US 3417005 A
Description (OCR text may contain errors)
United States Patent Office 3,417,005 Patented Dec. 17, 1968 3,417,005 NEUTRAL NICKEL-PLATING PROCESS AND BATH THEREFOR Muhammad L. Baig, Warren, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware N Drawing. Filed Dec. 27, 1965, Ser. No. 516,761 22 Claims. (Cl. 204-32) This invention relates to electroplating, and more particularly to the electrodeposition of nickel from a substantially neutral bath.
Nickel has been effectively electrodeposited from acid baths of less than 5.6 pH. The thus deposited nickel has served both as an undercoat for subsequently applied coatings of nickel, chromium, etc., and as a final finish in its own stead. Unfortunately, the acidity of the baths have made the direct plating of nickel onto metals such as zinc and iron impractical. The acid attacks the metal immediately upon immersion therein causing contamination of the bath and etching and disruption of the surface of the metal whereby its specular characteristics are deleteriously affected.
It has heretofore been necessary, therefore, to electroplate zinc alloys and the like in alkaline or substantially neutral solutions so as to avoid the deleterious effects of the acid baths. However, nickel has not heretofore been practically electrodeposited from such baths. For a variety of reasons peculiar to. the particular bath, attempts at neutral and alkaline nickel deposition have not proved to be feasible from a commercial or production standpoint. Some baths have used additives which complex or tie up the nickel so tightly at the higher pHs that the throwing power of the bath is substantially reduced. Hence while such baths might be acceptable for deposting on plates or otherwise planar surfaces, they have not proved adequate for plating complex articles having recessed surfaces with associated high and low current density areas. Other prior art baths have produced coatings which have unsatisfactory properties, typical of which are cracks, high stress, etc.
It is an object of my invention to provide a neutral nickel-plating bath whereby strike, semi-bright, or bright nickel may be deposited directly on such acid-sensitive metals as aluminum, zinc and iron without any deleterious effects resulting therefrom.
It is a further object of my invention to provide a neutral nickel-plating bath having exceptionally good throwing power.
It is a further object of my invention to provide a neutral nickel-plating bath which produces a crack-free nickel plate having low stress and superior adhesion characteristics.
It is a further object of my invention to provide a process for electrodepositing crack-free, adherent nickel coatings on such acid sensitive metals as zinc and iron.
These and other objects and benefits will become apparent'from the following description of a specific embodiment encompassed within the scope of my invention.
Briefly stated, my invention is a nickel-plating bath which can be operated at any pH within the range of -9.5 comprising:
At least 0.53 mole per liter of nickel ion (Ni++) At least 0.45 but less than 0.54 mole per liter of citrate H (C6H5OI1E) At least 0.023 mole per liter of gluconate s n 'f) An ion from the group consisting of (Cl) and (F') for inducing anode corrosion And an additive from the group consisting of (NHJ) ion and (triethanolamine) for relieving stresses in the deposit.
All molar concentrations have been rounded off to the nearst second significant figure.
A particularly significant bath composition comprises:
Ions Grams per Liter Moles per Liter Nickel (Ni++ 31-62 0. 53-1. 06 Citrate CsH5O7E) 85-103 0. 45-0. 54 Gluconate (CsH1i07-) 4. 5-40 0. 023-0. 21 Chloride (Cl-) 1-24 0. 028-0 68 Ammonium (NH4+) 1-24 0. 055-1. 33
Salts Grams per Liter Moles per Liter NiSOl 6H20 140-280 0. 53-1. 06 Na3CsH507 21120 132-160 0. 45-0. 54 Ct 501- 85-103 0. 45-0. 54 NaC5H11O1 5-45 0. 023-0. 21 0511110 4. 5-40 0. 023-0. 21 Cl- 1. 0-24 0. 028-0. 68 NH4+ 1. 0-24 0. 055-1. 33
H+ or (011-) to adjust pH to 5.0-9.5.
