US2451426A - Bright zinc plating - Google Patents

Description

baths and chloride baths.

Patented Oct. 12, 1948 I nnron'r'zmo PLA'mIG Robert R. Hair and Barry L. Bonner, Niagara Falls, and Joseph Frederic Walker, Lewiston, N. -Y., asslgnors to E. I. du Pont de Nemours It Company, Wilmington, Del., a corporation of Delaware No Drawing. Application November 25,1948,

Serial No. 511,686

This invention relates to zinc plating and, more particularly, to the production of bright zinc electrodeposits.

A number of processes have been developed 4 for plating bright zinc from cyanide baths by means of which excellent bright deposits can be obtained, provided the proper conditions such 4 Claims. (Cl. 201-55) ""1; polyvinyl alcohol addition agent, excellent bright deposits can be obtained over a wide range of current densities at temperatures from C.

to as high as 60 C. and even higher. In other 'words, the addition of the condensation product widens-the current density bright range at the as temperature andcurrent density are observed.

An example oi a successful bright zinc plating process is that described and claimed in U. 8. Patent 2,171,842 wherein the bright deposit is obtained by the addition of a small amount of polyvinyl alcohol to the plating bath. This proc-, ess. produces very bright deposits of zinc and over a wide current density range at low temperatures such as to C. However, it is often desired to operate such a bath at higher temperatures, such as 50 to 60 0., whereby more rapid electroplating may be obtained. At such high temperatures the polyvinyl alcohol still funchigher temperatures.

The aforesaid condensation product is described and claimed in a copending patent application, Serial No. 507,182, filed October 21, 1943' by Joseph F. Walker, now Patent No. 2,411,396. This condensation product may be made by the addition 1 sodium cyanamide to aqueous formaldehyde so utions, the reaction being-carried out under alkaline conditions at a temperature below 0., preferably inthe range of 15to 20 C. The reaction between the cyantions as an excellent brightening agent, but the current density bright range at such high temperatures is materially limited. It is desirable, therefore, to have a brightening agent which will produce the bright deposits obtainable by means of polyvinyl alcohol and which can be employed for that purpose at elevated temperatures and over a wide current density range at such elevated temperatures. 3

Addition aBents are often used heretofore in various acid zinc plating baths such as sulfate The effect oi such additions is to whiten the electrodeposit, but heretofore it has not been possible to produce lustrous, bright zinc plate from acid baths.

An object oi the present invention is to provide an improved process for electroplating bright zinc deposits. I A further object is to provide an addition agent for bright zinc plating baths which is operable over a wide range oi temperatures and current density, and which is eiiective in alkaline, acidic and neutral zincplating solutions.

A further object is to provide an electroplating bath addition agent composition as hereinafter set forth. from the following description of our invention.

We have discovered that a certain polymeric condensation productoi sodium cyanamide-and formaldehyde, which is soluble in alkaline solutions having a pH above 9 and insoluble in water at a pH oi B to 9, is an excellent brightening agent for zinc plating baths, whether alkaline, acidic or neutral. We have further discovered that when this condensation product is added to an alkaline zinc plating bath whichalready contains Still other objects will be apparent amide and the formaldehyde solution is exothermic and cooling is generally necessary to maintain the temperature below 35 C. After the reaction has been completed, the condensation product is precipitated from the reaction mixture by adding sumclent acid to reduce the alkalinity of the solution to a pH within the range 8 to 9. It may then be filtered ed and dried, the pure product being a white powder.

The product is probably a partially hydrated polymer of methylene cyanamide (CI-Ia=NCN) in which approximately two-thirds of the ON radicals have been converted to amide groups.

Analyses for carbon, hydrogen, .and nitrogen indicate that its composition closely approximates.

the empirical formula, CsHmNoOn. The characteristics of the product are those of an amorphous resin, and it is probably not a simple compoundoi low molecular weight but rather a polymer or mixture of polymers made up of units corresponding to the CsHioNsO: formula. It is believed that thestructural iormula oi this polymer unit is as follows:

.The new condensation product is a white, apparently non-crystalline solid which chars when heated to temperatures above 300 C. Although it is apparently stable when heated below 300 C. when dry, it undergoes further resiniilcation. when heated in the moist state and should be dried'quickly and at relatively low temperatures to avoid undesirable alteration. On long heating in the moist state-it combines with additional water and tends to liberate ammonia. I A sample heated in this way was found to have an embeing in the form of dry powders.

pirical composition corresponding to Cs'HnNsOs.

