US2712525A - Electropolishing of gold alloys - Google Patents

Description

United States Patent ELECTROPOLISHING OF GOLD ALLOYS Johannes Fischer, Hanan, Germany, assignor to Deutsche Goldand Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany N0 Drawing. Application June 18, 1954, Serial No. 437,856

Claims priority, appiication Germany June 27, 1953 6 Claims. (Cl. 204140.5)

The present invention relates to improvements in the electropolishing of gold alloys, and more particularly is directed to a modification of the process for electropolishing and deburring gold and gold alloys described in the copending application of Marianne Reichert, Serial No. 310,540, filed September 19, 1952 and the corresponding German Patent No. 888,492.

In the process described in application Serial No. 310,- 540, gold and gold alloys of a fineness of at least 8 carats are anodically electropolished and deburred in an acidic aqueous electrolytic bath containing at least g. of thiourea, preferably to 60 g., per liter, at temperatures up to 100 C. The baths used for such electropolishing are predominantly aqueous and contain only a minor pro portion of acid for the acidification thereof. Both or ganic and inorganic acids can be employed for such acidification, such as, for example, sulfuric acid, phosphoric acid, perchloric acid, phenol sulfonic acid, tartaric acid and the like, or mixtures thereof. Especially good results have been obtained using minor proportions of sulfuric acid and of mixtures of sulfuric and tartaric acid.

The current densities employed for such process are sufficiently high to cause formation of a film upon the surfaces being anodically treated.

In practicing the process described in Serial No. 310,- 540, it was found that, while excellent electropolishing and deburring effects could be achieved thereby, it was in some instances necessary to employ relatively high current densities to achieve the optimum polishing elfects, which lead to an undesirably high removal of gold from the surfaces being polished. Furthermore, the baths had insufiicient throwing power for some purposes.

It is an object of the present invention to provide a process for electropolishing gold alloys which will produce optimum polishing eifects at substantially lower current densities and potentials than are required with the baths described in copending application Serial No. 310,540.

It is a further object of the invention to provide electropolishing baths for gold alloys having greater elasticity in their use and range of polishing effects obtainable, as well as having improved throwing power.

In accordance with the invention, it was found that these objects could be attained by incorporating minor proportions of metal compounds containing ions of at least one of the metals contained in the alloy to be polished which is less noble than the gold, that is above gold in the electromotive series, in the acidic thiourea containing bath employed for the electropolishing treatment according to application Serial No. 310,540.

Preferably, such metal ions are introduced into the electropolishing bath in the form of their salts. It was found that especially good results are obtained when compounds of the least noble metal contained in the alloy are employed, that is, the metal with the largest normal negative potential.

The quantity of such metal compound incorporated in the electropolishing bath depends upon concentration of Cir 2,712,525 Patented July 5, 1955 the preparation of the electropolishing baths containing an addition of copper, either solutions of thiourea and copper sulfate can be admixed or these can be admixed in solid form and then simultaneously dissolved in the baths. It has been found particularly expedient to incorporate copper in the electropolishing baths by way of the copper salt of thiourea.

It was furthermore found that the addition of the metal salts to the anodizing baths upon reaction with the thiourea imparted the necessary acid character to the baths so that it was unnecessary to add further acidifying agents thereto. Consequently, in practicing the present invention, it is not necessary that such baths be specially acidified with other acids, such as, sulfuric and tartaric acids.

It has also been found advantageous for the polishing effect obtained to move the article being electropolished periodically in the baths after the surface roughnesses originally present have been substantially evened out. Evidently, such movement partially dissolves the anode film formed during the anodic treatment, so that the electric current can again act upon such locations where the anode film has been dissolved.

The following examples will serve to illustrate several modifications of the process according to the invention and the baths employable therefor.

