11GS Reddy and P Taimsalu Trans Inst Met Finish Vol 47 1969 p 187193 Rhodium Plating

Rhodium in its solid form is hard (microhardness about 800 to 1000 HV) and tough. It is nearly as tarnish resistant as platinum and palladium. However, because of its rare occurrence in PGM ores and market speculation, it is much more expensive, limiting its engineering use. Like silver, it has one of the highest reflectivities of all metals, making it ideal for use as a counterpoint to cut diamonds in jewelry and as a nontarnishing reflective coating for mirrors. Its excellent wear resistance and its superb contact resistance prompt its frequent use for rotating electrical contacts.

The electrolytes for deposition of rhodium from aqueous solutions are similar to those for ruthenium insofar as they are either based on simple rhodium salts or on special rhodium complexes (Ref 12, and 13). Because, in most cases, only layer thicknesses of 1 pm or less are specified, most commercial electrolytes have been developed to produce layers in this thickness range. The deposits have a high concentration of nonmetallic impurities (e.g., up to 1000 ppm H and/or O) (Ref 14), which causes high hardnesses and internal stresses, which easily lead to cracks. This thin and highly porous layer of rhodium, coupled with the high electrochemical nobility of the metal, limits its use as a corrosion protection layer. Therefore, an electroplated base coating must be used. Silver and silver-tin alloys (with varying concentrations of tin) have exhibited excellent field service behavior and are now applied for decorative as well as engineering purposes. Nickel is not recommended for use as a base coating. For decorative use the color (better reflectivity) is most important. It changes from electrolyte to electrolyte, many of which are commercial solutions. Deposition conditions must be carefully controlled for best results.

The complex rhodium salts of solutions cited in the literature are based on sulfate, phosphate, sulfate-phosphate, sulfatesulfite, sulfamate, chloride, nitrate, fluoroborate, or perchlorate systems. Properties of the layers are strongly influenced by the chemistry of their salts as well as by impurities present (Ref 15). Three solutions for decorative rhodium plating are given in Table 2.

Table 2 Solutions for decorative rhodium plating

Solution type

Rhodium

Phosphoric acid

(concentrate) fluid

Sulfuric acid

(concentrate)

fluid

Current density

Voltage, V

Temperature

Anodes

g/L

oz/gal

mL/L

oz/gal

mL/L

oz/gal

A/dm2

A/ft2

°C

°F

Phosphate

2(a)

0.3(a)

40-S0

5-10

2-16

20160

4-S

4050

coated®

Phosphatesulfate

2(c)

0.3(c)

40-S0

5-10

2-11

20110

3-6

4050

coated®

0.3(c)

40-S0

5-10

2-11

20110

3-6

4050

105120

Platinum or platinum-

(a) Rhodium as metal, from phosphate complex syrup.

(b) Platinum-coated products are also known as platinized titanium.

(c) Rhodium, as metal, from sulfate complex syrup

A typical, widely used production bath is based on rhodium sulfate (Ref 15). With use of proper additives, especially sulfur-containing compounds, crack-free layers may be obtained in thicknesses of about 10 pm and microhardnesses of 800 to 1000 HV (Ref 15). The deposition temperature of such baths is about 50 °C (120 °F), the current density is between 1 and 10 A/dm2 (9 to 93 A/ft2), and current efficiency is approximately 80%. Insoluble anodes are normally used.

For electronic applications where undercoatings are undesirable, special low-stress compositions have been developed. One electrolyte contains selenic acid and another contains magnesium sulfamate (Table 3). Deposit thickness obtained from these solutions range from 25 to 200 pm (1 to 8 mils), respectively. The low-stress sulfamate solution is used for barrel plating of rhodium on small electronic parts. Operating conditions for various plating thicknesses using this solution are given in Table 4.

