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Historical Articles


Monthly Review

November 1929


EDITORIAL

This month is one in which we celebrate our annual and memorial Thanksgiving that is so sacred in the history of our country, and should be likewise in our Society.

Much has been accomplished by this Society since its institution and similarly to our Pilgrim ancestors, and with many sacrifices to the pioneers of this organization. Yet in the great progress and strides of our industry, are we not prone to forget to be thankful for the foresight of the great men who so valiantly fought to promote our welfare by the organization of the American ElectroPlaters Society, with its unselfish and high ideals.

In previous editorials there has been much said about research and help to the industry, and it is surely in keeping with the original ideas of our founders; but we, as members of the Society, are also entitled to many congratulations, for our insight to their thoughts, and the earnestness with which we have continued to carry on.

Your editorial staff thanks you all for your kind indulgence; and hopes that our offerings have been received with the spirit they are offered, purely educational, and if you do, well it’s a real Thanksgiving for all of us

DETROIT CORRECTION

October 25, 1929. Mr. H. J. Jameson Detroit Testing Laboratories 554 Bagley St. Detroit, Mich.

Dear Mr. Jameson:

I have seen a copy of the article on the determination of boric acid in plating baths, as published in the monthly REVIEW of the American Electroplaters Society, Vol. 16, No. 8, for August, 1929.

The correction of which I spoke the other evening should be as follows: In the last line at the bottom of page 9, after the words: ”bromothymol blue” should be added ”and 5 g. of mannitol.”

I am writing to you because I think you will know the proper person to insert a correction in the next publication of the Society.

Very truly yours
H. H. WILLARD.

Editor: ”This was an omission by author on the original paper.”



THROWING POWER IN CHROMIUM PLATING

Report of H. L. Farber, Research Associate’ American Electroplaters’ Society, Detroit, July, 1929

I—Introduction
Although numerous authors have compared the throwing powers of different chromium plating solutions, as measured for example, with bent cathodes, no numerical ‘data on throwing power have been previously published The definition and methods used in this study are those described in previous papers from the Bureau of Standards.

II—Principles
(a) Throwing Power:
The throwing power is defined as the improvement in per cent`of the metal distribution ratio above the primary current ratio If the latter is 2:1, and the metal ratio is 1.5:1, the throwing power is +25 per cent. If the metal ratio, however, is 3:1, the throwing power is—50 per cent. In chromium the throwing power is always negative. A value of—25 per cent is good, while one of—100 per cent is poor.

It has been shown that in all solutions the throwing power depends only on the cathode polarization, the conductivity and the cathode efficiency. In chromium plating the cathode polarization and conductivity have negligible effects. Hence the throwing power depends almost entirely on the relation of the cathode efficiencies at the high and low current densities used. If these cathode efficiencies were equal, the throwing power would be zero. Actually the cathode efficiency in chromic acid baths is always less at low than at high current densities. This causes negative throwing power. Thus, if the cathode efficiencies are 16 per cent and 8 per cent at the high and low current ‘densities respectively, the throwing power is—100 per cent. The only way in which the throwing power can be improved is by making these cathode efficiencies more nearly equal.

(b) Methods of Improving the Primary Ratio:
As, at best, the throwing power in chromium plating is poor, it is desirable to so arrange the cathode and anodes as to have the current density on the cathodes as nearly uniform as possible. This can be accomplished in the following ways:

(1) Have the anodes and cathodes parallel or concentric.
(2) Have the cathodes a considerable distance from the anodes.
(3) Have conducting wires or rods connected to the cathodes so as to detract or ”steal” the current from points that tend to get too high a current density.
(4) Use insulating shields to deflect the current from projecting parts of the cathodes.

(c) Plating Range:
In chromium plating it is especially desirable to obtain bright deposits all over the cathode, as it is usually difficult to polish dull chromium. For each solution there is a limited range within which bright deposits can be obtained. The selection of this plating range is often more important than the actual thickness of the deposits, as expressed by the throwing power.

(d) Covering Power:
The best cathode test measures the extent to which the cathode is covered. As will be seen, the results are practically parallel to the throwing power.

III—Methods of Study
The throwing power measurements were made in a glass-lined steel box platinum gauze anode was used, and polished steel cathodes were used in most of the tests. The primary ratio was 2:1. The cathode efficiencies were calculated from the currents used and the weights of deposits. The latter were classified as (M), milky, (Br) bright, (F) frosty, and (Bu) burnt, in order to define the plating range. The bent cathode tests were made in the same glass box, and the ratio of distances from the anode to the far and near parts of the cathode was also 2 ;1.

