32RN Rhoda Plating Vol 49 1962 p 6971 Platinum Plating

The electrodeposition of platinum from aqueous electrolytes is of limited engineering value. The metal is very expensive, and the currently available plating solutions are not capable of consistently producing ductile and pore-free deposits at thicknesses above a few microns. Today, most of the deposits produced are less than 1 pm thick and are used primarily for decorative applications.

The main challenge when electroplating platinum from aqueous electrolytes is to obtain a clean, ductile platinum coating with a minimum of nonmetallic impurities, which act as hardeners and embrittle the platinum. This is rather difficult because platinum compounds tend to hydrolyze even at rather low pH levels. Therefore, close control of plating parameters is very important.

The three most common electrolytes used today are platinum chloride, diamino-dinitroplatinum (platinum "P" salt), and alkali hydroxy platinate. The current efficiency of the highly acidic baths is close to 90%, but the electrolytes are difficult to handle. Two platinum plating solutions are listed in Table 9.

Table 9 Platinum electroplating solutions

Constituent or condition

Amount or value

Solution A

Platinum (as sulfatodinitrito-platinous acid), g/L (oz/gal)

5 (0.7)

Sulfuric acid

To pH

pH

1.5-2.0

Temperature, °C (°F)

Room to 40 (100)

Current density, A/dm2 (A/ft2)

5-20 (5-20)

Anode

Platinum or platinized titanium

Cathode efficiency

10-20%

Solution B

Platinum (as diaminodinitrito salt), g/L (oz/gal)

10 (1.3)

Ammonium nitrate or phosphate, g/L (oz/gal)

100 (13.4)

Sodium nitrite, g/L (oz/gal)

10 (1.3)

Ammonium hydroxide (28% solution), mL/L (fluid oz/gal)

50 (6.4)

Temperature, °C ( °F)

90-100 (190-210)

Current density, A/dm2 (A/ft2)

3-10 (30-100)(a)

Anode

Platinum (insoluble)

Tank lining

Glass or plastic

Cathode efficiency

Source: Ref 18

A commercial process gaining more and more importance for engineering applications in the chemical, electronics, and glass industries is the electrodeposition of platinum from salt melts, because the process forms highly dense and ductile platinum layers. The platinum compound can be formed by electrolytic dissolution with alternating current in a NaCN/KCN fused-salt mixture, melting at 500 °C (930 °F). For deposition, a cyanide/cyanate mixture operating at about 450 °C (840 °F) is recommended.

For decorative platinum deposits, the use of a flash-plated base coat is recommended. Suitable layers include palladium-iron, silver, and copper-tin systems. Detailed information on platinum electroplating is available in Ref 33, 34, 35, 36, 37, and 38.

0 0

Post a comment