 percent copper, the cathode 93.5 percent copper. From this and a consideration of the final tin content, a good part of the tin went into the electrolyte in the unavailable form, most likely colloidal. The deposit on the platinum was brittle. However, about 50 grams of a compact deposit was obtained, which showed very nearly the same composition throughout. The solution above being used, runs were made with 6 , 18, and 30 amperes per square foot, but the deposits were too brittle t o get a measurement of the tensile strength. The relative amounts being the same, the solution was made five times more concentrated, in order t o increase the conductivity, and runs were made a t 1 5 0 and 250 amperes per square foot. The deposits were no better, however. The trouble here is most likely due t o the colloidal material. McMillan: “A Treatise on Electrometallurgy,” p. 2 8 7 .

Electrolytic Copper on a Rotating Cathode

309

This may be mechanically deposited with the bronze and prevent adherence, 'as the gelatine with the copper. Colloidal tin or tin oxide is no doubt present, for the tin content in the solution gradually goes up, showing that it is going into solution in an unavailable form. Tests applied for stannic tin were negative. When this is taken up again, the question of a solution is the first one to be answered. A systematic search should be made for a solution of high conductivity, where the chances for the formation of colloidal material and stannic tin are negligible, and for one, from which a deposit of constant composition can be gotten. Any attempt t o use the solution given above is useless if a bronze with good physical properties is desired. The next attempts at the application of the foregoing principle of rapid stirring and high current density were the study of brass. The solution used here was one of the double cyanides of copper and zinc with excess of potassium cyanide. This solution, 85 grams of ZnS0,.7H20, 35 grams of Cu(CN),, and 130 grams of KCN per liter, was electrolyzed with brass anodes containing 60 percent copper and 40 percent zinc. The current density varied from 12-150 amperes per square foot, with a rotation of 2500 R. P. M. Only one deposit was tough enough to test. A measurement was obtained on the run at 7 5 amperes per square foot. The average of two tests gave 9,500 pounds per square inch. Two pieces were annealed at 800' C for 8 hours and quenched, and then tested. Curiously the tensile strength went up. The average of two measurements gave 18,000 pounds per square inch. There has been a change in the piece other than the growth of the crystals, otherwise the tensile strength would have gone down. The fracture of the unannealed piece was interspersed with brownish black specks and lines which disappeared when the piece was annealed. It is possible that the piece as deposited had interspersed through it, probably as a film around the crystals, some decomposition product of the cyanide solution, which was driven out by annealing at the temperature used. At least, the curious reversal must

310

C . W . Bennett

have been due to the freakishness of the unannealed piece, since its tensile strength was very much too low for unannealed alpha brass. A sample was also heated t o redness by passing an electric current through it. The tensile strength was increased, as when the piece was annealed. The disturbing film around the metal was most likely burned out. Some runs were made with anodes, well over in the alpha field, 7 0 percent copper and 30 percent zinc, with more rapid stirring, but the deposit was very brittle. In practically all runs with brass, the first thin film of the deposit was very good, It was foil that could be bent into any shape. This was not tested because the results would have meant nothing, since the deposition could not be carried on indefinitely. The solution must change soon after the electrolysis is started. What this change is, or how it is brought about, is not known. What was said of the bronzing solution applies also to those for brass-plating. For this high current density work, it seems that an acid solution with very soluble salts of the metals will have to be found. While these experiments on the alloys have not been satisfactory in any respect, they a t least point out some lines along which work would be fruitless.

Conclusion These experiments have shown that: ( I ) Copper has been deposited electrolytically a t a current density of 4000 amperes per square foot or about 430 amperes per square decimeter. ( 2 ) In the electrolytic precipitation of a metal the crystal size decreases as the cathode is rotated more rapidly, other things being equal. (3) The crystal size decreases as the current density increases, and increase as the temperature rises. (4) The concentration of the electrolyte can be varied quite a little without changing the character of the deposit. ( 5 ) If the precipitation is carried on a t a high temperature an effect similar to annealing is accomplished during the electrolysis.

Electrolytic Copper o n a Rotating Cathode

311

(6) The tensile strength of metals varies inversely as the crystal size, and hence any factor tending to decrease the crystal size tends to increase the tensile strength. (7) The effect of rapidly rotating the cathode and of increasing the current density is to increase the tensile strength, the crystal size being decreased. (8) A good deposit of copper could apparently be obtained a t an infinite current density if the stirring were efficient enough to prevent impoverishment. (9) The current efficiency a t a high current density with rapid rate of rotation is high, it being 99.6 percent. ( I O ) “Hard drawn” copper can be deposited on a rotating cathode a t almost any current density, if the temperature be kept down; and likewise annealed copper, if the temperature be kept a t about 75’ C. ( I I ) Electrolytic copper has been obtained having a tensile strength of 68,000 pounds per square inch. ( 1 2 ) A deposit as good as the best can be obtained with a current density of 2400 amperes or more per square foot, with the rate of stirring used. (13) With the alloys, the trouble is most likely a question of colloidal material. (14) Acid solutions, with readily soluble salts, and no possibility for the formation of colloids should be sought for as electrolytes in alloy precipitation. This work was suggested by Professor Bancroft and carried out under his supervision, to whom, with Professor Upton of Sibley College, I wish to express my sincere thanks for advice and suggestions. Thanks are also due to Sibley College for cooperation by giving me the use of apparatus and machine shops. Cornell Universitji, Ithaca, N . 1‘.