IV—Results Obtained
The data obtained in several hundred experiments will require numerous tables and curves for their complete presentation. In the following table a few typical results have been assembled in order to show the principal effects of the different conditions.

V—Conclusions
From these experiments the following conclusions were drawn:

  1. The throwing power is determined almost entirely by the cathode efficiencies, and can be improved by making these efficiencies more pearly equal at the maximum and minimum current densities used.
  2. At a given current density an increase in temperature reduces the throwing power.
  3. At a given temperature, an increase in current density improves the throwing power.
  4. Good throwing power and bright deposits can be obtained more readily at high temperatures and current densities than at low. A higher voltage is required however.
  5. Dilute solutions of chromic acid give better throwing power for a given current density. As, however, their resistance is greater, a high voltage is needed. For a given voltage (e. g., 5 volts at the tank), better throwing power is obtained in the more concentrated solution.
  6. A low content of sulphate, e. g., CrO3/SO4 = 200, gives slightly better throwing power than a ratio of 100, and much better than a ratio of 50.
  7. Sodium dichromate, added in large amounts to a solution with a sulphate ratio of 100, slightly improves the throwing power, probably by changing the sulphate ratio. If the latter is 200, the addition of sodium dichromate is detrimental.
  8. Boric acid has no appreciable effect on the throwing power.
  9. Small concentrations of trivalent chromium have no effect on throwing power. High concentrations increase the throwing power, making it equal to that of a more dilute solution. As the resistance of the bath is thereby increased and the plating range decreased, trivalent chromium is not advantageous.
  10. Iron has practically the same effects on throwing power as trivalent chromium, but a greater detrimental effect on the resistivity and plating range.
  11. Polished metal surfaces yield better throwing power than rough surfaces.
  12. The throwing power is slightly better on steel and brass than on copper and nickel.
  13. The results for covering power on bent cathodes are practically parallel to the measured throwing powers.
  14. The best throwing power (—13 per cent) with bright deposits, was obtained in a solution containing 250 g/L (33 oz/gal) of CrO3, 1.25 g/L (0.17 oz/gal) of SO4, at a temperature of 55° C (131° F.) and an average current density of 35 A/dm2 (325 A/ft2). This required in the box, 6.4 volts. Such conditions are suggested for use when a voltage above 6 is available, and when the best attainable throwing power is desired.
  15. When the voltage is limited, and, e. g., only 5 volts is available at the tank, a good throwing power (about—30 per cent) can be obtained with 400 g/L (55 oz/gal) of CrO3 and 2 g/L (0.27 oz/gal) of SO4 at a temperature of 45° C. (113° F.) and an average current density of 15 A/dm2 (140 A/ft2).
  16. In the latter solution, at a temperature of 35° C. (95° F.) and an average current density of 7.5 A/dm2 (70 A/ft2) a throwing power of about 0 per cent is obtained. This is usually satisfactory if the articles are not of very irregular shape, or are favorably placed in the tank.
TYPICAL THROWING POWER DATA
I—EFFECT OF TEMPERATURE AND CURRENT DENSITY
250 g/L (33 oz/gal) CrO3; CrO3/SO4 = 100
Temp.
Av. C. D.
Appearance
°C
°F
A/dm2
A/ft2
Volts
Throwing Power %
Near
Far
25
35
35
45
45
55
55
55
77
95
95
113
113
131
131
131
5
7.5
15
15
25
25
35
45
47
70
140
140
230
230
325
420
3.9
3.9
4.8
4.6
5.8
5.5
6.4
7.5
-295
-108
-58
-65
-27
-46
-28
-14
Bu
F
F
Br
F
Br
Br
Br
M
M
Br
Br
Br
M
Br
Br
TYPICAL THROWING POWER DATA
II—CONCENTRATION OF CrO3 45° C (113° F); 16 A/dm2 (140 A/ft2)
Appearance
Conc. g/L
CrO3 oz/gal
CrO3/SO4
Volts
Throwing Power %
Near
Far
150
250
400
20
30
53
100
100
100
5.2
4.6
4.5
-50
-65
-85
Br
Br
Br
Br
Br
Br
III—CONCENTRATION OF SO4
45° C (113° F); 15 A/dm2 (140; A/ft2) 250 g/L (33 oz/gal) CrO3
Appearance
Conc. g/L
CrO3 oz/gal
CrO3/SO4
Volts
Throwing Power %
Near
Far
50
100
200
4.7
4.6
4.7
-154
-65
-52
Br
Br
Br
M
Br
Br
IV—TRIVALENT CHROMIUM AND IRON
45° C (113° F); 15 A/dm2 (140 A/ft2)
250 g/L (33 oz/gal) CrO3; CrO3/SO4 = 100
Cr III
Fe III
Appearance
g/L
oz/gal
g/L
oz/gal
Volts
ThrowingPower %
Near
Far
0
3
9
17
34