Table 3 Solutions for electroplating low-stress rhodium deposits for engineering applications

Solution

Selenic acid process

Magnesium sulfamate process

Rhodium (sulfate complex)

10 g/L (1.3 oz/gal)

2-10 g/L (0.3-1.3 oz/gal)

Sulfuric acid (concentrated)

15-200 mL/L (2-26 fluid oz/gal)

5-50 mL/L (0.7-7 fluid oz/gal)

Selenic acid

0.1-1.0 g/L (0.01-0.1 oz/gal)

Magnesium sulfamate

10-100 g/L (1.3-13 oz/gal)

Magnesium sulfate

0-50 g/L (0-7 oz/gal)

Current density

1-2 A/dm2 (10-20 A/ft2)

0.4-2 A/dm2 (4-22 A/ft2)

Temperature

50-75 °C (120-165 °F)

20-50 °C (68-120 °F)

Table 4 Plating parameters for producing low-stress deposits from a rhodium sulfamate solution

Required thickness

Thickness of plate

Apparent current density(a)

Calculated current density(a)

Plating time

fini

mil

mil

A/dm2

A/ft2

A/dm2

A/ft2

1

0.04

0.5-1.5

0.02-0.06

0.55

5.5

1.6-2.2

16-22

35 min

2.5

0.1

1.75-3.25

0.07-0.127

0.55

5.5

1.6-2.2

16-22

llh 4

(a) Calculated current density is an estimate of the amount of current being used by those parts that are making electrical contact and are not being shielded by other parts in the rotating load in the barrel. Calculated current density is considered to be about three times the apparent current density, that is, the actual current used for the load divided by the surface of that load.

Rhodium also can be electroplated from fused-salt electrolytes. This deposition process is interesting because the requirements are that the coatings must be highly ductile for high-temperature use (e.g., coatings on molybdenum for combustion engine parts or glass-making equipment). For fused-salt electrolysis, a variety of mixtures have been tested, ranging from cyanide to chloride melts (Ref 16).

Thickness class designations for engineering applications of electroplated rhodium are given in Table 5. Table 5 Thickness classifications for rhodium plating for engineering use

Specification

Class

Minimum thickness

^m

mil

ASTM B 634-78

0.2

0.2

0.008

0.5

0.5

0.02

1

1

0.04

2

2

0.08

4

4

0.16

5

6.25

0.25

MIL-R-46085A

1

0.05

0.002

2

0.3

0.01

3

0.5

0.02

4

2.5

0.10

5

6.4

Source: Ref 17

References cited in this section

12. G.R. Smith, C.B. Kenahan, R.L. Andrews, and D. Schlain, Plating, Vol 56, 1969, p 804-808

14. Ch. J. Raub, unpublished research

15. F. Simon, Degussa-Demetron, Information Sheet, and article in GMELIN Handbook of Inorganic Chemistry, Platinum Supplement, Vol Al, 1982

17. L.J. Durney, Ed., Electroplating Engineering Handbook, 4th ed., Van Nostrand Reinhold, 1984, p 276 Palladium Plating

Palladium has been electroplated since before the turn of the 20th century. However, it stirred little interest until the 1960s and 1970s, when the price of gold peaked, prompting a search for alternatives. Palladium plating is currently used for jewelry and electrical contacts; however, the decorative applications of palladium are limited due to the dark color of the metal. Three typical palladium plating solutions are listed in Table 6.

Table 6 Palladium electroplating solutions

Constituent or condition

Amount or value

Solution A

Palladium (as tetraamino-palladous nitrate, g/L (oz/gal)

10-25 (1-3)(a)

pH

8-10

Temperature, °C (°F)

40-60 (100-140)

Current density, A/dm2 (A/ft2)

0.5-2.2 (5-20)(b)

Cathode efficiency, %

90-95

Anodes

Insoluble; palladium, platinum, or platinized titanium

Tank lining

Glass or plastic

Solution B

Palladium (as diamino-palladous nitrite), g/L (oz/gal)

10 (1)

Ammonium sulfamate, g/L (oz/gal)

110 (15)

Ammonium hydroxide

To pH

pH

7.5-8.5

Temperature

Room

Current density, A/dm2 (A/ft2)

0.5-2.2 (5-20)(b)

