Silver-Bismuth Electrolyte for Separating Hard Silver Layers

Abstract

The present invention relates to an electrolyte for deposition of hard silver layers, wherein the element bismuth is alloyed to the silver. The invention also relates to a method for deposition of a corresponding silver-bismuth alloy from an electrolyte according to the invention and to a correspondingly deposited layer.

Claims

1. An aqueous electrolyte for electrolytic deposition of silver-bismuth alloys onto conductive substrates, the electrolyte having the following features: 0.5-200 g/l based on the metal of a silver compound or a soluble an-ode comprising silver; 0.1-50 g/l based on the metal of a soluble bismuth compound; 5-200 g/l of a soluble cyanide, in particular potassium cyanide; 0.05-2 mol/l of a soluble di-, tri- or tetracarboxylic acid; >0-5 g/l of a soluble brightener A which is a reaction product of ke-tones or dithiocarbamates with carbon disulfide; >0-5 g/l of a further soluble brightener B selected from the group of condensation products of arylsulfonic acids with formaldehyde; 1-1000 mg/l of a soluble wetting agent; and a pH of 10-14.

2. The electrolyte according to claim 1, wherein the silver compound is selected from silver methanesulfonate, silver carbonate, silver phosphate, silver pyrophosphate, silver nitrate, silver oxide, silver lactate, silver fluoride, silver bromide, silver chloride, silver iodide, silver thiocyanate, silver thiosulfate, silver hydantoins, silver sulfate, silver cyanide and alkali silver cyanide.

3. The electrolyte according to claim 1, wherein the bismuth compound is selected from bismuth(III) oxide, bismuth(III) hydroxide, bismuth(III) fluoride, bismuth(III) chloride, bismuth(III) bromide, bismuth(III) iodide, bismuth(III) methanesulfonate, bismuth(III) nitrate, bis-muth(III) tartrate, bismuth(III) citrate, in particular ammonium bismuth citrate.

4. A method for electrolytic deposition of silver and silver alloy coatings from an electrolyte according to claim 1, wherein an electrically conductive substrate is immersed in the electrolyte and a current flow is established between an anode in contact with the electrolyte and the substrate as cathode.

5. The method according to claim 4, wherein the temperature of the electrolyte is 20 C. to 90 C.

6. The method according to claim 4, wherein the current density during electrolysis is 0.2 to 150 A/dm2.

7. The method according to claim 4, wherein the pH value during electrolysis is continuously set to a range between 10 and 14.

8. The method according to claim 4, wherein a silver-bismuth coating is deposited which has a thickness of 0.1-50 m.

9. The method according to claim 4, wherein a soluble silver anode and/or an insoluble anode is used as the anode.

10. A silver-bismuth alloy layer having a thickness of 0.1-50 m produced according to claim 4 and having a hardness of >200 HV after annealing of the coating at 150 C. for 1000 h.

11. The alloy layer according to claim 10, wherein said alloy layer is deposited on a nickel or a nickel alloy layer or a copper or copper alloy layer.

Description

EXAMPLES

[0052] 1 liter of the electrolyte specified in the respective exemplary embodiment are heated to the temperature specified in the exemplary embodiment by means of a magnetic stirrer, while being stirred with a cylindrical magnetic stirring rod 60 mm long at at least 200 rpm. This stirring and temperature is also maintained during the coating.

[0053] After the desired temperature has been reached, the pH value of the electrolyte is set using a KOH solution (c=0.5 g/ml) and a suitable acid such as sulfuric acid (c=25%) to the value specified in the exemplary embodiment.

[0054] Silver plates or mixed metal oxide-coated titanium are used as anodes.

[0055] A mechanically polished brass plate with a surface area of at least 0.2 dm.sup.2 serves as cathode. This can be coated beforehand with at least 5 m of nickel from an electrolyte which produces high-gloss layers. A gold layer approximately 0.1 m thick may also be deposited on the nickel layer.