9
1.3
0
0.4
1.2
2.3
4.5




1.2
.2
0




4
9
19
37
9
0.9
0




0.5
1.2
2.5
5.0
1.2
.12
4.6
4.6
4.8
5.5
7.0
4.6
4.7
5.1
7.8
5.5
4.6
-65
-64
-57
-43
-29
-71
-56
-55
-37
-58
-71
Br
Br
Br
Br
Br
Br
Br
Br
F
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br

(Applause.)

CHAIRMAN O’CONNOR: Are there any questions you would like to ask Mr. Farber?

MR. J. HAY: I would like to ask the speaker if, when he made those tests, he cleaned the parts before plating or if they were done without cleaning.

MR. FARBER: They were all cleaned by the use of an electrolytic cleaner, that is, an alkaline cleaner.

MR. HAY: I will ask another question. Has the gentleman found the electric current used in electric cleaning before chromium plating has any effect on throwing power of a chromium solution.

MR. FARBER: No measurements of this kind were made, as far as I know. Probably in cast iron the effect would be negligible

CHAIRMAN O’CONNOR: Any other questions?

DR. BLUM: Just perhaps to start some more discussion, I want to call attention to the relation between this covering power test, these specimens passed around, and the results which Mr. Pinner reported this afternoon. I think it is an interesting development, we hope in the right direction, in that we find that last year, Mr. Sizelove reported the fact that he had used the bent cathode for comparing the throwing power of different solutions, and incidentally he reported that the low sulphate gave the better throwing power. Then, Professor Baker and Mr. lnner, quite independently, used the bent cathode test under certain conditions, and used it to study and thereby to define, the best conditions of, or get the best throwing power.

Now we, of course, made use of all the information they had obtained, and what has been done in these experiments is to attempt to standardize the bent cathode test, because of course, if you hang a bent cathode in a beaker and you don’t have it in exactly the same position in the beaker, or the beaker is a little bit better one time than the other, or the anode is a different size, you can’t expect to get just exactly the same degree of covering.

Another thing, as I called attention to this afternoon in those experiments of Dr. Baker and Mr. Pinner, they used conditions under which the cathode could be completely covered rather easily; in other words, they would have a rather wide range of sulphate ratio, ‘or other conditions, in which it would be completely covered. Now we deliberately chose the conditions, putting the bent cathode in this box so as to fix it absolutely in position at a certain distance away from the anode so we are sure we get reproducible results, and we chose the size and position of the cathode so that it would be almost impossible to plate into the corner. In other words, you have to get the best throwing power shown in those cards, the amount is about—13 or —14 per cent, and with that it is not quite plated into the corner.

Now you see that is better than if the cathode plates in the corner with say a—50 per cent throwing power, because when it once plates in the corner, you can’t tell how much better than that it is, and that is the criticism I made, of course in a friendly spirit, of the specimens that were passed around this afternoon, that you couldn’t distinguish after it once covered as to, whether one was better than another.

Now the value of this, I think, is shown in this way. A lot of work’ and a lot of trouble is involved to make actual throwing power measurements the way Mr. Farber did in this long investigation, to actually weigh the plates before and after and measure the currents exactly and measure the cathode efficiencies. That is a lot of trouble. It was worth while in order to get definite figures. Now, having gotten those figures and compared them with what we got with the bent cathodes, then if you will carry the bent cathode test out in exactly the same way as it was carried out here, all you have to do is look at the bent cathode and say, ”That is a—50 per cent throwing power,” or ”That is a —75 per cent or—25 per cent throwing power.” In other words, this bent cathode test becomes as nearly as you can judge by looking at it, an actual measure of the throwing power of the solution. So it is interesting to know that that was one of the very last things that we did in this investigation; in other words, Mr. Farber had gotten his results on throwing power—we felt that they were right, but we didn’t know whether you would believe it or not, and so we made up all these bent cathodes just to prove, first to ourselves, and then to you people, that these results on throwing power actually’ mean what they say, because that bent cathode certainly represents a typical, even though a simple, case of plating any irregularly shaped article. So I felt it was worth while emphasizing because it means a new tool in the hands of the plater—A bent cathode test, standardized quantitatively so it actually gives you practically a method of determining throwing power.

