US 3879270 A
A cyanide free electrolyte for the electrodeposition of metals comprises an aqueous solution of (a) a metal ion, (b) a cyanidine compound and optionally, (c) a phosphonic acid compound.
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Description (OCR text may contain errors)
United States Patent [1 1 Kowalski [451 Apr. 22, 1975 1 COMPOSITIONS AND PROCESS FOR THE ELECTRODEPOSITION OF METALS  Inventor: Xavier Kowalski, Saint Louis. Mo.
 Assignee: Monsanto Company, St. Louis, Mo.
 Filed: Jan. 10, 1974  Appl. No.: 432,377
 U.S. Cl 204/52 R; 204/D1G. 2; 204/43 R. 204/44; 204/45 R; 204/46 R; 204/46 G; 204/48; 204/49; 204/50 R; 204/51; 204/54 L.
204/55 R  Int. Cl. C23b 5/02; C23b 5/18; C23b 5/46  Field of Search 260/248 A, 248 R.
204/52 R, 52 Y, 45 R, 45 M, 45.5, 46 R, 46 G, 47, 48, 49, 50 R, 50 Y, 51. 53,54 R, 54 L, 55 R, 55 Y, 43, 44, 1.5
 References Cited UNITED STATES PATENTS 2,849,352 8/1958 Kirstahler ct a1, 204/44 OTHER PUBLICATIONS B. Egeberg et 11]., Trans. Electrochem. Soc., vol. 66, pp. 211-227, (1938).
Primary Examiner-G. L. Kaplan Attorney, Agent, or Firm-W. R. Eberhardt; T. B. Leslie  ABSTRACT A cyanide free electrolyte for the electrodeposition of metals comprises an aqueous solution of (a) a metal ion, (b) a cyanidine compound and optionally, (c) a phosphonic acid compound.
16 Claims, No Drawings COMPOSITIONS AND PROCESS FOR THE ELECTRODEPOSITION OF METALS BACKGROUND OF THE INVENTION l. Field of the Invention The present invention relates to the electrodeposition or electroplating of metals, and to the composition of the electroplating bath from which the metal is deposited. More particularly. the present invention relates to electrolyte compositions useful as a strike bath for copper and other metals.
2. Description of the Prior Art The electrodeposition of metals from aqueous solutions of metal ions is an old art. The typical electroplating system consists of an electroplating bath and two or more electrodes. The cathode of the electrode system is the object to be plated. The anode may be carbon or a solid piece of the metal to be plated upon the cathode, A solid metal anode is consumed in the electro- 7 Copper (as metal) 1.8 -3.5 oL/gal Free cyanide .75-1 .5 do. Rochelle salt 2 5 do. Soda ash 2 8 do. pH ll.5 I2.5 do. CD. l575 amptlftf Temperature l-l40F.
Time /1-3 minutes. depending on CD. used In making the copper strike, the current must be on when the parts are immersed in the solution. Following the copper strike. the parts are transferred to the regular copper plating bath for the balance of the copper deposit.
Electroplating baths and strike baths comprising aqueous alkaline solutions of metal cyanides have several disadvantages associated with the toxic nature of cyanide. The use of metal cyanide solutions can be hazardous if the pH of the electroplating medium should drop to neutral or below since there is a danger of poisonous hydrogen cyanide gas being produced. Also, the Toxic metal cyanides present a disposal problem, and removing cyanides from used baths prior to disposal requires special equipment and adds to the cost of the electroplating operation.
The prior art has suggested certain techniques for electroplating without the use of cyanides. US. Pat. No. 3,475,293, for example, suggests the use of certain diphosphonates or monoamino lower alkylene phosphonates in electroplating divalent metal ions. US. Pat. Nos. 3,706,634 and 3,706,635 suggest combinations of ethylene diamine tetra(methylene phosphonic acid). l-hydroxy ethylidene-l,l-diphosphonic acid, and amino tri(methylene phosphonic acid) as particularly useful complexing compositions. US. Pat. No. 3,6l7,343 is similarly directed toward a nickel plating system employing certain phosphonic acid compounds.
While the cyanide'free electroplating systems of the prior art are suitable for some plating operations. there is a tendency for immersion plating to occur on iron or zinc base stocks unless a metal strike is first applied. Metal deposited by immersion is spongy and adheres poorly so that subsequent metal deposited electrically has a tendency to crack and peel. It is accordingly an object of the present invention to provide a cyanidefree electrolyte composition suitable for use as a strike bath for depositing copper and other metals directly onto iron or zinc. It is a further object of the present invention to provide a method and cyanide-free electrolyte composition for electrically depositing metallic copper from a solution containing monovalent copper Cu(l).
