US2726969A - Chemical reduction plating process - Google Patents

Description

United States atent CHEMICAL REDUCTION PLATING PROCESS Robert A. Spaulding, Huntington Woods, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application December 3, 1953, Serial No. 396,082

9 Claims. (Cl. 117-130) This invention relates to improvements in the deposition of nickel from chemical reduction plating solutions and more particularly to an improved plating bath, its control and maintenance.

In the past, nickel has been deposited from chemical reduction plating solutions which generally comprise an aqueous solution of a water-soluble nickel salt, a reducing agent such as sodium hypophosphite or potassium hypophosphite, and various additives. Prior plating solutions, both acid and alkaline, generally have been employed at temperatures of about 90 C. or higher. In many instances, depending upon the article to be coated, certain catalytic metals are employed to initiate the oxidationr eduction plating solution.

One of the principal difliculties in prior chemical reduction plating operations has been the accumulation in the plating solution of undesirable by-products, chiefly alkali metal chlorides, soluble phosphites and an acid, the particular acid depending on the source of nickel employed. In the past it has been necessary to dispose of chemical plating solutions after a relatively short period of use because of the presence of these undesirable by-products. Moreover, these reaction products reduce the speed of the plating reaction and cause pitting and roughness in the deposited nickel before it is otherwise necessary to dispose of the bath.

I have now discovered that the aforementioned difliculties can be eliminated and an improved'nickel deposit obtained by utilizing a water-insoluble nickel salt as a source of nickel and by employing an ion exchange resin to remove accumulated phosphite impurities from the solution. More particularly, the water-insoluble nickel salts contemplated in the present invention are those adapted, when dissolved in the plating solution, to neutralize the by-product acid and to thereby regulate the bath pH, thuseliminating the necessity of separate additions of alkali to maintain the desired pH.

Examples of suitable water-insoluble nickel salts are: nickel oxy salts such as nickel monoxide, nickel peroxide, nickel sesquioxide, nickelous nickelic oxide, and nickel superoxide; hydroxy nickel salts such as nickelic hydroxide and nickelous hydroxide; and carbonates such as nickel carbonate and basic nickel carbonate. By employing one or more of these water-insoluble nickel salts as a source of nickel, either in replenishing a conventional chemical reduction bath, or in original bath makeup, not only is the desired nickel content provided, but also the bath pH is regulated by the alkaline radical liberated in the plating solution and chloride by-products are not formed.

The practice of the present invention contemplates treating a plating solution of the foregoing type with an In combination with the use of I anion exchange material. the above-described type of water-insoluble nickel salts substantially all of the reaction products detrimental to continuous plating are removed. Moreover, nickel and its reducing agent, the essential bath ingredients, are m ne us repl sh ice The anion exchange material to be used in any particular application will depend on a number of factors. In general, I prefer to employ an ion exchange material which is weakly basic so as to avoid removal of weakly acidic bath constituents. Suitable ion exchange materials include inorganic substances such as dolomite (calcium-magnesium-carbonate); synthetic inorganic materials such as heavy metal silicates; as well as various organic ion exchange resins including phenolic or phenolic derivative'type resins prepared by the polymerization of an aromatic amine, such as aniline or metaphenylen diamine and formaldehyde; or by the reaction of a polyamine, phenol and formaldehyde. Other satisfactory anion exchange resins include those formed by nitration and reduction of copolymers of styrene, divinylbenzene and/or other unsaturated compounds. Resins of the anion exchange type generally are characterized by reactive hydroxy groupings or amine type groupings as follows:

R 1- Rail-R 1% RN-'H ('J=NR RAT-H Where R is an aromatic or alkyl substituent group.

Examples of commercially available anion exchange resins are the following:

Typical of a nickel reduction plating bath which may be regulated in operation by the practice of the present invention is the following composition where the quantities expressed are per liter of water:

Nickel chloride grams 30 Sodium hypophosphite do 10 Glycollic acid milliliters 35 Sodium hydroxide grams 15 It will be understood, of course, that the above bath composition is intended only as an illustration of a typical reduction plating bath employing a Water-soluble nickel salt and that the present invention is applicable to a wide range of bath compositions. For example, plating baths containing about 5 to 50 grams per liter of a water-soluble nickel salt such as nickel chloride, nickel acetate, nickel sulfate, etc.; approximately 5 to grams per liter of a reducing agent, such 'as hypophosphorous acid, sodium hypophosphite or other soluble hypophosphites; and about 15 to 100 grams per liter of a buffer such as glycollic acid, sodium hydroxy acetate, sodium acetate, sodium citrate, etc. and/or other bath additives can similarly be replenished in accordance with the present invention.