MR. J. HAY: Mr. Chairman, I would like, if time would allow, just to illustrate how cleaning affects the throwing power of a chromium solution, regardless of the composition of the chromium bath. If you will allow me, I would like to illustrate on the blackboard

(Draws diagram of electric cleaner, as follows:)

For instance, this is our tank. (I am’ talking about cleaning strictly, now, gentlemen. ) We have a conveyor, and we are operating this cleaner at 7 volts. An alkali cleaner, using the tank-as the anode and the conveyor and head lamps as your cathode. Now through the current passing from one of those head lamps (at top of conveyor) to the other one (directly opposite), the effect is, through, I believe, arcing through the screw holes, that it turns the metal there electro-positive from electro-negative, and then this part (indicating outside of lamp around screw hole) will not plate.

DR. BLUM: What are they .7—Steel ?

MR. HAY: They are brass. I don’t know exactly what happens, although it was explained to me once.

Now in order to eliminate that, we changed the position of the head lamps and faced them the other way (indicating a half turn of the articles) and we have obtained good throwing power, plating them 100 per cent.

DR. BLUM: Well, in your electrolytic cleaner, Mr. Hay, it is possible those parts were shielded so that you got very little current there, so that you are doing very little electrolytic cleaning on that part.

MR. HAY: I was under the same impression, and changed the head lamps so they were exactly opposite to the way they were at first, but the same thing held true.

MR. HOGABOOM: In the work that Mr. Farber has done in the throwing power box, does he have the same condition as would be had in a plating solution? In the paper that was read this afternoon, written by Mr. Rowe, he states that he gets his best results by having an anode ratio of 3 to 1 of the cathode. If you are attempting to get throwing power with that ratio of anode to cathode, and then you attempt to measure throwing power with a throwing power box, are you getting results that will be reproducible in commercial practice?

Then, again, in his results, where he- gives a definite voltage, the voltage used is materially affected by the relation of the anode to the cathode. Now, if we plot volts like this, 6, 5, 3, 2, 1 (drawing on blackboard), and we plot here anode area ratio to cathode, and have here 4, /2, 3/4, 1 and 2, the result will be this: Draws as follows:

If you have one-quarter of your anode to your cathode to get a given current density, say 150 amperes per square foot, and having a solution of 250 grams of chromic acid per liter, you will have to use about 6 volts, but if you have it here (marks on curve) why you will only have to use probably about 36 to 3.8 volts. So that the relation of the anode to the cathode has a material effect upon the voltage, and when we use anything like that throwing power box, and then compare it with the actual conditions where it is known that you get a better plating condition by having a ratio of say 3 to 1, then is that reproducible.

DR. BLUM: Mr. Hogaboom thought things were getting entirely too tame around here; there wasn’t any argument. I am going to take the liberty of speaking for Mr. Farber; perhaps I can cover it more rapidly. The first question that was asked was regarding the relation of the anode and cathode areas in the box with respect to throwing power. Now the thing that we want to emphasize, and I an, very glad that it has been brought out in the discussion, is that these measurements of throwing power are simply comparative measurements. In other words, they do not tell you what the throwing power will be on any object plated in any position, in the plating tank, but they simply tell you that if you have one condition in the plating tank, with a given article, and you change that condition in a certain direction, whether you change the temperature, current density, sulphate ratio or anything else, that you will change the throwing power in a certain direction, and of a certain magnitude. In other words, these questions of the shape and size of the anode and cathode are all taken care of in the primary ratio, that is, they determine whether your current is more or less uniform over the article that you are plating. We have adopted arbitrarily here in all of these figures, a ratio of 2 to 1. Now that 2 to 1 we use because it is a ratio that can be rather easily obtained, and as a matter of fact you can’t get, if you use a ratio much larger than that, like 5 to 1—it will be only a very few sets of conditions in chromium plating that you can get a deposit both on the high and low current density. So that with respect both to the relative size of the anode and cathode and to the values given for voltage it should be distinctly understood (and of course that is the difficulty in a summary like this, you can’t go into detail), that these voltages only apply to that box. But the point is this: that if in one case the voltage in the box is 5 volts and in another case it is 6 volts, then you can be perfectly sure that if you have a tank—I don’t care what size it is, or what; shape the anode or cathode is, or the size at all—that in one condition it is going to take a greater voltage than in the other condition.
In other words, we don’t want you to think for an instant that because this table says it used 6.4 volts, that it is going to take you 6.4 volts to plate a certain thing. You might do it at 3 volts or you might do it at 10, depending entirely on the position and shape and size of your electrodes, and that is one of the reasons that in this outline we emphasized the fact, and that was brought out very splendidly in that paper of Mr. Rowe’s this afternoon, of which I only heard a part, that the biggest problem in chromium plating is the mechanical ingenuity to get the current where you want it.

