US 3929595 A
There is provided an electrolytic gold bath for the deposition of thicker, bright gold coatings at high rates of deposition. The bath contains a heterocyclic azohydrocarbon sulfonic acid or salt thereof, e.g. pyridine sulfonic acid.
Description (OCR text may contain errors)
United States Patent Biberbach et al.
[451 Dec. 30, 1975  Assignee: Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Germany  Filed: Nov. 4, 1974  Appl. No.1 520,844
 Foreign Application Priority Data Nov. 7, 1973 Germany 2355581  U.S. Cl 204/44; 204/46 G [5 l] Int. Cl. C25D 3/48; C25D 3/62  Field of Search 204/46 G, 43 G, 44, 109, 204/123 [5 6] References Cited UNITED STATES PATENTS 2,905,601 9/1959 Rinker et al. 204/43 G 2,967,135 1/1961 Ostrow et al. 204/43 G 2,986,498 5/1961 Strauss et al 204/46 G 3,023,150 2/1962 Willmund et al. 1 204/44 3,149,058 9/1964 Parker et al. 204/46 G Primary ExaminerG. L. Kaplan Attorney, Agent, or Firm-Cushman, Darby & Cushman  ABSTRACT There is provided an electrolytic gold bath for the deposition of thicker, bright gold coatings at high rates of deposition. The bath contains a heterocyclic azohydrocarbon sulfonic acid or salt thereof, e.g. pyridine sulfonic acid.
21 Claims, No Drawings ELECTROLYTIC BURNISIIED GOLD BATH WITH HIGHER RATE-OF DEPOSITION The invention is directed to an electrolytic gold bath for the deposition of thick, bright gold coatings at higher rates of deposition which contain sulfonic acids of heterocyclic nitrogen containing hydrocarbons, i.e. heterocyclic azohydrocarbons.
It is known to deposit bright (burnished) gold coatings from weakly acidic electrolytic baths (German Pat. No. 1,111,397, and corresponding Rinker US. Pat. No. 2,905,601, and German Pat. No. 1,213,697, and corresponding Parker U.S. Pat. No. 3,149,057). By coprecipitation of at least 0.2% of a non-noble metal as for example nickel or cobalt these coatings become bright, hard and abrasion resistant by adding to the baths largeamounts of the named metals. The addition of the non-noble metal normally amounts to more than 1 gram/liter although baths are also known which contain the non-noble metal in an amount down to 100 mg/l. These baths, however, no longer give satisfactory coatings, especially if the non-noble content in the bath lies below 100 mg/l.
These known baths have the disadvantage that the attainable current density and rate of deposition are relatively small. Thus the rate of deposition for example is 0.5 m per minute and the current density 0.5-1.5 A/dm A further disadvantage is that organic polymeric compounds become coprecipitated in such baths (Munier, Plating (1969), pages 1151-1157, Antler, Plating (1973), pages 468-73, and Holt, Plating (1973), pages 918-921) which polymers are formed from the cyanides contained in the bath under the catalytic action of the added non-noble metals such as, for example, nickel or cobalt. This coprecipitation of Organic polymer compounds increases with increasing non-noble metal concentrations in the bath and very strongly impairs the-soldering properties of the separated gold layers (NASA TM X-229O (1971)).
It is further known to bind the non-noble metals by addition' of strong complex builders, as for example ethylenediamine tetraacetic acid, and thereby to reduce the polymer formation (Werkstoffe und Korrosion Vol. 23 (1972), page 643). The addition of such complex builders is limited, however, because of industrial waste water grounds.
It was therefore the problem of the present invention to find an electrolytic bright gold bath which produced the deposition of thick, bright gold layers practically without non-noble metal additions and without use of additional strong complex builders and which possesses a good solderability.
This problem was solved by the invention by adding to known electrolytic gold baths chemical compounds which are N-heterocyclic hydrocarbon sulfonic acids (i.e., the compounds consist of carbon, hydrogen, the hetero-N-atom and the sulfonic acid group), or salts of such N-heterocyclic sulfonic acids.
Especially desirable are the addition of pyridine sulfonic acid, e.g. pyridine-3-sulfonic acid, pyridine-4-sulfonic acid and pyridine-2-sulfonic acid and/or their salts, e.g. water solubles such as salts with alkali metals or ammonium, e.g. the sodium or potassium salts, e.g. the sodium salt of pyridine-3-sulfonic acid and the potassium salt of pyridine-3-sulfonic'acid and/or their simple ring alkyl derivatives, as for example the picoline sulfonic acids, e.g. 2-methylpyridine-3-sulfonic acid, 2-methylpyridine-4-sulfonic acid, 3-methylpyridine-2-sulfonic acid, 3-methylpyridine-4-sulfonic acid and 4-methylpyridine-3-sulfonic acid.
