US3902978A - Electroplating bath for preparation of glossy rhodium coating - Google Patents

Abstract

There is provided a bath for the electroplating of a glossy rhodium coating which bath contains rhodium sulfate or rhodium phosphate, free acid and at least 50 mg/liter of chloride in the form of a chloride of main groups I and II of the periodic system or aluminum chloride or chromium III chloride.

Description

United States Patent [1 1 Zilske et al.

Sept. 2, 1975 ELECTROPLATING BATH FOR PREPARATION OF GLOSSY RHODlUM COATING Inventors: Wolfgang Zilske, Grossauheim;

Wolfgang Engelhardt, Hanau, both of Germany Assignee: Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Germany Filed: Apr. 9, 1974 Appl No.: 459,370

Foreign Application Priority Data June 9, 1973 Germany 2329578 US. Cl. 204/47 Int. Cl. C25D 3/50 Field of Search 204/47, 43 N, 44

[56] References Cited UNTTED STATES PATENTS 1,949,131 2/1934 Shields 204/47 2,461,933 2/1949 Smith et a1 204/47 X OTHER PUBLICATIONS Edward A. Parker, Plating, Vol. 42, pp. 882-892, (1955).

Primary Examiner-G. L. Kaplan Attorney, Agent, or FirmCushman, Darby & Cushman 12 Claims, No Drawings ELECTROPLATING BATH FOR PREPARATION OF GLOSSY RHODIUM COATING The present invention is directed to a rhodium electroplating bath to which a metal chloride is added to obtain a glossy coating.

For the deposition of thin rhodium layers, chiefly for decorative purposes baths are used which contain rhodium sulfate or rhodium phosphate and sulfuric acid or phosphoric acid. These baths have the disadvantage that only very thin glossy rhodium layers (0.1 to 0.2 mp.) can be deposited therefrom. Trifling thicker layers already become milky. Slight differences in thickness of the layer on the same parts, however, are practically unavoidable in electroplating. As is known, the local current density at the parts to be coated can depart substantially from the calculated average current density and be either higher or lower. The result of this is the deposition of the metal in different layer thickness. In this manner in the rhodiumizing on the parts at unfavorable places the layer thickness can be easily exceeded at which point the rhodium still deposits as a glossy or shiny coat. As a result, there occurs on the parts in addition to the glossy areas, other areas of a milky-matte appearance.

The problem of the present invention, therefore, was to develop a bath from which glossy rhodium layers of over 0.2 mu thickness could be deposited.

This problem was solved according to the invention by adding to known rhodium sulfate and rhodium phosphate electroplating baths chlorides of metals of main groups I and ll of the periodic system or aluminum and- /or chromium (lll) chloride in amounts of more than 50 mg/liter of chloride.

The rhodium electroplating bath for example can contain 2 to 10 grams per liter of rhodium phosphate, and 10 to 40 grams per liter of phosphoric acid or 2 to 10 grams per liter of rhodium sulfate and 20 to 60 grams per liter of sulfuric acid.

Until now chlorides were considered as disturbing impurities in rhodium baths which caused gray and matte coatings. It has now been surprisingly found that by the addition of chlorides of metals of main groups I and ll of the periodic system as well as aluminum chloride and/or chromium (lll) that it is possible to deposit glossy rhodium layers having a thickness of 1 my. (one millimicron) or greater from otherwise conventional rhodium electroplating baths. It has been found advantageous to have 0.1 to 5 grams/liter of chloride in the form of the named metal chlorides.

As metals of main groups I and II whose chloride can be used there may be mentioned sodium, potassium, lithium, rubidium, cesium, magnesium, calcium, strontium and barium. Thus, there can be used sodium chloride, potassium chloride, lithium chloride, rubidium chloride, cesium chloride, magnesium chloride, calcium chloride, strontium chloride and barium chloride as well as the above-mentioned aluminum chloride and chromium (lll) chloride. It has been found especially advantageous to add either lithium chloride or magnesium chloride.

At higher chloride contents, for example at grams/liter at first there is obtained unobjectionable rhodium coatings. However, after a relatively short time the coating becomes matte gray. Apparently very high chloride contents lead to changes in the rhodium complex, whereby rhodium compounds unsuitable for the electrolytic deposition are formed.

The rhodium coatings can be applied to steel and other metals as well as other substrates conventionally coated with rhodium.

Unless otherwise indicated all parts are by weight.

EXAMPLE:

To an aqueous rhodium bath containing 2 grams/liter of rhodium in the form of rhodium sulfate and 40 grams/liter of free sulfuric acid, there was added I gram/liter of chloride in the form of magnesium chloride. From this solution there was deposited on polished copper with a bright nickel deposit a 1 mp. thick rhodium layer using a current density of l A/dm Upon observation in light striking on it the coating exhibits a haze free high gloss.