Although various chemical reactions occur during the plating process and the bath replenishment, the exact interdependence of which is not clearly understood at present, the following reactions illustrate generally how the water-insoluble nickel salts of the present invention serve to replenish the nickel content and regulate the bath pH.

Employing nickel hydroxide, hydrochloric acid in the bath is neutralized and nickel is liberated as follows:

2HCl+Ni(OI-I)2 NiClz+2HzO Using basic nickel carbonate:

As shown in these reactions, when a nickel salt of the type contemplated in the present invention is added to the plating bath, a soluble nickel salt is formed therein to replenish the nickel consumed in plating. Simultaneously, the hydrochloric acid or other acid which had been accumulating in the bath is neutralized.

By employing water-insoluble nickel salts, the necessity of making frequent bath analyses for nickel content is eliminated. Moreover, the plating solution is not contaminated with by-product alkali metal chlorides and, since no foreign alkali need be added to the bath to regulate its pH, accurate pH determinations are no longer necessary.

The ion exchange treatment in accordance with the present invention may be better understood from a consideration of the following: An ion exchange resin, indicated as R(OH)2, is treated with a solution of a soluble hypophosphite or hypophosphorous acid as follows:

Used plating solution, contaminated with phosphite, is then passed through the resin treated according to Reaction 1 or 2: a I

(3) R(H2PO2)2+NaI-I2PO3 (used plating solution) RHPO3+(HaPO2+NaI-I2PO2) plating solution In many instances, instead of adding acid to the solution as is indicated by Reaction 3, it is desirable to activate or regenerate only a portion of the ion exchange resin. Hence, in a preferred bath treatment the exchange reaction is as follows:

4 R(OH)2+R(H2PO2)z+NaH2POs (used plating solution) 2R(HPOs)+2NaH2PO2 (plating solution) The ion exchange resin, now contaminated by phosphite, may be stripped with caustic soda generally as follows:

(5) R(H2PO3 2+2NaOH- R(OH) 2 ZNaHPOa (waste) Instead of caustic soda,-it will be understood, of course, that the ion exchange material may be stripped or re; generated with sodium carbonate or ammonium hydrox ide, as well as other basic materials.

The following are illustrative of ion exchange treatment in accordance with the present invention:

Example I A nickel reduction plating solution, having a pH of 4.82 and containing 110.8 g./l. of NazHPOs and 5.7 g./l. 0f NaHzPOz, was treated with 14.5 g./l. of hypophosphorous acid (in terms'of NaHzPOz). The resultant solution was then passed through Permutit W anion exchange resin which is an addition polymer having amine type active groupings. The effluent solution had a pH of 4.72 and contained 50.4 g./l. of NaI-IPO: and 11.0 g./l. of NaHzPOz. Thus the phosphite content was reduced from 110.8 g./l. to 50.4 g./l.

Example II A nickel reduction plating solution containing 70.6 g./l. of NazHPOa and 9.0 g./l. of NaHzPOz was treated with sutficieut hypophosphorous acid to produce a hypophosphite content of 18.6 g./l. The resultant solution was then passed through Permutit 'W anion exchange resin. The efiluent solution contained 32.8 g./l. of NaHPOs and 15.9 g./l. of NaHzPOz. Thus the phosphite content was reduced from 70.6 g./1. to 32.8 g./l.

t will be understood, of course, that the ion exchange resin instead may be treated with hypophosphorous acid, sodium hypophosphite or the like in a separate step, as shown in Reactions 1 and/ or 2, prior to passing the plating solution through the resin.

In general, I have found that electroless nickel plating baths of thetype contemplated herein operate most satisfactorily at a pH within the range of about 3.5 to a pH at which a nickel salt is precipitated, a preferred pH range being from about 5.0 to 5.5. In most instances,

an electroless nickel bath is operated most satisfactorily at a temperature of at least about F. although a temperature within the range from approximately F. to 210 F. is preferred. Any chemically resistant material such as stainless steel, glass, ceramic ware, rubber, or plastics such as polyethylene, saran, various phenolic and vinyl resins may be employed in the fabrication of containers for the ion exchange material. Although the temperature at which the ion exchange treatment is conducted may be varied in particular applications, it is generally desirable to employ as low a temperature as possible, preferably about room temperature, in order to avoid possible chemical attack on the ion exchange material.