Cathode efficiency, %

70

Anodes

Insoluble; platinum or platinized titanium

Tank lining

Glass or plastic

Solution C

Palladium (as palladous chloride), g/L (oz/gal)

50 (7)

Ammonium chloride, g/L (oz/gal)

30 (4)

Hydrochloric acid

To pH

pH

0.1-0.5

Temperature, °C (°F)

40-50 (100-120)

Current density, A/dm2 (A/ft2)

0.5-1.1 (5-10)

Anodes

Soluble palladium

Tank lining

Rubber, plastic, or glass

Source: Ref 18

Source: Ref 18

Palladium alloys such as palladium-nickel, palladium-iron, and, to a lesser extent, palladium-cobalt are also electroplated. The plating solutions for palladium alloys are generally based on the same or similar complexes as the ones for palladium alone. The main application at present for these alloy electrodeposits is for electrical connectors (Ref 19, 20, 21, 22). A solution composition for depositing palladium-nickel is given in Table 7.

Table 7 Palladium-nickel electroplating solutions

Constituent or condition

Amount or value

Palladium as Pd(NH3)2 (NO2)2, g/L (oz/gal)

6 (0.8)(a)

Nickel sulfamate concentrate, mL/L (fluid oz/gal)

20 (2.6)(b)

Ammonium sulfamate, g/L (oz/gal)

90 (12)

Ammonium hydroxide

To pH

pH

8-9

Temperature, °C (°F)

20-40 (70-100)

Current density, A/dm2 (A/ft2)

0.5-1.0 (5-9)

Anodes

Platinized

Note: Formulation is for plating an alloy of about 75 wt% Pd. A strike of gold or silver is recommended for most base metals prior to plating.

Source: Ref 23

The properties of palladium electrodeposits are generally similar to those of gold, but it has higher receptivity and hardness. Soldering, crimping, and wire wrapping present no serious problems. The sliding and wear behavior of palladium are similar to those of hard gold. Palladium coatings may be slightly less porous than gold coatings, and they resist tarnish and corrosion. On the other hand, the chemical properties of palladium are quite different from those of gold, which may explain why an effective agent for stripping palladium and palladium alloy electrodeposits has not yet been developed.

In service, palladium and palladium alloys tend to exhibit what is called a brown powder effect, in which a "brown polymer" catalytically forms on the contact surface upon exposure to organic compounds in the environment. This effect can be minimized by application of flash plating a layer of fine gold on top of the palladium surface. The biggest challenge when electrodepositing palladium is avoiding hydrogen embrittlement. Palladium in electrodeposition may dissolve fairly large amounts of hydrogen, and this expands the palladium lattice, especially if the so-called P-Pd/H phase is formed. However, this hydrogen diffuses out of the palladium during storage at room temperature, and the lattice contracts again. This expansion/contraction generates stresses in the deposit that cause cracks and pores. Furthermore, palladium promotes diffusion of atomic hydrogen, which may cause secondary reactions (e.g., hydrogen embrittlement of underlying steel bases or blister) if the base material does not take up the diffused hydrogen.

Electrolytes have been developed that effectively solve the problem of hydrogen embrittlement. The most economical are based on palladium chloride. In these solutions, the palladium ion is complexed by ammonia or amines. Other systems using other complexes have also been developed (Ref 19, 20, 21, 22, 24). Currently, no electrolyte for the deposition of palladium-silver or palladium-copper alloys is available. The influence of organic and inorganic impurities on palladiumnickel deposits has been studied extensively (Ref 19).

Thickness class designations for engineering applications of electroplated palladium are given in Table 8. Table 8 Thickness classifications for palladium plating for engineering use

Specification

Class

Minimum thickness

mil

ASTM B 679-80

5.0

5.0

0.20

2.5

2.5

0.10

1.2

1.2

0.05

0.6

0.6

0.02

0.3

0.3

0.01

F

0.025

0.0010

MIL-P-45209

1.3(a)

Source: Ref 17

(a) Unless otherwise specified.

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