Now that has nothing to do with throwing power as we have defined it. In other words, that is getting the best you can first in current distribution. Then we tell you afterwards, after you have done that, how you can get the best throwing power for a given current distribution. I think that answers the question.

MR. HAY: I would like to say that Dr. Blum hit the nail right on the head when he said that. I just want to tell the boys here tonight that it is very important to watch all the conditions before you get to the chromium bath, because the chromium bath itself is not as peculiar as most of you think it is, but the conditions of the operations before you get there are more important and have more to do with it than the chromium bath itself.



THE THICKNESS OF PLATED METAL COATS

(Presented to the Detroit Branch, A. E. S., Oct. 5, 1929)

Since the advent of chromium plating, the plating profession has become aware that any information which increased the efficiency and quality of a plating bath was important. With the use of scientific data, plating shops have speeded up production and improved the appearance of their goods. Likewise, the buyers of plated goods have realized that a noticeable difference exists between the best plated ware and the ordinary or poor goods, and they have set about to draw up specifications as to the amount of plate they desired and as to the corrosion resistance of the plated articles they were buying. Guesswork fails in gauging the thickness of a plate, but the importance of knowing the thickness is evident since the acceptance of a shipment of work may depend on meeting thickness specifications.

Knowledge of the thickness of metal plated enables a plater to test the efficiency of his bath, and also to determine the limits of the working of a bath which should be wide.

Faraday’s Law states that the amount of a metal deposited is directly proportional to the current passing through a solution, that is, double the amount of current should double the amount of deposited metal. By means of this law, the table of electrochemical equivalents has been calculated, which gives the amount of any metal deposited by one ampere of current flowing for one second. Such tables have now been elaborated to include such information as grams and ounces or pounds of metal per ampere hour. All such data is based on 100 per cent efficiency.

The terms frequently used in specifications are ounces per square foot, thickness in fractions of an inch, and ampere minutes of plate per square foot. The first two are not puzzling; the last term signifies the equivalent of the metal deposited in terms of the current used and the length of time plated, in other words, the amperes multiplied by the minutes of current flow. Thus, 200 ampere minutes can mean 10 amperes for 20 minutes, or 20 amperes for 10 minutes.

Should a square foot of metal cathode be carefully weighed and its thickness measured before plating, and the current and time of plating noted, followed by reweighing and re-measuring the thickness, there would be possessed all the factors for determin-ing the weight of plated metal per fraction of an inch thick per square foot of surface, and also the data for calculating the efficiency of the bath and the ampere minutes of metal plated. In many instances, such determinations are made in laboratories to establish the weight of metal per 0.001 in. per square foot of surface. However, without much time or trouble expended, such a relation can be calculated from data on the metallic elements already acquired and found in scientific handbooks.

Using the data on the metal chromium, the above relationship can be calculated as follows. The specific gravity of chromium is 6.92, that is, it is 6.92 times as heavy as water, which weighs 62.34 pounds per cubic foot. A simple multiplication gives the weight of chromium as 431.43 pounds per cubic foot. To simplify the calculation one must imagine a cube one foot square on all sides and one foot high. The base is equal to one square foot and by dividing the above weight by 12, the weight of a mass one inch high and one foot square is obtained, which is 35.95 pounds. To obtain the weight of one square foot of metal 0.001 in. high, 35.95 is divided by 1000, resulting in 0.03595 pounds of chromium per .001 in. per square foot of area, or 0.575 ounces per .001 in. per square foot. By experiment or calculation, the above relationship can be secured for any metal.

The actual determination of the thickness of plate on a production sample is accomplished either by direct measurement or by chemical analysis. It is fairly easy to produce a non-adherent plate, which can be stripped from the base metal and measured with a micrometer, and a piece of definite size weighed.
To produce non-adherent plate on colored brass or copper, dip the test piece in chromic acid before nickel plating. On die castings, dip the piece in the cleaning tank, followed by a quick rinse before plating; an alkaline film is left on the casting which produced a non-adherent film of plat, and which is easily loosened from the base metal by reversing the current on the sample in a cleaning tank. Oxidizing dips on copper and brass and possibly on steel, will produce subsequent non-adherent plates. Tincture of iodine will serve in silver plating. Also one can buy solutions for such a purpose in the market. The production of non-adherent coats of chromium, cadmium, copper and zinc plates is not so general, and most usually the determination of the thickness of these metals is a laboratory procedure.

