May 16, 1961 R. DUVA ETAL 2,984,604

PLATINUM PLATING COMPOSITION AND PROCESS Filed Aug. e, 1958 lid 0 INVENTORs IFOBERT .P :1 M4 [aw/v (mm-u 1mm? United States Patent PLATINUM PLATING COMPOSITION AND PROCESS Robert Duva, Paramus, and Edwin C. Rinker, Morristown, N.J., assignors to Sel-Rex Corporation, Nutley, N.J., a corporation of New Jersey Filed Aug. 6, 1958, Ser. No. 753,525

5 Claims. (Cl. 204-47) This invention relates to a platinum electrolyte from which it is possible to plate thick layers of stress free platinum and to the process for making the same.

Up to the present time the only commercially useful platinum plating baths have depended on chloroplatinic acid (H PtCl -6H O) or platinum diamino dinitrite Pt(NH (NO for the platinum ions. The chloroplatinic acid baths, though suitable for flash plating, are unwieldy for heavier deposits, which deposits become spongy as the thickness is increased. The useful life of this bath is very short because of the rapid accumulation of chloride ions as the platinum metal is replenished. Platinum diamino dinitrite (also known as P-salt) which is disclosed in US. Patent No. 1,779,436 is dissolved in an electrolyte containing ammonium hydroxide. This bath should be maintained at a temperature of 200 F. which causes a rapid loss of ammonia and a subsequent loss of electrodeposition efliciency. In order to obtain heavier than flash deposits with these baths it is necessary to remove the article from the bath from time to time, scratch brush it and only then continue the plating. Thcse baths operate erratically, apparently due to inconsistent cathode efficiency and (in the case with the P-salt bath) occasionally cease operating altogether.

Among the objects of this invention is to provide a platinum containing electrolyte free of the defects of the prior art baths and suitable for plating stress-free platinum of any thickness desired.

Among other objects of the invention is to provide a platinum plating bath suitable for making electroformed platinum articles plated in one continuous operation.

Among other objects of the invention is to provide a method for making the electrolyte of the invention.

One phase of this invention is based on the discovery that when the P-salt, Pt(NH (NO) is heated with an aqueous solution of sulfamic acid it dissolves therein.

Another phase of the invention is based on the discovery that the solution obtained by heating the P-salt with the sulfamic acid can be diluted to provide a stable electrolytic solution for electroplating stress free platinum or for electroforming platinum articles.

Thus, the objects of the invention are attained by heating the platinum diamino dinitrite in a sulfamic acid aqueous solution in the proportions of about 10-40 or more g. of P-salt per -200 cc. of sulfamic acid solution containing about 2 to about 400 g./liter of sulfamic acid, until a clear solution is obtained. Thereafter, the solution is diluted to provide a solution containing at least about 6 g. per liter of platinum (which corresponds to about 10 g. per liter of the P-salt). Articles are then electroplated or electroformed from this solution by conventional processes.

Plating of thick deposits of platinum in this low stress bath can be done in one operation, i.e., there is no need for intermittently scratch brushing or burnishing to produce a smooth deposit. Platinum plating with this low stress bath can be performed on a specification basis, i.e.,

constant electrodeposition eificiency is obtained at constant temperature and platinum metal content. For example, uniform deposits will be obtained on a series of articles by electroplating each in the bath for one minute.

The following are very satisfactory operating conditions for such a bath.

Platinum metal content- 6-20 grams/ liter or more. Sulfamic acid 20-100 grams per liter.

Bath container Pyrex glass.

Anodes Platinum.

Anode to cathode ratio..- 1 to 1 or higher.

Operating temperature-.. 65 to 100 C.

Current density 20-100 amperes per sq. foot. Agitation rate Moderate. Electrodeposition rate 5-30 milligrams per ampere minute.

The sulfamic acid above is calculated as pure acid although it may be added as a solution, if desired.

Platinum metal replenishment is accomplished by the addition of the electrolyte as such to maintain the platinum metal at the desired concentration. Solution level is maintained by the additiot f both this replenisher and water.

Below 6 -g./l. of platinum metal the deposits are .spongy. There is, however, no actual upper limit for the platinum metal content except possibly the limit of solubility of the P-salt. No advantage is obtained by increasing the amount of Pt above 20 g./l., however, since even at 15 g./l. (at 99 C.) an efficiency of about 30 milligrams per ampere minute is attained which is practically the theoretical maximum of about 30.4 mg/amp. On the other hand, increasing the Pt content also increases the losses due to drag out.

The drawing is a reproduction of a photomicrograph of a specimen of platinum plated graphite made according to the invention.

The following example illustrates how the invention 15 put into practice.

Example 50 grams of a sulfamic acid (NH SO H) in 200 cc. of water is heated to about. C. whereupon 20 g. of platinum diamino dinitrite are added with stirring. The stirring and heating is continued until a clear solution is obtained. It has not been possible to determine the composition of the resulting salt in the solution at the present time but it is probably a fairly complex compound. The resultant solution is diluted with water to make 1 liter of solution and is then ready for electroplating.

The platinum is plated at a voltage of about 1 volt to 3 volts and at a current density of about 20 amps. per square foot.

The proportion of sulfamic acid of P-salt can vary from about 1:10 to about 20:1. The electroplating bath should have a concentration of at least about 6 g. of Pt per liter for otherwise spongy deposits result.

The bath of the invention is stable as long as it is operated.

In electroforming conventional practice is followed. The form may be coated with resist material where no plating is desired, the plating on the exposed surface is continued without interruption until the desired thickness is built-up and thereafter the original form is separated from the plated deposit. With this bath, the removal of the basis metal from the deposit results in the production of a continuous film of platinum metal showing the lack of internal stress of the plated metal. In other words the platinum metal does not disintegrate into flakes or small pieces. With the process of the invention as described in Example 1, an adherent layer of platinum has been plated on graphite. The throwing power of the bath is excellent as can be seen from the fact that the platinum penetrated into the pores of the graphite as shown in the drawing. A protective layer of nickel was deposited thereon before cutting the section which was photographed.

The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific feature or details thereof.

We claim:

1. A process for electroplating. relatively thick layers of stress-free platinum comprisingdissolving platinum diamino dinitrite inan aqueous solution of sulfamicacid containing a sufiicient amountof sulfamic acid to dissolve said platinum diamino dinitrite, adding waterto the resultant solution to provide a solution containingat least 6 g./ liter of platinum metal andelectrolyzing said solution to plate out platinum therefrom.

.2. The process as claimed in claim 1 comprising forming the aqueous solution by heating the acid solution to dissolve the platinum salt.

3. As an electrolyte for the plating of platinum the composition coinprising water and the solution obtained by heating about 10-40 g. of platinum diamino dinitrite with 15-200 cc. of aqueous sulfamic acid solution containing about 2 to about 400 grams per liter of sulfamic acid.

4. A process for electroplating relatively thick layers of stress free platinum comprising providing a solution of about 10-40 g./liter of platinum diamino dinitrit'e in 15200 cc. of aqueous sulfamic acid solution containing about 2 to about 400 g./liter of sulfamic acid, adding water to said solution to provide a solution containing 6 g./ liter of platinum and electrolyzing said solution to plate out platinum therefrom. I V

5. The electrolyte as claimed in claim 3, wherein said electrolyte comprises at least about 6 g./liter of platinum metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,779,436 Keitel Oct. '28, 1930 1,921,941 Powell et a1. u .7...... Aug. 8, 1933 2,027,358 Powell et al Jan. 7, 1936