US 3748237 A
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3,748,237 ZINC PLATING Hans Gerhard Creutz, Westiand, Mich., assiguor to Oxy Metal Finishing Corporation, Warren, Mich. No Drawing. Filed July 14, 1971, Ser. No. 162,690 Int. Cl. C23b 5/12 US. Cl. 204-55 R 24 Claims ABSTRACT OF THE DISCLOSURE A non-cyanide zinc electroplating bath contains an organic compound which contains within this structural formula a sulfonic acid and a sulfide, mercapto or polysulfide group. The additives are introduced to obtain a bright zinc plate, as well as to obtain a leveling effect.
This invention relates to an improved composition and process for the production of bright zinc electrodeposits and more particularly, relates to compositions and processes for forming bright zinc electrodeposits from non-cyanide zinc electroplating baths.
Heretofore, various processes have been utilized for the electrodeposition of zinc to provide both corrosion resistant and decorative coatings, praticularly on ferrous metals, such as iron and steel. ln general, the zinc electroplating baths have been either acid zinc baths, which are generally operated in a pH range of from about 3 to 5, and alkaline zinc baths, which are generally operated in a pH within the range of from about to 14. Although the acid zinc plating baths are more inexpensive to operate, because of their relatively poor throwing power, their use has been largely restricted to the plating of relatively simple shapes, such as steel strip, pipe, wire, and the like, in which the plating is carried out over a relatively narrow current density range. Additionally, because of the corrosive nature of the acid zinc baths, corrosion resistant linings such as rubber, lead, ceramics, synthetic resins and the like, must be used in the plating tanks. For these reasons, the acid zinc plating baths have not been widely used.
The alkaline zinc plating baths have commonly been of two types, the cyanide-free baths, which contain sodium zincate, and the more conventional bright zinc baths which contain sodium cyanide. The sodium zincate baths, however, have a relatively low efiiciency and produce soft-spongy electrodeposits. Moreover, even with various additives, the zinc deposits produced are not as bright as those obtained from cyanide baths and, hence, there has been little commercial use of such baths.
With the alkaline cyanide zinc plating baths, however, generally acceptable electrodeposits are obtained, the baths are stable and relatively easy to control and the cyanide-containing solution is not corrosive to ferrous metal plating tanks. Accordingly, such baths have been widely used commercially. Nothwithstanding, this widespread commercial acceptance, however, in recent years, there has been an increasing awareness that these plating baths containing sodium cyanide constitute both a health hazard to operating personnel and a source of industrial pollution. Accordingly, and particularly because of the increased expense encountered in the installation and operation of the special processing equipment to overcome the waste disposal problem, significant numbers of platers having discontinued the use of zinc and switched to other metals as substitutes.
It is, therefore, an object of the present invention to provide an improved bright zinc plating bath which is free of cyanide.
Another object of the present invention is to provide a cyanide-free bright zinc plating bath which was excellent throwing power and will still produce a smooth, bright, adherent zinc deposit.
United States Patent 0 A further object of the present invention is to provide an improved process for the electrodeposition of a smooth, bright, adherent zinc deposit from a cyanidefree zinc plating bath.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.
Pursuant to the above objects, the present invention is directed to an aqueous cyanide-free zinc electroplating bath, which contains as brightening additives, an organic compound wherein the molecule contains a sulfonic acid group and a sulfide or mercapto, or polysulfide group hereinafter referred to as brightener #1. More precisely, the brightening additives are of the structure:
wherein: n is from 1 to 3; f is from 0 to 3; f is from 0 to 3;
R and R independently may be hydrogen, alkyl of 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, cyclo hexyl, phenyl, benzyl, naphthyl, thiazolyl or benzothiazolyl, providing that f, and f when added together is at least one and that when R is hydrogen, 1, is 0 and when R; is hydrogen, f is 0.
Some compounds which are brightener #1 are of the following structures: (1) HS(CH -'SO H prepare by reacting sodium sulfide with propane sultone. (2) HO S(CH -S-S(CH SO H prepare by reacting #1 with hydrogen peroxide.