To determine the thickness of plate by analysis, a piece is cut from the sample submitted and its area carefully measured. If plated with Chromium, Nickel and Copper, the thickness of all three can be obtained from one sample. The plated section is placed in a beaker and covered with concentrated sulphuric acid. The dissolving of the Chromium begins at once and with the addition of a very small amount of water, the Chromium is dissolved off completely, leaving the nickel untouched. The sample is washed and allowed to dry, and placed in another beaker, and concentrated nitric acid added with a few drops of concentrated sulphuric. The action is slow to start but small additions of water hasten it and soon the nickel and copper are dissolved, leaving the base metal clean. In the case of non-ferrous base metals, the dissolving of small amounts of zinc and other constituents is possible, as iron is dissolved from a steel base. Water must be used very sparingly, since concentrated acids have only a slight effect on the base metals, while dilute acids act rapidly.

The Chromium is determined analytically by electrometric titration with ferrous sulphate after oxidation. The copper and nickel solution must have the iron removed by precipitation with ammonium hydroxide. The copper is then determined by plating analytically from the mixed solution slightly acid with nitric acid. The nickel remains in the solution and is determined by titration with potassium cyanide in a slightly ammoniacal solution.

Cadmium is easily stripped with nitric acid, and determined from a solution made alkaline with sodium hydroxide and sodium cyanide, by analytical plating.

An ingenious test for Chromium plate is in use in several laboratories. Concentrated hydrochloric acid will dissolve Chromium but not Nickel and the chemical action results in bubbles of hydrogen being evolved as long as the metal is being acted upon. If the same acid is used and the drop of acid is of the same size, the amount of Chromium dissolved is proportional to the length of time the bubbles indicating reaction are observed. The relation between ounces per square foot and thickness of plate to seconds of reaction has been determined so as to provide an accurate test. A drop of acid is placed on the sample and the instant bubbles are observed a stop watch is started, to be stopped the instant the bubbles cease. The seconds are recorded and by reference to a table the thickness and weight of the Chromium plate is obtained. The table for this test is an arbitrary one and must be determined for standard conditions of the test. This test could possibly be applied to zinc and cadmium plating.

The results of all analysis in a laboratory are in grams, which are easily converted to ounces, and by dividing tho ounces of metal determined by the area in square inches of the sample, the ounces per square inch are obtained. Multiplying by 144 the figure becomes ounces per square foot. Referring to tables or previous calculations we find that the weight in ounce per .001 inch per square foot is .575 for Chromium, .742 for Nickel, and .747 for Copper. By dividing the weight of plate determined in ounces per square foot by the above number the thickness is obtained.

From the table of electrochemical equivalents the weight of a metal deposited by one ampere minute is obtained, and by dividing the weight per square foot of plate on the sample by this figure the ampere minutes necessary to deposit that eight are secured. In the case of Nickel or Acid Copper solutions, whose efficiencies approach 100% this figure is correct as obtained. In the case of Chromium with an efficiency of about 13% or Cyanide Copper of about 47%, the result must be divided by .13 or .47 to obtain the correct result. The result is expressed in ampere minutes per square foot of plated surface.

Various tables are published on the relation of ampere minutes per square foot to the thickness of metal plated, and from these the ampere minutes necessary to plate the metal of the thickness as determined, can be ascertained. One of the most complete and correct tables was published by Baker and Pinnet in January, 1928, in an article on ”The Protective Value of Chromium Plate,” presented-before the Society of Automotive Engineers.

A typical analysis of Chromium over Nickel gave 0.011 ounces per square foot. The table of electrochemical equivalents- gives 0.00019 ounces of Chromium per ampere minute. By division, 57.9 ampere minutes is obtained, which would be correct at 100% cathode efficiency. Since the efficiency is about 13.3, another division by .133 gives 435 ampere minutes of Chromium per square foot. If the thickness is to be determined, the weight 0.011 ounces per square foot is divided by 0.575, giving a result of 0.019. Since .575 is the weight per .001 inch per square foot, 0.019 must be multiplied by 0.001, and the thickness of the plate is 0.000019 inch. From a perusal of tables on thickness and ampere minutes, we find 0.000019 is equivalent to 435 ampere minutes per square foot.

The specifications of one automobile plant in Detroit are, Chromium, 0.00002” thick and 0.01175 ounces per square foot; Nickel on steel base, 0.20 ounce per square foot and on non-ferrous base, 0.10 ounce per square foot.