[0056] Prior to introduction into the electrolyte, these cathodes are cleaned with the aid of electrolytic degreasing (5-7 V) and an acid dip containing sulfuric acid (c=5% sulfuric acid). Between each cleaning step and before introduction into the electrolyte, the cathode is rinsed with deionized water.

[0057] The cathode is positioned in the electrolyte between the anodes and moved parallel thereto at at least 5 cm/second. The distance between anode and cathode should not change.

[0058] In the electrolyte, the cathode is coated by applying a direct electric current between anode and cathode. The current intensity is selected such that at least 0.5 A/dm.sup.2 is achieved on the surface area. Higher current densities can be selected if the electrolyte specified in the application example is intended to produce layers that can be used for technical and decorative purposes.

[0059] The duration of the current flow is selected such that a layer thickness of at least 0.5 to 1 m is achieved on average over the surface area. Higher layer thicknesses can be produced if the electrolyte specified in the application example is intended to produce layers of a quality that can be used for technical and decorative purposes.

[0060] After coating, the cathode is removed from the electrolyte and rinsed with deionized water. The drying of the cathodes can take place via compressed air, hot air, or centrifugation.

[0061] The surface area of the cathode, the level and duration of the applied current, and the weight of the cathode before and after coating are documented and used to determine the average layer thickness as well as the efficiency of deposition.

TABLE-US-00001 TABLE 1 Exemplary embodiments Example no. 1 2 3 4 5 6 Ag [g/l] 22 22 40 30 50 22 Bi [g/l] 0.5 2 2.5 2.0 3 0.5 Di-potassium 0 0 60 30 0 5 tartrate [g/l] Tripotassium cit- 100 60 0 0 0 150 rate [g/l] Di-potassium 5 0 0 10 0 0 oxalate [g/l] Di-potassium 0 10 0 0 50 0 malonate [g/l] Brightener A.sup.a) 7.5 0 50 0 40 0 [mg/l] Brightener A.sup.b) 0 25 0 40 0 0 [mg/l] Brightener A.sup.c) 250 0 0 0 0 10 [mg/l] Brightener B.sup.d) 1000 0 500 250 400 500 [mg/l] 2-ethylhexyl sulfate 0 0 100 0 150 0 [mg/l] Lauryl methyl- 0 0 0 50 0 0 glycinate, Na salt [mg/l] Fatty alcohol poly- 0 15 0 0 0 0 glycol ether [mg/l] Cocosamidopropyl- 5 5 3 0 0 2 dimethylammo- nium2-hydroxypro- pane sulfobetaine [m/l] pH 13 12.8 13 13.2 13.5 12.5 Temperature [ C.] 30 35 40 40 50 30 Anodes Ag Ag Ag Ag MMO Ag Current density 3 2 5 2 10 2 [A/dm2] Layer thickness 1.8 2 3 2.5 1.5 2 [m] Bi [wt. %] 2.48 0.84 1.13 1.28 2.0 1.53 Gloss Yes/Haze Yes/Haze Yes Yes/Haze Yes/Haze Yes Color: L* 96.05 98.2 98.11 not 97.76 97.7 known a* 0.10 0.11 0.09 not 0.11 0.1 known b* 4.3 2.09 2.25 not 2.35 2.49 known Hardness [HV] 260 205 220 not 250 240 known .sup.a)reaction product of 2-butanone with carbon disulfide in accordance with DE2731595 .sup.b)reaction product of 2,5-hexanedione with carbon disulfide in accordance with DE2731595 .sup.c)reaction product of potassium phenyldithiocarbamate with carbon disulfide in accordance with DE959775 .sup.d)naphthalenesulfonic acid-formaldehyde condensation product in accordance with DE2731595

[0062] Coatings obtained from an electrolyte according to Example No. 3 (Table 1) were aged at 150 C. for 100 and 500 hours and the hardness values were then determined. The results are shown in FIG. 1.