Additional brightening additives may be added to supplement the activity of the aforementioned additive such 3 as those described in US. 'Ser. No. 20,855, filed Mar. 18, 1970. In that case, brightening additives are described containing a combination of linear polyether and a heterocyclic nitrogen compound. These baths may also contain an organic chelating agent which have been found to produce bright, lustrous zinc electrodeposits over a wide range of operating conditions.
More specifically, the baths of the present invention are aqueous, cyanide-free zinc electroplating baths having a pH of from about 7.0 to 9.6, preferably 7.5 to 8.0 and containing the aforementioned brightener #1 in amounts sufficient to produce a zinc electrodeposit having improved brightness. These baths are aqueous solutions of zinc salt, such as zinc sulfate, zinc chloride, zinc fiuo borate, or the like. Additionally, the bath may also contain ammonium chloride, which has been found to improve the bath conductivity and throwing power. Desirably, these baths contain the zinc in an a mount within the range of about 10 to 30 grams/liter being preferred. Additionally, where ammonium chloride is included in the bath it is desirably present in amounts within the range of about 20 to 300 grams/liter, with amounts within the range of about 100 to 200 grams/liter being preferred. Moreover, compounds such as ammonium hydroxide, and hydrochloride acid may also be included in the zinc plating baths in amounts which will provide the desired pH of the bath. Accordingly, the specific amounts used will vary depending upon the specific make-up of the particular zinc plating bath which is being used.
The organic sulfo and sulfide compound which is added as a brightening additive, is present in amount from about 0.002 to about 10 grams/liter with an amount within a range of about 0.01 to about 1.0 gram/liter being preferred.
As additional brightening additives, it has been found that the cyanide-free electroplating baths of the present invention may also contain at least a linear polyether and a heterocyclic nitrogen compound. Desirably, the polyether compound is present in the bath in an amount within the range of about 0.2 to 20 grams/liter, with an amount within the range of about 0.5 to grams/liter being preferred. The polyether compounds which have been found to be suitable for use in the plating baths of the present invention are linear, mono or difunctional polyethers which are substantially free of said chains which terminate in a functional group, such as OH group. Suitable linear polyethers of this type include the difunctional polyethers, such as the polyglycols, exemplary of which are the polyethylene glycols, polypropylene glycols, mixed polymers of polyethylene and polypropylene glycol, and the like, as well as the monofunctional linear polyethers, such as the ethoxylated alkyl phenols, ethoxylated fatty alcohols, and the like. Typically, these linear compounds will have a molecular weight within the range of about 500 to 1,000,000. In the case of the monofunctional linear polyethers such as the ethoxylated alkyl phenols, these may contain up to about 18 carbon atoms in the alkyl group and up to about 50 moles of ethylene oxide. Of the various linear polyethers which are suitable for use in the present electroplating bath, the polyethylene glycols having a molecular weight of from about 1000 to 100,000 and the condensation products of nonyl phenol with about 30 moles of ethylene oxide, have been found to be preferred. It is to be appreciated, however, that these materials are merely exemplary of the linear polyethers which may be used and are not to be taken as a limitation on these materials.
The heterocyclic nitrogen compound additives, which are used in conjunction with the polyether materials, are desirably present in the present zinc electroplating bath in amounts Within the range of about 2 milligrams to grams/liter, with amounts within the range of about 10 to 100 milligrams/liter being preferred. These additive materials are bath-soluble quaternary nitrogen compounds which contain at least one substituting group selected from carboxylic esters, carboxamides, carboxy, and nitrile groups which have been quaternized with agents such as benzyl chloride, allyl bromide, methyl or ethyl and propyl nicotinate; for example, n-benzyl-3 alkyl carboxylate pyridinium chloride or the quaternary resulting from reacting benzyl chloride and nicotinic acid. The suitable heterocyclic nitrogen compounds of this type are described in detail in U.S. Pat. 3,318,787. Although in this patent, these compounds are disclosed as producing smooth, bright zinc deposits when added to alkaline cyanide zinc electroplating baths, suprisingly it has been found that when used alone in the non-cyanide zinc electroplating baths of the present invention, they have little eifect. It is only when these heterocyclic nitrogen compounds are used in conjunction with the linear polyether materials that full bright zinc electrodeposits are obtained from the non-cyanide baths.
