Platinum electroforming and platinum electroplating

Abstract: The invention relates to platinum electroforming and platinum electroplating capable of preparing a deposited platinum material having high hardness and increased thickness and size. The platinum electroforming or electroplating bath comprises at least one compound selected from the group consisting of chloroplatinic acid, chloroplatinates of alkali metals, hydrogen hexahydroxoplatinate, and hexahydroxoplatinates of alkali metals, 2-100 g/l as platinum and a hydroxylated alkali metal, 20-100 g/l.

Description: Field of the Invention The present invention relates to platinum electroforming. Background of the Invention Platinum has been widely used in ornaments and accessories because of its clear and subdued shine, although it has a less striking colour than gold. Platinum is also highly resistant to corrosion and is a valuable catalyst. However, its inherent tenacity means that it has poor workability; this is a problem for the production of elaborate accessories such as earrings or brooches. Further, its high specific gravity puts limits on the size of personal accessories made from platinum. Electroforming comprises forming, by electrodeposition, a thick layer on the surface of a die to which a release coat has been applied, and to obtain an electroformed product having opposite convex and concave surface to those of the die. A release coat may be applied to the surface of the resultant electroformed product, followed by electrodeposition to obtain a product having the same convex and concave surfaces as those of the die. Such a process might overcome the given problems, associated with the use of platinum, since it allows the preparation of hollow products or films of any thickness. However, no successful platinum electroforming process has yet been found, because the deposited layer must be about 10-50 times as thick as in conventional electroplating (see, for example, JP-A-107,794/1990). Deposited platinum has a tendency to occlude hydrogen, which increases the internal stress of the deposited layer, generating cracks or micro-crevices. In addition, platinum metal in general (not prepared by electroforming or electroplating) has a crystal structure of face-centered cubic lattice structure. It is soft (approximately 40 Hv) and ductile. This is a disadvantage for ornaments, e.g. rings or necklaces, since they are easily scratched and deformed. Platinum is therefore conventionally alloyed with other metals, to increase its hardness. However, intermetallic compounds are then generated, resulting in brittleness. An additional disadvantage is the generation of an oxide film during heating or brazing the platinum alloy, thereby reducing its external quality. Chemical Abstracts 53 No. 17, 1959, Abstract No. 15814i, discloses a bath that contains H 2 PtCl 6 .6H 2 O, NaOH and a carboxylate. FR-A-1273663 discloses a Pt electroplating bath containing H 2 Pt(OH) 6 (20 g), KOH (15 g) and H 2 O (1 litre). Summary of the Invention A platinum electroforming method according to the present invention comprises using a bath composition having 2 to 100 g/l, calculated as Pt, of at least one of hydrogen hexahydroxoplatinate and alkali metal hexahydroxoplatinates; and 20 to 100 g/l of an alkali metal hydroxide. Description of the Invention Hydrogen hexahydroxoplatinate [H 2 Pt(OH) 6 ], sodium hexahydroxoplatinate [Na 2 Pt(OH) 6 .2H 2 O] or potassium hexahydroxoplatinate [K 2 Pt(OH) 6 ] may be used in the invention. The hydroxide is preferably potassium hydroxide or sodium hydroxide; it is incorporated in order to dissolve platinum. The bath may also include a soluble carboxylate, such as potassium or sodium salts of acetic acid, oxalic acid, citric acid, malic acid, propionic acid, lactic acid, malonic acid or tartaric acid; a phosphate such as potassium phosphate, sodium phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, potassium hydrogen phosphate or sodium hydrogen phosphate; and a sulfate such as potassium sulfate or sodium sulfate. Such a soluble carboxylate acts as a stabilizer in the electroforming bath. It is preferably incorporated in an amount of 2-200 g/l. In addition to the above components, the electroforming bath of platinum may include additives such as various brightening agents and electroconductive salts. Additionally, a platinum alloy can be deposited by incorporating other metal salts in the electroforming or plating bath. Preferable examples of metals adapted to make an alloy with platinum are