A significantly effective bath for plating on zinc based die castings comprises:
Salts Grams per Liter Moles per Liter NiSO4-6HzO. 140-280 0.53-1.06 N33CGH507'2II2 132-155 0. 45-0. 53 C6 501 85-100 0. 45-0. 53 NaCaHnO 10-30 0046-014 40 CtHnOr 8. 9-27 0. 046-014 Cl 1 8-12 0. 23-0. 34 NI-l4+ 8-12 0. 44-0. 68 pH 6. 7-7. 5 Temperature, 130-150 The preferred bath can operate at different pHs and temperatures, depending on what material is to be plated. Hence for plating on zinc and alloys thereof, the pH is adjusted to between 6.7-7.5 by the addition of H+ or OH ions in a known fashion with NH OH being the preferred OH introduction means. Likewise for steel, copper or alloys of copper, baths may be adjusted between 59.5 without affecting the properties of the bath. The bath may be operated at temperatures ranging from -18 F. For normal plating operations, current densities in the order of 10-75 amperes per square foot are preferred. As a strike bath, however, current densities up to amperes per square foot have been employed without affecting the smoothness of the deposit. At higher strike current densities, roughness is apparent in the deposit.
In the past, relatively low concentrations of citric acid have been used in acid nickel baths as complexing and buffering agents. Nickel, which would otherwise precipitate out of citrate-free solutions as nickel hydroxide at pHs above about 5.6, is retained in solution by the presence of the citric acid which ties up the nickel in the form of a soluble nickel complex. The amount or extent to which a bath of given pH will retain the nickel in solution is dependent upon the concentration of the complexing agent. Unfortunately, to retain the nickel in solution under substantially neutral conditions the concentration of the citric acid is such as to produce side effects which interfere substantially with the baths performance.
I have found that at higher concentrations of citrate the acid form can no longer be practically used as its inherent acidity tends to counterbalance the benefits derived from its complexing and buffering properties. Hence the relatively neutral light alkali metal (Li, Na, K) salt must be used. Even with the use of the citrate salts, however, the high concentration of citrate ion required to completely complex the nickel at substantially neutral conditions produces undesirable side effects. In this connection, it was noted in experiments wherein a total current of 4.0 amperes was passed through one liter of solution maintained at a pH of 7.0 and a temperature of 145 and using 10.35 square inches of cathode surface and about 17.0 square inches of anode surface and spaced apart by about 2.8 inches, that at the citrate concentration required to adequately complex the nickel, a brownish film was found over the surface of the anode and the cell displayed a constant voltage of about six volts. It was noted that increasing the amount of sodium citrate above 152 g./l. increases gassing, which in turn reduces anode corrosion, current efficiency, and condenses the range of metal deposition current densities. It does, however, extend this range toward the lower current densities.
When the solution contains sodium citrate in amounts less than about 152 g./l., there is less gassing of the electrodes, the metal will deposit at higher current densities, and anode corrosion is better. However at these lower concentrations of sodium citrate, the throwing power of the solution diminishes and the pH becomes unstable. Precipitation of Ni(OH) is observed when the concentration drops below 132 g./l. It appears, then, that at least about 165172 g./l. of sodium citrate is required to adequately complex the nickel and stabilize the pH of the bath. However, such a concentration is undesirable, owing to the aforementioned side effects encountered at concentrations above 152 g./l. 160 g./l. then appears to be a maximum comprise upper limit. 132 g./l. is the minimum concentration required to hold the nickel in solution under substantially neutral conditions (pH 6.77.5).
I have found that the complexing and buffering benefits of the higher sodium citrate (165-172 g./l.) baths and likewise the low gassing, better throwing power and high etficiency benefits of the lower citrate baths (132-152 g./l.) can be obtained over an operating range of sodium citrate concentrations of between 132 g./l. and 160 g./l. by the addition of sodium gluconate to the bath. The addition of sodium gluconate permits retention of the favorable properties of sodium citrate to the exclusion of its unfavorable ones.
While I dont intend to be bound by this theory, it appears that the gluconate and citrate must be in a proportion which will effect a complexing and buffering equivalent of about 165172 g./l. of sodium citrate alone. Too little gulconate is ineffective in overcoming the disadvantages of the citrate. Yet higher concentrations of gluconate are equally ineffective if the citrate concentration falls below the critical 132 g./l. level.