In addition to the linear polyether material and the heterocyclic nitrogen compounds, in many instances it has also been found to be desirable to include in the plating baths an organic chelating or sequestering agent to help prevent the formation of zinc hydroxide. These chelating agents are used in amounts suflicient to chelate the zinc in the bath, and hence, the specific amounts used will vary in each instance, depending upon the composition of the bath. Typically, however, they are present in amounts up to about 250 grams/liter, with amounts within the range of about 10 to 150 grams/liter being preferred. Exemplary of the Various suitable chelating agents which may be used are ethylene diamine tetraacetic acid, nitrilotriacetic acid, ethylene diamine tetraethanol, citric acid, and the like, including the various salts of these compounds.
In formulating the zinc electroplating baths of the present invention, the various additive materials may be added separately to the bath in amounts which will provide the desired concentration of these components in the bath. Preferably, however, an aqueous concentrate composition is utilized which contains all of the desired additive material in the proper ratio and proportion to each other so as to provide the desired concentration of these materials in the bath. Thus, a suitable additive concentrate composition for use in formulating the zinc electroplating baths of the present invention may contain the following components in the amounts indicated:
Organic chelating agent (when present at least 10 g./l.) 0250 Brightener #1 0.002-10 In utilizing the plating baths of the present invention, the baths may be operated at temperatures up to about 60 degrees Centigrade, with temperatures in the range of about 15 to 30 degrees centigrade being preferred. The pH of the plating bath during the operation may be Within the range of about 7.0 to 9.6 with pHs from about 7.5 to 8.0 being preferred. Desirably, the average cathode current densities used will be within the range of about 5 to amps/square foot being preferred. Desirably, the average cathode current densities used will be within the range of about 5 to 100 amps/square foot, with average current densities of from about 10 to 50- amps/square tfoot being preferred. Thus, it is to be appreciated that the plating baths of the present invention may be used in various plating operations, including both barrel plating and rack plating processes.
In order that those in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percent are by weight and temperatures are in degrees centigrade. It is to be appreciated, however, that these are mere- 1y exemplary of the present invention and are not to be taken as limitation thereof.
EXAMPLE 1 A cyanide-(free electroplating bath was formulated containing the following components in the amounts indicated:
Component: Amounts Zinc chloride grams/liter 50 Ammonium chloride do 200 Tetraethanol ethylene diamine do.. 50 Ammonium hydroxide (29% by weight) milliliters/ liter- 25 Polyethylene glycol (molecular weight 4000) grams/liter 40 Ethylnicotinate-benzyl chloride quaternary milligrams/liter 50 dO 20 Using this plating bath, at a pH of 8.0, a bent J-shaped steel cathode was plated for 20 minutes at a current density of 30 amps/ square foot and a uniform full bright zinc electro-deposit was obtained.
EXAMPLE 2 A cyanide-free electroplating bath was formulated containing the following components in the amounts indicated:
Component: Amounts Zinc chloride grams/liter 30 Ammonium chloride do 220 Ammonium hydroxide (29% by weight) milliliters/liter -80 Polyethylene glycol (molecular weight 100,000)
grams/liter" 1O Isopropyl nicotinate-benzyl chloride quaternary milligrams/liter 30 With this bath, at a pH of 8.2, and an average current density of 20 amps/square tfoot, uniform full bright zinc electrodeposits were obtained, as in Example 1.
EXAMPLE 3 A cyanide-free electroplating bath Was formulated containing the following components in the amounts indicated:
Component: Amounts Zinc chloride grams/liter 21 Ammonium chloride do 125 Nitrilotriacetic acid .do 35 IGEPALCO 880 (trademark for nonyl phenol containing 30 molecules ethylene oxide, a product of General Aniline & Film do '10 Ethyl nicotinate quaternized with benzyl chloride mi1ligrams 30 The bath was ran at a pH of 8.