SUMMARY In accordance with the present invention there is employed as the electroplating bath an aqueous solution comprising (a) from 0.01 to about 5 percent by weight of a monovalent, divalent or polyvalent metal ion; (b) from about 0.0] percent to the solubility limit of a cyanidine compound containing the ring radical and optionally (c) an organodiphosphonic acid compound having the structure wherein M is hydrogen, alkali metal or ammonium, and R is an alkyl, alkylene or alkylidene radical having from 1 to 11 and preferably from 1 to 5 carbon atoms optionally substituted with OH or halide. Such electroplating bath compositions are useful in plating metals at a pH of from about 7 to 12 and at a temperature of from about 30 to C.
DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment of the present invention, a thin deposit of copper is electrically plated upon active base metals from a cyanide-free strike bath comprising monovalent copper Cu(l). While the ensuing description is directed primarily toward the plating of copper on zinc, it is contemplated that metals other than copper, particularly silver, tin, nickel, chromium and gold are also deposited in a like manner on zinc and other active base metals such as aluminum, magnesium and iron in accordance with the method and teaching of the instant invention.
Cyanidine compounds useful in the practice of the present invention are those compounds containing the trivalent ring radical where X is any cation which will render the compound at least slightly soluble in the electroplating bath composition. Preferably, X is selected from the group consisting of OM, halogen. -NH- and mixtures thereof, where M is hydrogen. alkali metal or ammonium. Most preferably, X is -OM and the cyanidine compound is cyanuric acid or an alkali metal salt thereof.
Organodiphosphonic acid compounds useful in the practice of the instant invention are those compounds having the general structure where R is an alkyl, alkylene or alkylidene radical of from 1 to about 11 and preferably from 1 to about 5 carbon atoms, or a said radical substituted with hydroxy or halide, and each M is individually hydrogen, alkali metal, or ammonium ions. Examples of preferred compounds include, but are not limited to, methylene diphosphonate, l.l-ethylene diphosphonate, l-hydroxy ethylidene-l l -diphosphonate, and l-chloroethylidenel, l -diphosphonate.
The electroplating bath or electrolyte of the present invention comprises an aqueous solution of the metal to be plated. the cyanidine compound. pH adjusters and optionally one or more additives including brighteners, buffering agents, levelers, and organodiphosphonic acid compounds which are intended to improve the performance or life of the bath or the quality of the metal deposit. or to impart other beneficial effect. It is common in the electroplating art to utilize many and varied additives which are selected by the skilled electroplater according to the particular system being used. The use of such additives in the electroplating bath compositions of the present invention in order to adapt these compositions to individual circumstances are included within the scope of the present invention.
The bath is prepared according to conventional techniques by simply dissolving the desired ingredients into a quantity of water. The water is preferably low in mineral content and may be deionized. The metal to be plated is often added in the form ofa water soluble salt such as a metal sulfate, chloride, phosphate, citrate, carbonate or acetate. Carbonate and acetate salts are often preferred because the anions of these salts may be thermally decomposed and thereby removed from the bath when the metal ion is released. In preparing a bath for copper striking, monovalent copper is most conveniently added as cuprous chloride.
The optimum concentration of metal salt and cyanidine compound in the electroplating bath will be largely determined by the identity of the metal and by the characteristics of the individual electroplating system. As a rule, the concentrations are limited primarily by the solubilities of the compounds. As an example, in a copper strike bath based upon Cu(l) there is usually employed from about 0.01 percent to about 0.5 percent by weight of metal in solution and from about 0.0l to about 0.25 percent by cyanuric acid, the maximums being the respective solubility limits. When plating Cu(ll) or other more soluble metals, the metal ion concentration may be increased to 5% or higher.
The copper strike bath is generally operated within a pH range of from about to 12. The optimum pH will depend to some extent upon the composition of the particular bath. Since pH is easily adjusted by the addition of alkaline materials such as alkali metal hydroxides, or acid materials such as mineral acids, it is a relatively simple matter to adjust the pH in either direction until optimum plating characteristics are achieved. As
a general rule it is often found that higher pH values, that is within the range of from about 1 l to 12, give better results when striking copper from Cu(l) than lower pH levels although exceptions to this rule may be found.
The use of the organodiphosphonic acid compound as defined above is found to enhance the performance of the electroplating bath, particularly with respect to depositing copper. A combination of cyanuric acid and l,l-ethylene diphosphonic acid (EDP) is particularly effective to produce bright, uniform, and tightly adhering deposits of copper on brass, zinc and steel by striking at a pH of about 11.5. The concentration of EDP used in the composition of this invention is sufficient to provide a mole ratio of EDP to copper of from about 0.5:1, although from about 1:1 to about 2:1 is preferred.