The new product dissolves readily in dilute aqueous acids or strong alkalies and is relatively insoluble in pure water, the product being least soluble in aqueous liquids having a pH of 8.0-9.0. when dissolved in aqueous alkalies or acids it may be precipitated therefrom, although the yields upon reprecipitation are not entirely quantitative. Analyses of the reprecipitated product indicate that its composition is substantially unchanged. v q I In using this condensation product as a brightvening agent, the amount to be added to the bath will vary upon the degree of brightening desired to be obtained. Generally. it is necessary to use at least about 0.1 ounce per gallon in order to obtain appreciable brightening effect. In amounts up to 2 ounces per gallon of the conden-.

- sation product, excellent brightening is obtained over a wide range of current densities. Within this range generally the best results are obtained with a concentration of about 0.5 ounce per gallon. At concentrations above 2 ounces per gallon the brightening eflect tends to occur only at relatively high cathode current density such as 50to 500 amps. per sq. ft. Thus, when recessed articles are electroplated with such high toncentration of the brightening agent, the areas of the cathodeclosest to the anode will plate bright, whereas the recesses which have a low current density tend to plate dull. With this limitation. our new brightening agent may 'be used in any concentration up to the limit ofits solubility. which is approximately 16 ounces per gallon. If more of the addition agent isv added than can dissolve in the plating bath, it is desirable to filter Example 1 A zinc plating bath was made by dissolvingthe following ingredients in water Ounces per gallon Zinc cyanide v 8.0 Sodium cyanide 5.59

' Caustic soda 10.0

The solution was purified was then operated to amps. per sq. ft. at a temperature of 25 C. with pure zinc anodes. The resulting electrodeposit. after bright dipping in dilute nitric acid, 'was white but not bright. 20 grams per gallon of the condensation prodnot of sodium cyanamide and formaldehyde was then added to the solution and when thishad dissolved, the solution was again operated at 40 amps. per sq. it. at 25 C. The electrodeposit after bright dipping had excellent brightness.

Example 2 Apurifled zinc plating solution was prepared as in Example 1 and to this was added 0.3 gram per gallon of polyvinyl alcohol in aqueous solution and 2.25 grams per gallon of the above-described posit had a hazy white color. At current denslties from 6.5 to 100 amps. per sq. ft. excellent "bright plate was obtained. The bath was operated at a temperature of 80 C.-

the bath to remove excess undissolved material.

the presence of which will tend to cause roughness in the cathode deposit. 1

In practicing our invention by the conjoint addition of polyvinyl alcohol and the condensation product to an alkaline zinc plating bath, the amounts oi polyvinyl alcohol suitable for zinc plating baths may be used for example, the amounts shown in U. S. Patent'2,171,842. The optimum amount of polyvinyl alcohol will vary,

and in most cases will lie in the range of 0.001 to r 0.1 ounce per gallon. Generally we prefer to add not more than about 0.03 ounce per gallon of polyvinyl alcohol, e. g., 0.001 to 0.03 ounce per gallon in plating baths like those shown in the examples given hereinafter.

In a preferred method for practicing our invention. we prepare a mixture of polyvinyl alcohol and the condensation product containing 1 to 5% by weight of the polyvinyl alcohol. both substances A conven: tional zinc cyanide plating bath is then prepared by dissolving in water, zinc cyanide. sodium cyanide and caustic soda and this solution is purified so as to remove heavy metal ions. for example. by treatment with sodium sulfide or metallic zinc powder. followed by filtration if necessary. About V. ounce per gallon of the abovedescribed mixture of polyvinyl alcohol and condensation product is then dissolved in the plating bath. If desired, the dry addition agent may first be dissolved in water containing suflicient caustic soda to give a pH of 10 to 12. The resulting electroplating bath maythen be operated in the conventional manner at any desired temperature, e. g. 25 to 60 C. and at cathode current densities up to 300 amps. per sq. ft.

