US2221562A - Electroplating - Google Patents

Description

Patented Nov. 12, 1940 ELECTROPLATING Christian John Wernlund, North Tonawanda, N. Y.. assignor to E. Ldn Pont de Nemours & Company, Wilmington, DcL, a corporation of Delaware No Drawing. Application December 24, 1937,

' Serial No. 181,617

6 Claims. (01. 204-40) This invention relates to electroplating nickel and more particularly to the electrodeposition of multiple layers of metal including a layer of nickel. I

This application is a continuation in part of my co-pending application Serial Number 120,315, filed January 12, 1937, issued November 28, 1939, as U. S. Patent 2,181,773.

Nickel plating is utilized'for coating a large variety of meta-l articles. While the nickel is often deposited directly on the base metal, it is general practice first to deposit a layer of copper or yellow brass on the base metal and then to electroplate nickel on this intermediate layer. An important use for nickel plating is to coat metal surfaces composed ofzinc or a zinc alloy containing at least by weight of zinc, e. g., zinc alloy die castings. In these utilizations, the nickel either may be the final decorative coating or it may serve as an undercoating for chromium or other metal. In the electroplating of zinc alloy die castings as practiced heretofore, three methods have been employed. In one method, the nickel is electroplated directly onto the die casting. In this method, it is necessary to add sodium sulphate or other conducting salt to the nickel solution in fairly large quantities and the at a high current density, then follow with electrodeposition at the normal current density.

Satisfactory nickel deposits by this method require careful control on the part of the operator. Also, unless the nickel deposit is exceptionally dense and relatively thick, the zinc or zinc alloy underneath the nickel tends to corrode causing the formation of blisters and eventual peeling of the deposit. p

A second method which has been proposed is to electroplate yellow brass as an undercoating for the nickel deposit. While this method usually gives satisfactory results if properly done, the electroplating of the brass is difficult and requires exceptionally careful control to obtain uniform results. If the bath temperature and current density are not maintained substantially constant, the composition of the electrodeposited brass will vary considerably. Moreover, it is necessary to use a low current density which results in a slow deposition of the brass.

The third method consists of electroplating copper as the undercoat for the nickel electrodeposit. The difliculty with this method is that in time the copper undercoating often tends to alloy with and diffuse into the zinc alloy casting,

so that finally the benefit of the undercoating disappears and the nickel is subject to the same blistering action whic'h occurs when nickel is deposited directly on the zinc alloy. To prevent this occurrence, it is necessary to plate a relatively thick layer of copper, adding to the manufacturing cost. A further-disadvantage is that unless the nickel deposit is exceptionally dense and non-porous, corrosion products from the copper undercoat appear on the surface of the mckled article, causing green discoloration.

An object of the present invention is to provide an improved method for producing nickel plated articles, which method is free of the disadvantages enumerated above. A further object is to provide an improved method for producing nickel plated articles made of zinc or zinc alloy, for example, nickel plated zinc alloy die castings. Other objects will be apparent from the following description.

The above objectsjmay be attained in accordance with the present invention by first electroplating the article to ibe nickel plated with a layer of a zinc copper alloy which contains from 19 to 31% of copper and then electroplating nickel on to the zinc copper alloy from an acidic solution of a nickel salt. By this method I am able to produce nickel plated zinc die castings or other zinc articles which will stand for long periods of time under corrosive conditions of the atmosphere without blistering or peeling of the nickel coating and without any tendency for copper staining. I have found that theelectro plated coating of zinc copper alloy containing not more than 31% of copper undergoes no reaction with the underlying zinc; that is to say, there is no tendency for thecopper in the alloy coating to diffuse, into the zinc alloydie casting. I have further found that the zinc copper alloy undercoating is suiliciently resistant to corrosion so that there is substantially no tendency for blistering and peeling of the nickel coating to occur. Also, any corrosive products from the zinc copper alloy which may be formed in small amounts apparently are white or colorless, for no green stains appear upon the nickel coatin when the latter is subjected to strenuous corrosion tests.

Another advantage of my nickel copper undercoating is that it readily may be brought to the proper degree of brightness required for electroplating a bright nickel deposit thereon. In fact, if desired, the zinc copper alloy'may be directly alloy surface.

plated in a bright condition so that substantially no bufling. is required. I have also found that the zinc copper alloy is considerably harder than either pure copper or yellow brass and that consequently in the bufiing operation there is much less danger or removing too much of the alloy to expose base metal at corners and edges of the article. Also, there'is no difliculty in obtaining a dense adherent electrodeposit of nickel on the zinc alloy undercoating and the control pf the nickel plating solution is easier than, for example, when nickel is plated directly on to a zinc or zinc In electroplating my zinc alloy undercoating, I prefer to utilize the'method described and claimed in my above mentioned Patent No. 2,181,773. In this method a cyanide bath containing the double alkali metal cyanides of copper and zinc and an alkaline material such as caustic soda is used.

The following is an example of a solution suitable for this purpose: Per liter The composition of the zinc copper alloy may be controlled as desired withinthe range 19 to 31% of copper by varying the amount of copper cyanide in the electrolyte and the temperature of the electroplating solution. In general, the amount of zinc cyanide in the electrolyte should be equal to three to four times that of the copper cyanide. It is also preferred to maintain the concentration ratio of zinc cyanide to sodium hydroxide to about one to one; however, this ratio may be varied between the approximate limits of .66 to 2.0; that is, the "weight of caustic soda may be equal to from 50 to 150% of the weight of the zinc cyanide. By thus maintaining the concentration ratio of zinc cyanide to caustic soda as above stated, the composition of the zinc. copper alloy will remain substantially constant throughout long periods of operation under varying conditions of current density. The electrolyte may be operated at any desired temperature. from room temperature, e. g. 20 C., up to the boiling point of the solution. I generally prefer to operate the bathat a temperature of about 45 C. It should be noted that the composition of the electrodeposit will vary somewhat as the temperature is changed, the copper content of the electrodeposit increasing as the temperature is raised. Hence, by varying the temperature of operation, the composition of the electrodeposit may be varied as desired.