While the aforegoing description is directed primarily to the composition of the electroplating or strike bath, the present invention further provides a process for the electrodeposition of monovalent, divalent or trivalent metals which comprises the steps of electrolyzing an aqueous solution of any of the metal ions hereinbefore described, a cyanidine compound, and any of the optional additives hereinbefore described. By the process of this invention, metal such as copper, iron, nickel, zinc and cadmium may be electrically deposited upon a cathode such as steel, aluminum, brass, zinc and the like with little or no chemical deposition.
During the electrodeposition process, the electroplating bath is maintained at a temperature within the range of the freezing point to boiling point of the bath, generally within a range of from about 30C. to about 90C. For reasons of current efficiencies it has been found preferable to maintain the temperature of the electroplating bath within the range from about 40C. to about C.
The amount of current employed in the electrodeposition may vary widely, depending upon the particular metal being plated, the temperature of the bath and whether or not the bath is agitated duringthe electroplating process. In general, the amount of current employed will be sufficient to provide a current density of from about 1 to 300 amperes per square foot of electrode surface. Ordinarily when the electroplating bath is quiescent or unagitated, the current density will be in the range of from about 15 to amperes per square foot, while when the electroplating bath is agitated current densities up to about amperes per square foot may be utilized. The optimum or preferred currentdensity for any particular electroplating situation will depend upon the individual characteristics of the operation and is readily determined by employing conventional electroplating techniques.
The time required to electroplate or to electrically deposit the metal will vary with the kind of metal, the current density, and bath composition and concentration, as well as upon the thickness of the plate or deposit desired. Generally, the greater the current density, the shorter will be the time required to produce a metal strike or a plate of a given thickness.
In accordance with a preferred embodiment of the present invention, a copper strike is electrically deposited upon a wide variety of base metals or substrates such as zinc, iron, brass, steel, aluminum and the like. This preferred process comprises passing an electric current, at a density in the range of from about 15 to about 75 amperes per square foot of cathode surface. through an aqueous solution containing monovalcnt copper ions and cyanuric acid or a mixture of cyanuric acid and EDP and having a pH in the range of from about 10.0 to about 12.0 and preferably about 11.5. The concentration of copper in the electroplating bath composition is preferably from about 0.01 to about 0.5 percent by weight, based on the weight of the solution. The temperature of the solution is preferably maintained within the range of from about 40C. to about 70C. during the electroplating operation.
As stated above, the electroplating and strike solutions of the present invention can contain known brighteners, buffers, and leveling agents and other additives commonly used in electroplating operations. Boric acid and its salts are compatible buffers for many formulas of the invention, and selenites and arsenites are useful brighteners for copper plating baths while aldehydes and ketones are useful for zinc plating. Other additives which may be employed in the electroplating solutions of the present invention include those disclosed in the 39th Annual Edition of Metal Finishing Guidebook Directory, 1973, published by Metals and Plastics Publications. Inc., 99 Kinderkamack Road, Westwood, NJ.
Certain preferred embodiments of the present invention concerning the electrodeposition of copper as illustrated by the following Examples which are not limiting of the invention. All parts and percentages are by weight unless otherwise specified. Plating tests were conducted according to conventional procedures in a Hull Cell.
The Hull Cell is constructed substantially as the electrolysis cell described in US. Pat. No. 2,149,344. This type of Hull Cell is standard equipment for the evaluation of electroplating solutions by the subjective determination of brightness rating as based on the width of the brightness range, uniformity of brightness, and the presence or absence of smudges, stains and discoloration. ln addition, the effectiveness of the bath is also evaluated on the basis of adhesion of plated metal to the cathode. The particular cathodes utilized in this test were brass or zinc as indicated and were each 5 X 3-% inches in size. The anode utilized in these tests was made of copper and was 2-% X 2- /8 inches in size. The Hull Cell utilized in this test had a capacity of 1,000 milliliters.
EXAMPLE 1 An electrolyte was made according to the following formulation:
0.99 g. C uC 1 12 g. CYA (cyanuric acid) 300 ml. DI. Water EXAMPLE 1] An electrolyte was made according to the following formulation:
1.5 g. CuCl 20 g. K- ,HCYA (partial K salt of CYA) 5 g. NaH- .CYA(partial Na salt of CYA) 5 g. EDP.-l Na (Na salt of 1.1-ethylene diphosphonic acid) 301) ml. D. 1. Water The pH was adjusted to l 1.5 by the addition of NaOH and the solution filtered to remove undissolved solids. A brass panel was cleaned and plated in the Hull Cell at 1 amp. for 2 minutes at 45C. A very good bright deposit of copper was obtained.
A zinc panel was cleaned and immersed in the solution for 30 seconds at 45C. No evidence of immersion plating was observed. The zinc panel was electroplated in the Hull Cell at 1 amp. for 2 minutes at 45C. A very good, thin deposit of copper was obtained.