Example 1 An acidic aqueous bath was prepared by dissolving 50 g. of thiourea and g. of copper sulfate in 1000 cc. of water, boiling the solution until the precipitated sulfur had been converted to a form in which it was easily filtered and then filtering off the sulfur. The thus prepared bath was suitable for the electropolishing of the usual gold alloys containing at least 33.3% gold, and silver and copper in addition to the gold. A typical gold alloy of this character, for example, was one containing 83.3% Au, 10% Ag and 6.7% Cu. For the anodic electropolishing of the gold alloys, a current density of 6 A./dm. at 3 volts was employed. The duration of the anodic treatment to produce a good polished effect varied from 5 to 10 minutes, depending upon the original roughness of the articles treated.

Example 2 An acidic aqueous bath was prepared by dissolving g. of thiourea and g. of thiourea-copper sulfate in 1000 cc. of hot Water. The thiourea-copper sulfate was obtained by reacting thiourea and cuprous sulfate according to Beilstein, vol. III, page 186. The equation for the reaction is as follows:

This bath gave excellent electropolishing with gold-silvercopper alloys under the conditions specified in Example 1, except that the bath temperature was C.

Good results were also obtained by replacing the thiourea-copper sulfate compound with the acid thiourea salt of acid copper sulfate of the formula obtained by reacting two mols of thiourea with acid copper sulfate.

Example 3 An acidic aqueous bath was prepared by dissolving 75 a g. of thiourea and 10 g. of silver nitrate in 1000 cc. of water, boiling the solution and filtering off the precipitated sulgfur.

This bath was especially adapted for anodically electropolishing copper free gold-silver alloys, even, those produced electrolytically, at current densities of 5 A./dm.

at 3 volts. For polishing electrolytically produced gold- E silver alloys the duration of the.electropolishing was about 10 minutes.

Example 4 An acidic aqueous bath was prepared by dissolving 50 g. of thiourea and 15 g. of nickel sulfate in 1000 cc. of

water, boiling such solution, and filtering to remove precipitated sulfur. This bath was especially adapted for electropolishing the so-called white gold alloys in which nickel is the least noble alloying component. White golds usually contain 33% to 75% Au and the remainder essentially nickel with minor amounts of silver and copper.

In electropolishing such white gold alloys, a current density of 6 A./clrn. was employed, with the bath maintained at a temperature of 60 C. The time required for the anodic electropolishing was about 5 to 10 minutes.

I claim: 1. A process for polishing the surfaces of gold alloys of a fineness of at least 8 carats which comprises anodically electropolishing such surfaces in an aqueous acidic bath containing at least 5 g. of thiourea per liter and to' which 0.03 to 0.3 mols of a salt furnishing ions contain- 3 ing at least one of the metals contained in the gold alloy to be polished which is less noble than gold has been added from an external source, at a current density causing the formation of a film upon the surfaces treated.

2. A process in accordance with claim 1 in which said salt is a metal salt which furnishes ions of the least noble '2 alloying component in the gold alloy.

faces being electropolished are periodically moved in the bath.

References Cited in the file of this patent FOREIGN PATENTS 888,492 Germany Sept. 3, 1953

Claims (1)

1. A PROCESS FOR POLISHING THE SURFACES OF GOLD ALLOYS OF A FINENESS OF AT LEAST 8 CARATS WHICH COMPRISES ANODICALLY ELECTROPOLISHING SUCH SURFACES IN AN AQUEOUS ACIDIC BATH CONTAINING AT LEAST 5 G. OF THIOUREA PER LITER AND TO WHICH 0.03 TO 0.3 MOLS OF A SALT FURNISHING IONS CONTAINING AT LEAST ONE OF THE METALS CONTAINED IN THE GOLD ALLOY TO BE POLISHED WHICH IS LESS NOBLE THAN GOLD HAS BEEN ADDED FROM AN EXTERNAL SOURCE, AT A CURRENT DENSITY CAUSING THE FORMATION OF A FILM UPON THE SURFACES TREATED.