Since electroless nickel plating baths frequently contain ingredients other than the source of nickel, and reduc-. ing agents, it is preferred to select an ion exchange resin which will not adsorb desirable bath constituents, such, for example, as glycollic acid. In regenerating the ion exchange resin as indicated in Reaction 3, I have found that it is desirable to employ hypophosphorous acid in an amount equivalent to about /5 the weight of sodium phosphite which is to be removed from the plating solution.

While the present invention has thus far been described in some detail as being applicable for the treatment of chemical reduction plating baths employing water-soluble nickel salts, the present invention also contemplates the formation'of an improved chemical reduction plating bath utilizing a water-insoluble nickel salt as an initial ingredient. ture of a reducing agent and a water-insoluble nickel salt selected from the group consisting of oxy nickel salts, hydroxy nickel salts and nickel carbonates. In most instances it is desirable to also include glycollic acid, sodium hydroxy acetate, sodium citrate and/ or sodium acetate. A preferred bath of this type comprises nickel carbonate, sodium hypophosphite, and glycollic acid.

A bath of the above-mentioned type can be produced by dissolving the desired water-insoluble nickel salt, or salts, in glycollic acid and thereafter adding the reducing agent. If sodium acetate, sodium citrate or sodium hydroxy acetate is used, the nickel salt may first be dissolved in hypophosphorous or phosphorous acid, enough acid being used to obtain the desired initial bath pH.

The following is an example of a reduction plating bath utilizing a water-insoluble nickel salt as an initial ingredient. The quantities expressed are per liter of water.

Nickel carbon grams" 15 Glycollic acid (70%) millimeters 35 Sodium hypophosphite ..grams 10 Preferred pl-l 5.0

Such a bath comprises an aqueous mix-,

ampere stantially insoluble in water, even at elevated temperatures, the use of these salts in the replenishment of conventional reduction nickel plating baths involves certain difliculties which I have succeeded in overcoming by the following method: A water-insoluble nickel salt is employed in the form of a slurry formed by stirring the finely divided nickel salt into water. A filter, conventionally employed in chemical reduction plating systems, is then coated with the finely divided water-insoluble nickel salt by passing the slurry into the filter intake. The used plating solution, before or after ion exchange, is then passed through the same filter whereby a quantity of the water-insoluble nickel salt is dissolved from the filter in the plating bath. In practice, the amount dissolved from the filter media will be approximately equivalent to the amount of nickel consumed in the plating. Hence, the plating bath will constantly replenish itself until the charge of the insoluble nickel salt has been consumed. Moreover, since the amount of nickel consumed in the reduction plating operation corresponds to the amount of acid produced, the alkaline radical liberated in solution by the gradual dissolution of the nickel salt will neutralize an equivalent amount of acid. Thus the pH will remain substantially constant.

In general, electrodes nickel reduction plating baths of the foregoing type operate most satisfactorily at a pH within the range from about 3.5 to a pH at which nickel hydroxide is precipitated. A preferred pH range is from 5.0 to 5.5. Plating baths embodying the present invention generally should be operated at a relatively high temperature for optimum results. I have found that the bath temperature in most instances should be at least 160 F. although a temperature within the range from about 190 F.2l0 F. is preferred.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. In a chemical reduction plating operation utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt and hypophosphite reducing agent, the improvement which consists of maintaining the desired nickel concentration in the bath by additions thereto of a water-insoluble nickel compound adapted to regulate the bath pH, and simultaneously removing phosphite by-products formed during plating by treating said bath with an anion exchange material.

2. In a chemical reduction plating operation utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt and hypophosphite reducing agent, the improvement which consists in maintaining the desired nickel concentration in the bath by additions of water-insoluble nickel compound which is adapted to regulate the bath pH, and removing phosphite by-products formed in said bath during plating by treating said bath with an anion exchange material, said anion exchange material being capable of adsorbing phosphite ions without substantial adsorption of weakly acidic bath constituents.

3. In a chemical reduction nickel plating operation of the type utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt and a reducing agent selected from the group consisting of sodium hypophosphite, potassium hypophosphite and hypophosphorus acid, the improvement which'consists of adding to said bath at least one water-insoluble nickel compound selected from the group consisting of oxy nickel compounds, hydroxy nickel compounds, and carbonate nickel compounds, and removing phosphite ions which accumulate during said plating operation by treating said bath with an anion exchange material.

