US2181773A

US2181773A - Brass plating

Info

Publication number: US2181773A
Application number: US120315A
Authority: US
Inventors: Christian J Wernlund
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1937-01-12
Filing date: 1937-01-12
Publication date: 1939-11-28
Anticipated expiration: 1956-11-28
Also published as: DE685630C

Description

Patented Nov. 28, 1939 UNITED STATES PATENT OFFICE BRASS PLATING aware No Drawing. Application January 12, 1937, Serial No. 120,315

7 Claims.

This invention relates to the electrodeposition of copper-zinc alloys to serve as protective and decorative coatings on base metals and the like and more particularly to the electrodeposition of such alloys from cyanide solutions.

Electrodeposition of brass from cyanide solutions as practiced heretofore has been characterized by being somewhat more diflicmt than the plating of a single metal such as zinc or copper. In the electrodeposition of the zinc-copper alloys the electroplater combines the electrodeposition of two difierent metals, each of which requires diiferent conditions for optimum practical results. As a consequence, current efiiciencies are usually relatively low, relatively low current densities must be used and-considerab'e difilculty is usually encountered in maintaining a constant composition of the alloy in the cathode deposit. The composition of the cathode deposit varies widely with changes in the bath composition and with varying operating conditions. An increase of the temperature of the plating bath, for example, usually greatly increases the copper content of the cathode deposit. The cathode composition also changes materially when the current density is changed by relatively small amounts. In a commercial electroplating operation it is relatively easy to maintain constant bath composition and constant bath temperatures may be readily obtained by thermostat control. However, it is practically impossible to maintain constant current density in a commercial electroplating installation where numerous articles of various shapes and sizes are continually passing through the electroplating bath. The current density will vary for difl'erent shapes and sizes of work and moreover, different current densities often will appear at difierent portions of the same article.

An object of the present invention is to provide an improved method for plating zinc-copper alloys, by means of which method the composition of the alloy electrodeposit can be more closely controlled than in the methods known heretofore. A further object isto provide a means of electroplating zinc-copper alloys at substantially constant composition under varied current density conditions. My invention also includes a novel type of electrodeposited zinccopper alloy which I term white brass, as described hereinafter. Other objects will be apparent from the following description of my invention.

of copper cyanide, zinc cyanide, sodium cyanide and sodium carbonate.- It has also been proposed to omit the sodium carbonate from such solutions but this actually makes little difference in prolonged operations because the free cyanide present eventually decomposes to form carbonate in a relatively short time. Various investigators have experimented with additions of caustic soda to such baths and have uniformly reported that such additions of caustic soda are de eterious and cause formation of zinc red cathodic deposits, i. e., the caustic appears to cause an undesirable changev in the cathode composition and makes control of the cathode composition extremely diflicult. I have now discovered, however, that by the addition of considerable amounts of caustic soda to a brass plating bath containing cyanide, greatly improved results are obtained, provided that the ratio of the caustic soda to the zinc cyanide present in the bath, expressed in unit weight per unit volume of solution, is maintained within the approximate limits of 2:3 to 2:1. For example, in preparing a brass plating bath in accordance with my invention, for each gram per liter of zinc cyanide in the electroplating bath I may add approximately from 0.5 to 1.5 grams per liter of caustic soda. I prefer to maintain this ratio at approximately 1:1. Ihave found that when such a bath is electrolyzed, there will be very little variation in the cathode composition over wide ranges of change in current density, as long as the temperature is maintained reasonably constant. In such a bath having a given ratio of copper to zinc in the bath, I can vary the composition of the cathode deposit within a fairly wide range by merely varying the temperature of the plating bath. Having selected the particular temperature required to deposit the desired composition,

I may apply various current densities over a wide range without materially changing the composition of the cathode deposit. As a result, the bath is excellently adapted for commercial plating of a variety of articles of various shapes and sizes in the same installation and, if a reasonably constant bath temperature is maintained, all of the articles coming from the plating bath are identical in color. Since the only control necessary is to control the bath temperature and bath composition, my improved bath is easily controlled.