The following examples further illustrate O invention:

Example 3 To a zinc plating solution prepared as in Example 1, 4 grams per gallon of polyvinyl alcohol and 15 "grams per gallon of the condensation. product was added. With a bath temperature of 50 C; cathodic deposits were obtained at current densities from 20 to 180 amps. per sq. it. which had excellent brightness after bright dipping in Example 4 The following zinc cyanide plating solution was made'up:

Ounces per gallon Zinc cyanide 8.0 Sodium cyanide 3.4 Caustic soda 7 10.0

This was purified by addition of ,4 ounce per gallon of sodium sulfide and ounce per gallon of a mixture of powdered polyvinyl alcohol, containing 3% by weight of polyvinyl alcohol, was dissolved-in the bath. This bath then was utilized to electroplate strip steel at cathode current densities of 200 to 300 amps. per sq.-ft. After bright dipping, the resulting electrodeposits were bright and pleasing in appearance. The cathode current efiiciencies were 90.5% at 200 amps. per sq. ft. and 69.5% at 300 amps. per sq. ft.

L's-ample 5 To the zinc plating solution of Example 1 was.

added 9 grams per gallon of a mixture of equal parts by weight of the aforesaid condensation product and anisaldehyde. Excellent bright deobtained at current densities of 3 to ampspel sq. ft. With this bath good bright by the addition of As ounce per gallon of sodium suliide. The bath the condensation product and deposits could be obtained without bright dipp Example 6 I The method of Example 5 was repeated except that the zinc plating solution had the following formula:

Ounces per gallon Zinc cyanide Q. 8.0 Sodium cyanide v 4.49 Caustic, soda 10.0

Example 7' The following acidic zinc plating solution was prepared:

Zinc chloride -ounces per-gallon..- Ammonium chloride do 28.0 AlCh ..do pH adjusted to elcctrometric 3.5

The above solution, operated at 25 0., plated a white, crystalline, non-lustrous zinc deposit at current densities up to 150 amps. per sq. it. An

X 80' O. The deposits were dark gray at current densities of to 25 amps. per sq. ft. and spongy and non-adherent at higher current ensities. The addition of 1 ounce per gallon of t e mixed addition agent of Example 8 gave pleasing white,

lustrous zinc deposits at current densities of 10 to 60 amps. per sq. ft.

addition of 0.5 ounce per gallon of the hereindescribed condensation product gave an'excellent bright zinc deposit at current densities of about 40 to 125 amps. per sq. ft.

' Example 8 A sodium zincate bath was prepared as follows:

Ounces per gallon Zinc hydroxide 7.5 Caustic soda 20.0

This solution, operated at 0., produced a white, crystalline. non-lustrous zinc plate at current densities of 4 to 10 amps. per sq. ft. At higher current densities, the deposits were gray and spongy. An addition of 1 ounce per gallon of a mixture of 97 parts by weight of the condensation product and 3 parts by weight of polyvinyl alcohol gave a semi-bright, lustrous deposit at current densities of 10 to 100 amps. per sq. ft.

Without addition agent deposits plated from this .solution at a bath temperature of 25 C. were gray to white matte and at current densities in excess of. approximately amps. per sq. ft.. the deposits were dark gray. The addition of 0.5 ounce per gallon of the condensation, product gave brilliant, lustrous deposits at current densities of 10 to above 125 amps. per sq. ft.

Example 10 A zinc pyrophosphate plating bath was prepared as follows:

Zinc pyrophosphate ounces per gallon.. 8.0 Sodium sulfate do.. 4.0 NH4OH to a pH of -electrometric.. 11.5

This bath was operated at a bath temperature of Our invention also includes the novel zinc plating addition agent which comprises a mixture of the condensation product and polyvinyl alcohol in suitable proportions. The preferred composition is made by mixing the solid, pulverulent condensation product with anhydrous, finely divided polyvinyl alcohol so that the mixture contains 1 to 5% by weight ofthe polyvinyl alcohol. This may be dissolved in caustic soda solution or other aqueous alkaline liquid to prepare a. stock solution. We have found the two ingredients to be mutually compatible, and substantially non-reactive towards each other, both in the anhydrous state and in solution. If desired, other substances may be added to the mixture, either in solid or in solution form, for example, plating bath ingredients, other addition agents, dyes or pigments, or inert materials. Excellent results have been obtained with a pulverulent mixture containing approximately 3% by weight of polyvinyl alcohol and 97% by weight of the condensation product.

As shown by the foregoing examples, our novel brightening agent may be used in zinc plating baths either alone or in combination with other brightening agents, including both organic agents and metallic compounds such as salts of molybdenum, manganese, chromium and the like. As the condensation product has a low solubility at a pH of 8 to 9, it is preferable that the plating bath have a pH below 8 or higher than 9, This presents no difilculty' as alkaline zinc cyanide plating baths usually have the required high pH and the acid to neutral baths have a pH below 8. While we prefer to utilize our addition agent in fine cyanide baths, which have been purified to remove heavy metal ions, the invention'is not restricted thereto as improved electrodepositsare obtained in baths which have not been so purified.