What is claimed is:

1. An aqueous bath suitable for the electroplating of glossy rhodium coatings, said bath containing either (a) 2 to 10 grams/liter of rhodium phosphate and 10 to 40 grams/liter of phosphoric acid or (b) 2 to 10 grams/liter of rhodium sulfate and 20 to 60 grams/liter of sulfuric acid and at least 50 mg/liter of a chloride in the form of a metal chloride or main group II of the periodic system, lithium chloride, aluminum chloride or chromium (lll) chloride.

2. A bath according to claim 1 wherein the amount of chloride is between 0.1 and 5 grams/liter.

3. A bath according to claim 2 containing lithium chloride.

4. A bath according to claim 2 containing magnesium chloride.

5. An aqueous bath according to claim 1 suitable for the electroplating of glossy rhodium coatings, said bath containing (1) either 2 to 10 grams/liter of rhodium phosphate and 10 to 40 grams/liter of phosphoric acid or 2 to 10 grams/liter of rhodium sulfate and 20 to 60 grams/liter of sulfuric acid and (2) at least 50 rng/liter of lithium chloride or magnesium chloride.

6. An aqueous bath according to claim 5, containing l 2 to 10 grams/liter of rhodium phosphate and 10 to 40 grams/liter of phosphoric acid or 2 to 10 grams/liter of rhodium sulfate and 20 to 60 grams/liter of sulfuric acid and (2) 0.1 to 5 grams/liter of lithium chloride or magnesium chloride.

7. An aqueous bath according to claim 6 containing 2 grams/liter of rhodium as rhodium sulfate, 40 grams/- liter of sulfuric acid and 1 gram/liter of chloride as magnesium chloride.

8. A bath according to claim 1 wherein the chloride is lithium chloride or magnesium chloride.

9. A bath according to claim 1 wherein the chloride is a metal chloride of a main group ll metal.

10. A bath according to claim 1 wherein the chloride is chromium (lll) chloride.

11. A bath according to claim 1 wherein the chloride is aluminum chloride.

12. A bath according to claim 1 wherein the amount of chloride is from 50 mg/liter to 10 grams/liter.

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Claims (12)

1. AN AQUEOUS BATH SUITABLE FOR THE ELECTROPLATING OF GLOSSY RHODIUM COATINGS, SAID BATH CONTAINING EITHER (A) 2 TO 10 GRAMS/LITE OF RHODIUM PHOSPHATE AND 10 TO 40 GRAMS/LITER OF PHOSPHORIC ACID OR (B) 2 TO 10 GRAMS/LITER OF RHODIUM SULFATE AND 20 TO 60 GRAMS/LITER OF SULFURIC ACID AND AT LEAST 50 MG/LITER OF A CHLORIDE IN THE FORM OF A METAL CHLORIDE OR MAIN GROUP 11 OF THE PERIODIC SYSTEM, LITHIUM CHLORIDE, ALUMINUM CHLORIDE OR CHROMIUM (111) CHLORIDE

2. A bath according to claim 1 wherein the amount of chloride is between 0.1 and 5 grams/liter.

3. A bath according to claim 2 containing lithium chloride.

4. A bath according to claim 2 containing magnesium chloride.

5. An aqueous bath according to claim 1 suitable for the electroplating of glossy rhodium coatings, said bath containing (1) either 2 to 10 grams/liter of rhodium phosphate and 10 to 40 grams/liter of phosphoric acid or 2 to 10 grams/liter of rhodium sulfate and 20 to 60 grams/liter of sulfuric acid and (2) at least 50 mg/liter of lithium chloride or magnesium chloride.

6. An aqueous bath according to claim 5, containing (1) 2 to 10 grams/liter of rhodium phosphate and 10 to 40 grams/liter of phosphoric acid or 2 to 10 grams/liter of rhodium sulfate and 20 to 60 grams/liter of sulfuric acid and (2) 0.1 to 5 grams/liter of lithium chloride or magnesium chloride.

7. An aqueous bath according to claim 6 containing 2 grams/liter of rhodium as rhodium sulfate, 40 grams/liter of sulfuric acid and 1 gram/liter of chloride as magnesium chloride.

8. A bath according to claim 1 wherein the chloride is lithium chloride or magnesium chloride.

9. A bath according to claim 1 wherein the chloride is a metal chloride of a main group II metal.

10. A bath according to claim 1 wherein the chloride is chromium (III) chloride.

11. A bath according to claim 1 wherein the chloride is aluminum chloride.

12. A bath according to claim 1 wherein the amount of chloride is from 50 mg/liter to 10 grams/liter.