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Historical Articles, October, 1924

DETROIT BRANCH AMERICAN ELECTROPLATERS SOCIETY

SPECIAL REPORT NO. 17--RESEARCH COMMITTEE

THE NICKEL PLATINC OF ZINC BASE DIE CASTINGS

by B. E. Miller and R. L. Shepard

Read by Mr. Woodmansee

(Oct. 1924, American Electroplaters’ Society Monthly Review)

The following outline and notes as given on the plating of die castings are based on our experience in handling 30,000 to 50,000 pieces per day during the past two years: Actual figures of the past six months show that our rejects run less than .5%, which is quite satisfactory from a production standpoint. Yet we are constantly experimenting to improve details of our practice, and do not yet feel that we have reached a point where we can outline a practice as standardized.

Success in the plating room starts with the raw materials used in the manufacture of the alloy, is very much affected by melting room and die-casting practice, absolutely dependent on the finishing operation, and determined finally by the procedure and skill of the plater. So much can be said on the effect of the alloy, casting and finishing, that detailed discussion on these phases is reserved for a future paper on "The Manufacture of Die Castings for Plating." The scope of the present paper is therefore confined to the actual plating.

Castings received from the polishing room with adhering huffing compound must be soaked or scrubbed in gasoline or a mixture of equal parts gasoline and carbon tetra chloride. It is much better practice, however, to insist that they shall be left clean from the wheel, in which case the gasoline cleaning can be eliminated. We are at present using successively two hot electric cleaners. The first is a caustic cleaner strong in soap which does the heavy cleaning; the second is a weak alkaline cleaner for removing the film of soap left from the first cleaner.

Caustic Soda: 2 oz. per gal.

Tri Sodium Phosphate: 2 oz. per gal.

Soda Ash 1 oz. per gal.

Whale Bil Soap: oz. per gal.

In use the caustic soda is converted to soda ash and the soda is precipitated as scum. To strengthen this solution, therefore, it is only necessary to add caustic soda at the rate of two pounds per 100 gallons every few days, and about two pounds of soap about once a week.

The second cleaner has the following composition:

Tri Sodium Phosphate:2 oz. per gal.

Soda Ash: 1 oz. per gal.

In use it becomes contaminated with soap which may cause peeled plate. At the first sign of trouble due to this cause it should be thrown out and a new solution made up.

In addition to the cleaners, we use a muriatic dip (1/8 pint per gallon) and a cyanide dip (2 oz. per gallon).

The plating tank is 30 inches wide, 24 inches deep and 10 feet long, with agitated cathode rods with a 6-inch stroke, moving ten to twenty times per minute.

The solution need not be of any particular formula. The factors governing its composition are the nickel content of 2.10 oz. per gallon, conductivity of 15 amperes per square foot of cathode at 3 volts, and acidity of 5.7 to 5.8 by the Bureau of Standards drop ratio method. The following solution answers to above requirements, and has given very satisfactory service:

Double Nickel Salt: 10 oz. per gal.

Single Nickel Salt: 3 oz. per gal.

Epsom Salt: 3 oz. per gal.

Common Salt: 4 oz. per gal

Sodium Citrate: 1/2 oz. per gal.

Total Solids: 20 1/2 oz. per gal.

Baume at 70° F: 11.8°

Another solution which is equal or better in every respect, and in addition is much simpler to control from the laboratory, or by the plater, is made up as follows:

Single Salt: 10 oz. per gal.

Sal Ammoniac: 2 oz. per gal.

Baume at 70° F: 11.8°

It is particularly recommended that the solution be analyzed at regular intervals of one to two weeks, to determine its exact composition. Varying conditions of concentration by evaporation, and dilution by dips and fillings up the tank may so affect the composition as to make it quite uncertain. On the other hand a periodical analysis and adjustment of the composition to the proper formula will forestall many troubles which otherwise may be misinterpreted.

It is even more important to control the acidity exactly by routine tests, It is our practice to test all solutions daily, late in the afternoon, and make proper additions after the tanks are shut down. If a solution is too alkaline, one pint of hydrofloric acid is added; if too acid, one pint by routine tests. It is our practice to test all solutions of ammonia is added. The proper range is pH = 5.7 to 5.8. Above this the plate will tend to be duller, the solution muddy, the anodes passive and the plate pitted. Below this point the plate will tend to be brittle and peel, thc cathode efficiency low, and the plate streaked and pitted.

Proper racking and wiring of the work is important. If wired the pieces should be ½ to 1 inch apart, so as not to shade. If racked they should be arranged similarly to avoid overlapping, and also to avoid high points, which take a strong direct current. All contacts must be tight to avoid streaking.

In carrying on regular production the work should be handled by a crew of two, three, or four men, one of whom is an experienced operator who directs the others and regulates the current. The current required will be from 2 1/4, to 3 1/2 volts, which will be determined by experience for different classes of work. The time required will be 10 to 20 minutes, similarly determined.

During the course of production it frequently happens that castings fall from the racks to the bottom of the tank. The tanks should be carefully fished every night to remove these castings. However it frequently happens that some castings are not removed, and in the course of several months the solution becomes contaminated with zinc, giving a streaked nickel. If time and solution storage space are available the zinc may be precipitated with sodium bicarbonate, allowed to settle and the clear solution decanted

The sequence of operations is as follows:

  1. Soap cleaner, 5 to 10 seconds
  2. Cold water rinse
  3. Alkaline cleaner, 5 seconds
  4. Cold rinse
  5. Muriatic dip
  6. Cold rinse
  7. Cyanide dip
  8. Cold rinse
  9. Nickel plate, 10 to 20 minutes
  10. Cold rinse
  11. Hot rinse

References:

Bureau of Standards Letter Circular No. 82, "Control of Acidity in Nickel Deposition." Blum and Thompson.



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