At least 5 .g./l. and preferably to g./l. of sodium gluconate should be added to the bath. I have never found a need to add more than g./l. to the bath of my invention. In fact, I have found that in the lower concentration citrate baths (less than 132 g./1.), gluconate additives even as high as 60 g./l. would not prevent precipitation of the nickel. For the ideal compounding of a bath within the scope of my invention, the proportion of the citrate and gluconate may be approximated by reducing an initial concentration of 45 g./l. of sodium glu conate by 1.5 g./l. of sodium gluconate for each 1 g./l. increase of sodium citrate above about 132 g./l. of sodium citrate. However, the sodium citrate concentration should not exceed 160 g./l., nor the sodium gluconate concentration fall below about 5 g./l. This is tantamount to saying that for every 0.01 mole per liter increase of citrate ion above 0.45 mole, 0.02 rnole per liter of gluconate ion may be withdrawn from an initial 0.21 mole concentration thereof within the limits of 0.450.54 mole per liter of citrate ion and 0.0230.21 mole per liter of gluconate ion. Another way of expressing this relationship is by the mathematical expression y=1.082x wherein y is the concentration of the gluconate ion in moles per liter and x is the concentration of the citrate ion in moles per liter.
The pH of the baths generally decreases with an increase in operating temperature. Hence in one particular bath which was generally representative of them all, a pH of 7.5 at 76 F. dropped off to 6.6 at 150 F. It was noticed, however, that this pH variation with temperature could be minimized by utilizing nickel acetate rather than nic kel sulfate.
To determine the effect of sodium citrate and sodium gluconate on the cathode current efficiency, tests were conducted at various current densities and with baths of varying concentrations. A mean efficiency value of 96.0:16 percent resulted. Generally, therefore, the current effieiency of the solution was not affected significantly by variations of the sodium citrate and gluconate concentrations tested.
Other tests were conducted to determine the covering power of the respective baths. It was noted that all the bath combinations studied had nearly the same covering power, with baths containing 150 g./l. or more of sodium citrate and 15 g./l. or more of sodium gluconate having slightly better covering power in the low current density plating range.
Chloride ion addition to the baths reduced cell voltage, prevented formation of the aforementioned bro'wn anodic film, reduced gassing even further and increased anode corrosion. It was determined that from 1 to 24 g./l. could be added to effect theaforementioned improvements but that optimum results are obtained at about 10-12 g./l. This amount appeared to be independent of the concentration of nickel and citrate ions. Fluoride ions are equally satisfactory for these purposes.
Increasing the chloride and fluoride ion concentrations increases the stress in the deposit when added in excess of 10 g./l. However, by adding triethanolamine or ammonium ions to the solution, the stress in the deposit is reduced considerably. Solutions containing chloride and ammonium ions in equal amounts by weight produce the best results. Though acceptable in small quantities, if triethanolamine is added in excess of 8 ml./l. for stress reduction, the deposit appears to have a network of healed over cracks. An excessive amount of triethanolamine also increases the cell voltage. However, unlike triethanolamine, the concentration of ammonium ions can be increased to obtain stress reduction without the production of cracked deposits. Several tests were conducted to determine the relative effects of the (Cl and NHJ). The concentrations of each were varied but always holding the weight ratio of 1:1. The results indicate that in all cases the deposits obtained are smooth and have about the same level of stress, independent of the total concentration of the chloride ions. It appears then that when the molar ratio of (Cl) to (NHJ) ions in the bath is 1:2, the deposit obtained has a minimum amount of stress. NH ions, like triethanolamine, increase the throwing power of the solution at low current densities and produce grain refinement at high current densities. Any soluble ammonium salt can be used as a source of NHJ.
EXAMPLE A 96 gallon plating bath was prepared. The bath contained:
G./l. Nickel sulfate (hexahydrate) 142.2 Nickel chloride (hexahydrate) 29.92 Sodium citrate Sodium gluconate 29.92 Ammonium sulfate 33.9
The bath was prepared by filling the tank with water, heating to 140 F. and dissolving the above-mentioned chemicals therein, in the sequence cited. The pH was adjusted to the operating range by the addition of a 15% by weight sodium hydroxide solution. About 3.5 oz./.gal. was needed to obtain a pH of 7.0. The solution was then filtered.