Surprisingly it has been found that this addition of the sulfonic acid derivatives of nitrogen containing heterocyclic also considerably increases the rate of deposition of the gold layer since the baths can be operated at higher current yields at higher current densities. Thus the current density can range from 0.5 to 12 A/dm preferably from 2 to 6 Aldm Besides pyridine sulfonic acids and picoline sulfonic acid there can also be added the sulfonic acids of other substituted nitrogen containing heterocyclics, as for example quinoline sulfonic acid, e.g. quinoline sulfonic acids such as quinoline-5 -sulfonic acid and quinoline-8- sulfonic acid. However pyridine sulfonic acids, especially pyridine-3-sulfonic acid, has proven best.
Preferably the pyridine sulfonic acid or salt and/or derivative is employed in weakly acid gold baths, e.g., pH 3 to 5.5. Such gold baths according to the invention, for example, have the following composition.
2-15 g/l gold as alkali gold cyanide, e.g. sodium gold cyanide or potassium gold cyanide or ammonium gold cyanide.
30-200 g/l of a weak organic acid, preferably citric acid or tartaric acid; hydroxide, e.g. sodium hydroxide or potassium hydroxide or ammonium hydroxide to adjust the pH to between 3.5 and 5.0 or 5.5. Additional suitable weak organic acids include formic acid, lactic acid, kojic acid, itaconic acid, citraconic acid, gluconic acid, glutaric acid, glycolic acid, acetic acid and propionic acid.
1-20 g/l of pyridine sulfonic acid, preferably 1-10 g/l.
The buffering action of the weak organic acids added can also be produced by addition of or the use of only other buffers known to be usable in the named pH range as for example phosphates and borates, e.g. dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, sodium metaborate and potassium metaborate.
The bright range of the burnished gold baths of the invention in regard to working parameters such as concentration, temperature, pH-value or current density can be widened considerably by small additions of non-noble metals such as iron, cobalt, nickel, chromium, cadmium, copper, zinc, tin, indium and/or antimony in an amount of 3 to mg/l, preferably 10 to 50 mg/l. These non-noble metals are normally added in the form of soluble salts, e.g. as salts of the acids set forth above such as citric acid and tartaric acid for example.
Although a coprecipitation of the foreign metal takes place in only a very small amount the overcoats deposited from these baths possess outstanding properties in regard to hardness, abrasion resistance and low porosity.
The amounts of foreign metals used in the baths of the invention are considerably below the amounts necessary to produce brightness in the known baths.
Above all there have proven good the addition of cobalt in amounts of 10 to 30 mg/l and iron in amounts of 10 to 50 mg/l. The addition of iron has the further advantage that in the operation of such a bath according to the invention there can be used as anode material fine steel and therethrough there can be eliminated the deliberate addition of iron content to the bath since the small amounts of iron will come from the anode.
deposited gold layer in a wide working range of the baths.
By a high current density up to A/dm there is a deposition rate of the gold layers of l um in 0.75 to 1.5 minutes, depending on the current density and agitation of the bath. The baths possess an outstanding throwing power.
Through a substantially reduced addition of nonnoble metals in comparison to the known baths there is obtained a higher current efficiency and therewith an again higher rate of deposition of the gold layers with negligible amounts of non-noble metal or organic polymer compounds codepositing.
Unless otherwise indicated all parts and percentages are by weight.
The compositions can comprise, consist of'or consist essentially of the materials set forth.
In the examples the potassium gold cyanide employed was potassium gold (I) cyanide (potassium cyanoaurite). There can also be used, however, other alkali gold cyanides such as sodium gold (I) cyanide,
potassium gold (In) cyanide and sodium gold (lll) cyanide or ammonium gold (I) cyanide,
Also while bathtemperatures of 45C were used in the working examples, this can be varied as is conventional in the art, e.g. 30 to 50C.
The following examples further explain the method of operation and the advantages of the baths of the invention. All of the baths were aqueous baths.
EXAMPLE 1 Bath composition l0 g/l Au as potassium gold (I) cyanide 60 g/l citric acid 80 g/l KH P0 4 g/l pyridine-3-sulfonic acid Potassium hydroxide was added until a pH of 4.5 was established.
At a current density of 2 A/dm bath temperature of 45C, bath and articles motion of 4 cm/sec. there were obtained excellent bright gold coats of more than um thickness with a 60 percent current efficiency.
The deposition of a 1 pm layer took place in 1.25 minutes. At a pH of 4.0 and a current density of 4 A/dm the bath also supplies bright coats, which in spite of the high current density have a very low porosity. The overcoats deposited from this type of bath are free of foreign metals and have a Vickers hardness of about 150 kp/mm EXAMPLE 2 Bath composition 8 g/l Au as potassium gold (I) cyanide 60 g/l citric acid 5 g/l pyridine-3-sulfonic acid Potassium hydroxide was added until a pH of 4.0 was established. mg/l iron as ferric citrate.
Working temperature 45C, articles motion 4 cm/sec. Current density 2 A/dm There was obtained an outstandingly bright coat having an iron content of 0.2 weight percent. In 10 minutes there was deposited at 7 urn Au layer.