I have further discovered that a white zinccopper alloy containing about 19 to 31%- of cop'- per can be plated by the method of the hereinafter described invention with excellent control of the cathode composition. This white alloy which I term white brass" has been found to have excellent resistance to corrosion and is well adapted for plating on steel, brass or other base metal, both for the purpose of protection against corrosion and for decorative purposes. The white brass electrodeposit plated according to my method'is white and semi-bright and is easily buffed to form a highly polished plate of pleasing appearance. brass similar to polished metal also may be obtained by purifying the bath to remove heavy metal compounds therefrom, for example, the addition of a small amount of an alkali sulphide and bright dipping the resulting in acidic oxidizing solutionsuch as nitric acid, chromic acid, acidic hydrogen peromde or the like. Bright deposits may also be obtained by the addition of various mown brightening agents in the bath.

To produce a bright white brass coating by the above mentioned bright dipping operation, it is necessary to add suflicient sulfide or otherwise treat the solution to substantially completely precipitate the traces of heavy metal impurities usually present in the bath, e. g., lead salts. Various methods, alternative to sulfide addition may be utilized for such bath purification. For example, various reducing agents, e. g., zinc dust, may be added to the bath. Another effective method consists in allowing the bath to stand over night with zinc anodes placed therein. Since heavy metal impurities ordinarily occur in the anode and tend to enter the bath during electrolysis, I prefer to efiect bath purification by addition of a soluble sulfide or other soluble sulfur compound capable of precipitating heavy metals and to maintain a small excess of such sulfide or sulfur compound in the bath at all times.

By the application of my invention, i. e., by maintaining the ratio of zinc cyanide concentration to causticconcentration in the plating bath within the limits stated above and by maintaining reasonably constant bath temperature, I am able to consistently electrodeposit the white brass described above over a wide range of current densities without substantial variation of the composition of the alloy electrodeposit. As a result, a

' wide variety of articles may be plated in a given installation without danger of having certain portions of the work ofi-color and the composition of the electrodeposit may be readily maintained at any desired point in the range of 19 to 31% copper.

The herein described white brass is especially well adapted as an undercoat for nickel and chromium plating. It has the same advantages as those possessed by yellow or red brass for this purpose. In addition the color of the white brass is close to that of nickel or chromium, so that very s amounts of the nickel or chromium can be plated with no danger of a yellow cast showing through.

ishes produced; on steel by this method have 1m.- usually high corrosion resistance.

In orderto electrodeposit brass according to my inventiomll'may prepare a plating solutionby dissolving-sodium cyanidefcopper cyanide, zinc cya nide and -caustic, sodajin water. The relative 9?! Very bright deposits of my white invention I have obtained very satisfactory chrome finishes by plating a thin layer of chromium directly onto the white brass. Chrome finover the copper cyanide concentration, preferably about four timesas great. The sodium cyanide concentration must beat least suiiicient to dissolve all of the zinc cyanide as the double cyanide and preferably an excess of sodium cyanide is added. The concentration of the caustic soda should be approximately equal to that of the zinc cyanide. The following formula for plating white brass will serve as an example:

For plating yellow brass, the copper-cyanide concentration preferably is much greater than that of the zinc cyanide. The following formula is suitable for plating yellow brass according to my invention:

Gramsper liter Sodium cy Copper cyanide -1 60 Zinc cyanide"-.. 8

Caustic soda The anodes used preferably are of approximately the same composition as the desired. alloy electrodeposit. If desired, combinations of copper and zinc anodes may be used, as in known methods. I prefer to use alloyanodes.

While I prefer to maintain the ratio of zinc cyanide to caustic soda concentrations close to 1:1, this ratio may be varied within reasonable limits without materially afiecting the results. In general, the ratio of zinc cyanide to caustic soda may be varied between the approximate limits of 2:3 to 2:1 (0.66 to 2.0) That is, the weight of caustic soda may be equal to 50 to 150% of the weight of the zinc cyanide. I have found that as the zinc cyanide concentration is increased, fthe range over which the zinc cyanide-caustic ratio may vary also increases. For example, in a bath containing 8 grams per liter of zinc cyanide, the

.ratio Zn(CN)-z:NaOH may vary from 2:4 to 4:3

(0.50 to 1.33), while in a bath cong 16 grams per liter of zinc cyanide, the ratio may vary from 2:3 to 2:1 (0.66 to 2.0). In a bath containing only 6 grams per liter of zinc cyanide, the ratio-must be within the range 3:4 to 5:4 (0.75 to 1.25). However, it is preferable to maintain this ratio as close to 1:1 as operating conditions will permit, because any substantial deviation from the 1: 1 ratio correspondingly decreases the effectiveness of the control of the cathode composition.