gold, silver, palladium, iridium, ruthenium, cobalt, nickel, copper, and the like. The number of other metals being incorporated is not restricted to one. Two kinds of metals can be incorporated to make an alloy with platinum, for example, an alloy of platinum-palladium-copper. A preferable operating temperature for the electroforming bath is not lower than 65°C, with a temperature of not lower than 80°C being particularly preferable. Generally, a current density is preferably 1-3 ASD, when platinum is contained in the amount of 20 g/l, though it depends on plating conditions. A platinum metal produced by means of electrodeposition from the platinum electrolytic bath has a reduced crystal size. The platinum metal has also a hardness of at least 100-350 Hv. Such hardness is much higher than that of platinum metal, i.e. about 40 Hv, prepared by general melting procedures. There is the following relationship between the purity and hardness of the platinum material prepared by the method of the present invention: Purity (wt%) Hardness 99.9 Above 100 H v 95.0 - 99.9 Above 200 H v 90.0 - 95.0 Above 250 H v 85.0 - 90.0 Above 300 H v Microscopic and macroscopic stresses are involved in the platinum metal obtained by means of electrodeposition. The microscopic stress which is a non-uniformed stress corresponding to an expanded width of X-ray diffraction lines causes the increased hardness of the deposited metal. While the macroscopic stress is a residual tensile or compressive stress involved in the deposited platinum metal and makes a cause of strain or cracks. The macroscopic stress of platinum is very large. The macroscopic stress, however, can be restrained by adopting an alkaline platinum electrolytic bath or by annealing (heat treatment) for each additional thickness of about 5-10 µm of a deposited layer. The annealing is performed under heating, preferably, at 400-900°C for 30-120 min. By the annealing, the hardness of the platinum metal may be reduced. Such degree of the reduced hardness is nevertheless higher than that of conventional platinum metals. Accordingly, the deposited layer can have sufficiently large thickness and size that platinum products having high hardness can be manufactured by electroforming. As a platinum electrolytic bath when adopting a means of platinum electroforming to improve the hardness of platinum, an alkaline bath is very advantageous from the aspects of deposition efficiency and macroscopic stress. Additionally, annealing is not necessary when using a platinum electrolytic bath in accordance with the invention. Other features of the invention will become apparent in the course of the following description of the exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof. EXAMPLES Example 1 A preferable example of the electroforming of the present invention is herein illustrated. (Composition of a platinum electroforming bath) Hydrogen hexahydroxoplatinate [H 2 Pt(OH) 6 ] 30 g/l Potassium acetate [KCH 3 CO 2 ] 40 g/l Potassium hydroxide [KOH] 60 g/l pH: 13.5 A test was performed using the above electroforming bath shown in Table 1 under the different conditions with respect to the time and the current density to deposit a deposition layer of platinum on the surface of a test piece of brass. The results are shown in Table 2. The deposition layers obtained all exhibited an excellently glossy appearance. Observation under microscope showed no existence of cracks. Further, the deposition layers had an increased thickness in proportion to the electroforming time. These results demonstrate that the bath can be used as an electroforming bath. Accordingly, light and large-sized earrings or brooches with a hollow construction can be produced by the method using the electroforming bath of the present invention. Also, elaborate works can be achieved without using high technical skill. No. Electro-Forming min Current Density ASD Deposition Efficiency mg/A·min Thickness of Deposition µm 1 4 3 29.3 1.64 2 4 3 29.6 1.66 3 60 3 29.6 24.8 4 153 2 29.2 41.7 5 240 2 29.3 65.6 6 265 2 29.5 72.9 7 180 3 29.4 74.0 8 480 2.3 29.5 150 Example 2 In this example, an experiment of producing an insoluble platinum electrode was performed by plating platinum on titanium. A plating bath having the same composition as that of the electroforming bath shown in Table 1 was used in this example. The plating was carried out using this plating bath under the following operating conditions. Plating method: dip plating Bath temperature: 80°C Current density: 3 ASD Plating time: 10 min