In the same bath composition but without the pyridine sulfonic acid the overcoats were dull brown and dirty. Nearly glossy, but still haze exhibiting coatings were first produced at a minimum iron content of 100 mg/l. The coatings then contained 0.75 percent iron and were correspondingly brittle.
EXAMPLE 3 acid resulted only in dirty brown coatings. An improvement also was not produced by variation of the current density, pH-value and temperature which showed the decisive influence of the pyridine sulfonic acid in the baths of the invention.
EXAMPLE 4 Bath composition 11 g/l Au as potassium gold (I) cyanide g/l citric acid 5 g/l pyridine-3-sulfonic acid Potassium hydroxide was added until a pH of 4.2 was established.
50 mg/l iron as ferric citrate Working temperature 45C, article motion 4 cm/sec.; current density 3 Aldm There was obtained a high glossed coating having 0.4 weight percent iron at a rate of deposition that provided a 8 pm thick coating in 10 minutes.
An advantageous use of the baths consists in the use of fine steel anodes. On account of the continuous resulting small dissolution of iron from the anodes the bath could be operated in a continuous experiment with 10 times the gold deposition without the addition of iron. I
EXAMPLE 5 The bath composition and temperature were the same as in Example 4 but at an article motion of 25 cm/sec.
At 4 A/dm density there was deposited 1 pm thick very bright gold in 48 seconds (current efficiency 55percent).
EXAMPLE 6 The bath composition and temperature were the same as in Example 4 but at an article motion of 25 cm/sec. and simultaneous vibration movement of the bath.
At 5 A/dm current density there was deposited a l um'thick very bright gold coating in 36 seconds (current efficiency 52 percent).
The coatings in Examples 5 and 6 were pore free. Both of these examples show that by increasing the movement of the bath the rate of deposition of the gold layers can be increased considerably.
- We claim:
1. In an aqueous, acidic alkali gold cyanide bath suitable for the electrolytic deposition of gold the improvement comprising including in the bath a heterocyclic sulfonic acid selected from the group consisting of pyridine sulfonic acid, picoline sulfonic acid and quinoline sulfonic acid or a water soluble salt of such heterocyclic sulfonic acid in an amount sufficient to provide improved bright gold deposition from the bath.
2. A bath according to claim 1 wherein the heterocyclic sulfonic acid is pyridine sulfonic acid or quinoline sulfonic acid.
3. A bath according to claim 2 wherein the pH is 3.5 to 5.5.
4. A bath according to claim 2 wherein the heterocyclic sulfonic acid is pyridine sulfonic acid.
5. A bath according to claim 4 containing 2-15 g/l gold as alkali gold cyanide, 30-200 g/l of a weak organic acid and 1-20 g/l of a pyridine sulfonic acid, said bath having a pH of 3.5 to 5.5.
6. A bath according to claim 5 including a buffer capable of maintaining the pH within the range 3.5 to 5.5.
7. A bath according to claim 5 also including 5-100 mg/l of iron, cobalt, nickel, chromium, cadmium, copper, zinc, tin, indium or antimony in the form ofa water soluble salt thereof.
8. A bath according to claim 7 containing 10-30 mg/l of cobalt.
9. A bath according to claim 7 containing 10-50 mg/l of iron.
10. A bath according to claim 5 wherein the weak organic acid is an alka'noic acid or a hydroxyalkanoic acid.
11. A bath according to claim 10 wherein the acid is a hydroxyalkanoic acid.
12. A bath according to claim 11 wherein the acid is citric acid or tartaric acid.
13. A bath according to claim 1 containing 1 to 20 g/l of the heterocyclic sulfonic acid.
14. A process of electrolytically depositing a glossy gold coating on an article comprising placing the article in the bath of claim 1 and passing a current therethrough at a current density of 0.5 to 12 A/dm 15. A process of electrolytically depositing a glossy gold coating on an article comprising placing the article in the bath of claim 1 and passing a current therethrough at a current density of 2 to 6 A/dm 16. A process of electrolytically depositing a bright gold coating on an article comprising placing the article in the bath of claim 5 and passing a current therethrough at a current density of 0.5 to 12 A/dm 17. A process of electrolytically depositing a bright gold coating on an article comprising placing the article in the bath of claim 5 and passing a current therethrough at a current density of 2 to 6 Aldm 18. A process of electrolytically depositing a bright gold coating on an article comprising placing the article in the bath of claim 7 and passing a current therethrough at a current density of 0.5 to 12 A/dm.
19. A process according to claim 18 wherein the bath contains 10-30 mg/l of cobalt.
20. A process according to claim 18 wherein the bath contains 10-50 mg/l of iron.
21. A process of electrolytically depositing a bright gold coating on an article comprising placing the article in the bath of claim 7 and passing a current therethrough at a current density of 2 to 6 A/dm