The following examples serve to illustrate my Emmplel a plating bath is made with the following formula:

Grams per liter Sodium cyanide 60 Copper cy i7 Zinc cyanide 60 Caustic sod 60 To the bath, prior to electrolysis, about 0.5 gram per. liter of. sodium sulfide is added-to precipitate heavy metal impurities. The bath is electrolyzed at current densities ranging from 10 to p amperes per square foot,"-depending on'the bath 4 t m i es were;

At about 25 0., current density of to 40 amps/sq. it.

At about 40 0., current density of 10 to 60 amps/sq. it.

At about 80 0., current density of 10 to 100 amps/sq. it.

The anodes will contain about 30% copper to 70% zinc and may be of the conventional forms known to the plating art. Preferably, the anodes are enclosed in cotton bags or the like to prevent solid impurities on the anodes from entering the bath.

In a typical run with this bath, operated at bath temperature of 25 to 30 C., the following results were obtained:

Copper clon- Cathode current density fg g deposit Perceniz9 1 20 am also. it 30 amga/sq. it 27. 1 40 amps/sq. it 28. 8

Operating at bath temperatures of 25 to 80 C.

and current densities of 10 to 100 C., the cathode current efliciencies varied from 72.4% to 89.7%.

Example 2 Two electroplating baths for yellow brass were made as follows:

. Ingredients Beth A Bath B Grams/liter Grams/liter Sodium cyanide 135 90 Copper cyanide 90 60 Zinc Cyanide.-. l2 8 Cuastic soda 12 I 8 These baths were operated at d ifierent bath temperatures and current densities, using alloy anodes'containing 70% copper and 30% zinc. 'I'he'results tabulated below were obtained:

Bath A (operated at 28" C.)

The cathode current efliciencies in these baths vary from about 64 to 82%.

Various modifications and adaptations of my invention will be apparent to those skilled in electroplating. For example, while I have described baths containing sodium cyanide and caustic soda, other alkali metal cyanides may be used in place of sodium cyanide and the caustic soda may be replaced by a chemically equivalent weight of another alkali metal hydroxide.

I claim: 1. A process for electroplating brass comprising electrolyzing a solution containing zinc and copper cyanides and an alkali metal hydroxide, the ratio of the concentration of zinc cyanide to the concentration of said hydroxide in said solution being maintained chemically equivalent to a zinc cyanide to caustic soda ratio of approximately 1:1, said concentrations being expressed as weight of solute per unit volume of solvent, and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes occur in the current density.

2. A process for electroplating brass comprising electrolyzing a solution containing zinc and copper cyanides and an alkali metal hydroxide, the ratio of the concentration of zinc cyanide to the concentration of said hydroxide in said solution being maintained chemically equivalent to a zinc cyanide to caustic soda ratio within the range of 2:3 to 2: 1, said concentrations being expressed as weight of solute per unit volume of solvent, and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes occur in the current density.

3. A process for electroplating brass comprising electrolyzing a solution containing zinc and copper cyanides and caustic soda, the ratio of the concentration of zinc cyanide to the concentration of the caustic soda in said solution being maintained at approximately 1:1, said concentrations being expressed as weight of solute per unit volume of solvent, and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit whenchanges occur in the current density.

4. A process for electroplating brass comprising electrolyzing a solution containing zinc and copper cyanides and caustic soda, the ratio of the concentration of zinc cyanide to the concentration of the caustic soda in said solution being maintained within the range of 2:3 to 2:1,

said. concentrations being expressed as weightof solute per unit volume of solvent, and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes ocamount of alkali metal hydroxide which is'chemicaliy equivalent to approximately one part by weight of caustic. soda for each part by weight of the zinc cyanide in said solution and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes occur in the current density.

6. A process for electroplating white brass comprising'electrolyzing a solution containing copper cyanide and an amount of zinc cyanide greater than the amount of copper cyanide present, while maintaining in said solution that amount of alkali metal hydroxide which is chemically equivalent to an amount of caustic soda which is about 50 to 150% of the weight ofthe zinc cyanide in said solution and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes occur in the current density.

I. A process for electroplating white brass comprising preparing a solution containing copper cyanide and an amount of zinc cyanide greater than the amount of copper cyanide present, while maintaining in said solution that amount of caustic soda which is approximately equal to the weight of the zinc cyanide in said solution and maintaining said solution at a reasonably constant temperature, so as to maintain a substantially constant composition of electrodeposit when changes occur in the current density.