Continuous plating at 3 0' amperes was maintained on the solution with a dummy cathode load of 0.75 square feet of wrought zinc (four percent aluminum) panels. When specific tests were made in regard to adhesion, cathode efficiency, or coverage, the dummy load was removed from the tank and representative die castings were plated.
It was noted that conventional methods of cleaning zinc die castings were not particularly good for this bath. In this regard, the surface preparation of unbuifed zinc die casting was found to be an important factor. The normal zinc cleaning cycles of cathodic and anodic treatment followed by a dilute sulfuric acid etch proved to be less desirable than the preferred cleaning cycle which follows:
(1) Anodic clean at 6 volts for one minute in alkaline cleaner (e.g. Northwest #371) at 150 F.
(2) Water rinse.
(3) Dip for seconds at room temperature in 4 oz./ gal. of an acid salt (e.g. Northwest Actisalt #1) solution, 1211s a wetting agent (e.g. Northwest Addition Agent (4) Water rinse.
(5) Dip for -30 seconds in 100 ml./ gal. of triethanolamine.
(6) Without rinsing, enter plating solution live.
An alternate method is:
(A) Same as steps 1-4 of the preferred method.
(B) Cathodic treatment at 5 volts for one minute in 1 oz./ gal. sodium cyanide at room temperature.
(C) Same as steps 2-4 of preferred method.
(D) Enter bath live.
After the neutral nickel plating the parts were plated with 12-15 minutes of Harshaw Nubrite nickel at a current density of 40 amperes per square foot followed by a 3-5 minute conventional chromium at a current density of one ampere per square inch.
Excellent solution stability was obtained from the bath during ten weeks of continuous operation in which 50 ,000 ampere hours of plating were registered. During this time the temperature was maintained essentially constant at 140 F., though the pH varied from about 6.8-7.2. Current densities in the order of 150 and 40 amperes per square foot were used for striking and plating operations respectively. Precipitate formation was not encountered at any time. Although constant filtration was used for eight of the ten weeks, noappreciable pickup of impurities or slowdown of flow rate was observed. One week of plating without the use of filtration did not present any problems such as roughness or pitting of deposits. Cathode bar agitation in the order of 1.0 inch per second was used.
A gradual loss of ammonia resulting in a drop in pH was experienced during the test period. Weekly additions of 1-2 liters of ammonium hydroxide kept both the pH and ammoniumv ion concentrations within the recommended range. Indications are that two milliliters per gallon of ammonium hydroxide added in dilute form (1:5) each day to the bath would be most desirable from an operating standpoint. Loss of chemicals from the bath was limited to ammonia. Weekly analysis of the other compone-nts indicated there were no significant changes in sodium gluconate, sodium citrate, nickel sulfate, or nickel chloride during the test period.
The throwing power of the bath was excellent and compared favorably to a copper cyanide bath. Live entry into the plating tank at a strike current density of 150 amperes per square foot was found to be satisfactory for opti- TABLE Brighteners Concentra- Remarks tion, g. ll.
4 1) 2-butene-1,4-diol 0. a2. 0 Mirror bright.
(2) 2-butyne-1,4-diol 0. 5-2. 0 Do. (3) Sodium allyl sulfonate-.. 1. 0-4. 0 Semi-bright. (4) Combination of 1 and 3 Mirror Bright. (5) Combination of 2 and 3 Do. (6) Coumarin Semi-bright.
5. (7) Hydrolyzed coumarin 2 0. 5-3. 0 (8) saccharin (sodium salt) 0. 0-2. 0 (9) Combination of 3, 6 and 8 (10) Combination of 3, 7 and 8 (11) Allyl quinaldinium bromide Mirror bright.
0.01-0.03 Fine grain, dullsemi-bnght at 20 amps/it.