4. In a chemical reduction nickel plating operation utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt and a reducing agent selected from the group consisting of hypophosphorus acid, sodium hypophosphite, and potassium hypophosphite, the improvement which consists of adding to said bath at least one water-insoluble nickel compound selected from the group consisting of nickel oxide, nickel hydroxide, nickel carbonate and basic nickel carbonate, and removing phosphite impurities accumulated during said plating operation by treating said bath with an anion exchange resin, said resin being capable of adsorbing phosphite impurities without adsorbing weakly acidic bath constituents and being characterized by active groupings selected from the class consisting of hydroxy groupings, hypophosphite groupings and amine groupings.

5. In a chemical reduction nickel plating operation utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt and a hypophosphite reducing agent, the improvement which consists of adding to said bath at least one water-insoluble nickel compound selected from the group consisting of nickel oxide, nickel hydroxide, nickel carbonate and basic nickel carbonate, and removing phosphite impurities accumulated during said plating operation by treating said bath with an anion exchange resin, said resin being capable of adsorbing phosphite impurities without adsorbing weakly acidic bath constituents and being characterized by active groupings selected from the class consisting of hydroxy groupings, hypophosphite groupings and amine groupings.

6. In a chemical reduction nickel plating process in the type utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel salt, a reducing agent selected from the group consisting of sodium hypophosphite, hypophosphorous acid and potassium hypophosphite, and a bufier, said bath maintained at a pH generally within the range of 3.5 to 6.0 and operated at a temperature of at least F., the improvement which consists of maintaining the desired nickel content in the bath, regulating the bath pH, and removing undesirable by-products by adding to said bath at least one waterinsoluble nickel compound selected from the group consisting of nickel oxide, nickel hydroxide, nickel carbonate and basic nickel carbonate and treating said plating bath with an anion exchange resin capable of adsorbing phosphite ions from said solution and releasing a reducing agent in the solution.

7. In a chemical reduction nickel plating operation of the type utilizing an aqueous bath containing a hypophosphite reducing agent, a soluble nickel compound and a water-insoluble nickel compound to replenish the soluble nickel compound and regulate the pH, the improvement which consists in removing phosphite from said bath and replenishing the hypophosphite reducing agent during plating by treating said bath With an anion exchange material which has been previously treated with a source of hypophosphite ions.

8. In the chemical reduction deposition of nickel utilizing an aqueous solution of soluble nickel compound and a hypophosphite reducing agent, the improvement which consists of passing said solution through a filter containing a water-insoluble nickel compound to maintain the desired nickel content in solution and removing phosphite by-products formed during plating by treating said solution with an anion exchange resin, said resin being characterized by active groupings selected from the class consisting of hydroxy groupings, hypophosphite groupings and amine groupings.

9. In a chemical reduction plating process utilizing a plating bath comprising an aqueous solution containing a water-soluble nickel compound and a hypophosphite reducing agent, the improvement which consists in maintaining the desired nickel concentration in the bath by addition of water-insoluble nickel compound, maintaining the desired hypophosphite concentration in the bath by addition of hypophosphorous acid, and removing phosphite formed in the bath during plating by treating said bath with an anion exchange material. 7

References Cited in the file of this patent UNITED STATES PATENTS Brenner et a1. Dec. 5, 1950 8 OTHER REFERENCES Y Paulso'n et al.: Plating, vol. 40, No. 9, September 1953, pp. 1005-4009.

Sussman et 211.: Industrial and Engineering Chemistry, vol. 37, No. 7, July 1945, pp. 618-624.

Claims (1)

1. IN A CHEMICAL REDUCTION PLATING OPERATION UTILIZING A PLATING BATH COMPRISING AN AQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE NICKEL SALT AND HYPOPHOSPHITE REDUCING AGENT, THE IMPROVEMENT WHICH CONSISTS OF MAINTAINING THE DESIRED NICKEL CONCENTRATION IN THE BATH BY ADDITIONS THERETO OF A WATER-INSOLUBLE NICKEL COMPOUND ADAPTED TO REGULATE THE BATH PH, AND SIMULTANEOUSLY REMOVING PHOSPHITE BY-PRODUCTS FORMED DURING PLATING BY TREATING SAID BATH WITH AN ANION EXCHANGE MATERIAL.