Stabilization of the Deposition Rate of Platinum Electrolytes

Abstract

The present invention is directed toward a method for stabilizing the electrolytic deposition of platinum from an electrolytic bath. In particular, the present invention relates to a corresponding method in which the platinum electrolytic bath has platinum in the form of a sulfamate complex.

Claims

1. A method for stabilizing the deposition of platinum from an acidic, aqueous, cyanide-free electrolytic bath containing a platinum sulfamate complex, which comprises destroying amidosulfonic acid released from the platinum sulfamate complex in the electrolysis bath during electrolysis.

2. The method according to claim 1, further comprising adding a quantity of a soluble nitrite salt corresponding to the amidosulfonic acid to the bath.

3. The method according to claim 1, further comprising detecting the free amidosulfonic acid in the bath during the electrolysis.

4. The method according to claim 1, wherein the destruction takes place during the electrolysis.

5. The method according to claim 1, wherein the amidosulfonic acid is hydrolyzed from time to time by heating.

6. The method according to claim 5, wherein the hydrolysis takes place at up to 100 C.

7. The method according to claim 1, wherein the pH value of the bath is <7.

8. The method according to claim 1, wherein in the event of the electrolysis being interrupted, the electrolytic bath is reused after the amidosulfonic acid has been destroyed.

Description

FIGURES

[0021] FIG. 1: Deposition rate of Pt from a Pt sulfamate electrolyte, without the addition of nitrite

[0022] FIG. 2: Deposition rate of Pt from a Pt sulfamate electrolyte, with the addition of nitrite

EXAMPLE

[0023] The deposition rate can be determined as follows:

[0024] 1 liter of an electrolyte (as in EP737760A1) is heated to the temperature mentioned 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.

[0025] Platinum-plated titanium or titanium coated with mixed metal oxide is used as an anode material. A respective anode is attached parallel to the cathode on both sides of the cathode. 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 2 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.

[0026] 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.

[0027] The cathode is positioned in the electrolyte between the anodes and moved parallel thereto by at least 3 m/min. The distance between anode and cathode should not thereby change.

[0028] In the electrolyte, the cathode is coated by applying a direct electric current between anode and cathode. The amperage is thereby selected such that the current density predetermined for the test is achieved over the surface area, e.g. 20 mA/cm.sup.2. The duration of the current flow is selected such that the layer thickness predetermined for the test (e.g. 1 m) is achieved on average over the surface area. 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.

[0029] 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 for determining the average layer thickness as well as the efficiency or rate of deposition.

Results:

[0030] At an operating temperature of 55 C., platinum can initially be deposited with 0.25 m/min at 2 A/dm.sup.2 in a freshly prepared platinum electrolyte (as in EP737760A1) with 10 g/l of Pt as a Pt sulfamate complex and 20 g/l of sulfuric acid. After a deposition of 10 g/l of Pt, only a deposition rate of about 0.12 m/min is now achieved. This corresponds to a reduction to 45% of the original rate.

[0031] Given a corresponding deposition with 10 g/l of Pt at 60 C., in the initial state layers 2 m thick are deposited as shiny and homogeneous layers. Without added nitrite, an increasing deterioration in appearance occurs over the course of throughput. After a deposition of 10 g/l of Pt, the deposited coatings are milky, brown, and blotchy. If electrolytic platinum deposition takes place with the addition of nitrite such that the deposition rate is kept approximately stable, the layers will be practically invariantly glossy and homogeneous.

[0032] The porosity of the deposited layers likewise deteriorates during electrolytic platinum deposition without the addition of nitrite. This is manifested by significantly poorer corrosion results, detectable given platinum layers 1 m thick under anodic stress in a 1% sodium chloride solution at 40 C. and 5 V voltage and a Pt counter-electrode. In the initial state, given a platinum layer thickness of 1 m on 2 m of glossy nickel-plated copper substrate, it can be observed via optical microscope under 20 magnification that more than 120 minutes will pass without the layers being torn open by corrosion products. After a throughput of 10 g/l of platinum without the addition of nitrite, this value falls to below 5-10 minutes, caused by the increased porosity of the deposited platinum layer. If, given the same throughput, the deposition rate is kept constant via regular addition of nitrite, the deterioration of the corrosion resistance then is not observed.