Likewise, the invention is not restricted to the bath formulas shown in the above examples, which have been given merely by way of example. Our novel brightening agent is effective for electroplating from any aqueous solution containing a zinc compound in solution. In baths which inherently produce inferior 'zinc deposits, the appearance of the deposit is invariably improved by addition of our condensation product. Baths which inherently produce superior zinc deposits generally are markedly improved by meansof our addition agent and are preferred in practicing our invention. Thus, to produce bright, lustrous zinc deposits according to our invention, we generally prefer to utilize the zinc chloride and zinc sulfate acid baths and the zinc cyanide alkaline baths.

Suitable zinc cyanide baths may be made, for

example, by dissolving zinc cyanide, oxide or carbonate in a solution of an alkali metal cyanide, While we prefer to use sodium or potassium cyanide, the cyanides of other alkali metals,"

cesium, ribidium and lithium may be used if desired. The bath may contain the ions of one or "several of the alkali metals in any desiredproing processes, the electrodeposit obtained from a cyanide bath by our process may have more orless of a brownish or bronze-like film which can readily be'removed by conventional bright dipping procedures to obtain maximum brightness.-

Ir. some cases the film is so slight that bright dipping is not necessary or desirable, and our invention is not restricted to the use of a bright dip ride or zinc sulfate solutions, we prefer to have present in the bath a small amount of an aluminumsalt, for example, 1 to ounces per gallon of aluminum chloride or aluminum sulfate.

While we prefer to use the herein-described invention to electrodeposit pure zinc, the invention is not restricted thereto. If desired. vari: ous metal salts may be included in the plating bath to brighten or otherwise modify the cathodic depositv and the electrodeposit then may contain up to 10% by weight of such added metal. Thus, the invention may be used to produce improved electrodeposits composed of 90% or more by weight of zinc. Representative examples of such other metallic components which may be thus codepositedare molybdenum. chromium,

manganese, copper, silver. mercury and cadmium.

We claim:

1. The process of electrodepositing zinc from an aqueous zinc cyanide electroplating solution having a pH above 9.0 which comprises electrodepositing said zinc from said solution containing dissolved therein about 0.1 to 2.0 ounces per gallon of the condensation product of sodium cyanamide and formaldehyde obtained by reacting substantially equimolecular amounts of sodium cyanamide and formaldehyde in aqueous alkaline solution at a temperature below 35 C.

2. The process of electrodepositing zinc from an aqueous zinc cyanide electroplating solution having a pH above 9.0 which comprises electrodepositing said zinc from said solution containing dissolved therein about 0.1 to 2.0 ounces per gallon of the condensation product of sodium cyanamide and formaldehyde obtained by reacting substantially alkaline solution at a temperature below 35' 0.. and about 0.001 to 0.1 ounce per gallon of polyvinyl alcohol.

3. An aqueous zinc electroplating solution containing the necessary components. for the electrodepositlon of zinc, said solution having a pH. above 9.0 and containing dissolved therein as a brightening agent, 0.1 to 2.0 ounces per gallon of the condensation product of sodium cyanamide and formaldehyde obtained by reacting subetantially equimolecular amounts of sodium cyanamide and formaldehyde in aqueous alkaline solution at a temperature below 35 0.

"4. An aqueous zinc electroplating solution containing the necessary components forthe eleetrodeposltion of zinc, said solution having n. pl! above-9.0 and containing dissolved therein as a brightening agent, 0.1 to 2.0 ounces per gallon of the condensation product of sodium cyanamide and formaldehyde obtained by reacting substantially equimolecular amounts of sodium cyanamids and formaldehyde in aqueous alkalinesolution at a temperature below 35 C., and 0.001 to 0.1 1 olmceper gallon of polyvinyl alcohol.

ROBERT R. mm. nanny L. BENNER. JOSEPH rannmrc wanna.

ass-Ea" ENOES orrnn The following references are of record in the file of this patent:

UNITED STATES m'rau'rs Metal 1ndustry, Nov'. 29,1940, page 443.

equimolecular amounts of sodium cyanamide and formaldehyde in aqueous-