Typical analyses of the plating produced by one shop in Detroit are as follows:

 
Oz., sq. ft.
Inches thick
Amp. min.
Ni over cu on steel
0.350
.00034
370
Ni on die cast
.138
.00018
200
Cr. over Ni on die cast
.011
.000019
435
Ni on brass
.101
.00013
153

General practice calls for about 300 ampere minutes of Nickel and between 400 and 500 amp re minutes of Chromium.

If the time of plating is known and the ampere minutes calculated, the current density is obtained in amperes per square foot by dividing the ampere minutes per square foot by the time in minutes. The current density on a production tank can be roughly calculated from a knowledge of the total square feet of cathode surface and the current flowing, but such a figure does not usually include racks and wires. From the data on thickness -of plate, the actual current density on the plated article is obtained, and the knowledge as to whether a bath is operating at its best-current density is obtained.

Also, from a knowledge of the ampere minutes of plate on an article and the ampere minutes actually put into the tank, the loss on the racks and wires can be calculated. It follows then that the current necessary to plate at a certain current density can be calculated from the per cent of ampere minutes actually going on the work.

With data on the ampere minutes of plate per square foot and ampere minutes actually put into the tank, the efficiency of a bath can be calculated. Suppose one square foot of surface to be plated with chromium at 100 amperes per square foot current density for three minutes, and the resultant plate found to be 0.0074 ounces per square foot in weight. From the tables it is ascertained that one ampere minute of chromium per square foot at 100 per cent efficiency weighs 0.00019 ounce. This figure multiplied by 3 X 100 or 300 ampere minutes is 0.057 ounce. The actual weight determined, 0.0074 ounce, is divided by the weight at 100 per cent efficiency, and the result is 0.13, which multiplied by 100 is 13 per cent, which is the efficiency of the bath.

In conclusion, I would impress on you that a knowledge of the thickness of plate is important, and has many applications which lead to economy, efficiency and a more complete understanding of the working of a plating bath.

O. A. STOCKER,
Bohn Aluminum & Brass Corp.



A. E. S. PAGE
Assembled Expert Scraps With and Without Significance

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Don’t fool away so much time in toil. If you die and go to Heaven you’ll have the work habit so bad that you can’t enjoy resting. And if you go to Hell you’ll regret that you didn’t have more fun when you had a chance.

—————

Sentry Duty
The owner of a big plant, addressing a new employee:
“Did my foreman tell you what you will have to do?”
“Yes, sir, he told me to wake him up when I see you coming.”—Forbes Magazine

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Getting Peeled
Wild and disheveled, watery of eye, and trembling of limb, he burst into the dentist’s consulting room and addressed the molar merchant in gasping tones:
“Do you give gas here ?”
“Yes,” replied the dentist.
“Does it put a man to sleep ?”
“Of course.”
“Nothing would wake him ?”
“Nothing. But—”
“Wait a bit; you could break his jaw or black his eye without him feeling it?”
“My dear sir, of course, I—”
“It lasts about half a minute, doesn’t it?”
“Yes.”
With a war-whoop of joy and relief the excited man threw off his coat and waistcoat.
“Now,” he yelled, as he tugged at his shirt, ”get yer gas-engine ready. I want you to pull a porous-plaster off my back.”—Credited to “Exchange” by the Christian-Evangelist.

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No Incentive
Employer—“Sam, I hear you and George almost had a fight.”
Sam—“Yassah, boss, we all would ‘a’ had a terrible fracas, only they wasn’t nobody there to hold us apart.” —Life.

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Did you hear about the smallest woman in the world? She is so small that she can sit and sew on a button.

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Famous Pairs
Hug & Kiss
Neck & Neck
Night & Day
Yes & No
Win & Lose
Free & Easy
Up & Down
Jack & Jill



SOME OBSERVATIONS ON CHROMIUM PLATING

By E. D. Bedwell, Los Angeles Branch, A. E. S.

As I am a practical plater and not a chemist, in this paper I will not attempt the use of chemical or technical terms, but will endeavor to give in the platers’ every-day language an outline of our experiences and conclusions during the past 18 months of the more or less successful operation of a chromium solution.

The first and most important thing I believe in chromium plating is the cleaning of work for nickeling before chrome plating, as we all know that chrome is only as good as the deposit of nickel underneath. The most essential thing in cleaning brass or copper for a chrome resisting nickel is a medium strength cleaner that will not tarnish but will still clean (and there are several such cleaners on the market3 then dip directly into clean muriatic acid (no water in this dip), rinse in clean water, cyanide, rinse and into your nickel with as little delay as possible to avoid any chance of stain or oxidizing.