Cathode deposits from such baths had a tendency to have a dull, rough plate at current densities higher than about 20 amps/square foot. When a similar bath had added 50 milligrams/liter of mercapto propane sulfonic acid thereto the high current density plate is of a better quality as well as the overall leveling and brightness was improved.
In a similar manner, the compound of the formula, using 30 milligrams/ liter gave results equivalent to mercapto propane sulfonic acid,
6 EXAMPLE 4 In a manner similar to mercapto propane sulfonic acid of Example 3, the compound of the following formula gave equivalent satisfactory results using 0.1 gram/liter.
What is claimed is:
1. An aqueous zinc electroplating bath comprising as a source of zinc ions, a zinc salt selected from a group consisting of zinc sulfate, zinc chloride, and zinc fiuoborate and as a brightening agent in amounts sufiicient to produce a zinc electrodeposit of improved brightness, a compound of the formula:
wherein n is from 1 to 3; f is from 0 to 3; f is from 0 to 3; R and R are independently selected from the group consisting of hydrogen, alkyl of 1 to 6 carbon atoms, cyclo hexyl, phenyl, benzyl, naphthyl, thiazolyl and benzothiazolyl, providing that f and f when added together is at least one and that when R is hydrogen, f is 0 and when R; is hydrogen, f is 0, wherein the bath has a pH of from about 7.0 to 9.6 and also contains a linear polyether in an amount within the range of 0.2 to 20 grams/ liter and a heterocyclic nitrogen compound present in an amount within the range of about 0.002 to 10 grams/liter.
2. The bath of claim 1 wherein the bath also contains a metal chelating agent present in an amount up to 250 grams/liter.
3. The bath of claim 1 wherein zinc chloride is the source of zinc in the bath.
4. The bath of claim 1 wherein n is 1.
5. The bath of claim 1 wherein n is 2.
6. The bath of claim 1 wherein R is alkyl of 1-6 carbon atoms.
7. The bath of claim 1 wherein R is hydrogen.
8. The bath of claim 1 wherein R is phenyl.
9. The bath of claim 1 wherein f and f total 2.
10. The bath of claim 1 wherein R is benzothiazolyl.
11. The method of depositing a smooth, adherent, bright zinc deposit which comprises electrodepositing zinc from an aqueous cyanide free electroplating bath as claimed in claim 1.
12. The method as claimed in claim 11 wherein the bath also contains a metal chelating agent present in an amount up to 250 grams/liter.
13. The method as claimed in claim 11 wherein zinc chloride is the source of zinc in the bath.
14. The method as claimed in claim 11 wherein n is 1.
15. The method as claimed in claim 11 wherein n is 2.
16. The method as claimed in claim 11 wherein R is alkyl of 1-6 carbon atoms.
17. The method as claimed in claim -11 wherein R is hydrogen.
18. The method as claimed in claim 11 wherein R is phenyl.
19. The method as claimed in claim 11 wherein f and f total 2.
20. The method as claimed in claim 11 wherein R is benzothiazolyl.
21. An aqueous zinc electroplating bath comprising as a source of zinc ions, a zinc salt selected from the group consisting of zinc sulfate, zinc chloride and zinc fluoborate and as a brightening agent in an amount sufiicient to produce zinc electrodeposit of improved brightness, a compound selected from the group consisting of and --O3SR1S--SRZ SO3 Whfein R1 and R2 are independently selected from the group consisting of alkyl of 1 to 6 carbon atoms; wherein the bath has a pH from about 7.0 to 9.6 and also contains a linear poiyether in an amount within the range of 0.2 to 20 grams/liter and a nitrogen heterocyclic compound present in the amount within the range of about 0.002 to 10 grams/liter.
22. The bath of claim 21 wherein the sulfonate compound is present in an amount ranging from about 0.002 to 10 grams/liter.
23. The bath of claim 22 wherein the bath also con- References Cited UNITED STATES PATENTS 2,849,351 8/1958 Gundel et ai. 20455 R X 3,023,150 2/1962 Wilhnund et a1. 204-55 R X 2,830,014 4/1958 Gundcl et a1. 204-55 X F. C. EDMONDSON, Primary Examiner US. Cl. X.R.
l5 204Dig. 2