1 P. 704, Organic Chemistry, 2nd ed., Frank G. Whitmore; D. Van Nostrand Co., Inc.
2 Based on coumarin.
Different combinations of the above-cited compounds have been found to produce a variety of nonpredictable results. Hence, for example, the combination of sodium allyl sulfonate and N-allyl quinaldinium bromide produces a bright deposit up to current densities of only 35 amps./ ft. However, if small amounts (0.05-0.1 g./1.) of either 2-butene-1,4-diol or 2-butyne-1,4-diol is added along with the combination, the current density range for the deposition of bright nickel increases considerably. Likewise, when saccharin (sodium salt) is added to the combination of sodium allyl sulfonate and either coumarin or hydrolyzed coumarin, the brightness range increases from about 40 amps/ft. to about amps/ft. current density in addition to forming a less stressed deposit.
It was also surprisingly noted that it was necessary to have ammonium ions present to effect the mirror bright deposits accredited to the aforementioned brightener system.
It has been observed that a simple brightener system containing either 2-butyne-1, 4-diol alone or in combination with sodium allyl sulfonate is quite satisfactory. The deposit is rather stressed, however, and may not be suitable for zinc die cast parts if the thickness of the nickel deposit is very small and has a subsequent deposit of bright chromium, which is also highly stressed. Nevertheless, for most other ferrous and non-ferrous metals and their alloys, the deposit obtained from such a system would be both adequate and economical. On the other hand, where ductility of the bright nickel deposit is very important, as in the case of zinc die cast parts, the somewhat more involved systems of brighteners (Examples 9 and 10 of the table) appeared to be quite suitable. The optimum concentration of coumarin or hydrolyzed coumarin (based on coumarin) appears to be about 1.5 g./l.
Generally speaking, all the additive compounds tested except the ammonium salts were detrimental to the brightening action of the acetylenic compounds. In this regard it was noted that though triethanolamine was incompatible with brightening systems based on 2-butyne-1, 4-diol, a small amount could be tolerated in the coumarin based systems.
Although the invention has been described in connection with certain specific examples, it is to be understood that no limitation is intended thereby except as defined in the appended claims.
1. An aqueous bath for the electrodeposition of nickel having a pH of 5.0-9.5 and comprising a nickel ion concentration of at least about 0.53 mole per liter, a citrate ion concentration of at least about 0.45 but less than about 0.54 mole per liter, a gluconate ion concentration 0. Smooth and dull of at least about 0.023 mole per liter, at least one ion for inducing anode corrosion from the group consisting of chloride and fluoride, and at least one additive for reducing stress in the deposit from the group consisting of ammonium ions and triethonalamine.
2. A bath as defined in claim 1 wherein the citrate ion and gluconate ion concentrations result from the dissolution of the corresponding salt of an alkali metal from the group consisting of sodium, lithium and potassium.
3. A bath as defined in claim 2 wherein the concentration of said citrate ion is 0.450.53 mole per liter and the concentration of said gluconate ion is 0.0460.14 mole per liter.
4. A bath as defined in claim 2 wherein said corrosion inducing ion is the chloride ion, said stress reducing additive is ammonium ions and the ratio of the concentration by weight of said chloride ion to the concentration by weight of said ammonium ion is about 1.0.
5. A bath as defined in claim 4 wherein the concentration of the gluconate ion in moles per liter is approximately y as defined by the expression y=1.082x in which x is the concentration of the citrate ion in moles per liter and x varies from about 0.45 to about 0.54 mole per liter.
6. A bath as defined in claim 4 wherein the concentration of said citrate ion is 0.450.53 mole per liter and the concentration of said gluconate ion is about 0.046- 0.14 mole per liter.
7. A bath as defined in claim 2 wherein the concentra tion of the gluconate ion in moles per liter is approximately y as defined by the expression y=1.08-2x in which x is the concentration of the citrate ion in moles per liter and x varies from about 0.45 to about 0.54 mole per liter.
8. A bath as defined in claim 7 wherein x varies from about 0.45 to about 0.53 mole per liter.
9. An aqueous bath for the electrodeposition of nickel having a pH of 5.0-9.5 comprising a nickel ion concentration of at least about 0.53 mole per liter, a citrate ion concentration of at least about 0.45 but less than about 0.54 mole per liter, a gluconate ion concentration of at least about 0.23 mole per liter, at least one ion for inducing anode corrosion from the group consisting of chloride and fluoride, at least one additive for reducing stress in the deposit from the group consisting of ammonium ions and triethanolamine and at least one additive from the group consisting of 2-butene-1,4-diol, 2-butyne-1,4-diol, sodium allyl sulfonate, coumarin, hydroylzed coumarin, saccharin (sodium salt), and allyl quinaldinium bromide.