Inspection of the insoluble platinum electrode obtained revealed that an adhesive platinum layer having a glossy surface with a thickness of 4 µm was formed. The surface of the platinum layer was observed under a microscope to show that any pin hole or crack did not occur. It was confirmed that a uniform current distribution could be obtained when this insoluble platinum electrode was used as an electrode in practice and also that the platinum layer on the surface of the electrode was never peeled off from titanium which was a metal underneath over a prolonged period of time. The platinum plating according to the present invention, however, is not restricted to use in a field of the above insoluble platinum electrode, but can be applied to, for example, the formation of a platinum layer on a heat resisting section of a jet turbine. Example 3 Electroforming was carried out using the electrolytic baths No. 1-5 having the compositions and conditions as tabulated below to deposit platinum on a test piece of brass, while deposited layers were annealed during the above procedures when their microscopic stresses were high. The deposited layers (platinum material) obtained had high hardness, the surface thereof being smooth. Also, the flexibility of the deposited layer stood comparison with that of ordinary platinum. Electrolytic bath No. 1 Composition Pt [as H 2 Pt(OH) 6 ] 13 g/l CH 3 COONa 0.5 mol/l EDTA-4H 0.05 mol/l NaOH 40 g/l NiSO 4 ·6H 2 O 0.04 mol/l Condition pH 13 Temperature 65°C Current density 1.0 A/dm 2 Deposition efficiency 31.0 mg/A·min Electrolytic time 360 min Deposited layer Thickness 48 µm Purity 96.2 wt% Hardness 440 H v Electrolytic bath No. 2 Composition Pt [as H 2 Pt(OH) 6 ] 13 g/l CH 3 COONa 0.5 mol/l EDTA-4H 0.05 mol/l NaOH 40 g/l NiSO 4 ·6H 2 O 0.04 mol/l Condition pH 13 Temperature 65°C Current density 1.0 A/dm 2 Deposition efficiency 31.0 mg/A·min Electrolytic time 180 min Deposited layer Thickness 14 µm Purity 97.0 wt% Hardness 450 H v Electrolytic bath No. 3 Composition Pt [as H 2 Pt(OH) 6 ] 20 g/l KOH 50 g/l K 2 C 2 O 4 ·H 2 O 30 g/l Condition pH 13.5 Temperature 90°C Current density 3 A/dm 2 Deposition efficiency 30 mg/A·min Electrolytic time 240 min Deposited layer Thickness 100 µm Purity 99.9 wt% Hardness 350 H v Electrolytic bath No. 4 Composition Pt [as H 2 Pt(OH) 6 ] 20 g/l KOH 40 g/l Sn [as K 2 SnO 3 ·3H 2 O] 30 g/l Potassium tartrate·1/2H 2 O 100 g/l Condition pH 13.3 Temperature 90°C Current density 2 A/dm 2 Deposition efficiency 20 mg/A·min Electrolytic time 300 min Deposited layer Thickness 60 µm Purity 85 wt% Hardness 650 H v Electrolytic bath No. 5 Composition Pt [as H 2 Pt(OH) 6 ] 20 g/l KOH 100 g/l Zn [as ZnO] 0.8 g/l Condition pH 14 Temperature 90°C Current density 2 A/dm 2 Deposition efficiency 30 mg/A·min Electrolytic time 180 min Deposited layer Thickness 50 µm Purity 95 wt% Hardness 450 H v

Claims: A method for the electroforming of platinum from a bath, wherein the bath composition comprises: 2 to 100 g/l, calculated as Pt, of at least one compound selected from hydrogen hexahydroxoplatinate and alkali metal hexahydroxoplatinates; and 20 to 100 g/l of an alkali metal hydroxide. A method according to claim 1, wherein the composition further comprises a compound selected from soluble carboxylates, phosphates and sulfates. A method according to claim 2, wherein the composition comprises about 30 g/l of hydrogen hexahydroxoplatinate, about 40 g/l of potassium acetate, and about 60 g/l of potassium hydroxide. A method according to any preceding claim, wherein the composition further comprises an alloying metal salt. A method according to any preceding claim, which is conducted at a temperature of at least 65°C. A method according to claim 5, for the preparation of an electroformed product of platinum or platinum alloy having a thickness of at least 10 µm. A method according to any preceding claim, for preparing a platinum material having (i) a purity above 99.9 wt% and a hardness above 100 H v ; (ii) a purity of at least 95.0 wt% and less than 99.9 wt% and a hardness above 200 H v ; (iii) a purity of at least 90.0 wt% and less than 95.0 wt% and a hardness above 250 H v ; or (iv) a purity of at least 85.0 wt% and less than 90 wt% and a hardness above 300 H v .

Inventors: KATSUTUGU KITADA, SOUMEI YARITA, KATSUTUGU, KITADA, SOUMEI, YARITA3-90, SOUMEI, YARITA, DIA-PALACE FUCHINOBE III, 10-13-90
Applicants: ELECTROPLATING ENG, ELECTROPLATING ENGINEERS OF JAPAN LIMITED
Application year: 1991
Main classification: C25D3
IPC codes: C25D3, A44C27