Secondly, the nickel bath should be of the single salts, Epsom salts and salamoniac variety, very low in acid, and current should be run very low (I should say not more than half the current the average plater uses ordinarily for nickel plating), so as to obtain the very softest deposit possible.

If these two rules are observed very closely we have found little or no trouble with chrome pulling the nickel; and as I will explain later we have had an excellent opportunity to observe the action of chrome on different nickel deposits.

Third, we come to the chrome bath itself, and the operation of same We have carried on experiments for the past three years with a number of different chrome baths and about 18 months ago we arrived at the conclusion that very good results were to be had from the very common formula of chromic acid and sulphuric acid in the proportion of about 100 to 1, and let me say right here that we have found that about, is as near as you can give proportions, as in some cases we have only been able to use 1-1/2 oz. of sulphuric to 100 lbs. of chromic acid. The proportion of sulphuric to be used, however, may readily be determined by the bent cathode test, which we are nearly all familiar with by now.

For the benefit of those who are not familiar with the test I will refer to the paper on the bent cathode test by W. L. Pinner and E. M. Baker in the September issue of the monthly REVIEW.

We have had the best success with a chrome bath having 3 lbs. chromic acid to the gallon, but very satisfactory results may be had with 2 lbs., and here let me say that if you make your bath up with 3 lbs. to the gallon you must maintain it at that for best results because the sulphate is not used out of your solution in anywhere near the proportion that chromic acid is, therefore you must make frequent addition of chromic acid without the addition of sulphuric, and in no case should sulphate be added without first making a test as it is very easy to add but practically impossible to remove. However, if you do get too much sulphuric there are several ways to adjust that condition. Obviously you may add ,more chromic acid, which will reduce the proportion of sulphate to chromic acid, or you may remove part of your bath and add water and chromic acid enough to increase the gravity of original point or you may add barium carbonate. None of these should be added until you have made a test in small solution to determine amount required, and then add to or reduce your large bath in proportion.

In regard to the use of barium carbonate for the reduction of the excess of sulphates in a chrome bath we have found by a number of very careful tests that the striking in power of the bath will never be quite as good as it was before the use of barium. Personally, I would not recommend the use of it, but the more simple method of adding chromic acid or if the excess of sulphuric is too great the reduction of the bath with water and then addition of chromic acid.

The regulation of current is also a very important factor in obtaining a bright uniform deposit of chrome, especially in lower temperature baths is this true, because the cooler baths have a slower cleaning power Incidentally, the tendency is downward in the last year in regard to the temperature of the bath. Several platers I have talked to lately that are operating the better known patented baths, tell me they are instructed to run the bath at about 100 degrees and we are operating ours at very much lower degree than that.

The current should be started about 12 to 2 volts and gradually advanced after about 15 seconds to 5 volts in case of small easy work to strike in on; but on large difficult pieces, 7 to 9 volts if you have it. I wish to explain here that I talk in voltage, because I practically never read the ammeter for the very simple reason that I found that at a certain voltage, regardless of amount of work in the bath I got a certain result, and I didn’t have to figure the surface to be covered.

Last I believe in importance are the anodes, as there are a number of different metals that may be used with about equal satisfaction and results. Nickel, lead, chrome and iron are some of them, and there seems to be considerable difference of opinion as to which are best. Our experience along this line is this: we started in 18 months ago with lead anodes in a 200 gallon bath at 120 to 130 degrees and used them about three weeks. At the end of this period we had come to the conclusion that even if iron anodes did ruin our solution in three months, as we had been told it would, we would still be ahead of the game to throw it out and make a new one, as we had taken our lead anodes out at least twice a day to clean them and then if we had a large piece of work we could not get current enough to plate it. Accordingly, we put in iron anodes and have had them in the solution ever since. About three or four months later we reduced our temperature to about 90 degrees and have since run at that or cooler. We are still getting a bright uniform deposit from this bath, with at least 95 per cent of the striking in power of a bath we made up in the last two months and also know others that have run a year or more with iron anodes with the same results.

In closing I wish to refer to the nickel subject again. We are doing chrome for 8 to 10 different plating shops and have found that the nickel done by some of them almost always stands up under a chrome plate, while others in nearly every job have some that peels This, I think, is because of carelessness mostly in cleaning the work for plating, as if it was trouble in the bath itself or in regulation of the current the whole job would peel off in the chrome.

So, if you are having trouble with your nickel peeling when being chrome plated, look first to your cleaner, second to your current and last to your nickel bath.





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