10. A bath as defined in claim 9 wherein said additive is 2-butene-1,4-diol.
11. A bath as defined in claim 10 and a second of said additives consisting of sodium allyl sulfonate.
12. A bath as defined in claim 9 wherein said additive is 2-butyne-1,4-diol.
13. A bath as defined in claim 12 and a second of said additives consisting of sodium allyl sulfonate.
14. A bath as defined in claim 9 wherein said additive is sodium allyl sulfonate.
15. A bath as defined in claim 9 wherein said additive is coumarin.
16. A bath as defined in claim 15 and a second and third of said additives consisting of sodium allyl sulfonate and saccharin (sodium salt).
17. A bath as defined in claim 9 wherein said additive is saccharin (sodium salt).
18. A bath as defined in claim 9 wherein said additive is allyl quinaldinium bromide.
19. A bath as defined in claim 18 and a second of said additives consisting of sodium allyl sulfonate.
20. A bath as defined in claim 19 and a third of said additives from the group consisting of 2-butene-l,4-diol and 2-butyne-l,4-diol.
21. A process for the electrodeposition of nickel comprising the steps of preclaiming a part to be plated, immersing said part into an aqueous bath having a pH of 5.09.5 and comprising a nickel ion concentration of at least about 0.53 mole per liter of solution produced from at least one salt from the group consisting of nickel sulfate, nickel chloride, nickel fluoborate, nickel sulfamate and nickel acetate, a citrate ion concentration of at least about 0.45 but less than about 0.54 mole per liter of solu ion, a gluconate ion concentration of at least about 0.023 mole per liter of solution, at least one ion for inducing anode corrosion from the group consisting of chloride and fluoride, and at least one additive for reducing stress in the deposit from the group consisting of ammonium ions and triethanolamine, applying to said bath and said part a voltage such as to effect a current density of 10-150 am s/ft. removing and rinsing said part.
22. A process for the electrodeposition of nickel comprising the steps of preclaiming a part to be plated: immersing said part into an aqueous bath having a pH of 5.09.5 and comprising a nickel ion concentration of at least about 0.53 mole per liter of solution produced from at least one salt from the group consisting of nickel sulfate, nickel chloride, nickel fluoborate, nickel sulfamate and nickel acetate, a citrate ion concentration of at least about 0.45 but less than about 0.54 mole per liter of solution, a gluconate ion concentration of at least about 0.023 mole per liter of solution, at least one ion for inducing anode corrosion from the group consisting of chloride and fluoride, and at least one additive for reducing stress in the depot-it from the group consisting of ammonium ions and triethanolamine, and at least one additive from the grOup consisting of 2-butene-l,4-diol, 2-butyne-1,4-diol, sodium allyl sulfonate, coumarin, hydrolyzed coumarin, saccharin (sodium salt) and allyl quinaldinium bromide, applying to said bath and said part a voltage such as to effect a current density of 10150 amps/ft removing and rinsing said part.
References Cited UNITED STATES PATENTS 2,513,280 7/1950 Brown 204-49 2,523,190 9/1950 Brown 20449 2,773,818 12/1956 Moy et al 204-49 2,972,571 2/1961 Towle 20449 2,994,648 8/1961 DuRose 204-49 3,082,156 3/1963 Brown 20449 3,264,199 8/1966 Fassell et a1 204-49 XR FOREIGN PATENTS 615,036 12/1948 Great Britain. 354,325 11/1937 Italy.
11,207 2/1961 Japan.
OTHER REFERENCES Hammond, L. D.: The Electrodeposition of Nickel, Transactions of the American Electrochemical Society, vol. 30, pp. 103134, 1916.
HOWARD S. WILLIAMS, Primary Examiner.
G. KAPLAN, Assistant Examiner.
US. Cl. X.R. 204-49