It should be noted that the present invention is not restricted to the above method for electroplating the zinc copper alloy which has been described merely by way of example. Various solutions containing copper or zinc cyanides or other salts which are suitable for electroplating copper zinc alloys may be used to practice my invention. In using such solutions it is, of course, necessary to regulate the respective amounts of zinc and copper salts in the electrolyte and to regulate other factors such as temperature and current density so as to obtain an electrodeposit containing from 19 to 31% copper. The thickness of the electrodeposit of the zinc copper alloy may be varied considerably with good results and may be as low as 0.0001 inch in thickness. In general, I prefer to electroplate a layer of the zinc copper alloy about 0.0003 to 0.0005 inch thick. Much thicker coatings may be used if desired but ordinarily there is little or no advantage in a thickness of more than 0.0005 inch thick.

After the zinc copper alloy has'been plated on the zinc alloy gr other base metal, if it has not been plated in a bright condition, it should be buffed before plating it with nickel. The buffing operation is similar to bufling zinc prior to electroplating; that is, it is desirable to use a greaseless bufling compound to avoid the formation of insoluble zinc soaps on the surface of the metal.

If desired, bufilng may be eliminated by electroplating the copper zinc alloy in a bright condition according to the method described in the above mentioned co-pending patent application. In order to plate the metal in'the bright condition it is necessary to use a cyanide bath and, prior to electroplating; to subject the solution to a purification treatment which removes substantially all traces of heavy metal impurities such as soluble lead and cadmium coz'npoun'ds'which may be present therein. This may be done by addin a soluble sulphide or other soluble sulphur compound which will cause a precipitation of such heavy metals as sulphides. Alternatively, the heavy metal impurities may be removed by adding an emcient reducing agent, for example, zinc dust. After precipitation of the impurities, the

.bath may be allowed to stand or may be filtered.

' ing it in an acidic oxidizing solution, for example,

a dilutesolution of nitric acid, e. g. 0.5-5%, or a hydrogen peroxide solution acidified with sulphuric acid. The resulting surface is smooth and lustrous and usually is practically identical with a well buffed surface. k I

In accordance with my invention the above described zinc copper alloy coating. containing 19-31% of copper is electroplated with a nickel plate of any desired thickness, e. g. 0.0003 to 0.001 inch thick. It is essential that the nickel be electroplated from an acidic solution. Any of the various well known acidic nickel plating baths may be utilized for my purpose. The following formula is an example of a well known nickel plating solution which gives satisfactory results:

Ounces per gallon NiSO4.6H2O 16 NiChS g 4 Boric acid 4 Y 4 tained at a pH of 5 to 6. .The acidity may be adjusted by the addition of hydrochloric or sulphuric acid in the required amount. If the solution is inadvertently made too acid, it may be readjusted to the desired pH value by the addiponderantly of zinc, although castings may be made from pure zinc. For example, certain zinc die castings which were successfully coated with nickel by my method were found to contain about 95% by weight of zinc and about 5% by weight of aluminum and less than 1% by weight of cop per. In this description and in the appended claims, I use the term zinc die castings to include alloy castings containing 90% or more by weight of zinc and also castings of pure zinc.

It is further understood that my invention is not restricted to coating cast zinc articles, but is applicable to the coating of any metal surface composed of zinc or a zinc alloy containing at least 90% by weight of zinc. For example, zinccoated iron and steel articles, such as galvanized iron or articles having an electroplated zinc surface likewise may be coatedwith nickel by my method, with excellent results.

I claim:

1. An article of manufacture comprising a metallic base composed preponderantly of zinc having a multiple, electrodeposited metal coating comprising a layer of white brass at least 0.0001 inch in thickness containing 19 to 31% by.

electroplated onto said white brass layer.

2. An article of manufacture comprising a zinc alloy base containing at least 90% by weight of zinc, having a multiple, electrodeposited metal coating comprising a layer of white brass at least 0.0001 inch in thickness containing 19 to 31% 40 by weight of copper, the remainder being zinc,

electroplated onto said base and a layer of nickel electroplated onto said white brass layer and a coating of chromium on said nickel layer.

3. An article of manufacture comprising a zinc die casting having a coating at least 0.0001 inch in thickness of white brass containing 19 to 31% by weight of copper, the remainder being zinc, electroplated onto the surface of said die casting and having an adherent coating of nickel electroplated onto said white brass coating.

4. The process for coating a metal surface composed preponderantly of zinc comprising electroplating said surface with a white brass coating at least 0.0001 inch in thickness containing 19-31% byweight of copper the remainder being zinc and thereafter electroplating nickel onto the white brass coating from an acidic nickel plating solution.

5. The process for coating a metal surface containingnot less than by weight of zinc, comprising electroplating said surface with a white brass coating at least 0.0001 inch in thickness containing 19 to 31% by weight of copper the remainder being zinc and thereafter electroplating nickel onto the white brass coating from an acidic nickel plating solution, while maintaining the acidity of said solution within the pH range of 4 to 6.5.

6. The process for coating a zinc die casting comprising electroplating the surface of said casting with a white brass coating at least 0.0001 inch in thickness containing 19 to 31% by weight of copper the remainder being zinc from a solution containing zinc and copper cyanides, and thereafter electroplating nickel onto the white brass coating from an acidic nickel sulfate solution, while maintaining the acidity of said solution within the pH range of 4 to 6.5.

CHRISTIAN JOHN WERNLU'ND.