EXAMPLE Ill An electrolyte was made according to the following formulation:
1 g. CuCl 12.6 g. Melamine 12 g. NaOH 10 g. EDPA Na 200 ml. D,l. Water The pH was adjusted to l 1.0 by the addition of orthophosphorus acid, and the solution filtered to remove undissolved solids. Brass and zinc panels were individually cleaned and plated in the Hull Cell at 1 amp for 2 minutes at 35C. The copper depositions were judged as fair to good in brightness and uniformity. No immersion plating was observed.
Although the invention has been illustrated by reference to the electrodeposition of copper and particularly copper striking utilizing the method and compositions of this invention, the present invention is not limited thereto but is also useful in electrodepositing other metals upon other cathode materials as hereinabove described. Accordingly, the present invention is not to be limited except as defined in the claims appended hereto.
The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:
l. A cyanide-free process for the electrodeposition of monovalent, divalent and polyvalent metal ions which comprises the steps of electrolyzing an aqueous solution comprising a. a monovalent, divalent or polyvalent metal ion,
b. a cyanidine compound having the general formula c N c C \N x c \N \N c c -x wherein X is selected from the group consisting of -OM, --NH and halogen ions, where M is hydrogen or alkali metal ion, the concentration of (a) in said aqueous solution being from about 0.01 percent to 5 percent; the concentration of (b) being from about 0.01 percent to the limit of solubility; the pH of said aqueous solution being from about 7 to 12, and the temperature of said aqueous solution being from about 30 to 90C.
2. A process of claim 1 wherein said aqueous solution comprises. in addition to (a) and (b). a compound (c) having the structure wherein M is hydrogen. alkali metal or ammonium ion, and R is selected from the group consisting of alkylene or alkylidene radical having from 1 to about 1 1 carbon atoms and hydroxy and halogen substituted derivatives thereof, in a concentration sufficient to provide a mole ratio of at least about 0.511 of (c) to (a).
3. A process of claim 2 wherein said compound (c) is l.l-ethylene diphosphonic acid.
4. A process of claim 3 wherein (b) is cyanuric acid or an alkali metal salt thereof.
5. A process of claim 4 wherein said metal ion (a) is monovalent Cu(l). the concentration of said metal ion is from about 0.01 to about 0.5 percent by weight, and the pH of said aqueous solution is from about I l to 12.
6. A process of claim 1 wherein said metal ion (a) is monovalent Cu(l). the concentration of said metal ion is from about 0.0l to about 0.5 percent by weight and the pH of said aqueous solution is from about l l to 12.
7. A process of claim 6 wherein (b) is cyanuric acid or alkali metal salts thereof.
8. A process of claim 1 wherein said metal ion (a) is selected from the group consisting of copper, iron, nickel. zinc. cadmium. tin. silver, chromium and gold.
9. A cyanide-free electroplating bath composition comprising an aqueous solution of a. from about 0.01 to about percent by weight of a monovalent. divalent or polyvalent metal ion, and
b. from about 0.0l percent to the solubility limit of a cyanidine compound having the general formula wherein M is hydrogen, alkali metal or ammonium ion, and R is selected from the group consisting of alkylene or alkylidene radical having from 1 to about 1 1 carbon atoms. and hydroxy and halogen substituted derivatives thereof, in a concentration sufficient to provide a mole ratio of at least about 0.521 of (c) to (a).
11. A composition of claim 10 wherein (b) is cyanuric acid or an alkali metal salt thereof.
12. A composition of claim 11 wherein said metal ion (a) is monovalent Cu(l) and the concentration of said metal ion is from about 0.01 to about 0.5 percent by weight.
13. A composition of claim 10 wherein said compound (c) is l,l-ethylene diphosphonic acid.
14. A composition of claim 9 wherein (b) is cyanuric acid or alkali metal salts thereof.
15. A composition of claim 14 wherein said metal ion is monovalent Cu(l) and the concentration of said metal ion is from about 0.01 to about 0.5 percent by weight.
16. A composition of claim 9 wherein said metal ion (a) is selected from the group consisting of copper. iron, nickel, zinc, cadmium, tin, silver. chromium and gold.
UNITED STATES PATENT OFFICE @EHMQATE 0F coEeTwN a PATENT NO. 3,879,270
DATED April 22 1975 tNVENTOR S Xavier Kowalski It is certified that error appears in the ab0veidentihed patent and that sald Letters Patent are hereby corrected as shown below: O
Claim 9 column 8 line 9 "-OH should be OM,
Q fined an fif ii of August 1975 [SEAL] Arresr:
RUTH c. MASON c. MARSHALL DANN Arresting Officer (mmnissimu'r uj'lulcnlx and Trademarks