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 ^m 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 ^m (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

^m

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

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